United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-93/017
February 1994
&EPA
Guide to Cleaner
Technologies
Cleaning and Degreasing
Process Changes
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EPA/625/R-93/017
February 1994
GUIDE TO
CLEANER TECHNOLOGIES
CLEANING AND DECREASING
PROCESS CHANGES
Office of Research and Development
United States Environmental Protection Agency
Cincinnati, Ohio 45268
Printed on Recycled Paper
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NOTICE
This material has been funded wholly or in part by the United States Environmen-
tal Protection Agency under Contract No. 68-CO-0003, Work Assignment 3-49, to
Battelle. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
This Guide to Cleaner Technologies: Cleaning and Degreasing Process
Changes has been subjected to U.S. Environmental Protection Agency peer and
administrative review and approved for publication. Approval does not signify |
that the contents necessarily reflect the views and policies of the U.S. Environ-
mental Protection Agency.
This document identifies new approaches for pollution prevention in cleaning and
degreasing processes. Site-specific selection of a technology will vary depend-
ing on shop and manufacturing process applications. It is the responsibility of
individual users to make the appropriate application of these technologies.
Compliance with environmental and occupational safety and health laws is the
responsibility of each individual business and is not the focus of this document.
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FOREWORD
Today's rapidly developing and changing technologies and industrial products
and practices frequently carry with them the increased generation of materials
that, if improperly dealt with, can threaten both public health and the environ-
ment. The U.S. Environmental Protection Agency (EPA) is charged by Congress
with protecting the Nation's land, air, and water resources. Under a mandate of
national environmental laws, the agency strives to formulate and implement
actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. These laws direct the U.S. EPA to
perform research to define our environmental problems, measure the impacts,
and search for solutions.
Reducing the utilization or generation of hazardous materials at the source or
recycling the wastes on site is one of EPA's primary pollution prevention goals.
Economic benefits to industry may also be realized by reducing disposal costs
and lowering the liabilities associated with hazardous waste disposal!
This Guide to Cleaner Technologies: Cleaning and Degreasing Process
Changes summarizes information collected from U.S. Environmental Protection
Agency programs, peer-reviewed journals, industry experts, vendor data, and
other sources. The cleaner technologies are categorized as commercially
available or emerging. Emerging technologies are technologies that are in
various stages of development, and are not immediately available for purchase
and installation. For each technology, the Guide addresses its pollution preven-
tion benefits, operating features, application, and limitations. Elimination or
reduction in use of hazardous solvents applied in cleaning processes is the main
focus of the technologies covered in the Guide.
HI
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ACKNOWLEDGMENTS
This Guide was prepared under the direction and coordination of Douglas :
Williams of the U.S. Environmental Protection Agency (EPA) Center for Environ--
mental Research Information and Paul Randall of the U.S. EPA Risk Reduction
Engineering Laboratory (RREL), both located in Cincinnati, Ohio. Battelle .
compiled and prepared the information used for this Guide. \
The following people provided significant assistance in reviewing the Guide and
making suggestions: Robert Pojasek, AIPP, GEI Consultants, Winchester, '
Massachusetts; John Sparks, U.S. EPA, Stratospheric Protection Division, ;
Technology Transfer Branch, Washington,~D.C.; Johnny Springer, U.S. EPA,
RREL, Pollution Prevention Branch, Cincinnati, Ohio; and Charles Darvin,
Organics Control Branch, U.S. EPA, Research Triangle Park, North Carolina. ,
IV
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CONTENTS
Notice jj
Foreword ." jjj
Acknowledgments : iv
Section 1. Overview 1
What Is Cleaner Technology? ; 1
Why Clean and Degrease? '. 1
Pollution Problem . . i 1
Potential Solutions 2
What's In This Guide? .....'. 2
Other Questions Affecting Investment Decisions 2
Who Should Use This Guide? 2
Summary 3
Keywords 3
Section 2. Available Technologies : .-.-. 4
How to Use the Summary Tables ,...; 4
Add-On Controls to Existing Vapor Degreasers f 7
Completely Enclosed Vapor Cleaner 9
Automated Aqueous Cleaning 12
Aqueous Power Washing ; 16
Ultrasonic Cleaning 18
Low-Solids Fluxes ', 20
Inert Atmosphere Soldering 22
Section 3. Emerging Technologies 24
How to Use the Summary tables '. 24
Vapor Storage Technology 26
Vacuum Furnace 26
Laser Cleaning '. 27
Plasma Cleaning 30
Fluxless Soldering 31
Replacement of Tin-Lead-Solder Joints 32
Section 4. Pollution Prevention Strategy : 33
References .ซ 34
Sections. Cleaner Technology Transfer Considerations..... 35
Section 6. Information Sources 38
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FIGURES
t
Figure 1. Completely enclosed vapor cleaner .....10
Figure 2. Adsorption stage at the end of a cleaning cycle on the CEVC 12
Figure 3. Variation of cycle time for various metals in the CEVC 13
Figure 4. Automated aqueous rotary washing process. 14
Figure 5. Aqueous powerwasher. \.. 17
Figure 6. Ultrasonic cleaning tank 18
Figure 7. Equipment used in vapor storage technology. 26
TABLES
I
Table 1. Available Technologies for Cleaning and Degreasing: |
Descriptive Aspects .ป 5
Table 2. Available Technologies for Cleaning and Degreasing:
Operational Aspects 6
Tables. Estimated Capital Cost of Add-ons to
Existing Vapor Degreasers 8
Table 4. CEVC Cleaning Cycle 1-1
TableS. Waste Volume Reduction by Using the , ,
Automated Aqueous Washer 15
>r
Table 6. Emerging Technologies for Cleaning and Degreasing: j.
Descriptive Aspects 24
Table 7. Emerging Technologies for Cleaning and Degreasing: ;
Operational Aspects : 25
Table 8. Trade Associations and Technology Areas '...,. 38
vi
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SECTION 1
OVERVIEW
What Is Cleaner Technology?
A cleaner technology is a source reduction or recycling
method applied to eliminate or significantly reduce
hazardous waste generation. Source reduction in-
cludes product changes and source control. Source
control can be characterized as input material changes,
technology changes, or improved operating practices.
ns.
Pollution prevention should emphasize source reduc-
tion technologies over recycling but, if source reduction
technologies are not available, recycling is a good
approach to reducing waste generation. Therefore,
recycling should be used where possible to minimize or
avoid the need to treat wastes that remain after viable
source reduction options have been evaluated and/or
implemented.
The cleaner technology must reduce the quantity and/
or toxicity of the waste produced. It is also essential
that final product quality be reliably controlled to
acceptable standards. In addition, the cost of applying
the new technology relative to the cost of similar
technologies needs to be considered.
Why Clean and Degrease?
Cleaning and decreasing processes are applied in a
variety of industries to remove dirt, soil, and grease
(often referred to together as soil). Cleaning and
degreasing, are done as a final step in manufacturing a
product, as a preliminary step in preparing a surface for
further work (e.g., electroplating), or as a cleaning step
for forms or equipment between uses.
In preparing metals for finishing, the cleaning process
is the most important. Finishing processes depend on a
clean surface as a foundation. In selecting a cleaning
operation, the process to be performed as well as the
type of metal and contaminant are important consider-
ations.
Many parts manufacturers clean their own products,
whereas others send them out to companies whose
sole business is parts cleaning. Currently, the common
cleaning processes for metals include liquid solvent
cleaning (cold cleaning) and vapor degreasing. Liquid
solvent cleaning usually is done in large tanks contain-
ing solvent solutions in which the parts are immersed.
This usually is an automated process. Vapor degreas-
ing generally involves chlorinated solvents such as
methylene chloride, 1,1,1/trichloroethane, trichloro-
ethylene, or perchloroethylene. Parts are immersed in
the vapors of these solvents for degreasing. In the dry
cleaning industry, perchloroethylene is commonly used
for washing clothes.
<,
In the electronics industry, parts generally are cleaned
after soldering to remove contaminants. These con-
taminants originate from the fluxes used to promote the
wetting necessary for good solder joints to be formed.
The flux residue can interfere with future processes
and reduce the aesthetics and reliability of a part.
Traditionally, chlorinated, fluorinated, and other haloge-
nated solvents have been used to remove these
residues.
Pollution Problem
Cleaning and degreasing technologies generally
involve applying some form' of a solvent to a part.
Solvents are used in virtually every industry to some
extent. During the cleaning process, there is often an
environmental problem with air emissions from the
solvents. After the cleaning process, a waste stream-
composed of the-solvent combined with oil, debris, and
other contaminantsis leflfor disposal.
Halogenated solvents have been chosen in the past for
their stability, ease of drying, and effectiveness in
removing oils. Some of the same characteristics that -
make these solvents effective in cleaning processes
-------
have detrimental environmental effects. Solvent
evaporation has been investigated for its role in strato-
spheric ozone depletion, global warming potential, and
ground smog formation.
Using halogenated solvents to clean and degrease not
only generates hazardous solvent wastes but also
creates work conditions that may be detrimental to the
health and safety of workers. Questions concerning
safety and health issues include chronic and acute
effects, carcinogenicity, and teratogenicity.
* , - s^v f f *, s * *r
Many foetofiftfes imm begun & mduce #r
' use of hatogemted &?&&$$<
Because environmental laws restrict the use of such
solvents, many industries are attempting to reduce or
eliminate their use of halogenated solvents. Additional
restrictions can be expected in the future.
Potential Solutions
Cleaner technologies now exist or are being developed
that would reduce or eliminate the use of solvents for
many cleaning and degreasing operations. There are
two main focuses in describing cleaner technologies for
cleaning and degreasing:
Alternative cleaning solutions (e.g., aqueous-
based) replace solvents. These alternatives
could be used in existing processes that currently
use solvents.
Process changes use different technologies for
cleaning or eliminate the need for cleaning. The
capital costs may be greater for process
changes, but the reduced cost of buying and
disposing of solvents often makes up for this.
This application guide focuses on those cleaner
technologies that involve process changes. Process
changes can either eliminate the need for cleaning or
apply techniques that eliminate or reduce the use of -
solvents.
Another possibility is to combine the above two meth-
ods. Sometimes the cleaning effectiveness of a solvent
substitute is not adequate, and a process change can
improve the effectiveness of the substitute. In such a
case, a process change is combined with solvent
substitution to create a cleaner technology. In other
cases, the process change may involve reducing the
amount of solvent or making it amenable to recycling.
Alternative cleaning solutions are described in the
companion U.S. EPA publication, Guide to Cleaner
Technologies: Alternatives to Chlorinated Solvents for
Cleaning and Degreasing. Both alternative cleaning
solutions and process changes may have limitations
that should be carefully evaluated by potential users for
their specific applications. '
! '
What's In This Guide?
i
This application guide describes cleaner technologies
that can be used to reduce waste in cleaning and
degreasing operations. Its objectives are to help
identify potentially viable cleaner technologies that can
reduce waste by modifying the cleaning and degreas-
ing process. This guide also provides'resources for
obtaining more; detailed engineering information about
the technologies. The following specific questions are
addressed: '-
I ' . . ,
What alternative cleaning and degreasing
process changes are available or emerging that
could significantly reduce or eliminate pollution
being generated from current operations?
Under what circumstances might one or more of
these process changes be applicable to your
operations? i
What pollution prevention, operating, and cost
benefits could be realized by adapting the
technology?
Other Questions Affecting Investment
Decisions
These other considerations affect the decision to
explore one or more cleaner technologies:
> Might new pollution problems arise when imple-
menting cleaner technologies?
Are tighter and more complex process controls
needed? '
Will product quality and operating rates be
affected? ;
Will new operating or maintenance skills be
needed? .
What are the overall capital and operating cost
implications? ;
To the extent possible, these questions are addressed
in this guide. The cleaner technologies described in this
guide are applicable under different isets of product and
operating conditions. If one or morejare sufficiently
attractive for your operations, your next step is to
contact vendors or users of the technology to obtain
detailed engineering data and make an in-depth
, evaluation of its potential for your plant.
Who Should Use This Guide?
This application guide has been prepared for plant
process and system design engineers and for person-
nel responsible for process improvement. Process
-------
change descriptions within this guide allow engineers
to evaluate options and major plant expansions, so that
cleaner technologies can be considered for existing
plants and factored into the design of new cleaning and
degreasing operations.
Sufficient information is presented to select one or
more candidate technologies for further analysis and
in-plant testing. This guide does not recommend any
technology over any other. It presents concise summa-
ries of applications and operating information to
support preliminary selection of cleaner technology
candidates for testing in specific processes. Sufficient
detail is provided to allow identification of possible
technologies that can be applied immediately to
eliminate or reduce waste production.
The keywords listed below will help, you quickly scan
the available and emerging technologies covered in
this guide.
Summary
The cleaner technologies described in this guide are
divided into two groups based on their developmental
maturity:
Commercially available technologies
Emerging technologies.
Pollution Prevention Strategy, Section 4, discusses the
impact of regulations on the potential for cleaner
technologies. The Cleaner Techno logy Transfer Con-
siderations, Section 5, discusses the various technical,
economic, and regulatory factors that influence the
selection and use of a cleaner technology.
Keywords
Cleaner Technology
Pollution Prevention
Source Reduction
Source Control
Recycling , -
Cleaning :fc
Degreasing
Vapor Degreasing
Metal Finishing
Defluxing
Add-on Controls to Existing Vapor Degreasers
Completely Enclosed Vapor Cleaner
Automated Aqueous Cleaning
Aqueous Power Washing
Ultrasonic Cleaning
Low-Solids Fluxes
Inert Atmosphere Soldering
Vapor Storage Technology
Vacuum Furnace
Laser Cleaning
Plasma Cleaning
Fluxless Soldering
Replacement of Tin-Lead Solder Joints
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SECTION 2
AVAILABLE TECHNOLOGIES
How to Use the Summary Tables
Seven available cleaner technologies for cleaning and
degreasing are evaluated in this section:
Add-on controls to existing vapor degreasers
Completely enclosed vapor cleaner
Automated aqueous cleaning
Aqueous power washing
Ultrasonic cleaning
Low-solids fluxes
Inert atmosphere soldering.
Tables 1 and 2 summarize descriptive and operational
aspects of these technologies. They contain evalu-
ations or annotations describing each available cleaner
technology and give users a compact indication of the
range of technologies covered to allow preliminary
identification of those technologies that may be applica-
ble to specific situations.
Descriptive Aspects
Table 1 describes each available cleaner technology. It
lists the Pollution Prevention Benefits, Reported
Application, Operational Benefits, and Limitations
of each.
Operational Aspects
Table 2 shows key operating characteristics for the
available technologies. The qualitative rankings are
estimated from descriptions and data in the technical
literature and are based on comparisons to typical
technologies that cleaner technologies would replace.
Process Complexity is qualitatively ranked as "high,"
"medium," or "low" based on such factors as the
number of process steps involved and the number of
material transfers needed. Process Complexity is an
indication of how easily the technology can be inte-
grated into existing plant operations. A large number of
process steps or input chemicals, or multiple opera-
tions with complex sequencing, are examples of
characteristics that would lead to a high complexity
rating. ;
i
The Required Skill Level of equipment operators also
is ranked as "high," ''medium," or "low." Required Skill
Level is an indication of the relative level of sophisti-
cation and training required by staff td operate the new
technology. A technology that requires the operator to
adjust critical parameters would be rated as having a
high skill requirement. In some cases* the operator may
be insulated from the process by complex control
equipment. In such cases, the operator skill level is low
but the maintenance skill level is high'.
Table 2 also lists the Waste Products and Emissions
from the available cleaner technologies. It indicates
tradeoffs in potential pollutants, the waste reduction
potential of each, and compatibility with existing waste
recycling or treatment operations at the plant.
The Capital Cost column provides a [preliminary
measure of process economics. It is a quantitative
estimate of the initial cost impact of the engineering,
procurement, and installation of the process and
support equipment. Costs are given for a specific unit
or plant that has implemented the process change.
Costs will vary for each facility due to the diversity of
data and the wide variation in plant needs and condi-
tions. Cost analyses must be plant specific to
adequately address factors such as the type and age of
existing equipment, space availability, production
volume, product type, customer specifications, and cost
of capital. I
The Energy Use column provides data on energy
conversion equipment required for a specific process.
In addition, some general information on energy
requirements is provided.
[
The last column in Table 2 cites References to publica-
tions, that will provide further information about each
available technology. These references are given in full
at the end of the respective technology sections.
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Pollution
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Operational Benefits
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Testing must be done to obtain optimun
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Thick oils and grease may absorb ultras
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Energy required usually limits parts size
Wastewater treatment required if aqueo
cleaners are used
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Eliminates solvent hazards
Can clean in small crevices
Cost effective
Faster than conventional methods
Inorganics are removed
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The text further describes pollution prevention benefits,
reported application, operational benefits, and limita-
tions for each available technology. Technologies in
earlier stages of development are summarized to the
extent known in Section 3, Emerging Technologies.
Add-on Controls to Existing Vapor
Degreasers
Pollution Prevention Benefits
tost
The single largest use of halogenated solvents in the
United States is for vapor degreasing. This includes
batch-type open-top vapor cleaners (OTVCs) and
continuous-type in-line cleaners. As much as 90% (or
.more) of the solvent used in conventional open-top
vapor degreasers is lost due to air emissions. Control-
ling these air emissions from vapor degreasers is
therefore of fundamental interest from a pollution
prevention point of view. Air emissions from an OTVC
occur during startup/shutdown, working, idling, and
downtime. During startup, losses occur as the solvent
in the sump is heated and a vapor layer is established
in the open tank. Shutdown losses occur as this vapor
layer subsides when the unit is switched off. Downtime
losses occur due to normal evaporation of the solvent
when the OTVC is not in use. Idling losses occur by
diffusion from the vapor layer in the period between
loads.
of&ir
By far the most important losses are the working losses
or work load-related losses. As long as there are no
disturbances at the vapor-air interface in the OTVC
tank, air emissions occur but are limited by existing
features such as freeboard height above the vapor
interface and primary (water-cooled) condensing coils
on the freeboard. Most vapor degreasers maintain, a
freeboard ratio (FBR), i.e., the ratio of the freeboard
height to the width of the tank, of 0.75. However, as the
work load (basket of soiled parts) is inserted into the
tank or taken out after cleaning, this interface is dis-
turbed and considerable amounts of solvent vapor
escape to the ambient air by forced convection. Also, a
large amount of solvent condensate is dragged out on
the cleaned parts as the work load.is removed from .the
tank. This solvent residue evaporates from the parts
over time, leading to considerable air emissions. .
Additional controls can be incorporated into an existing
OTVC to reduce these air emissions. These add-on
controls are an important way of reducing solvent
emissions without changing the cleaning operation
dramatically.
How Do They Work?
Add-on controls are features that can be incorporated
into an existing degreaser to reduce air emissions.
These process changes include the following:
.,. Operating controls
Covers
Increased FBR ,
Refrigerated freeboard coils
Reduced room draft/lip exhaust velocities.
Operating Features
The add-on controls limit air emissions through
changes in operating practices or through equipment
modifications. Operating controls are practices that
reduce work load-related losses. These can be easily
incorporated into the operating procedure, but their
impact on emission reduction is significant. Air emis-
sions can be reduced by slowing down the rate of entry
of the work load into the OTVC tank. The more quickly
the work load is lowered into the tank, the greater the
disturbance or turbulence created at the vapor-air
interface and the greater are the air emissions as the
interface tries to reestablish itself. When the work load
is lowered manually into the tank it is difficult to achieve
a slow, steady rate of entry. Installing an electric hoist
above the OTVC allows greater control on the rate of
entry or removal of the work load. Reducing the area of
the horizontal face of the basket in proportion to the
area of the OTVC tank opening is another way of
reducing turbulence at the interface; this will however,
adversely affect the produiction rate.
- 5to&e/# cwKfen&ate ttra&wt&an he mduc&tin
Facilitating parts drainage also is an important operat-
ing control. Parts that have recesses in which solvent
condensate could accumulate must be placed in the
basket in such a way that the condensate drains out of
and not into the recesses.lThus, the amount of conden-
sate dragged out as the basket is removed from the
OTVC tank is limited, reducing subsequent air emis-
sions. Another way of reducing dragout is to install
electric-powered rotating baskets. The rotation allows
condensate to drain out of the recesses in the parts.
A simple flat or rolling cover can be installed on the top
of the OTVC tank to reduce air emissions. A cover
reduces drafts in the freeboard that may cause distur-
bances. A cover also reduces diffusion losses during
startup/shutdown, downtime, or idling. Covers should
-------
slide gently over the top of the opening to reduce
disturbances. Automatic biparting covers that enclose
the tank while the work load is in the process of being
cleaned also are available. Covers can reduce working
air emissions from an OTVC by as much as 35 to 50%
(U.S. EPA, 1989). The variations in the percent reduc-
tion reflect different initial design and operating condi-
tions of the OTVCs tested.
sir
Increasing the FBR from 0.75 to 1 .0 or 1 .25 can reduce
air emissions significantly. Increasing the freeboard
height that is, the height of the tank above the vapor-
air interface reduces the susceptibility of the interface
to room drafts and also increases the distance over
which diffusion has to occur. Raising the freeboard on
an existing OTVC may, however, reduce a worker's
accessibility to the tank. But a raised platform next to
the OTVC or an electric hoist can alleviate the problem.
Raising the FBR from 0.75 to 1 .0 reduces working air
emissions by up to 20%. Increasing the FBR from 1 .0
to 1.25 reduces emissions by another 5 to 10% (U.S.
EPA, 1989). Under idling conditions, air emissions can
be reduced by up to 40% when the FBR is increased
from 0.75 to 1.0.
reduce solvent concentrations in the region where
workers are exposed. However, this very feature
increases diffusion and solvent diffusion losses from
the OTVC sometimes are almost doubled. Although
most of the diffusing solvent is captured by the lip
exhaust and may be recovered later by carbon adsorp-
tion, some vapor escapes to the ambient.
Application ;
The attractiveness of these add-on controls is that they
can be applied to almost any vapor degreaser without
having to change the process completely. The basic
degreasing principle does not change. These controls
can be phased in gradually, improvements being made
one at a time. '
Existing OTVCs can be retrofitted with add-on controls
at a reasonable cost. Table 3 shows examples of costs
for retrofitting additional controls on typical small or
large OTVCs. Actual costs can vary ffom these aver-
ages depending on the types of features obtained and
the design of the existing OTVC. The|se costs indicate
that these controls are viable options;for small or
medium-sized plants. ;
Table 3. Estimated Capital Cost of Add-ons to Existing Vapor
Degreasers
Cost for a small 'Cost for a large
Add-on degreaserf ($) degreaser" ($)
Air emissions through diffusion can be reduced by
installing refrigerated coils on the freeboard above the
primary condenser coils. The refrigerated coils may be
designed to operate either above or below freezing
temperatures. Although theoretically the below-freezing
coils should work better, in practice, the below-freezing
coils have to be operated on a timed defrost cycle to
prevent ice from building up on the coils. This periodic
defrosting cycle reduces the efficiency of the coils to
some extent. Working emissions are reduced by
approximately 20 to 50%.for above-freezing coils and
by approximately 30 to 80% for below-freezing coils
(U.S. EPA, 1989). Under idling conditions, emissions
with below-freezing coils were reduced by approxi-
mately 10 to 60%. Some systems operate with the
primary condenser coils themselves refrigerated,
instead of having separate refrigerated coils.
Room drafts caused by plant ventilation can cause an
increase in air emissions by sweeping away solvent
vapors that diffuse into the freeboard region, leaving
behind a turbulence that promotes greater emissions.
Reducing room drafts can reduce these emissions.
One interesting case is when lip exhausts themselves
cause emissions. Lip exhausts are lateral exhausts
installed on the perimeter of the OTVC opening to
Automated work load handling
Bi-parting cover
Increasing FBR to 1.0
Refrigerated coils
2,000-3,000
8,000-9,000
1,000-2,000
5,000-7,OJDO
3,000-4,000
10,000-12,000
1 ,500-2,500
1 8,000-12,000
* A small degreaser would have a 4- to 5-ft2 opening.
b A large degreaser would have a 15-ft2 opening.
Benefits
The benefits of these add-on controls are
They can be retrofitted onto existing vapor
degreasers.
Simple add-ons such as a cover can reduce air
emissions significantly. >
Reduced air emissions mean reduced solvent
consumption and hence reduced operating cost.
Add-on controls are relatively Inexpensive.
They are easy to install and operate.
' Using add-on controls required no additional
labor or skills.
-------
Limitations
The limitations of add-on controls are
The performance of any one add-on control is
dependent on the design features already
available on the OTVC. For example, the control
efficiency of refrigerated coils varies depending
on the temperature and efficiency of the existing
primary condenser.
Air emissions can be reduced considerably but
not eliminated by using multiple controls. For
example, if adding a cover alone reduces air
emissions by 50% and adding refrigerated coils
alone reduces air emissions by 50%, adding both
the cover and the refrigerated coils will not give
100% reduction.
Work load-related losses can be reduced but not
eliminated.
Dragout of solvent with the work load cannot be
eliminated using add-on controls. Some residual
solvent will escape from the parts to the ambient
air.
Reference
U.S. Environmental Protection Agency. 1989. Alterna-
tive Technology Control Documents Halogenated
Solvent Cleaners.. August.
Completely Enclosed Vapor Cleaner
Pollution Prevention Benefits
The add-on controls described previously can signifi-
cantly reduce air emissions, but the completely en-
closed vapor cleaner (CEVC) virtually eliminates them.
Tests have shown over 99% reduction in solvent
emissions by using the CEVC. This technology was
first developed in Germany, where vapor degreasers
are regulated as a point source. Some companies have
recently started selling this technology in the United
States.
How Does It Work?
In a CEVC, the work load is placed in an airtight
chamber, into which solvent vapors are-introduced.
After cleaning is complete, the solvent vapors in the
chamber are evacuated and captured by chilling and
carbon adsorption. Once the solvent in the chamber is
evacuated, the door of the chamber is opened and the
work load is withdrawn. The cleaned work load is also
free from any residual solvent and there are no subse-
quent emissions.
i'
Operating Features
Th$ &ฃVO f&toaifis etnctased during $&
' "
Figure 1 shows the CEVC unit configuration. Approxi-
mately 1 hour before the shift begins, a timer on the
CEVC unit switches on the heat to the sump. When the
solvent in the sump reaches vapor temperature, the
vapor is still confined to an enclosed jacket around the
working chamber. The parts to be cleaned (work load)
are placed in a galvanized basket and lowered by hoist
from an opening in the top into the working chamber.
The lid is shut, the unit is switched on, and compressed
air (75 psi) from an external source hermetically seals
the lid shut throughout the entire cleaning cycle.
in &
-------
Electric
Heat
Water
Legend
Desorption Stage
Adsorption Stage
Liquid Solvent
Figure 1. Completely enclosed vapor cleaner.
Unlike a conventional degreaser, there are no signifi-
cant idling losses between loads or downtime losses
during shutdown. The CEVC can be operated as a
distillation unit to clean the liquid solvent in the sump.
To distill, the unit is switched on without any work load
in the chamber. After most of the solvent is converted
to vapor, the residue in the sump is drained out and the
vapors in the chamber are condensed in the chiller to
recover the solvent. CEVC thus provides a good
alternative for meeting pollution prevention objectives.
Energy requirements of the CEVC are higher com-
pared with a conventional degreaser. The CEVC
operates on a 480-V AC electric supply and consumes
approximately 22 kW of power. The higher energy is
required to generate, condense, and'move the vapor
during each load. j
One significant difference between a!conventional
degreaser and the CEVC is that, in the conventional
degreaser, there is always a solvent vapor layer
present in the degreasing tank. This layer is continu-
ously replenished with solvent vaporizing from the
sump. The work load therefore reaches vapor tempera-
ture very soon and the cleaning is completed. The
CEVC, on the other hand, goes through several stages
to evacuate and introduce vapors. Although most of the
stages have a relatively fixed time requirement, the
vapor-fill stage time varies. The vapor is introduced
anew near the bottom of the working chamber with
each work load. The vapor slowly wbrks itself up
through the work load bringing each!successive layer
of parts in the basket to vapor temperature. The time
taken for the entire load to reach vapor temperature is
10
-------
Table 4. CEVC Cleaning Cycle
Stage
Solvent heatup (once a day)
Solvent spray (optional)
Vapor fill
Degreasing
Condensation
Air recirculation
Carbon heatup
Desorption
Adsorption
Vendor- Recom m ended
Time Settings
Variable8
, 10-1 80 sec
Variable"
20-180 sec
1 20 sec
1 20 sec
Variable0
60 sec
60-240 secd
..winiMiijr i<^uifc;o ctppiuAuucutsiy i nuui un uays roiiowing overnignt
shutdown when sump solvent temperature drops to 70ฐC. After
weekend shutdowns, when sump solvent temperature drops to
20ฐC, it may take 1 1/2 hours for solvent to reach vapor
temperature. Time on unit allows automatic heat-up prior to
beginning of shift.
b Varies according to mass of work load and type of metal. Generally
varies between 8 to 40 min.
c Carbon heatup took approximately 22.5 min during testing.
d At 60 sec, if monitor shows that chamber concentration is above
1 g/m3, then the adsorption stage proceeds to the full 240-sec
stage. This sequence repeats if necessary.
described as the vapor-fill stage in Table 4. This vapor-
fill time, however, is highly dependent on the total mass
and type of metal in the work load. The factor that
governs the variation based on type of metal is the
thermal diffusivity of each metal. The thermal diffusivity
itself is a function of the thermal conductivity, specific
heat, and density of the metal.
i Gt&&ttf$g cyete Htm varies with
Based on a CEVC unit that is rated at 560 Ib of steel
parts per hour (1 cleaning cycle per hour), the total
cycle time required for various work load metals and
masses is shown in Figure 3. For any of the metals, as
the mass of the work load increases, the total cycle
time increases (mainly due to an increase in the vapor-
fill stage time). Parts made out of copper or aluminum
require a lower cycle time compared to steel. Alumi-
num, though, has a much lower density, and there, is a
limit as to the mass (or weight) of parts that can fit into
the basket for one cycle. Additional parts have to be
run through the next batch or cleaning pycle. Because
of the fixed portion of the cycle time involved in running
a fresh batch, the line for aluminum (Figure 3).shows a
jump after 375 Ib of parts.
Application
The CEVC can be applied wherever vapor degreasing
currently is being used. The degreasing principle is the
same; the pollution prevention features set this unit
apart from conventional OTVCs. This unit is an excel-
lent option for plants thai want to eliminate solvent
emissions but are unwilling to change over to aqueous
cleaning.
' Th& GEVChas a ttfatiwfr Mgbw capital cost
Given the longer vapor-fill time, carbon heatup, desorp-
tion, and adsorption stages of the CEVC, a much larger
cleaning chamber capacity (or batch volume) is re-
quired to maintain the same processing rate as with a
conventional degreaser. This and other emission
control features make the capital cost of the CEVC
significantly higher than that of an OTVC. A commer-
cially available CEVC with a capacity of 560 Ib/hr of
steel parts costs approximately $200,000 (Townsend,
1993). Savings in solvent consumption offset the initial
investment.
casts,
Compared to a conventional OTVC with the same
production capacity, the CEVC results in operating
savings of $25,000/yr from reduced labor costs (due to
larger, unattended batch sizes) and lower solvent
requirement (due to solvent recovery) based on a 40-hr
work week (Gavaskar et al., 1993). The CEVG does
not require much of the auxiliary equipment that may
be required for a standard conventional vapor de-
greaser, if the user is aimiing to reduce workplace
emissions to meet or anticipate increasingly stringent
environmental and worker safety regulations. Additional
control devices for standard conventional degreasers
(e.g., increased freeboard ratio, refrigerated coils, and
room ventilation control) v/ould add considerably to
capital and operating costs. In contrast, the CEVC is a
self-contained unit that would require no additional
facility modifications to achieve significant emission
reduction.
Benefits
The CEVC has the following benefits:
It reduces solvent emissions by over 99%
compared to a conventional OTVC.
Users who do not want to switch to aqueous
cleaning can still achieve significant pollution
prevention by using the CEVC.
Labor and skill level requirements are similar to
those for a conventional OTVC.
The CEVC lowers operating costs by reducing
solvent losses.
No additional facility modifications are needed to
meet OSHA requirements for plant ambient
solvent levels.
The CEVC has fully automated cycles and runs
unattended except for loading and unloading.
11
-------
10,000
1.000-
o,
Ul
2
100-
Cleaning
Chamber
Solvent'
Concentration
10-
300
Time (seconds)
Figure Z. Adsorption stage at the end of a cleaning cycle on the CEVC.
600
The unit adjusts automatically to any type of work
load and unseals the working chamber when the
cycle is complete.
Limitations
The CEVC has relatively high capital cost
compared to a conventional OTVC.
The CEVC has longer cleaning cycles for the
same capacity.
It has a relatively higher energy requirement
because of the alternating heating and cooling
stages.
References
i
Gavaskar, A. R., R. F. Olfenbuttel, arid J. A. Jones.
1993 (in press). On-Site Solvent Recovery. U.S.
Environmental Protection Agency 'Project Summary.
Townsend, D. 1993. Personal communication from
Dave Townsend of Durr Automation, Inc. in
Davisburg, Michigan, to Arun Gayaskar of Battelle,
Columbus, Ohio. January. i
Automated Aqueous Cleaning
t
Pollution Prevention Benefits ,
Automated aqueous cleaners use aqueous cleaning
solutions instead of solvents to achieve high-quality
12
-------
30
200
r
600
400 600 800
Mass of Parts Cleaned per Cycle (Ib)
Figure 3. Variation of cycle time for various metals in the CEVC.
1000
1200
cleaning. This available technology replaces the
hazardous solvent waste stream with a much less
hazardous wastewater stream. These automated
machines also have features for significantly reducing
the amount of wastewater generated. These machines
remove some of the contamination that comes out from
the parts being cleaned into the cleaning solution. The
cleaning solution can then be recirculated for cleaning
several times.
How Does It Work?
Small machined parts often are cleaned in batches of
thousands by immersion into a solvent solution or a
solvent vapor. Instead, the automated^ aqueous washer
sprays an aqueous solution across the parts to remove
oil and debris. Parts travel through a series of cham-
bers, each with different concentrations of cleaning and
rinsing solutions. Excess sprayed solution is recovered
and reused. Similar automated cleaners are also
available for semi-aqueous cleaning solutions.
Operating Features
Figure 4 shows, a typical configuration of the automated
washer. Not all users require the multitude of compart-
ments shown in the figure, and simpler versions of this
unit can be manufactured. The process unit shown in
the figure consists of a series of five compartments
through which the soiled metal parts are transported.
The parts are transported from one compartment to the
next by a helical screw conveyor. The parts are
sprayed successively with solutions from five holding
tanks (one for each compartment)."The first compart-_
ment sprays hot water on ;the parts. Because many
residual machining fluids on the parts are oil-water
emulsions, the hot water helps to break the emulsion.
The second and third compartments spray detergent
solutions at two different concentrations on the parts.
The fourth compartment is for a clean water rinse. The
fifth and final compartment sprays a rust inhibitor
solution if required. The fifth compartment is followed
by a dryer that vaporizes any water droplets remaining
on the parts. The cleaned parts drop out of the dryer
onto a vibrating conveyor from which they are col-
lected. ,
The automated aqueous v/asher also makes use of a
"closed loop" system, whereby the used solutions are
not disposed of daily but can be recirculated for a week
of relatively continuous operation. The cleaning solu-
tions are recaptured after use and sent to the separator
tanks shown in Figure 4. Qne such separator tank is
provided for each compartment. In these tanks, the oil
floats to the surface and is skimmed off by a pump. Dirt
and suspended particles settle down at the bottom of
13
-------
t . E
-If
Oil
Sump
L_
m 4
Helical
Screw
PH^i
4 -
Hopper/
Feeder
i i
; ,
Oil Separators
A Continuous
Y Draining
i
Product
Shaking T
Screen
mpartmenfll Compartment SI Compartment 31 compartment 4 1 compartment t> i Dry^r J
* 1 <=nrnW 1 "* 1 ^ 1 ** 1
Hot
Water
Tank
Non-
Phosphate
Detergent
Tank
Non- i
Phosphate "
Detergent Cold
Tanl< Water
i : 1 Rinse
Optional
Rust
Inhibitor
Tank
i! Gas
j: Heat
Rgure 4. Automated aqueous rotary washing process.
the tank. The bulk of the solution is recirculated back to
the holding tanks for reuse. Some makeup solution is
needed periodically to replace losses from evaporation
and dragout. Detergent chemicals are replenished
periodically.
alkaline tumbling or hand-aqueous (manual) washing.
These traditional processes have the disadvantage of
generating large amounts of wastewater. The auto-
mated washer, on the other hand, allows for recovery
and reuse of the cleaning solution. Wastage of both
water and cleaning chemicals is prevented without
compromising cleaning effectiveness.'
The same cleaning solution can be recirculated and
used for a week without changing. At the end of the
week (or whenever the contaminants reach a certain
level), the holding tanks are emptied and fresh solu-
tions are made up. Because recovery and reuse of the
cleaning solution is automatic, the unit requires very
little operator attention, in contrast to vapor degreasing
or traditional batch aqueous cleaning processes, the
continuous operation of this conveyorized unit enables
production efficiency. The only operator involvement is
for unloading a barrel of soiled parts into the hopper
that feeds the parts to the compartments.
Several variations of the washer shown in Figure 4 are
now commercially available. Different types of filters,
oil-water separators, and sludge thickeners are some
of the features offered. The main principle in this new
line of washers however is the same improved
contact between the part surface and the cleaning
solution and several recovery and reuse cycles of the
cleaning solution. Some new units claim zero Wastewa-
ter discharge, with fresh water added only to make up
for evaporation in the drier (Scapelliti, 1993).
Application
Automated aqueous washing provides a comparable
level of cleaning quality for most parts that normally
would be run through a-vapor degreaser. This technol-
ogy eliminates the need for using solvents. It also
provides a cleaning quality comparable to that from
traditional aqueous cleaning processes such as
Quality Rolling and Deburring (QRD) Company, Inc., a
medium-sized metal finishing company in Thomaston,
Connecticut, has been using an automated aqueous
washer similar to the one shown in Figure 4 since
February 1990. QRD added the autoijnated washer to
accommodate a growth in production/Therefore, the
traditional processes (vapor degreasing, alkaline
tumbling, and hand-aqueous washing) are still avail-
able in the plant, although much of the new work is run
through the automated washer. QRD was thus able to
expand the plant capacity without increasing solvent
consumption. :
The reaction of QRD employees andlcustomers to the
new washer has been positive. Cleaning quality is
comparable to that of the traditional processes
(Gavaskar et al., 1992). At the same time, additional
capacity has not resulted in additional solvent pur-
chases or significant increases in wastewater. The
automated washer was found to be using 90% less
water compared with alkaline tumbling and 80% less
water compared to hand-aqueous washing. The
additional wastewater generated through the expansion
in capacity was easily handled by QRD's existing
wastewater treatment plant. Table 5 shows the waste
volume reduction resulting from the use of the auto-
mated washer at QRD.
14
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Table 5. Waste Volume Reduction by Using the Automated Aqueous Washer
Conventional Clea'ning
Waste stream
Vapor Degreasing*
Wastewater in separator
Still bottom sludge
Alkaline Tumbling11
Wastewater
Hand-Aqueous Washing0
Wastewater
Volume
Generated
(gal/yr)
200
1,440
1,010,880
296,400
Automated Washing
Waste stream
Automated Washing3
Wastewater
Oily liquid
Automated Washing"
Wastewater
Oily liquid
Automated Washing0
Wastewater
Oily liquid
a Based on 5,200 barrels/yr run on automated washer instead of vapor degreaser.
b Based on 3,120 barrels/yr run on automated washer instead of alkaline tumbler.
0 Based on 2,080 barrels/yr run on automated washer instead of hand-aqueous washer.
Volume
Generated
(gal/yr)
143,000
962
85,800
577
57,200
385
Because cleaning solution is recovered and reused in
the automated washer, consumption of cleaning
chemicals (and their loss through wastewater) was
considerably lower. Chemical costs for the automated
washer were 40% lower compared to alkaline tumbling
and 95% lower compared to hand-aqueous washing.
I, energy requirements c&mpar&d to vapor
"'''
The energy requirement of the automated washer was
found to be comparable to that of the traditional aque-
ous cleaning processes (tumbling and hand-aqueous
washing), but was higher than the energy requirement
of the vapor degreaser for equivalent production. The
automated washer used by QRD (Figure 4) consumes
energy for a 5-hp motor for the helical screw (con-
veyor), four 3-hp motors on the circulation p'umps on
the holding tanks, a 1 .5-hp motor for the oil skimming
pump, and 150 cu ft of LPG gas for the drier. The
drying required after aqueous cleaning appears to drive
the energy requirement of the automated washer
above that of the vapor degreaser. The vapor de-
greaser does not require a drier because excess
solvent residual on the cleaned parts evaporates off to
the ambient overtime. However, this feature is one of
the main sources of emissions from vapor degreasing.
Labor requirement of the automated washer was
equivalent to that of the vapor degreaser but much
lower than for the alkaline tumbler or hand-aqueous
washer. The only labor required for the automated
washer was for unloading the parts to be cleaned into
the hopper once every 20-25 minutes. The hand-
aqueous washer had the highest labor requirement
because one person had to be in constant attendance
to move the barrel of parts from one cleaning tank to
the next with an overhead hoist.
geaaset but tower operating costs compared
By installing an automated washer instead of a vapor
degreaser or a traditional aqueous process, annual
savings of $60,000 were realized. The automated
washer shown in Figure 4 cost QRD approximately
$200,000 to purchase and install. The high initial
investment is therefore expected to be recovered in a
relatively short period. Note that the cost saving is
realized only when the automated washer is compared
to all three existing processes at QRD vapor degreas-
ing, tumbling, and hand-aqueous washing. When
compared with the vapor degreaser alone, the auto-
mated washer has higher operating costs, mainly due
to higher energy (drying) requirements.
for aff types of parts,
Some special jobs are still run through the old pro-
cesses of alkaline tumbling, vapor degreasing, or hand-
aqueous washing. For example, QRD still uses the
vapor degreaser for very delicate parts. Parts that are
particularly difficult to clean (for example parts with a lot
of crevices) are sent to the hand-aqueous washer. For
certain types of parts that tend to slide over each other
to form a close fit, QRD avoids aqueous processes
completely, because the surface tension of water at the
interface tends to hold the parts together and prevent
good cleaning access. However, except for such
special jobs, Inost types of parts are processed through
the automated washer. :
Benefits
The automated washer described above has several
benefits:
15
-------
Improved contact between cleaning solution and
parts being cleaned enables most types of parts
to be aqueous cleaned instead of solvent
cleaned.
Solvent usage at a metal finishing plant can be
drastically reduced or eliminated.
Cleaning effectiveness is comparable to vapor
degreasing or conventional aqueous cleaning
processes (alkaline tumbling or hand-aqueous
washing).
The amount of wastewater generated is very low
compared to the amount generated by traditional
aqueous processes. In some types of units,
wastewater is claimed to be completely elimi-
nated with fresh water added only to make up for
evaporation.
The automated aqueous washer is easy 'to install
and operate. The labor and skill requirements are
low.
This technology has lower cleaning chemicals
consumption compared to traditional aqueous
processes.
Continuous operation of the automated aqueous
washer enhances plant efficiency.
The technology realizes operating cost savings
compared to traditional aqueous processes.
Limitations
The limitations of the automated washer are as follows:
Wastewater generated has to be treated and
discharged.
Some types of parts cannot be cleaned as
effectively in the automated aqueous washer as
in a vapor degreaser or with a conventional
aqueous process.
The technology has a high energy requirement
compared to vapor degreasing, mainly due to the
energy required for drying.
The automated aqueous washer technology has
a relatively high initial capital requirement.
Drying can leave spots on aqueous-cleaned
parts if rinsing is inadequate or if rinsewater
contains a high level of dissolved solids.
References
Gavaskar, A. Ft., R. F. Olfenbuttel, and J. A. Jones.
1992. An Automated Aqueous Rotary Washer for
the Metal Finishing Industry. EPA/600/SR-92/188, ,
U.S. Environmental Protection Agency Project
Summary.
Scapelliti, J. 1993. Personal communication from J.
Scapelliti of Durr Automation, Inc., Davisburg,
Michigan, with A. R. Gavaskar of Battelle, Colum-
bus, Ohio.
i
Aqueous Power Washing
Pollution Prevention Benefits >
!
t
The aqueous power washer is similar to the automated
aqueous washer in that it combines innovative process
technology with the use of an aqueous (or semi-
aqueous) cleaning solution: Both technologies elimi-
nate the use of solvents for cleaning. When combined
with a "closed-loop" technology, in whiph the cleaning
solution is recirculated, aqueous power washing also
reduces water and cleaning solution disposal require-
ments. ;
How Does It Work?
Unlike the automated washer which has a continuous
operation, most power washers are batch units. Some
continuous (conveyorized) units also are available.
Whereas the automated washer is mote suitable for
smaller parts, the power washer is suitable for larger
parts. In a power washer, a large part or a group of
smaller parts is placed in a closed chamber and
blasted from all sides with water or cle'aning solution.
Parts to be cleaned are placed inside the power
washer unit on a turntable (Figure 5). As the turntable
rotates, the parts are blasted from all angles with water
at high-pressure (180 psi) and elevated temperature
(140ฐF to 240ฐF). The force of the spray jets, the heat,
and the detergent combine to strip oil; grease, carbon,
etc. The cycle time varies from 1 to 30 minutes de-
pending on the type of part, i '
Operating Features
Power or jet washers are available from a variety of
vendors with varying options and in various sizes. One
option available is a closed-loop system. The water is
collected and sent through a filtration |or sedimentation
unit or another method of contaminant removal and
then sent back to the unit for reuse, this can reduce
wastewater treatment and disposal requirements as
well as water consumption. Although most systems are
simple single-compartment batch units,.they are
available also as multiple-stage cleariing units or as
conveyorized automated systems. '
Energy requirements are simple. Most units run on .
220 V electrical power. Aqueous power washers are
stand-alone units and are available iri a range of sizes
to fit even in crowded plants. Depending on the type of
parts to be washed, an aqueous cleaner can be
selected for use in a power washer. !
16
-------
Water Line
Figure 5. Aqueous power washer.
Turntable
Like the equipment, the actual operating steps are
quite simple. The machine is loaded and the wash
cycle timer set. The operator can then leave the
equipment while the parts are cleaned.
The aqueous power washer is useful for parts that
normally would be run through a vapor degreaser,
alkaline tumbler, or hand-aqueous processes. Power
washing, with the correct selection of detergents, is
safe for metals, plastics, varnish coatings, etc. A power
washer also can be used for deburring and chip
removal of metal parts.
j
Costs vary widely depending on the size of the unit and
options selected. One firm is spending approximately
$70,000 to install a unit that can clean 6,000 pounds ofซ
material at one time; the energy load for this unit also is
quite high. An average unit might cost about $12,000
and clean 1000 pounds of parts per batch in 10 min-
utes with fairly low energy requirements. Still smaller
are portable units that clean 500 pounds per batch.
Application
Power washers are being used in a variety of industries
to clean jet engines, electric motors, metal stampings,
diesel engines, etc. The Seattle Metro Garage in
Seattle, Washington, uses a power washer to clean
parts removed from buses during overhaul and mainte-
nance (Evers and Olfenbuttel, 1993). Previously, they
used a combination of solvent baths/wash stations and
alkaline steam spray with the washing system. A
smaller system is to be put into operation at the same
plant for parts from cars and trucks. These units
eliminate solvent cleaning for the parts. The discharges
from the unit pass through an oil/water separator and
then to the Seattle sanitary sewer system. This type of
unit eliminates the cost of hazardous or oily waste
disposal.
Benefits
The benefits of the aqueous power washer are as
follows:
Aqueous cleaners can be used in applications
where solvent cleaning was used previously.
Aqueous cleaners provide more efficient cleaning
compared to manual aqueous tank cleaning.
Cleaning times are reduced.
17
-------
The most common, unit is a compact machine
with one chamber as opposed to several tanks or
compartments.
The small units are available also as portable
units.
Limitations
Aqueous power washers have certain limitations:
Wastewater generated has to be treated and
discharged.
Some parts, such as electronic sensors or
diaphragms, may not be able to withstand the
high pressure or temperature of the sprays.
It is also possible that jet washers will not be able
to remove baked-on dirt that cannot be removed
by scrubbing.
Drying can leave spots on aqueous-cleaned
parts if rinsing is inadequate or if the rinsewater
contains a high level of dissolved solids.
Reference
Evers, D. P., and R. F. Olfenbuttel. 1993 (in press).
Power Washer with Wastewater Recycling Unit.
Technology Evaluation Report prepared by Battelle
under Contract No. 68-CO-0003 for the Pollution
Prevention Branch of the U.S. Environmental
Protection Agency, Risk Reduction Engineering
Laboratory.
Ultrasonic Cleaning
Pollution Prevention Benefits
Ultrasonic cleaning makes use of cavitation in an
aqueous solution for greater cleaning effectiveness.
The efficiency of the technology greatly reduces or
eliminates the need for strong solvents. Although
solvents can be used with ultrasonic technology, an
aqueous or semi-aqueous solution can be substituted
for solvents, thereby eliminating solvents from the
waste stream. The wastewater generated can then be
treated on site and discharged.
How Does It Work?
In ultrasonic cleaning, high frequency sound waves are
applied to the liquid cleaning solution. These sound
waves generate zones of high and low pressures
throughout the liquid. In the zones of negative pres-
sure, the boiling point decreases and microscopic
vacuum bubbles are formed. As the sound waves
move, this same zone becomes one of positive pres-
sure, thereby causing the bubbles to implode. This is
called cavitation and is the basis for ultrasonic clean-
ing.
Cavitation exerts enormous pressures (on the order of,
10,000 pounds per square inch) and temperatures
(approximately 20,000ฐF on a microscopic scale).
These pressures and temperatures loosen contami-
nants and perform the actual scrubbing;of the ultra-
sonic cleaning process. j
Operating Features
Ultrasonic energy usually is applied to a solution by
means of a transducer, which converts electrical
energy into mechanical energy. The positioning of the
transducers in the cleaning tank is a critical variable.
The transducers can be bonded to the tank or mounted
in stainless steel housings for immersion in the tank.
The number and position of immersiblejtransducers are
determined by the size and configuration of the parts,
the size of the batch, and the size of the tank. It is
preferable to locate the transducers so [that the radiat-
ing face is parallel to the plane of the rack and the
ultrasonic energy is directed at the workpieces. Figure
6 shows the cleaning tank.
The part being cleaned must be immersible in a liquid
solution. For best cleaning results, testing must be
done with each set of parts to obtain the optimum
combination of solution concentration and cavitation
levels. i
lw& 1ft& most effect &n ultrasonic
gt&mftig.
Temperature is the operating feature trjat has the most
effect on the cleaning process (Fuchs.h 991): In-
creased temperature results in higher cavitation
intensity and better cleaning.This is tn|ie provided that
the boiling point of the chemical is not jtoo closely
approached. Near the boiling point, the liquid will boil in
the negative pressure areas of the sound waves,
resulting in no effective cavitation. j
How parts are loaded into an ultrasonic cleaner also is
an important consideration. For instance, a part with a
blind hole or crevice can be cleaned effectively if it is
placed so that liquid fills this hole and is therefore
60 Hz
Figure 6. Ultrasonic cleaning tank.
18
-------
subjected to cavitation action. If this hole is inverted
into a liquid with the opening of the hole facing down-
ward, it will not fill with liquid and will not be cleaned.
Overloading baskets with small parts can sometimes
result in ultrasonic energy being absorbed by the first
several layers of parts. Large volumes of small parts
can.be more effectively cleaned a few at a time with
relatively short cycles.
The actual basket design is another important con-
sideration. It should ensure that transmission of ultra-
sonic energy will be attenuated as little as possible. An
open racking method is best whenever possible.
There are three basic stages in ultrasonic cleaning.
The first is the presoak stage, which is vital to the
efficiency of the system. In this stage, the part is placed
in the heated cleaning solution which removes all
chemically soluble soil and gross contaminants. The
second stage is the primary stage of ultrasonic clean-
ing, in which scrubbing and cleaning are performed
through cavitation in the solution. The third stage is
rinsing of the cleaned part. Ultrasonics also can be
applied in this stage for increased efficiency.
The primary ultrasonic cleaning system has three
components: a liquid solution tank, an ultrasonic
generator which is the power source for electrical
energy, and a transducer which converts electrical
energy to mechanical energy. Most generators accept
standard AC input at 60 Hz and then convert it to DC.
Sizes range from 200-W tabletop units to large 1000-W
units. The optimum transducer frequency for most
applications has been found to be approximately to 20
kHz. Transducers can be bonded to the tank, or an
immersible transducer can be used. Immersible
transducers are convenient when a transition is being
made to ultrasonic cleaning and existing tanks are to
be used.
The use of ultrasonic equipment does not require any
special knowledge. The equipment can be selected
with the aid of the manufacturer and is simple io
operate. Additional discussion of ultrasonic cleaning
can be found in Burstein (1989), Magnapak (1988), and
Scott (1989).
Application
Ultrasonic cleaning uses conventional equipment
available from a wide variety of vendors. There are two
basic types of ultrasonic equipment available.
Electrostrictive ultrasonics use a ceramic crystal to
produce sound vibrations while magnetostrictive
ultrasonics use metallic elements.
Ultrasonic cleaning can be applied to almost any parts.
Materials such as ceramic, aluminum, plastic, and
glass, as well as electronic parts, wire, cables, and
rods and detailed items that may be difficult to clean by
other processes, are ideal candidates for ultrasonic
cleaning.
Printed circuit boards and other electronic components
can also be cleaned using ultrasonics. While there
have been complaints that the 20 kHz equipment can
damage fragile products such as electronic equipment,
there is now available 40 kHz equipment which is more
applicable to the electronics industry/This also reduces
the noise level associated with ultrasonic cleaning.
Although most available ultrasonic cleaning equipment
is designed for batch tanks, equipment does exist in
cylindrical form. A horizontal cylindrical tube or pipe is
fitted with peripheral transducers. The resonant-tuned
circuit focuses energy along the in-line centerline to
allow noncontact cleaning except for the cleaning
solution. It has a concentrated high power which results
in reduced cleaning times. It generally is used for
cleaning wire, strip, tube, cable, and rod configurations.
The cylindrical form allows items to feed through
withoutjDending and is easily adaptable to varying
customer line speeds. ,
Because of the simplicity of the equipment and the
decreased cleaning time, there is a savings in labor
costs when using ultrasonics. This savings along with
that from decreased solvent purchase and disposal
costs, offsets the capital cost of the equipment in a
short time. Although costs vary for specific equipment,
the cost for an ultrasonic cleaner console with a
25"x18"x15" chamber is approximately $10,000. A rinse
console and dryer console would add about $4,000
each. Of course, smaller units can be obtained and
existing tanks often can be used if a transducer is
added.
The Ross Gear Division of TFIW Inc. a manufacturer
of hydraulic motors, hydrostatic steering units, and
manual steering gears has been using an' ultrasonic
cleaner since December, 1987"(U.S. EPA, 1991).
19
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TRW uses an intensive machining process known as
lapping to improve the surface finish of parts. Lapping
uses an abrasive material that must be completely
removed after finishing. Prior to installation of the
ultrasonic cleaner, TRW used a solvent (trichloroethyl-
ene, TCE) vapor degreasing system to remove the
compound. In 1987, this resulted in approximately
14,090 Ib of TCE still bottoms, 3,740 Ib of filtration
powder, and 50,300 Ib of fugitive and stack emissions.
associated
with aof&tis can jtes
By using a three-stage ultrasonic system washer, TRW
has eliminated the use of TCE. The alkaline solution is
sent to an ultrafiltration unit to remove oils and then is
discharged to the sanitary sewer. This has resulted in a
50% reduction in the quantity of hazardous waste
generated at the plant, and thus a significant decrease
in disposal costs. Ultrasonic cleaning also has eliminat-
ed the potential health hazards associated with TCE.
ซ
A military avionics overhauler has converted several
processes to use ultrasonics. In one metal-cleaning
operation, the use of 1 000 gallons of 1 ,1 ,1-
trichloroethane and Freon 113 was eliminated. The
ultrasonic process uses 200 gallons of recoverable
water and results in savings of over $8,000 per month.
In another process, rings and gaskets were cleaned
manually. This labor-intensive method was replaced by
ultrasonic cleaning, resulting in a savings of more than
1800 labor hours per year.
Benefits
The ultrasonic technology offers many advantages:
Ultrasonic cleaning can reach into crevices and
small holes where conventional methods may not
reach.
Ultrasonics removes inorganic particles ais well
as oils.
Processing speed can be increased.
Health hazards are greatly reduced.
A lower concentration of cleaning solution can be
used and possibly fewer toxic agents such as
neutral or biodegradable detergents can be
employed.
Although capital costs may be higher with
ultrasonic cleaning, reduced solvents expense
can often pay for a system in a short period of
time.
Limitations
The ultrasonic technology has several potential limita-
tions:
ซ Wastewater generated has to be treated and
discharged. j
Ultrasonic cleaning requires that jthe part be
immersible in the cleaning solution.
Dryers may need to be employed to obtain a dry
part. ,
Testing must be performed to obtain the optimum
combination of cleaning solution ^concentration
and cavitation level. i
The electrical power required for|large tanks
generally limits part sizes that can be cleaned
economically. . ;
The tendency for thick oils and greases to absorb
ultrasonic energy may limit their removal.
Operating parameters have to be more closely
monitored. >
References
Burstein, E. 1989. "Ultrasonics and the Plater." Prod-
ucts Finishing. September, pp. 60-65.
Fuchs, J. 1991. "Ultrasonic Cleaning." [Metal Finishing
Guidebook and Directory. Metals and Plastics
Publications, Inc., Hackensack, New Jersey, pp.
1,35-140. ,
Magnapak. 1988. 20 kHz Magnapak and Magnatrak
Ultrasonic Cleaning Equipment. Brochure from
Magnapak Ultrasonics by Branson. June.
Scott, J. W. 1989. MagnaSonic Energy and Biodegrad-
able Solutions to Replace Chlorinated Solvent
Cleaning. MagnaSonic Systems, Inc., Xenia, OH.
U.S. Environmental Protection Agency. 1991. "TRW,
Ross Gear Division, Greeneville, Tennessee."
Achievements in Source Reductiori~and Recycling
for Ten Industries in the United States. Risk Reduc-
tion Engineering Laboratory, Off ice'of Research and
Development. September.
Low-Solids Fluxes
Pollution Prevention Benefits |
i
Traditionally, environmentally harmful jchlorofluorocar-
bon (CFC) compounds were used in the electronics
industry to clean the residue left behirjd by con-
ventional fluxes. Using low-solids fluxes (LSFs) prior to
soldering leaves little or no visible residue on printed
circuit boards (PCBs). Therefore, cleaning with sol-
vents is not needed. |
How Does It Work? i ,
Fluxes are used to promote the wettability needed to
produce a good solder joint. They also reduce the
20
-------
effect of the inevitable entrapment of air during paste
deposition by providing a barrier to the oxidation and
reoxidation of metals during the liquefaction stage of
soldering. The disadvantage of most fluxes is that they
leave residues. These residues can jeopardize the
functional reliability of solder joints or circuitry and can
interfere with subsequent process steps such as testing
or coating, or they may be aesthetically undesirable. A
low-solids flux (also known as no-clean flux) leaves
minimal residues that generally are considered noncor-
rosive and have high insulation resistance. Therefore,
cleaning is not necessary.
Operating Features
Low-solids fluxes contain only 1 to 10% nonvolatile
materials by weight, compared to the 15 to 35% found
in conventional fluxes. Low-solids fluxes leave little
residue. Any residue that does remain dries rapidly.
, Low-solids fluxes are noncorrosive and have high
insulation resistance. Therefore, trace residues need
not be removed in most cases. However, even trace
residues may affect the reliability of certain products. To
reduce residues even further and improve the reliability
of the component, two processes maybe considered:
the low-solids flux applicator and inert atmosphere
ovens. The low-solids flux applicator is available to
electronic manufacturing companies. Inert ovens are
widely available, and many manufacturers are now
making their standard ovens with options that allow
easy conversion to an inert atmosphere. These two
processes can be used together or separately.
\
The LSF applicator was developed by AT&T. The LSF
applicator is designed to apply less flux just prior to
wave-soldering than do conventional methods. The
applicator contains a spray fixture that can be adjusted
very precisely to achieve controlled uniform flux
coverage. The spray fixture produces a fine, precisely
directed spray pattern and oscillates at a speed deter-
mined by the operator or regulated automatically
against conveyor-line speeds. The operator controls
flux deposition over a wide range by adjusting the air
pressure applied to the flux tank.
The second method uses an inert atmosphere. Be-
cause the small quantity of organic solids in low-
residue solder pastes is volatile, reoxidation of exposed
surfaces during reflow is a major cause of poor solder-
ing. Eliminating oxygen by creating an inert atmo-
sphere (e.g., nitrogen) improves solder reliability with
low-solids fluxes. ;
One benefit of this technology is that it is relatively easy
to convert an existing system. A low-solids flux is
substituted for conventional fluxes. In some instances,
equipment will need to be added to improve product
quality. Examples are thei flux applicator described
above and the inert atmosphere oven described in the
next section. ;
The use of low-solids fluxes (Joes not require any
special knowledge beyond that required for use of
conventional fluxes. It does, however, require greater
process control, particularly in the area of component
cleanliness. Soils on circuit boards and components
that were not noticed previously may cause solder
defects with low-solids flux. Cleanliness requirements
for components should be investigated during the
evaluation of this technology. Additional discussion of
low-solids fluxes can be found in Hwang (1990) and
U.S. EPA (1990 and 1991).
Application
The lowrsolids flux technology is applicable to the
electronics industry where fluxes are needed to pro-
mote wettability so that sound solder joints can be
formed. The LSF applicator would be applicable for
through-hole component circuit boards only.
Equipment may need to be altered to use low-solids
fluxes, especially if an inert atmosphere (described in
next section) is needed for best results. The purchase
and application costs of low-solids fluxes are com-
, parable to those for conventional fluxes. Economic
benefits result from eliminating the cleaning step.
The AT&T plant in Columbus, Ohio converted com-
pletely to a low-solids flux system in August, 1988.
AT&T's system consists oif a low-solids flux used with
their patented LSF-2000 flux applicator. With this
system, the plant has eliminated post-solder cleaning
and the use of 30,000 gallons of perchloroethylene
(PCE) annually (U.S. EPA; 1991). Using the flux
applicator also has reduced the amount of flux material
product used by approximately 2,000 gallons per year.
Cost savings at the plant are estimated at $145,000 per
year as a result of the decreased need to purchase,
treat, track, and report on this solvent.
Benefits
Benefits of using this technology are as follows:
It is easy to convert to this technology.
Low-solids flux eliminates the need for defluxing
and for the use of solvents.
21
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"Bed of nails" testing on printed circuit board
assemblies can be performed immediately after
wave soldering, without the problems created by
the presence of rosin residues.
This technology has low capital costs.
Limitations
The limitations of low-solids fluxes are
ซ Special equipment, such as an LSF applicator,
may be required in some cases.
Even limited residues are unacceptable in many
military specifications.
The activity of these fluxes is usually limited to a
short dwell time.
Lack of adhesion caused by the washing effect of
a jet wave sometimes leaves too little active flux
at the exit point in the wave to achieve accept-
able soldering results. Use of a spray fluxer and
a single surface-mount solder wave application
can mitigate these problems.
There may be tighter cleanliness requirements
for components and circuit boards.
References
Hwang, J. S. 1990. "No-Clean Soldering and Solder
Paste." Circuits Manufacturing. September, pp. 41-
46.
U.S. Environmental Protection Agency. 1990. "Low-
Solids FluxesrNo-Clean' Assembly." Manual of
Practices to Reduce and Eliminate CFC-113 Use in
the Electronics Industry. March.
U.S. Environmental Protection Agency. 1991. "AT&T
Bell Laboratories/AT&T Network Systems,
Princeton, New Jersey/Columbus, Ohio." Achieve-
ments in Source Reduction and Recycling for Ten
Industries in the United States. Risk Reduction
Engineering Laboratory, Office of Research and
Development. September.
Inert Atmosphere Soldering
Pollution Prevention Benefits
Traditionally, environmentally harmful chlorofluorocar-
bon (CFC) compounds have been used to clean the
residue left behind by the fluxes in the electronics
industry. An inert soldering atmosphere can eliminate
the need for flux and, consequently, for cleaning
(no-clean soldering). Without the cleaning process,
solvents are not needed. Due to the volatile nature of
even low-solids fluxes, reoxidation of exposed surfaces
during ref low is a major cause of poor [soldering. One
solution is to eliminate oxygen.
How Does It Work? '
When the solder station is placed in ah oxygen-free
environment, there is no need for traditional flux to
keep the solder wave oxide-free. This approach
requires an inert atmosphere. Because of its availability
and low cost, nitrogen often is used.
Operating Features !
The function of an inert atmosphere in the no-clean
process is to create a solder wave upon which no
permanent oxide film can form. The inert atmosphere
thus eliminates the need for flux to cle^an the surface of
the wave There are two no-fiux machine concepts on
the market: open and closed. The open-concept
machine, which employs flaps leading into a tunnel, will
not reach the desired oxygen rate of linder 10 ppm by
continuous nitrogen flow alone. This system uses
formic acid to reduce the oxygen level. Although this
system has the advantage of mechanical simplicity,
formic acid is potentially hazardous, and therefore is
undesirable or, in some companies, prohibited. A
closed system can prevent oxidation without the use of
aggressive chemicals.
In a closed system, there are vacuurn chambers at the
entrance and exit of the process area, which is con-
stantly flushed with nitrogen to keep the oxygen level
within limits. This concept also uses a tunnel, but this
tunnel is absolutely sealed to the outride environment.
The vacuum chambers allow a continuous flow of
PCBs through the system while maintaining a nitrogen
atmosphere. ;
Gas consumption depends on the design and operating
parameters of the reflow equipment. [The key operating
parameters for atmosphere in a furnace are gas flow
rate, oxygen level, water-vapor level, belt speed, and
temperature profile. ;
Most no-clean applications will yield exceptional results
in the 500- to 1 ,000-ppm range. Zonjsd forced-convec-
tion reflow soldering ovens can efficiently maintain inert
atmospheres below 500 ppm oxygen and approaching
250 ppm. Achieving oxygen levels below 250 ppm
requires the use of nitrogen volumes so large that they
negate any potential cost benefit. Additional discussion
on inert atmosphere soldering can be found in Hwang
(1990), Morris and Conway (1991), Trovato (1991), and
Tuck (1991). > i
22
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Application
This technology is applicable in the electronics industry.
The use of nitrogen is beneficial in some applications,
particularly with fine-pitch assemblies and certain no-
clean formulations. A large number of new furnaces are
now available that have inert gas capability, including
the popular forced-convection type. It may also be
possible to use existing or retrofitted equipment.
The operator skill level is somewhat higher than that
needed for existing operations because the operating
parameters require greater control. Costs vary widely
depending on existing operations. If .existing or retrofit-
ted, equipment cannot be used, it may be necessary to
purchase a new oven capable of maintaining an inert
atmosphere.
8ft-
Tradeoffs can be seen in terms of cost. When the need
for cleaning is eliminated, costs are reduced because
there is no need for costly solvents or for time spent on
cleaning. In addition, there is no solder waste through
oxidation and possibly no need for flux. These costs
are offset by the cost of nitrogen. Industrial nitrogen
with less than 3 ppm oxygen costs around $0.15/m3.
Consumption is approximately 10 to 20 m3/hr, depend-
ing on the production rate.
Another consideration is oxygen level and nitrogen
consumption rate. The better enclosed the oven and
the higher the gas flow rate, the lower the overall
oxygen content. These levels affect the quality of the
finished product as well as the economics of the
system.
Rather than complete elimination of flux, the use of an
inert atmosphere combined with low-solids fluxes is
somewhat common. For example, as of July, 1991 ,
AT&T had five production lines capable of ref lowing no-
clean paste under a nitrogen blanket, and that figure
was expected to double by the end of the year.
The use of an inert atmosphere to eliminate flux is still
undergoing evaluation by many plants. One such
company performed tests on 500 production boards to
compare soldering in an inert atmosphere to conven-
tional soldering. Short circuits and solder bridging on
small outline integrated circuit (SOIC) leads decreased
in inert atmosphere soldering. The bridging between
closely spaced metal electrode faces (MELFs) in-
creased due to the higher surface tension of the solder
in the presence of nitrogen. Bridging could be avoided
by improving the board layout.
Benefits
Inert atmosphere soldering has the following benefits:
Eliminating flux eliminates the need for solvent
cleaning.
Eliminating both the use of flux and cleaning
results in a simpler process, resulting in eco-
nomic and process time savings.
Eliminating flux also eliminates any flux residue
that may cause reliability problems.
Reduced solder dross may be achieved
Limitations !
The limitations of inert atmosphere soldering are as
follows:
Compared to conventional methods, the no-clean
alternative requires tighter control and higher
precision in the refilow process. The temperature
profile, gas flow rate, and other operating param-
eters have to be controlled closely.
A preparation fluid containing adipic acid additive
is sometimes required to achieve wetting. This is
similar to the use of a low-solids flux, except that
only a small amount of the organic acid is
needed. It is believed that this will be washed off
by the solder wave, therefore eliminating the
need for solvent cleaning. Of course, it may still
be necessary to meet military specifications for
cleaning.
A slightly higher skill level is required for opera-
tion.
Inert atmosphere soldering may involve an initial
, capital outlay for a new furnace.
References
Hwang, J. S. 1990. "No-Clean Soldering and Solder
Paste." Circuits Manufacturing, September, pp. 41-
46.
Morris, J. R., and J. H. Conway. 1991. "No-Clean
Reflow Process Implementation." Circuits Assembly,
August, pp. 28-35. ;
Trovato, R. A., Jr. 1991. "Inerting the Soldering Environ-
ment." Circuits Assembly, April.
Tuck, J. 1991. "A New No-Clean World?" Circuits
Assembly, July, pp. 18-19.
23
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SECTION 3
EMERGING TECHNOLOGIES
How to Use the Summary Tables
Six emerging cleaner process changes for cleaning
and degreasing are evaluated in this section:
Vapor storage technology
Vacuum furnace
Laser cleaning
Plasma cleaning
Fluxless soldering
Replacement of tin-lead solder joints.
Tables 6 and 7 summarize descriptivejand operational
aspects of these technologies. The tables contain
evaluations or annotations describing each emerging
technology and give a compact indication of the range
of technologies covered to allow preliminary identifi-
cation of those that may be applicable! to specific
situations. !
Descriptive Aspects |
Table 6 describes each emerging clearer technology. It
lists the Pollution Prevention Benefits, Reported
Table 6. Emerging Technologies for Cleaning and Degreasing: Descriptive Aspects
Emerging
Technology
Type
Vapor Storage
Technology
Vacuum
Furnace
Laser
fMaaninn
Pollution Prevention
Benefits
Reduces amount
of solvent used
Eliminates solvent
use for cleaning
Eliminates solvent
use for cleanina
Reported Applications
Vapor degreasing
Dry cleaning
Removal of oils
from metals
Cleans metallic or
nonmetallic surfaces
Benefits
Decreases potentially .
hazardous emissions from
vapor degreasers
One-step process
Newer processes collect
the oil for recycling;
therefore
waste stream
is eliminated
Can clean with high
spatial selectivity
Limitations
solvent use
Typical processes do
not allow for oil re-
cycling. If oil is not
collected, it can de-
grade the diffusion
pumps; so frequent
cleaning would be
necessary
Requires a special
cleaning chamber
Plasma
Cleaning
Fluxless
Soldering
Eliminates solvent
use for cleaning
Reduces solvent
use in cleaning
Reduces hazardous
fluxes
Cleans metallic or
nonmetallic surfaces
Electronics industry
Cleaning process is
fast and energy-
efficient
Performs ultrafine
cleaning
Reduces process steps
i Requires a special
J cleaning chamber
i Relatively slow
|process
1 Some materials could
I be degraded by
certain fluxless
processes
Replacement
of Tin-Lead
Solder Joints
Eliminates solvent
cleaning and
hazardous fluxes
Electronics industry
Reduces process steps
Eliminates hazards of
lead compounds
May replace hazards
F of lead compounds
! with hazards of silver
24
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Table 7.
Emerging Technologies for Cleaning and Degreasing: Operational Aspects
Emerging
Technology Type
Vapor Storage Technology
Vacuum Furnace
Process
Complexity
Medium
Medium
Required
Skill Level
Medium
Medium
Waste Products
. and Emissions ,
Residual air losses and still
bottom residues
Oils, or if recycling is used, no
waste products
References
Hickman and Goltz, 1991
Mitten, 1991
Laser Cleaning
Medium
Medium
Only the contaminants removed
from the part
Plasma Cleaning
Medium
Medium
Only the contaminants removed
from the part
Allen, 1991
Allen etal., 1992
Kuperand Brannori, 1991
Lee etal., 1992
Peebles etal., 1990
Peebles et al., 1991
Tarn etal., 1992
Walters et al., 1993
Watanabe and Bison, 1992
Zapkaetal., 1991,1992
Baker, 1980
Brunner, 1992
Cobum, 1991
Horwath and Moore, 1983
IBM, 1986
Kominiak and Mattox, 1977
Liston, 1989
O'Kane and Mittal, 1974
Ward and Buss, 1992
Fluxless Soldering
Replacement of Tin-Lead
Solder Joints
Varies
Medium
' Varies.
Low
No waste products
No waste products
Hosking, 1990
' Werther, 1990
Application, Operational Benefits, and Limitations
of each.
Operational Aspects
Table 7 shows key operating characteristics for the
emerging technologies. The qualitative rankings are -'
estimated from descriptions and data in the technical
literature.
Process Complexity is qualitatively ranked as "high,"
"medium," or "low" based on such factors as the
number of process steps involved and the number of
material transfers needed. Process Complexity is an
indication of how easily the technology can be inte-
grated into existing plant operations. A large number of
process steps or input chemicals, or multiple opera-
tions with complex sequencing, are examples of
characteristics that would lead to a high complexity
rating.
The Required Skill Level of equipment operators also
is ranked as "high," "medium," or "low." Required Skill
Level is an indication of the level of sophistication and
training required by staff to operate the new technology.
A technology that requires the operator to adjust critical
parameters would be rated as having a high skill
requirement. In some cases, the operator may be
insulated from the process by complex control equip-
ment. In such cases, the operator skill level is low, but
the maintenance skill level is high.
Table 7 also lists the Waste Products and Emissions
from the emerging cleaner technologies. It indicates
tradeoffs in potential pollutants, the waste reduction
potential of each, and compatibility with existing waste
recycling or treatment operations at the plant.
The last column in Table 7 cites References to publica-
tions that will provide further information about each
emerging technology. These references are given in full
at the end of the respective technology sections.
The text further describes operating characteristics,
application, benefits, and limitations for each technol-,
ogy. Technologies in more advanced stages of develop-
ment are discussed in Section 2, Available
Technologies.
25
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Vapor Storage Technology
fe w
-------
signs, these vapors are then condensed and collected
for later removal to be reprocessed or recycled. This is
essentially a one-step process. Another recent design
of the vacuum furnace for deoiling is a hot-wall design
that eliminates furnace wall oil deposits caused by
condensation. There is no condensation because the
walls are at a temperature above that which will
condense the vapor.
The temperature and time requirements are based on
the parts material and the oil being removed. Operating
temperatures range from 2iO to 650ฐF, and the vacuum
range is 100/20,000 u.. Atypical cycle time is approxi-
mately 15 to 20 min. The operator loads the parts into
the chamber, selects the cycle based on the type of
parts and oil, and then removes the cleaned parts.
Periodically, the operator must defrost the condenser
and drain the oil for recycling.
The use of a vacuum furnace to deoil metal parts
produces a small waste stream consisting only of the
oil removed from the part. With the proper equipment,
the oil can be recycled and reused or sold. Either
option would result in no waste stream. The pollution
prevention benefits of such a technology are great.
Both a solvent waste stream and a contaminant waste
stream are eliminated.
Vacuum furnaces are available from a variety of
vendors. However, newer equipment, especially that
designed for deoiling parts, is less common but is
available from some vendors.
" " Vgcvum 'iHeoilfng vwbe af&f
Vacuum furnace deoiling pan be applied where vapor
decreasing typically is used to clean metal parts. Other
typical applications include removal of paint solvents;
drying of ink/paint designs; and precleaning for brazing,
plating, or heat treating. The technology also can be
used to remove oil from nonmetallic parts.
One study (Mitten, 1991) conducted an economic
comparison of vacuum furnace deoiling with vapor
degreasing. Although capital costs for vacuum furnace
deoiling were higher, the estimated operating costs
were lower, resulting in a payback of approximately 2
years for a 4,000 hr/yr operation. The capital cost for
vacuum deoiling was $1 92,000 for a system that would
accept a work load with dimensions 30"W 36"H 48"L,
and the operating cost was estimated at $5.20/hr.
A number of companies already use vacuum furnace
deoiling to clean metal parts. However, newer versions
of the technology are available. For example, newer oil
collection and recycling equipment as well as new hot
wall design techniques that eliminate problems
associated with condensation on the walls are now on
the market.
The major benefits of vacuum furnace deoiling over
vapor degreasing are the, pollution prevention benefits
and the health and safety benefits resulting from
solvent elimination. Another benefit of vacuum furnace
deoiling compared to other cleaner technologies is that
the cleaned parts do not become water soaked and
therefore do not need to be dried after the cleaning
process.
' One limitation is that the processing time and tempera-
ture depend on the material to be cleaned and the oil to
be removed. Therefore, adjustments may be needed
for each new material, oil, or combination thereof. Also,
the part must be able to withstand the required tem-
perature and vacuum pressure.
Reference
s -
I
Mitten, W. 1991. "Vacuum Deoiling for Environmentally
Safe Parts Cleaning." Metal Finishing, 80(9):29-31.
Laser Cleaning
The use of laser cleaning to clean material surfaces is
being explored by Sandia National Laboratories in
Albuquerque, New Mexico. Short pulses of high-peak-
power laser radiation are used to rapidly heat and
vaporize thin layers of material surfaces. These layers
of surface material form a dense cloud of hot vapors
that will condense and reeontaminate the surface if not
removed immediately. To prevent recontamination, the
vapors are removed by eritrainment into a flowing gas
stream. Laser cleaning must be carried out in an inert
gas environment to avoid further contamination.
Localized cleaning is an operational advantage of laser
cleaning. "With this technology, a small area can be
cleaned without affecting the entire part. Laser cleaning
contributes directly toward meeting waste minimization
goals no solvents or even aqueous solutions are
needed. The only waste is the small amount of material
removed from the surface of the item being cleaned.
27
-------
Laser cleaning has been contemplated since the
1960s, but has been implemented in only a limited
number of applications. Most notably, lasers have been
used, and continue to be used, for cleaning statuary
and aging paintings. Also, lasers have been used to
strip paint from metal and composite substrates and to
strip insulation from conductors.
On$fafm^f laser cf&atfng,
tttfn /a/ers
Laser cleaning can be performed on either metallic or
nonmetallic surfaces. There are at least two mecha-
nisms in laser cleaning of metallic surfaces: laser
ablation of absorbing contaminants and laser-driven -
blowoff of transparent contaminants. Laser ablation
vaporizes thin layers of contaminants at the air-con-
taminant interface. Efficient use of this requires that the
contaminant be strongly absorbing at the laser wave-
length. Typically, in the absence of a strong absorption
peak, far ultraviolet (UV) wavelengths are preferred
because broadband adsorption occurs in most materi-
als in the far UV wavelength region. Also, more efficient
contaminant removal is found for short-pulse-width,
high-peak-power lasers.
laser. This type of laser can be used to (advantage in
reducing chemical usage in processes such as fluxless
soldering (Peebles et al., 1991), which js discussed in
this Emerging Technology section. The [threshold for
efficient film removal varies only slightly with film
composition, but typically is about 1 J/cm2 in all cases
reviewed. Laser cleaning typically is done in a cleaning
chamber that is either evacuated or pressurized with an
inert gas such as helium or argon, witlrthe laser beam
introduced through a window. However, it is feasible to
perform cleaning in a clean room with a fume hood and
a flowing inert gas. '
The bulk of literature on laser cleaning|of nonmetallic
surfaces has been directed toward the;cleaning of
semiconductor materials. The primary ^semiconductor
contaminants that have been laser cleaned are oxide
films and absorbed metal ions. Over trie past 25 years,
ruby, Nd:YAG, alexandrite, and CO2 lasers have been
used for cleaning, but more recently, excimer lasers
have been the focus of most laser cleaning of semicon-
ductor substrates. '
parities]" actually gets urt\^ ^.^w, we* ^ '
The blowoff mechanism occurs when the material is
mostly transparent to the laser beam, which passes
through the contaminant and initiates a microexplosion
at a subsurface site, either at small absorbers within
the contaminant or at the metal surface. The trapped,
expanding vapors generated from the microexplosion
pop off relatively large pieces of contaminants. The
blowoff mechanism is, in general, more efficient than
ablation because less energy goes into the heat of
vaporization to remove trie same mass of material.
A third possible mechanism has been conjectured by
Walters et al. (1993) that is believed to be responsible
for a cured epoxy removal process discovered recently.
In this process, the semitransparent epoxy material
transmits enough of the pulsed laser beam to heat the
metal substrate. Differential thermal expansion leads to
a debonding of the material in one pulse. No damage
to the substrate occurs, and no significant effluent is
generated.
"t "V X-> """ v ' ' " '. - <
A pulsed laser c4nb& used & advantage in
'"
Because of the lack of high surface reflectivity of many
nonmetallic substrates, the only laser cleaning mecha-
nism identified in the literature reviewed is the ablation
process discussed above. Consequently, UV lasers,
such as krypton-fluoride excimers, have been used
most for this cleaning application (Kuper and Brannon,
1991 ; Watanabe and Bison, 1992). UV wavelengths
can clean more efficiently than other laser wavelengths
because the absorption depth into oxides is much
smaller (typically about 10 nm) in the UV region.
The most common contaminants found in the literature
consist of oxide films that have been effectively re-
moved using a pulsed yttrium aluminum garnet (YAG)
*- - " " ta&er dotting.
' \
Liquid-assisted laser cleaning is a variation in the use
of lasers for cleaning. Many repair an|d maintenance
tasks require final cleaning of the surfaces in a clean
room environment using methods that achieve very low
numbers of residual particles on the part surface.
These micron-size particles are extremely difficult to
remove because the binding forces (Van der Waals,
capillary, and electrostatic) holding them on the surface
are much greater than gravitational and inertial forces
at this particle size. Traditionally, a filtered Freon
wash performed in a laminar flow clean station is used
for this step. A similar problem arises in semiconductor
device microfabrication, where microh-size particles
cause defects on the same scale as that of the micro-
structure being produced in the process. Two groups
have developed benign-liquid-assisted laser-cleaning
techniques that have successfully aqhieved paniculate
28
-------
removal without Freon or harsh solvents. It is impor-
tant to note that the liquid does not reside on the
surface for longer than a second or so.
Zapka and colleagues at IBM (Tam et al., 1992; Zapka
et al., 1991 and 1992) have developed a technique
wherein a very thin volatile liquid layer (water, ethanol,
methanol, isopropanol, and mixtures thereof) is formed
on the surface to be cleaned just before delivery of a
short laser pulse. The liquid works its way under the
particles by capillary action and is explosively evapo-
rated by conduction of heat from the substrate which is
heated directly by'the laser pulse. The IBM researchers
conducted most of their research on silicon surfaces
exposed to 16-ns laser pulses from a KrF excimer laser
(0.248 urn wavelength). They consider the process a
dry-cleaning process because 'of the short residence
time of the liquid on the surface.
In similar work at the University of Iowa, Allen and
colleagues (Allen, 1991; Allen et al., 1992; Lee et al.,
1992) have cleaned micron- and submicron-size
particles from silicon substrates using a slightly differ-
ent approach. They use a laser wavelength that is
absorbed directly in the liquid that is deposited as the
assist layer rather than in the substrate itself. Water
was found to be the best liquid and the CO2 TEA laser
(100-ns pulse, 1-u.s tail) with 9.6- and 10.6-u.m wave-
lengths was used in most of their research. Both this
method and the IBM approach appear to work well in
most cases.
There are several advantages of laser cleaning:
The area to be cleaned can be highly selective
and sharply defined.
The process generally is very fast and energy
efficient.
No foreign atoms are introduced to the surface
as in ion bombardment techniques.
If cleaning is done in vacuum, the vacuum is not
compromised because the laser source can be
located outside of the cleaning chamber.
Thermal diffusion of bulk impurities to the surface
is avoided because of the extremely large
quenching rate afforded by very short pulses
available.
The removal rate can be easily controlled by
changing the beam fluence or pulse repetition
rate.
Laser cleaning is amenable to "dry" effluent
control through cover gas filtering.
Liquid-assisted laser cleaning removes micron-
size particles.
The blowoff mechanism is more efficient that ablation,
but the blowoff cleaning process is self-limiting at laser
29
wavelengths that are reflected by the metal surface,
provided that the beam fluence is below the damage
threshold of the substrate. Self-limiting behavior has
been observed by Peebles et al. (1991) using a YAG
laser to clean oxide from stainless steel (SS) 304, but
the limiting behavior should be dependent only ori the
reflectivity of the substrate to the laser wavelength and
generally is applicable.
References
Allen, S. D. 1991. "Method and Apparatus for Remov-
ing Minute Particles From a Surface." U.S. Patent
#4,987,286, January 22.
Allen, S. D., S. J. Lee, and K. Imen. 1992. "Laser
Cleaning Techniques for Critical Surfaces." Optics &
Photonics News, June, pp. 28-30.
Kuper, S., and J. Brannori. 1991, "Krypton Fluoride
Laser Ablation of Polyurethane." SPIE, 1598, Lasers
in Microelectronic Manufacturing, pp. 27-35.
Lee, S. J., K. Imen, and S. D. Allen. 1992. "CO2 Laser
" Assisted Particle Removal Threshold Measure-
ments." >4pp//ec/P/?ys/cs Letters, 67(19):2314-2316.
Peebles, H. C., D. M. Keicher, F. M. Hosking, P. F.
Hlava, and N. A. Creager. 1991. "Laser Ablative
Fluxiess Soldering (LAPS): 60Sn-40Pb Soldering
Wettability Tests on Laser Cleaned OFHC Copper
Substrates." ICALEO, pp. 186-202.
Tam, A. C., W. P. Leung, W. Zapka, and W. Ziemlich.
1992. "Laser-Cleaning techniques for Removal of
Surface Particulates," Journal of Applied Physics
77(7) :3515-3523. '
Walters, C. T, J. L. Dulaney, and B. E. Campbell. 1993.
Advanced Technology Cleaning Methods for High-
Precision Cleaning of Guidance System Compo-
. nents. Summary Report, Contract No.
F04606-89-D-0034/DO'Q807, Department of the Air
Force, Aerospace Guidance and Metrology Center,
Newark AFB, Ohio.
Watanabe, J. K., and U. J/Bison. 1992. "Excimer Laser
Cleaning and Processing of Si(100) Substrates in
Ultrahigh Vacuum and Reactive Gases," Journal of
Vacuum Science Technology A, 10(4)-.823-828.
Zapka, W., A. C. Tam, and W. Ziemlich. 199.1. "Laser
Cleaning of Wafer Surfaces and Lithography
Masks." Microelectronic Engineering, 13:547-550.
Zapka, W., A. C. Tam, G. Ayers, and W. Ziemlich. 1992.
"Liquid Film Enhanced Laser Cleaning." Microelec-
tronic Engineering, 77:473-478.
-------
Plasma Cleaning
Plasma cleaning is one type of surface processing,
among several, that depends on production of a low-
pressure steady-state plasma in a special vacuum
chamber. These processes include sputter deposition,
ion plating, plasma-enhanced chemical vapor deposi-
tion, etching, cleaning, and surface modification. These
techniques are fairly well developed and are used
widely in industry. Plasma cleaning is, by its nature, a
batch process and has a relatively low cleaning rate
that is most appropriate to removal of thin contaminant
films. A review article by Coburn (1991) presents a
good basic overview of plasma surface processing and
a guide to terminology used in the field.
In typical plasma processing arrangements, the excita-
tion of the gas may be from a direct current (DC),
radiofrequency (RF), or microwave power source. A
simple DC low-pressure glow discharge can clean
effectively if electrodes can be permitted in the dis-
charge chamber. If contamination from electrode
sputtering is a concern, inductive or capacitive coupling
from an RF circuit can produce an electrodeless
plasma discharge that will clean a part surface. The
typical frequency for the RF discharge is 13.56 MHz.
Additional processing control can be achieved by
separating the plasma generation function from the
surface interaction control function as in the microwave
plasma generation approach. All of these geometries
may be used to clean surfaces, but selection of one or
another will depend on the substrate material (conduc-
tor, insulator, etc.) and the nature of the contaminant.
affApsma cleaning ate *fte
hose used fo plasma eteMn$*
interested in increasing the adhesion of a lubricating
film to the gyro's shaft and rotor, which required
furnishing a clean surface to the lubricant. The main
alternative to glow discharge cleaning of a gyro surface
was to employ organic solvent, detergent, and water
baths followed by a light abrasive buffing to prepare
bearing surfaces for the lubricant. Thisitime-consuming
process involved days to complete. By contrast, glow
discharge could clean bearing surfaces in minutes. In
the cleaning tests, an argon plasma produced in an
industrial plasma cleaner was used. Typically, the
contamination layer to be removed was only 1 to 3
molecules in depth. ;
Plasma cleaning works by the same principles as
etching. If an inert gas is used, the ions and neutrals in
the plasma bombard the surface to be cleaned and
sputter off the contaminant film molecule by molecule,
in a purely physical manner. By using a reactive gas in
the plasma, the bombarding ions also may react with .
the contaminants and form gaseous species that
evaporate from the surface. For energetic ions, the
process known as reactive ion etching is used in
microfabrication as well as in cleaning. Examples of
plasma cleaning processes for metallic and nonmetallic
surfaces are discussed in the following paragraphs.
Plasma cleaning has been used since 1968, when it
was found to be effective in guidance system compo-
nent cleaning. Initial research in this area reported an
investigation of the effectiveness of glow discharge
cleaning of gas-bearing gyros. The researchers were
O'Kane and Mittal (1974) compared traditional solvent
cleaning with RF plasma cleaning for preparing
rhodium and iron-cobalt surfaces to repeive a vapor-
deposited polymer film. They used water wettability and
Auger electron spectroscopy to measure the cleanli-
ness of the surfaces. Their results showed that argon
or helium-oxygen plasma cleaning was more effective
than solvent cleaning in removing sulfur and carbon
contamination. No damage to the magnetic properties
of the surface was observed.
!
In another study motivated by the need for good
adhesion and interface bonding in depositions (in this
case an aluminum deposition), Kominiak and Mattox
(1977) found that a reactive plasma cleaning was most
effective for their titanium, SS-304, Kqvar, and Ni-Co
steel substrates. Using soft X-ray appearance potential
spectroscopy to check cleanliness, they obtained the
best results in carbon residue reduction with low-
voltage RF sputtering (300 V) with an'Ar-HCI mixture
forming the plasma. ' j.
i -,
Baker (1980) studied the reactive plasma cleaning of
copper, aluminum alloy, and Inconel 625 with DC and.
RF discharges in argon-oxygen mixtures. He used a
mass spectrometer set on CO2 to mohitor carbon
evolution from the surface and calibrated the measure-
ment by etching a pure carbon film. Baker found that
reactive plasma cleaning was most elective at remov-
ing the deeply bonded carbon when the workpiece was
at cathode potential to enhance the ion impact energy.
"toasjG minimization teal
Of critical importance is the work of Ward and Buss
(1992), who recently studied plasma cleaning as a
30
-------
waste minimization tool. They experimentally investi-
gated the effect of the process parameters on the
plasma removal of thin films (1.5 to 7 urn) of pblym-
ethyl-methacrylate (PMMA) and poly-2-vinylpyridine
from a substrate located in a research chamber using
parallel plate electrodes with RF excitation. They found
that by using a 40% SFJO2 plasma that had an opti-
mized plasma pressure (250 Torr) and power density, a
removal rate of approximately 5 nm/sec (19 u.m/hr) for
PMMA could be obtained. This was an enhancement of
two orders of magnitude over the removal rates of
commercial plasma cleaners. By contrast, an optimized
argon plasma produced a removal rate of only 0.08 nm/
s.
Additional experiments addressed the removal of A-9
aluminum cutting fluid oil from substrates. The A-9 was
a mixture of hydrocarbon solvents, waxes, fatty acids,
fragrances, and dyes. In their report they did not
specify the amount of oil present in the substrates that
were tested. Using a 40% SFg/O2 plasma they were
able to obtain a removal rate of 7.5 nm/s.
^ Pt&ma cfemmgm $ m&ifo
" '
Recent research indicates that plasma cleaning also
can be of value in cleaning circuit boards. IBM re-
searchers (Horwath and Moore, 1983; IBM, 1986)
discovered that RF reactive cleaning with oxygen-
carbon tetrafluoride may be used to remove epoxy/
glass particle/copper drill smears in drilled through-
holes in printed circuit boards. Both nonreactive and
reactive plasma cleaning have been studied as a
means to prepare hybrid integrated circuits prior to wire
bonding (Brunner, 1992; Listen, 1989). Performance
exceeding that of standard solvent cleaning in remov-
ing adhesive vapor residues has been achieved as
measured in terms of wire bond yields-Plasma clean-
ing accomplishes ultrafine cleaning.
References
Baker, M. A. 1980. "Plasma Cleaning and the Removal
of Carbon From Metal Surfaces." Thin Solid Films
69:359-368.
Brunner, R. J. 1992. "Oxygen-Plasma Cleaning of
Hybrid Integrated Circuits." AT& T Technical Journal,
March/April, pp. 52-58.
*,:
Cobum, J. W. 1991. "Surface Processing With Partially
Ionized Plasmas." IEEE Transactions on Plasma
Science, 79(6):1048-1.062.
Horwath, R., and H. Moore. 1983. "Gas Plasma '
Cleaning Process for Multiwire Boards." IBM Techni-
cal Disclosure Bulletin, 25(11 AJ.-5391 .
IBM. 1986. "Board-Cleaning Technique Using Hollow
Cathode Plasma Discharge." IBM Technical Disclo-
sure Bulletin, 29(4)-.-\848--\85Q.
Kominiak, G. J., and D. M. Mattox. 1977. "Reactive
Plasma Cleaning of Metals." Thin Solid Films
40:141-148.
Liston, E. M. 1989. "Plasma Cleaning of Hybrids."
Hybrid Circuit Technology, September, pp. 62-65.
O'Kane, D. F., and K. L. Mittal. 1974. "Plasma Cleaning
of Metal Surfaces," Journal of Vacuum Science
Technology, 11(3), May/June, 1977. pp. 567-569.
Ward, P. P., and R. J. Buss. 1 992. Rapid Plasma
Cleaning as a Waste Minimization Tool. Report
#DE92-015395, Sandiia National Laboratories, -
Albuquerque, New Mexico.
Fluxless Soldering
In the electronics industry, conventional soldering
requires fluxing to promote wetting and to remove
oxidation from surfaces to be soldered. Halogenated
solvents must then be used to remove the flux resi-
dues. Saridia National Laboratories (SNL) in Albuquer-
que, New Mexico, is exploring methods of fluxless
soldering. With this approach, no flux residue is cre-
ated, and the cleaning step is eliminated. Therefore, no
solvent is needed. i
Under contract to the U.S. Department of Energy
(DOE) for the DOE weapons complex, SNL is devel-
oping four technologies that could eliminate the need
for flux (Hosking, 1990). These technologies reduce or
prevent surface oxidation prior to or during soldering,
which is the main function of flux.
Technologies
Controlled atmosphere soldering is discussed under
Inert Atmosphere Soldering in Available Technologies,
Section 2. Besides inert atmospheres, SNL is exploring
reactive plasma and a dilute acid vapor-inert gas
mixture that functions as either a protective or a
reducing cover during processing.
31
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Thermomechanlcal surface activation soldering
uses kinetic or directed thermomechanical energy to
spall or ablate the surface oxide and facilitate wetting.
Laser, solid-state diffusion, and ultrasonic soldering are
typical ways to accomplish this. These processes can
be done in air or in a controlled atmosphere.
Metallization technology involves using silver as a
nonoxidizing, readily wettable surface. The application
of silver is an exacting process and must be controlled
precisely. A thick layer generally is required to guaran-
tee complete coverage and wettability. However, extra
silver will produce a brittle solder joint. The approach is
to apply a thinner layer of silver, although this has the
risk that the coating will be porous, exposing the
underlying metallic surface to oxidation and degrading
wettability. Methods of inhibiting these effects are being
investigated.
Inhibitor technology involves protecting these porous
metallizations by applying inhibitors. SNL, in conjunc-
tion with the State University of New York at Stony
Brook, is studying the bonding behavior of organic
inhibitors on metallic surfaces and their effect on
subsequent solder wetting.
Possible limitations of these technologies include
incompatibility with processes and other materials as
well as potential underperformance of the finished
product.
Reference
Hosking, M. F. 1990. "Reduction of Solvent Use
Through Fluxless Soldering." In: Solvent Substitu-
tion:A Proceedings/Compendium of Papers. The
U.S. Department of Energy, Office of Technology
Development, Environmental Restoration and
Waste Management and U.S. Air Force, Engineer-
ing and Services Center.
Replacement of Tin-Lead Solder Joints
One electronics manufacturer has developed a method
of replacing tin-lead for soldering with a combination of
organic polymers (epoxies, thermoplastics, or silicones)
and conductive fillers (carbon, copper, pr silver). This
replacement would eliminate the need for fluxes and,
consequently, the need to remove flux residues
(Werther, 1990). Because the electronics industry is the
largest user of Freon-based cleaning solvents,
fluxes, and (possibly) lead, this approaph would have
significant pollution prevention benefits.
An indication of the potential magnitude of the pollution
prevention benefits can be seen at Interconnect
Systems Incorporated (ISI), an electronics manufac-
turer. Current production levels at ISI require that
approximately 5 million solder joints be made annually.
With tin-lead soldering, this would require several
thousand grams of solder paste, several dozen gallons
of flux, and more than 50 gallons of Frieon per year.
By replacing tin-lead with organic polymers and con-
ductive fibers, the use of flux and Frebn would be
eliminated. .-.!'...
ISI is currently examining the feasibility of this replace-
ment in terms of functional qualities such as the
electrical resistance in the joint, the mechanical and
electrical integrity of the joint over time, the physical
form of the adhesives, and the cost-effectiveness of the
replacement. !
Reference
Werther, W. 1990. "Definition of Electrically Conductive
Adhesives for the Replacement of Jin-Lead (Solder)
Joints in Electronic Systems." Pollution Prevention
By and For Small Business Application Submittal.
Interconnect Systems, Inc. |
32
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SECTION 4
POLLUTION PREVENTION STRATEGY
The main federal environmental regulations influencing
the application of new cleaning technologies are the
Clean Air Act Amendments (CAAA), the Resource
Conservation and Recovery Act (RCRA), the Right to
Know provisions of the Superfund Amendment and
Reauthorization Act (SARA), and the emphasis on
eliminating pollution at the source in the Pollution
Prevention Act of 1990. Solvent cleaners also increase
the potential workplace exposures to volatile organic
compounds (VOCs) regulated under the Occupational
Safety and Health Act (OSHA). There are a wide
variety of state and local limits on VOC, hazardous,
and aqueous wastes that also are of concern.
The requirements for cradle-to-grave management for
solvent waste established by RCRA create several
incentives to seek solvent-free alternatives. Disposal of
RCRA wastes (including solvent waste) is costly and
carries continued liability. RCRA also requires the
waste generator to maintain a waste minimization
program. Converting all possible plant applications to a
cleaning technology that eliminates or reduces solvent
use helps to demonstrate an effort to minimize hazard-
ous waste.
repotted scoter T&te
Since 1988, manufacturing facilities have been report-
ing emissions of more than 300 chemicals or chemical
categories. The reporting requirements are established
under Title III of SARA. The toxic chemical release
reporting usually is referred to as the Toxics Release
Inventory (TRI). The reporting rule requires annual data
on direct releases to all environmental media. Facilities
meeting the following conditions must file TRI data:
A Standard Industrial Classification (SIC) code in
the range of 20 to 39
10 or more employees
Manufacture or processing of more than 25,000
pounds or use of more than 10,000 pounds of a
chemical on the TRI list.
The reporting requirements were expanded to include
data on recycling as required by the Pollution Preven-
tion Act. The effort required to track and report chemi-
cal usage as required by these legislations is
significant. For plants that exceed the reporting thresh-
old, reducing chemical use below the threshold elimi-
nates the requirement to prepare a report for the
chemical. Commonly used cleaning solvents1,1,1-tri-
chloroefhane (TCA), trichloroethylene (TCE), methyl-
ene chloride (MC), and perchloroethylene (PCE)also
are TRI chemicals. Therefore, reducing or eliminating
the use of any such solvent will eliminate the need to
complete a TRI reporting form for that solvent.
The EPA also encourages the voluntary reduction of 17
priority toxic chemicals identified in the 33/50 Program
for early pollutant reductions (U.S. EPA, 1991 , 1992).
Several cleaning solvents are on the list of priority toxic
chemicals identified by the EPA Administrator for early
reduction in the 33/50 Program. Switching from con-
ventional solvent cleaning to a cleaner technology will
assist in meeting the reduction goal.
Another consideration is that the organic solvents used
in cleaning may result in sufficient vapor concentrations
to cause concern for workers in the area. The National
Institute for Occupational Safety and Health (NIOSH)
recommends that occupational exposure to, carcin-
ogens be limited to the lowest feasible concentration.
OSHA regulations for workplace emissions are also
becoming increasingly stringent.
33
-------
Title 111 of the CAM requires adoption of Maximum
Achievable Control Technologies (MACT) for control of
189 hazardous air pollutants (HAPs). Cleaning pro-
cesses using solvents are considered major sources of
HAPs and are subject to MACT standards. Vapor
degreasing is the single largest use for solvents,
followed by dry cleaning (clothes cleaning) and cold
cleaning (liquid solvent cleaning). Based on 1987 U.S.
EPA estimates, approximately 25,000 to 35,000 batch
vapor degreasers and 2,000 to 3,000 continuous
cleaners were used in the United States.
or operational practices. All sources in ;a source cat-
egory or subcategory will have to implement MACT.
Unless the owner of the source is eligible for the 6-year
extension (for 90% reduction), all industrial sources are
expected to be in compliance within 3 years of promul-
gation of the MACT standards. A 5-year compliance
extension also may be granted for prior installation of
Best Available Control Technology (BACT) or Lowest
Achievable Emissions Rate (LAER). ;
ฃA4A. MACT dftflcfeftfe b'm te at*
v ..., <., *.
Th& Pafktt&nPrsv&ntton
soufce recfacltoff &s)heprefefFetf method for
The Pollution Prevention Act establishes pollution
prevention as the preferred method for pollutant
management. The processes described in this docu-
ment provide promising alternatives to conventional
processes for potential users, i.e., the metal-finishing,
dry-cleaning, electronics, and any other industiy that
uses cleaning processes. Under programs such as the
U.S. EPA's 33/50 Program, industries are encouraged
to reduce pollutants voluntarily in anticipation of future
regulations, which are expected to become increasingly
stringent. The CAAAof 1990 allows the U.S. EPA to
grant a 6-year compliance extension on the MACT
compliance date to any existing source of air toxics that
reduces emissions voluntarily by 90% (95% for particu-
lates) below 1987 levels before January 1 , 1994.
MACT standards will be issued by the U.S. EPA for
new and existing sources, using the best controlled
similar sources as a measure. MACT can include
control equipment, process changes, material substitu-
tions, equipment design modifications, work practices,
' Under Title III of the CAAA, the U.S. E-PAon July 16,
1992 (Federal Register, 1992) added halogenated
solvent cleaners as an area source category. Thus,
halogenated solvent cleaners are considered a major
source category emitting at least 1 0 tons/year of any
one air toxic or 25 tons/year of any combination of air
toxics. Therefore, MACT standards can be expected to
be promulgated for these cleaners. Vapor degreasing
constitutes the single largest use of solvents in the
United States, and therefore is an important area
targeted for pollution prevention. j:
i
References
Federal Register. 1 992. The Clean Air Act Amend-
ments, Title III. Federal Register, 57(137). July,16.
i
U.S. Environmental Protection Agency. 1991. The33/
50 Program: Forging an Alliance for Pollution
-Prevention (2 ed.). Special Projects Office, Office of
Toxic Substances, Washington, D.C. July.
U.S. Environmental Protection Agency
50 Program Second Progress Report.
Office of Pollution Prevention
1992. ERA'S 337
TS-792A,
and Toxics. February.
34
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SECTIONS
CLEANER TECHNOLOGY TRANSFER CONSIDERATIONS
Alternative cleaning technologies are important for
users who want to meet or anticipate new regulations.
Such users have two options:
Use new equipment that significantly reduces
solvent emissions.
Use a semi-aqueous or aqueous cleaner.
The latter option is more attractive from the pollution
prevention standpoint, but for a variety of technical or
economic reasons, the user may choose to go with the
former. A third option, using processes that eliminate
the need for cleaning, may also be viable.
fmm &
In the metal-finishing industries with conventional open-
top vapor cleaners (OTVCs), the user could implement
an incremental approach to pollution prevention. Such
an approach would involve gradual phasing in of add-
ons such as installing OTVC covers, increasing free-
board height, changing from water-cooled to
refrigerated coils (assuming that a non-CFC refrigerant
is being used), and controlling room ventilation or
exhausts. It could be possible to achieve 90% reduc-
tion with a combination of these standards. If the
eventual MACT standard promulgated is limited to
these add-ons to existing OTVCs, the user would then
be in compliance. If the MACT standard turns out to be
more stringent (enclosed vapor degreasing), the user
could still be eligible for the 6-year extension if the add-
ons were implemented before January 1 , 1 994.
become the MACT, especially given the high capital
cost of the CEVC. However, on the assumption that
environmental regulations become more stringent over
time, the user could consider investing in a CEVC. The
CEVC brings about savings in operating costs that
would offset the higher capital cost overtime. Some
European countries regulate vapor degreasers as point
sources, and CEVC is the required technology in, those
countries. Future models of the CEVC are expected to
incorporate two carbon beds (instead of the one bed in
the current model) so that one can be desorbed while
the other is adsorbing. The extra bed would eliminate
carbon heatup and desorption stage times and in-
crease processing speed significantly for greater
operating savings.
There currently is no indication that the completely
enclosed vapor cleaner (CEVC) would immediately
The above considerations jassume that the user wants
to retain vapor degreasing; in some form and not switch
over to aqueous or semi-aqueous cleaning. Industries
have been reluctant to eliminate vapor degreasing
completely because of its advantage with certain types
of parts or simply because of tradition and ease of
operation. However, aqueous and semi-aqueous
cleaning technologies have advanced in recent years in
terms of both the type of cleaning chemicals used and
the type of equipment used. A fairly broad range of
equipment such as the automated washer (for smaller
parts) and the power washer (for larger parts) is
commercially available. These washers are much more
efficient compared to the traditional aqueous cleaning
processes of alkaline tumbling and hand-aqueous
washing. Ultrasonics affords another method for
improving cleaning efficiency in hard-to-reach places of
the workpiece. Because many aqueous washers
involve either high-pressure sprays or tumbling action
to expose all surfaces of th0 workpiece, users often
prefer vapor degreasing for delicate parts. A combina-
tion of aqueous cleaning and ultrasonic cleaning holds
a lot of promise for such applications.
35
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Most of the cleaner technologies discussed in this
guide result in reduced operating costs, and users can
expect a payback on their initial investment. However,
the reluctance to switch-over to a new cleaning technol-
ogy results from an understandable concern over the
cleaned product quality. Will the new process provide
the same cleaning effectiveness as the conventional
technology for the desired range of applications?
f ., f f f ffff f f % f -
Ventf&rs can h&fc in the selection process.
Before switching to a new cleaning technology, users
must first ensure that the new technology is suitable for
their particular application. This may involve testing a
pilot unit at the plant or at the vendor's location. Many
vendors have test units at their manufacturing loca-
tions, where typical soiled parts can be cleaned to
evaluate the effectiveness of the technology. Some
users switch one of their traditional cleaning lines
(vapor degreasing, alkaline tumbling, etc.) to new
aqueous cleaning methods as a means of gradually
phasing in the technology. If the new technology is
found to be unsuitable for certain types of parts, the
user can still clean those parts using traditional pro-
cesses. Plants that plan to increase their existing
capacities could consider adding new cleaning] tech-
nologies instead of traditional ones, as a step toward
gradual phase in.
When evaluating the effectiveness of a new cleaning
technology, the user must often evaluate how clean the
part is after washing. Evaluating the cleanliness of the
workpiece can involve anything from simple visual
observation to sophisticated surface analysis depend-
ing on the requirements of the application. For ex-
ample, many metal finishing industries use a relatively
simple test called the water-break test to evaluate a
clean surface. The test involves dipping the cleaned
workpiece or a cleaned test panel into a beaker of
water and pulling it out. If the water film forms a con-
tinuous layer on the workpiece surface that can be
sustained for about a minute, the surface is considered
clean enough for further product finishing steps such as
electroplating. Evaluating the cleanliness of an irregu-
larly shaped part with crevices or blind holes may be
more difficult.
the same. The process similarities eliminate the
detailed cleaning effectiveness testing that may be
required when switching over to an aqueous process
and thus alleviate product quality concerns. Vendors,
however, can work with users to design suitable
aqueous cleaning equipment. When cleaning effective-
ness of aqueous systems falls below Expectations, the
vendor often can bring about improvements by making
design modifications. For example, in aqueous clean-
ing, just changing the angle of the sprays can some-
times improve the cleaning effectiveness dramatically.
Aqueous cleaning systems also can bฃ custom de-
signed for a particular application.
Most dry d&tntog &$tabfistimettf$ atrmdy
'
- Gleaning principle*
Unlike switching to aqueous processes, switching to a
CEVC has the advantage that the cleaning effec-
tiveness can be expected to be similar to that of the
OTVC that it replaces, because the basic process is
-msflfc
In the dry cleaning industry, most establishments
already incorporate some form of closed-loop equip-
ment. Solvent vapors in the cleaning chamber typically
are evacuated and recovered by either refrigeration or
carbon adsorption, so that only a ver^ tiny amount of
solvent vapor is emitted at the end of the cycle. Re-
search in this area is focused on developing better
adsorbents that can remove this tiny fraction by captur-
ing more of the solvent from the circulating air stream
then is possible with refrigeration or carbon adsorption..
In the electronics industry, chlorinated solvents and
CFCs traditionally have been used to; remove the flux
residue during soldering. One option is to use organic
fluxes that can be washed off with water. However, a
highly acidic solution may be required to avoid tin and
lead hydroxide deposition, and this e^n lead to corro-
sion. Also, unlike conventional rosin fluxes, water-
soluble organic fluxes do not encapsulate impurities
resulting in electromigration. !
I
Alternatively, several users have switched over to using
water fortified with surfactants to clean conventional
rosin fluxes. Although this is an important step in
eliminating solvent use, the surfactarjit may not always
be able to remove all the flux. Aqueous cleaning and
ultrasonics can be used together for difficult cleaning
applications, but as in all aqueous cleaning, drying is
required. Also, the wastewater generated must be
treated before discharge. !
The low-solids flux (LSF) technology, is increasingly
being used during soldering to eliminate solvent use.
New options being developed range|from controlled
atmospheres to alternative solder aljoys. Here again,
the industry is taking a cautious approach of in-house
testing and gradual phase-in of technologies for
pollution prevention. |
36
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One concern about cleaner technologies that has not
yet been sufficiently addressed is the impact of second
ary pollution sources on pollution prevention. Some-
times, pollution prevention technologies have higher
energy requirements than the conventional technolo-
gies that they replace. For example, both the auto-
mated aqueous washer and the completely enclosed
vapor cleaner (CEVC) described in Section 2 were
found to have higher energy requirements than a
conventional vapor degreaser. In the case of the
automated aqueous washer, She higher energy require-
ment arose from the need for drying the parts after
cleaning. In the case of-the CEVC, the higher energy
requirement arose from the consecutive heating and
cooling cycles needed to generate and recover solvent
vapors. The issue of higher energy consumption versus
in-plant reduction or elimination of a hazardous pollut-
t ant is an issue that needs to be addressed in the
future.
37
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SECTION 6
INFORMATION SOURCES
Table 8 shows the trade associations and the technology areas they cover. Readers may contact; these trade
associaSrand request their assistance in identifying one or more companies that could prov.de the des.red
technological capabilities. ,
TablaS. Trade Associations and Technology Areas
Trade Association Technology Areas Covered
AT&T Manufacturing Tech Group (MTG)
Contact
American Electroplaters' and Surface
Finishers1 Society
institute of Metal Finishing
National Association of Metal Finishers
Electronics Industry Association
Low-solids flux applicator
Metals finishing
Metals finishing
Metals finishing
Electronics industry
Magit Elo-Gunther/Head of MTG
(Developer)
tel. (609) 639-2238
12644 Research Parkway.
Orlando, FL 32826
tel. (407)281-6441
f
Exeter Housjs
Holloway Head
Birmingham :B1 1VQ England
tel. (021).622-7388
111 E. Wacker Drive
Chicago, IL ''60601
tel. (312)644-6610
1711 "I" Street N.W., Suite 3000
Washington, DC 20006
38
ฃl).S. GOVERNMENT MUNTING OFFICE! mi - tSM06f2MK
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