A Cooperative Project
between the
U.S. Environmental
Protection Agency
and PWB
 December 1996
 EPA 744-F-96-024
                                     PRINTED WIRING
                                     BOARD  PROJECT
                                  A  Continuous-Flow
                                  System for Reusing
                               ^ • lo stay competitive in the Printed Wiring
                                    Board (PWB) industry, many manufac-
                                JL turers have developed cost-effective
                               methods that not only improve environmental
                               performance, but also make production more
                               efficient. Even minor process changes can
                               result in more efficient chemical use, which in
                               turn reduces waste and chemical costs, and
                               improves process control. As PWB Case Study
                               3 (Acid Recovery and Management) illustrates,
                               a sulfuric acid-hydrogen peroxide
                               microetchant can be regenerated and reused
                               by plating out the copper electrolytically from
                               the spent solution. That system maximizes use
                               of microetchant, improves process control,
                               and saves money.
                                    Power Circuits of Santa Ana, California,
                               has adopted an alternative approach to con-
                               serving microetchant. With a relatively small
                               initial investment, they have implemented a
                               continuous-flow system for reusing sulfuric
                               acid-potassium persulfate microetchant.
                               Spent microetchant from one process flows
                               to the microetch tank for another process
                               that requires a lower etch rate and thus a
                               weaker solution. Spent solution from this
                               second process  line can then be reused to
      clean plating racks and tanks. In this way, a
      single tank of fresh microetchant is used and
      reused in a sequence through four different
      cleaning processes. As a result, Power Circuits
      has realized immediate waste reduction and
      cost savings.
           This case study describes how a contin-
      uous-flow system works, the benefits of
      reusing microetchant, and the costs and poten-
      tial concerns a company may face when
      implementing this type of system.

      Power Circuits —
      Who Are They?	

           Power Circuits, established in 1985,
      manufactures nearly 700,000 surface square
      feet of double-sided and multilayer printed
      wiring board annually. With 165 employees,
      this company specializes in complex printed
      circuit boards in a short lead-time environment
      primarily for the computer, medical, and
      telecommunications industries.
           Although Power Circuits has been
      engaged in pollution prevention activities for a
      number of years, the company created a for-
      mal pollution prevention program in January
      1995 and began to document more fully their
      accomplishments in waste reduction and water
      conservation. For their achievements, Power
      Circuits received the 1996 "Industry of the Year
      Award" for the Southern California region from
      the California Water Environment Association.
           Before implementing the continuous-
      flow system, the company had been dumping
      a total of 800 gallons of spent microetchant per
      week from four different baths. Each of the
      tanks was monitored separately by different
      individuals. The operator of each process
      would take a sample of the microetchant man-
      ually and send it to the lab for analysis of
      available oxygen, which determines etch rate.
      Chemical additions were made as needed by
      each operator. When the microetch solution
      was spent, it was pumped to the electrolytic
      plate-out cell, which removed copper from the
      spent solution prior to sending it to the waste-
      water treatment unit.

Why Reuse Microetchant?

      Microetching is a ubiquitous process found as a pre-
clean step for many of the stages of PWB manufacturing.
Microetching removes anywhere from 4 to 70 microinches of
copper to  rid the panels of oxidation prior to the subsequent
process, such as soldermask application, oxide treatment,
electroless copper plating, pattern plating, or hot-air-solder-
leveling. This cleaning process is also used to strip copper
build-up on plating tanks and racks. Power Circuits uses a
sulfuric acid-potassium persulfate microetchant.
      Although Power Circuits does use an electrolytic recov-
ery process to plate-out copper from spent microetch solu-
tion, the sulfuric acid-potassium persulfate microetchant
cannot be regenerated due to the buildup of sulfates that
results from the breakdown of the microetch chemistry. A
hydrogen peroxide-based microetchant, on the other hand,
can be regenerated using this technology because the break-
down product is simply water. For Power Circuits, therefore,
electrolytic recovery serves to remove copper before the solu-
tion goes to the wastewater treatment unit, but not to regen-
erate microetchant.
      Power Circuits was initially motivated to conserve
microetch solution because their electrolytic plate-out unit fre-
quently failed to meet capacity needs for processing spent
solution. Excess waste had to be placed in claims until capac-
ity was available. Because the amount of copper that must be
removed from the board varies among the different process
lines, a microetch considered spent for the purposes of one
process line may still be useful for microetching in a line
requiring a lower etch rate. At Power Circuits, for example,
the microetch step for an electroless copper line must remove
40 to 60 microinches of copper from the PWB, whereas the
preclean step for pattern plating requires a microetch rate of 4
to 6 microinches. Using this process knowledge, a Power Cir-
cuits manufacturing engineer designed a scheme to reuse
microetch solution.
      The continuous-flow system developed by Power Cir-
cuits required little more than installing some new plumbing
and pumps. It was not even necessary to interaipt production
because most of the installation was done on a Saturday by
Power Circuits' maintenance  personnel.

How  the  Continuous-Flow
System Works

(1) Day Tank.
The continuous-flow system begins with a single tank of
microetch solution, prepared daily. This "day tank" is formu-
lated to the specifications for the electroless copper line's pre-
clean step, during which 40 to 60 microinches of copper must
be removed from the panels.
(2) Microetch for Electroless Copper Plating.
A photocell provided by the electroless copper equipment
vendor measures copper ion concentration in the microetch
bath and automatically feeds fresh microetchant from the day
tank to the bath when the copper concentration reaches a
threshold of 10 g/1. Before this automated system •was
installed, the bath was dumped three times per week, the
copper concentration in the bath ranged from 0 to 13 g/1, and
the etch rate ranged from 14 to 60 microinches. The autofeed
arrangement maintains copper concentration between 9 to 11
g/1, allowing for better process control and a more stable etch
rate of 33 to 57 microinches. Although the flow of microetch
could be triggered manually (i.e., without using a photocell),
frequent analysis would be required to feed fresh solution
into the microetch bath for electroless copper at the appropri-
ate time. The benefits would not be as substantial.

(3) Microetch for Pattern Plating.
Next, as additions are made to the microetch tank for the
electroless copper line, the excess overflows and  is gravity-
fed to the microetch prior to pattern plating. This  preclean
process needs  to remove only 4 to 6 microinches of copper.
Therefore, a weaker solution — one that has a higher copper
concentration and less oxygen available — can be used.
Because the etch rate is determined by temperature, concen-
tration, and dwell time, the latter has been adjusted to
achieve the desired etch rate based on the copper concentra-
tion of the incoming solution.

(4) Electroless Copper Rack Strip.
The excess from the microetch bath for pattern plating flows.
in turn, to a tank used for electroless copper rack stripping.
During the electroless plating process, copper is deposited on
the wire racks  that hold parts to be plated, as well as on the
inside of the plating tank. It is not necessary to use fresh
microetch solution because etch rate is not a critical parame-
ter in cleaning racks and tanks. Microetch solution can be
reused from other processes to remove copper build-up.

(5) Electroless Copper Tank Strip.
When the rack-strip tank is full, the microetchant  is then
pumped to a holding tank. Each weekend, the solution is
pumped back  into the electroless copper tank to  remove cop-
per build-up from its walls.

(6) Electrolytic Recovery.
After cleaning  the electroless copper tank, the microetchant is
pumped to the electrolytic plate-out cell, where copper is
plated out and sold to a recycler at S0.80/lb.

(7) Wastewater Treatment.
The remaining spent microetchant. consisting of sulfates.
sulfuric acid, residual copper, and water, is sent to wastewater

                            ELECTROLESS COPPER
                                         PATTERN PLATE
                                                      ELECTROLESS COPPER
                                                        TANK STRIP
                                                          ;  5
                                                      ELECTROLESS COPPER
                                                         RACK STRIP
Designing a Continuous-Flow

     The first step in implementing a continuous-flow sys-
tem for reusing microetchant is to identify the microetching
processes to include. For Power Circuits, it was a logical
choice to begin the flow with fresh  microetchant on the elec-
troless copper line, due to the solution volume, precision,
and relatively high etch rate required. Spent rnicroetch flows
to the preclean step for pattern plating because of its proximi-
ty to the electroless line and its lower etch rate.
     Other microetching processes were not incorporated
because of the quality of microetchant required and tank
location. The oxide line was not incorporated into the system
because the preclean step requires essentially the same etch
rate as for the electroless line. Microetch solution is also more
fully utilized on the oxide line before it is considered spent
because the cleaning process for the oxide line does not
require the same level of precision as does the process for the
electroless line. When the bath is considered spent for the
purposes of the oxide line, it is essentially unusable for
microetching elsewhere.
     It was not practical to include the microetchant for the
hot-air-solder leveling process in the continuous-flow system
because of its remote location in the facility. In designing a
continuous-flow system for your facility, you will need to
consider microetch rates and other process requirements,
tank location, and floor layout.
                                                      Benefits of Reusing Microetchant

                                                           Better process control is a key objective shared by most
                                                      successful PWB manufacturers. In addition to improving
                                                      process control by stabilizing etch rates, the continuous-flow
                                                      system for reusing microetchant yields other benefits through-
                                                      out the production process.
                                                           First, process line operators no longer have to prepare,
                                                      maintain, and dump four microetch baths several times per
                                                      week. Instead, a single tank of microetchant is prepared daily
                                                      by one individual. This change in materials handling saves
                                                      about two hours of labor time per day and decreases chemi-
                                                      cal exposure risk.
                                                           Next, by reusing microetchant, Power Circuits reduced
                                                      the weekly volume of spent solution flowing to wastewater
                                                      treatment from 800 gallons to 120 gallons. Also, because this
                                                      smaller volume of spent  solution has a higher copper concen-
                                                      tration than before the continuous-flow system was installed,
                                                      a lower electrical current is needed to plate out the copper.
                                                      Less waste translates into reduced chemical use and less labor
                                                      time for treatment as well as lower electricity costs.
                                                           Finally, Power Circuits was able to reduce annual use
                                                      of persulfate  by 16,460 pounds and sulfuric acid use by 1,646
                                                      gallons. By reusing microetchant through the continuous-flow
                                                      system, they  needed less virgin microetchant for the same
                                                      level of production.

Savings and Costs
Annual Savings
     Reduced materials handling/labor time
     associated with bath maintenance
     Potassium persulfate purchases
     Sulfuric acid purchases
     Waste treatment chemical purchases
Capital Costs
     Plumbing and pumps
     Installation (labor costs)
     'Photocell was provided by a vendor; commercial cost
      is approximately $1,200.

     "Estimated value.

     -12 days

Other Issues to Consider

  •  Power Circuits uses gravity feed for some flows. Your
    facility may require more pumps.
  •  A spread out, complex floor layout may limit the feasibil-
    ity of installing additional plumbing. This system may be
    best suited for smaller shops, or a facility may design a
    smaller continuous-flow system that reuses microetchant
    only one or two times.
  •  Some facilities have sales agreements with chemical sup-
    pliers that guarantee purchase cost based on square feet
    of boards produced, provided that process chemicals
    with certain specifications are used. Ask your supplier
    whether reusing microetch on the process line will affect
    your purchasing agreement.

What is the Design for the
Environment (DfE) Printed
Wiring Board Project?	
     Representatives of the printed wiring board industry
and other stakeholders entered into a partnership with the
U.S. Environmental Protection Agency (EPA), called the
Design for the Environment (DfE) Printed Wiring Board Pro-
ject. This project is a cooperative, non-regulatory effort in
which EPA, industry, and other interested parties are working
together to develop technical information on pollution pre-
vention technologies specific to the PWB industry. This infor-
mation includes comparative data on the risk, performance,
and cost of alternative manufacturing options.
     To date, the DfE Project has focused on conducting a
comprehensive evaluation of alternative technologies for
making through-holes conductive. The Project is also begin-
ning to evaluate alternatives to the hot-air-solder-leveling
process. By publishing the results of these evaluations, DfE
is able  to provide PWB manufacturers with the information
they need to make informed business decisions that take
human health and environmental risk into consideration, in
addition to performance and cost. The Project is also identi-
fying and publicizing other pollution prevention opportuni-
ties in the industry through the development of PWB case
studies, like this one.
     EPA's Design for the Environment Program would like
to thank Power Circuits for participating in this case study,
and DfE PWB Project participants from the following organi-
zations, who provided advice and guidance: Circuit Center,
Inc., Concurrent Technologies Corp., DuPont Electronic
Materials, Electrotek Corp., Hadco Corp., H-R Industries, Inc.,
and IPC.
                    Additional Pollution Prevention
                    Resources for the PWB Industry

                In addition to this case study, the DfE PWB Project has pre-
                pared other case studies that examine pollution prevention
                opportunities for the PWB industry, including:
                      Pollution Prevention Work Practices
                          On-site Etchant Regeneration
                        Acid Recovery and Management
                                Plasma Desmear
                These case studies, and other documents published by the
                DfE Project, are available from:

                  Pollution Prevention Information Clearinghouse (PPIC)
                          U.S. EPA 401 M Street, SW (3404)
                               Washington, DC 20460
                        Phone: 202-260-1023 Fax: 202-260-0178

                            e-mail: PPIC@epamail.epa.gov
                     DfE PWB information: http://www.ipc.org
                The DfE Program welcomes your feedback. If you have
                implemented any of the ideas in this series of PWB case
                studies, please tell us about it by calling the DfE Program at
                202-260-1678 or via email at oppt.dfe@epamail.epa.gov
                         r     Recycled/Recyclable

                        Printed with Soy/Canola Ink on paper containing at
                        least 50% recycled fiber.