United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/170  September 1994
Project  Summary
Electronic Component  Cooling
Alternatives: Compressed  Air
and  Liquid  Nitrogen
Stephen C. Schmitt and Robert F. Olfenbuttel
  The goal of this study was to evalu-
ate  tools used to troubleshoot elec-
tronic circuit boards with  known or
suspected thermally intermittent failure
modes. Aerosol cans of refrigerants,
which are commonly used in electron-
ics  manufacturing and  repair  busi-
nesses for this purpose, served as the
benchmark for the evaluation.
  One promising alternative technology
evaluated  in this study is a  com-
pressed-air tool that provides a con-
tinuous stream of cold air that can be
directed  toward specific components.
Another alternative technology that was
considered is a Dewar flask that dis-
penses cold nitrogen gas as the cool-
ing  agent. Critical parameters were
measured for each cooling  method to
provide a basis  for  comparing com-
pressed  air and liquid  nitrogen  with
spray cans  of refrigerant.  These pa-
rameters are accuracy, electrostatic dis-
charge  risk,  cooling  capability,
technician safety, pollution  prevention
potential, and economic viability.
  Newark Air Force  Base (NAFB), in
Ohio, was the site  at which  com-
pressed-air  and  liquid-nitrogen tech-
nologies were evaluated. The electronic
circuit boards that are tested and re-
paired daily at NAFB come from a vari-
ety  of Air Force Systems, such as
inertial guidance systems used in KC-
135, C-5, and C-141 aircraft and a fuel-
saver advisory system used  in the
KC-135. A percentage of these circuit
boards demonstrate thermally intermit-
tent failure modes and were used for
comparison testing. Both alternative
cooling technologies performed suffi-
ciently well to be considered for use in
trouble-shooting circuit boards. Both
reduced  pollution  and  cost less than
aerosol refrigerants typically used.
  This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).

Introduction
  The objective of the U.S. Environmental
Protection Agency (EPA) Waste  Reduc-
tion Innovative  Technology Evaluation
(WRITE) Program is to evaluate, in a typi-
cal workplace environment, examples of
prototype technologies that have potential
for reducing wastes at the source or for
preventing pollution. In general, for each
technology to be evaluated, three issues
should be addressed.
  First, it must be determined whether the
technology is effective. Because pollution
prevention or waste reduction  technolo-
gies usually involve recycling or reusing
materials  or using substitute materials or
techniques,  it is  important to verify that
the quality of the  materials and the quality
of the work product are satisfactory for
the intended purpose.
                                                Printed on Recycled Paper

-------
  Second,  it must be demonstrated that
using the technology has a  measurable
positive effect on reducing waste or pre-
venting pollution.
  Third, the economics  of the new  tech-
nology must be quantified and compared
with the  economics of the existing  tech-
nology. It should  be clear, however, that
improved economics  is  not  an absolute
criterion for the use of the prototype  tech-
nology. There may be justifications  other
than saving  money that would encourage
adoption of new operating  approaches.
Nonetheless, information about the eco-
nomic  implications of any such potential
change is  useful  for understanding the
overall effect of implementation.
  This study evaluated  the  use  of cold
compressed-air tools  and liquid  nitrogen
as methods for cooling electronic circuits
while searching  for causes  of thermally
intermittent circuit failures. Aerosol  cans
of refrigerant (i.e., CFC  R-12 and HCFC
R-22),  which commonly  have been  used
in electronics  manufacturing and repair
businesses for this purpose, served as
the benchmark for the evaluation. Six criti-
cal  parameters were  measured for  each
cooling method: accuracy, electrostatic dis-
charge risk, cooling capability, technician
safety, pollution prevention potential, and
economics. The first three parameters are
related to product quality, i.e., the accu-
racy with which circuit board failures can
be located,  and are discussed in that sec-
tion. The remaining parameters  are dis-
cussed independently.

Description of the Technology
  Aerosol cans of refrigerant, such as R-
12 and R-22, are commonly  used in the
electronics  manufacturing and repair in-
dustries for trouble-shooting circuit boards
that  have known  or suspected thermally
intermittent failure modes. Thermally in-
termittent failures  occur  when tempera-
ture  changes and material expansion or
contraction  aggravate the mechanical fail-
ure to create an  electrical  discontinuity
condition.  For example,  if an electronic
device works when first turned on but fails
as it warms up in operation,  a technician
may spray  refrigerant towards board ar-
eas or on specific components to reduce
temperatures until the device begins to
work again. The  component that, when
cooled, causes the failure mode to appear
or disappear is replaced.  If the circuit fail-
ure mode still exists, the troubleshooting
process is repeated.
  Aerosol cans  of refrigerant are  com-
monly used as trouble-shooting tools.  They
can be used easily to cool an entire circuit
board or a single  solder connection and
are portable  and  relatively  inexpensive.
As recognized in the Montreal Protocol of
1987, however, chlorine released by de-
composing chlorofluorocarbons  (CFCs),
such  as R-12, decreases stratospheric
ozone. The protocol calls for the elimina-
tion of CFC manufacture in the future. As
a result, many businesses  are  seeking
technologies that will replace current uses
of CFCs.  Hydrochlorofluoro-carbons
(HCFCs), such  as  R-22, also will  be
phased  out,  although they have  lower
stratospheric ozone-depleting potential.
  The first alternative technology  evalu-
ated was a compressed-air tool that pro-
vides a  continuous stream of cold air that
can  be directed  towards components.
Compressed air enters a tangentially drilled
stationary generator which forces the air
to spin  down the long tube's inner walls
toward  the hot-air control valve. A per-
centage of the air, now at  atmospheric
pressure, exits through the needle valve
at the hot-air exhaust. The remaining air
is forced back through the center  of the
sonic-velocity airstream where, still spin-
ning, it moves at a slower speed, causing
a simple heat exchange  to take  place.
The inner, slower-moving air gives up heat
to the  outer, faster-moving  air  column.
When the slower inner air column exits
through  the center of the stationary gen-
erator and  out  the cold  exhaust,  it  has
reached an  extremely low temperature.
To obtain temperatures in the  range of
-35°C to -40°C,  the  tool requires  clean,
dry,  room-temperature air flowing  at  15
scfm at  100 psi pressure.
  The  second alternative  technology
evaluated uses liquid  nitrogen.  A  1/2-L
Dewar flask can be used  with a release
valve that allows a stream of nitrogen gas
and liquid droplets to be directed through
a small-diameter stainless-steel nozzle. As
the  valve and nozzle are cooled by the
nitrogen flow, the  portion of the stream
that is droplets increases and the output
stream drops in temperature. A variety of
valves, nozzles, and heat exchangers are
available to tailor the delivery and cooling
characteristics of the  stream of nitrogen.
The  Dewar flask  can  be  refilled from a
bulk container of liquid nitrogen.

Description of the Site
  Newark AFB (NAFB) was the site at
which compressed-air and liquid-nitrogen
alternative technologies were evaluated.
Electronic circuit boards from a variety of
Air Force systems, such as  inertial guid-
ance systems, are tested and repaired at
NAFB daily. A percentage of the tested
circuit boards demonstrate thermally inter-
mittent failure modes; during the test pe-
riod, these boards became test articles for
comparison  testing.  R-12 was used  for
this study as the benchmark.
  Each repair shop  at NAFB is respon-
sible for specific systems. Because com-
pressed air is not typically available at the
test stations where cooling  materials are
needed,  it was necessary to select one
shop for the study. After evaluating sev-
eral  shops,  the  Carousel Shop  was se-
lected as the test site because:

  • The test stations included fixtures that
    were capable of reducing circuit board
    temperature (using  carbon  dioxide)
    while the board is tested. This feature
    provided confirmation that  thermally
    intermittent failure  mode existed but
    did  not  provide a  troubleshooting
    capability since the entire board was
    cooled at one time.
  • The systems repaired in  the Carousel
    shop contained  circuit  boards in  a
    variety of sizes, component densities,
    and component varieties.
  • Installation   costs   to   deliver
    compressed air  could be minimized
    because the three  test stations used
    for the study are in close proximity.

  The  compressed-air system  used for
the study consisted  of a large industrial
compressor  with a  refrigeration system to
chill the compressed air as it passed into
a storage tank.  The air  passed through
approximately 50  ft  of 1/2-in.  line with
nonrestrictive couplings to three outlets. A
filtration and drying system  was  installed
approximately 20 ft from the  test  stations.
A  5-hp  compressor  is the  minimum re-
quirement for continuous operation of an
air tool.

Product Quality  Evaluation
  Three  factors determine  how well  a
given cooling method will work to identify
failing circuit board components: accuracy,
electrostatic  discharge risk, and the cool-
ing rate and absolute temperature drop.
The  procedures used to evaluate these
factors and the conclusions  reached dur-
ing this study are described  briefly below;
additional detail is provided in the final
report.

Accuracy
  For this project,  accuracy  was defined
as the  capability of a technician using a
cooling method to identify a  specific com-
ponent with a thermally intermittent failure
mode causing a circuit  board to  have a

-------
thermally intermittent circuit failure mode.
An accurate  cooling method provides a
high component identification confidence
(CIC) level, which avoids the cost of erro-
neously  replacing nondefective compo-
nents, potential  damage created during
component replacement, and multiple it-
erations of testing and repair.
   An experiment was performed to  com-
pare the capability of each cooling method
to identify components with  thermally in-
termittent failure modes.  During the 5-
month test period,  17 circuit  boards  were
identified initially as having  thermally in-
termittent failure modes.  Four  of  these
were subsequently  removed from the
evaluation because they were found not
to be  thermally  intermittent or  because
the defective  components were known
from previous experience with a specific
model circuit  board. Each  of the remain-
ing 13 test articles were evaluated with
the use of each of the three cooling meth-
ods. Three technicians, working indepen-
dently  of each other,  evaluated  the test
articles following a randomized sequence
of cooling methods. For each evaluation,
the technicians assigned a CIC level which
reflected  their confidence  that they  had
been able to  isolate the cause of the cir-
cuit  failure using  the  assigned cooling
method.
  The number and variety of test articles
identified during  the test period  were not
as great as hoped for. Also, the results of
the test article evaluations do not support
comparisons of the accuracy of the  cool-
ing methods.  However, the results do in-
dicate that the compressed-air method was
able to reproduce circuit failures  in 12 of
13 test articles. This is significant because
it is  known that  the cooling  capability of
compressed  air  is less than  either refrig-
erants  or liquid nitrogen.
  Also, a potential problem related to liq-
uid nitrogen temperatures may have been
identified — for  one test article, the very
low temperature apparently caused a  RAM
chip to fail temporarily, masking the diode
that was  the actual defective component.
Potential users of liquid nitrogen may  want
to consider temperature control strategies
to avoid excessively low temperatures that
could temporarily change component func-
tions or even damage components;  pos-
sible  strategies are  discussed in an
appendix to the report.
  Detailed accuracy evaluation results are
provided  in the final report, including  pho-
tographs  of each test article  and, if avail-
able, the  results  of  the  component
replacement and retesting. This  informa-
tion is expected to help potential  users of
the alternative cooling materials determine
the applicability  of  study results to their
operations.

Electrostatic Discharge Risk
  The amount  of  electrostatic charge
buildup generated by the cooling material
as it is dispensed  is a concern because
components can be damaged  by electro-
static discharge. Two experiments  were
designed to compare the  electrostatic
charge generated by the following cooling
method/nozzle combinations:

  • R-12 aerosol with a plastic tube nozzle
  • R-12 aerosol with a steel tube nozzle
  • compressed-air tool with a  single-
    section plastic nozzle
  • liquid nitrogen  Dewar flask  with  a
    straight  stainless-steel  nozzle
    approximately 4-in. long

  The first experiment measured the elec-
trostatic charge generated on  the nozzle
during  release of cooling  material. During
a 10- to 12-sec material release, the nozzle
was held parallel to and  approximately 1
in. from  the  platen of an  Ion Systems,
Inc., Model 200 Charged  Platen Monitor*,
which measured charge buildup. Two mea-
surements  were taken for  each cooling
method/nozzle combination.
  The second experiment measured elec-
trostatic charge buildup when cooling ma-
terial was dispensed toward circuit boards
placed on the platen of an Ion Systems
Model  200 Charged Platen Monitor. The
dispenser was  held so that  the  nozzle
was approximately 0.5 in. from the  edge
of the circuit board,  both  horizontally and
vertically, and at approximately 45 degrees
relative to the horizontal surface of the
circuit board. Six circuit boards were evalu-
ated, with two  measurements taken for
each cooling  method/nozzle combination.
The six circuit boards were selected to
provide component and density variety.
  Averages of each  pair of measurements
indicate that both the compressed air and
the liquid nitrogen alternatives  generated
lower electrostatic charge buildup than did
R-12 through either plastic or steel nozzles.
Thus,  the  risk of  electrostatic charge
buildup is not increased by using either of
the alternative component  cooling tech-
nologies.  If  aerosol cans of  R-12  have
been used successfully, either compressed
air or liquid nitrogen  should be acceptable
alternatives.
  Mention of trade names or commercial products does
  not constitute endorsement or recommendation for
  use.
 Cooling Rate and Absolute
 Temperature Drop
   Cooling rate and absolute temperature
 drop  were measured  for each method.
 Understanding the characteristics of and
 differences between  cooling methods will
 enable technicians to use the compressed-
 air and  liquid nitrogen technologies effec-
 tively. For example, the distance between
 the applicator nozzle and the component
 does  not significantly  affect  the cooling
 rate of aerosol cans of  R-12; this distance
 is, however, expected  to be a significant
 factor in the cooling rate provided by com-
 pressed air.
   An  experiment was  designed to esti-
 mate  the rate of change of component
 temperature.  Two test  boards were  fabri-
 cated, one having  integrated  circuits and
 the other having wound-film capacitors.
 Each  test board contained  three compo-
 nents with thermocouples  buried  inside
 and one exposed  thermocouple. During
 tests,  all  four thermocouples on a test
 board were connected to  a Yokogawa
 LR4110 four-channel data  logger, which
 simultaneously recorded temperatures of
 all four thermocouples as cooling material
 was directed at the target component. For
 each test board, cooling material was ap-
 plied  from two  directions  and two dis-
 tances.  Two  measurements were taken
 for each combination of test board,  cool-
 ing method, direction,  and  distance. Be-
 fore each  measurement  for  R-12 and
 compressed air, the  cooling material was
 dispensed directly at  the exposed thermo-
 couple to determine  the absolute lowest
 temperature that could  be achieved given
 the test distance,  direction, and  cooling
 method. This  was not necessary for  liquid
 nitrogen  because it  was known that the
 thermocouple would reach the lowest mea-
 surement  limit of -175°C. Table  1 shows
 the temperatures achieved under one set
 of conditions.
   In all tests,  the cooling  material dis-
 pensers were positioned and aimed manu-
 ally. By using visual feedback from the
 data logger chart  to determine when  a
 stable minimum temperature was reached,
the technician adjusted the angle of el-
 evation  slightly to  ensure that  minimum
temperatures  were obtained for each ap-
 plication direction and  distance.  Different
 angles  of elevation  undersprayed  or
 oversprayed  the cooling material,  thus
 changing the  cooling rate and the differ-
 ence  in temperature between the target
component and other components on the
test fixtures. As a result, the absolute tem-
 perature drop data presented are used for
direct  comparison of cooling materials; but

-------
Table 1.  Minimum Temperature Achieved (at 1/4-in. Distance) and Elapsed Time for Three Cooling Points
                                       Aerosol R-12
Compressed Air
                                                                                                                 Liquid Nitrogen
Component
Type/Test
Integrated Circuit
Target Component
Exposed Thermocouple
Wound-Film Capacitor
Target Component
Exposed Thermocouple
Temperature
(°C)

-45.0
-54.5

-53.5
-59.5
Elapsed Time
(sec)

18.0
—

77.5
—
Temperature
(°C)

-27.5
-35.5

-11.5
-35.0
Elapsed Time
(sec)

29.0
—

121.0
—
Temperature
(°C)

-175.0
-175.0

-134.0
-175.0*
Elapsed Time
(sec)

31.0
—

51.0
—
* Minimum thermocouple temperature assumed to be — 175°C based on wound-film capacitor tests.
                      100   _
                 o
                 I
                 I
                 .5
                        o   -
                     -100   -
                     -200
                                                                                                 Compressed air
                                                                                                       R-12
                                                        Elapsed time (seconds)
Figure 1. Typical cooling rate comparison for integrated circuits: distance 1/4-in.

-------
cooling  rate and temperature  difference
data, while they indicate performance that
may be obtained  in actual use,  are  not
used for direct comparisons. The cooling
rates of the three methods under one set
of conditions are compared in Figure 1.
  The three cooling materials differed in
how they  cooled  components. As R-12
was sprayed towards components, it built
up  a "slush" on and around the  compo-
nent. When the spray of R-12 was stopped,
the  slush continued to evaporate and lower
the  component temperature even further.
The fastest initial cooling rates were  ob-
tained with R-12, although the cooling rate
decreased  as  component temperature
dropped. Liquid nitrogen provided the cold-
est  temperatures of the three cooling ma-
terials. In contrast to R-12, an accelerating
cooling rate was obtained when liquid ni-
trogen  was  used.  The  cooling material
consists of nitrogen gas and droplets of
liquid  nitrogen;  as  the dispensing  valve
and nozzle cools, the proportion of drop-
lets increases.  The increase in droplets
could be heard as  increased "sputtering"
of cooling material during material release.
Frost buildup on the components during
cooling was minimal. Compressed air pro-
vided the least cold temperatures and the
slowest  cooling rate. As with  R-12,  the
cooling rate decreased as component tem-
perature dropped.  Compressed-air cool-
ing  resulted in a slight frost buildup on the
components.
  The three cooling methods differed also
in their sensitivity to such parameters as
component type, application distance, and
application  direction. Evaluation of mini-
mum target component temperature data
indicates that:

  •  Component type is  affected by  the
    sensitivity  of the liquid nitrogen and
    of the compressed air, both of which
    provided  lower  temperatures  with
    integrated circuits  than  with  the
    wound-film capacitors. R-12 was not
    significantly sensitive to the type of
    component and provided  minimum
    temperatures for  capacitors  and
    integrated circuits  that  were  not
    significantly  different under  each
    application     distance/direction
    combination.
  •  Distance from  the target component
    affects the  component cooling
    capabilities of both compressed air
    and liquid  nitrogen. An examination
    of  the temperature data summarized
    in the final  report reveals that, as the
    distance from the component to the
    nozzle increased from 0.25  in. to  1
    in.,   the   minimum   component
    temperature  decreased  for both
    alternative methods. This relationship
    does not exist for R-12, indicating that
    it is not  as sensitive to distance.
   • A comparison of component minimum
    temperature data  for two different
    directions of application indicates that
    R-12  is not sensitive to  application
    direction. In contrast, compressed air
    provided     lower    component
    temperatures  for integrated  circuits,
    but liquid  nitrogen yielded  lower
    component temperatures for wound-
    film  capacitors.  The most  likely
    explanation of this difference is the
    variability  resulting  from  manual
    positioning of the dispensers.

Technician Safety
  Exposure  to sound  created  by opera-
tion  of the  compressed-air  tool  was a
safety concern. To  assess the potential
safety hazard, personnel from the Newark
AFB Bioenvironmental Engineering Office
took sound-level  measurements during
operation  of the compressed-air tool. A
sound level  of 81 dBA was  recorded  at
the operator work position. Because the
sound levels did not exceed 84 dBA, ad-
ditional measurement was not required by
the Air Force and, in accordance with Air
Force Regulation 161-35, hearing conser-
vation precautions were deemed  unnec-
essary.
  The safety concerns related to handling
liquid nitrogen  and  aerosols  are well-
known. Therefore, no  technician safety
testing was required.

Pollution  Prevention Potential
  The purpose of replacing aerosol cans
of refrigerant is to reduce the  amount of
pollutants  released into  the atmosphere.
During the  accuracy experiments, the
weight of R-12 released during  evaluation
of each.circuit board with thermally inter-
mittent failure modes was determined. The
procedures for collecting these data are
described  in the project report.
  These  data provide a  measure of the
average pollution per circuit  board that
could be prevented if either of the alterna-
tive cooling methods were adopted in place
of R-12. The average R-12 release/article
was 232.65  g (0.51 Ib). With the adoption
of either alternative technology, release of
R-12 would  be eliminated along  with  the
wastestream of empty aerosol  cans. Nei-
ther usage nor production information for
the United States was available when this
report was  written;  quantities  consumed
vary by user, ranging from a few cans/ mo
in repair shops to over a thousand cans/yr
in production operations.
Economics Evaluation
  To ass.ess the economics of replacing
R-12  use with either of the alternative
technologies, operating costs and invest-
ment costs were examined.
  The  approach to estimating operating
costs was to measure the volume of each
cooling  material used during test article
accuracy evaluations and calculate a per-
board  material cost. Although material
costs  are only one  aspect  of operating
costs, it was the only aspect that could be
measured during the tests. It was beyond
the scope of  this  study  to  measure all
potential effects of alternative component-
cooling materials on operating costs, par-
ticularly those for direct labor  and
materials. If an alternative cooling mate-
rial is less able to isolate the specific com-
ponent  causing a thermally intermittent
circuit, components may be replaced un-
necessarily. Each component replacement
adds cost in the form of direct labor for
replacement and  retesting, component
costs, and risk of circuit board damage. If
a cooling method is unable to identify the
defective component, a circuit board may
be condemned unnecessarily. Compari-
sons of the ability  of the various cooling
materials to  isolate thermally intermittent
components was addressed in the discus-
sions  of accuracy and absolute tempera-
ture drop/cooling rate above.

Cooling Material Costs
  Cooling material costs are based on the
use data  collected during  the accuracy
evaluation of the 13 test articles. The meth-
odologies for collecting the  data and cal-
culating the use are described in detail in
the project report.  Use data were con-
verted to cost data as follows:

  • R-12  cost  was based  on  a cost  of
    $7.50/16-oz aerosol can.  Purchase
    price  of   R-12  or  R-22  freeze
    compound ranges from $6  to $157
    can;  $7.50  was  selected as  a
    conservative estimate.
  • Compressed-air cost was  calculated
    by using an air tool consumption rate
    of 15 cfm at 100 psi and an estimated
    compressed-air generation  cost  of
    $0.26/1000 ft3. The generation  cost
    will vary based on power  costs and
    other factors  and should be verified
    by potential users.
  • Purchase cost of liquid nitrogen varies
    widely; $0.25/L was used as a typical
    cost.  Potential users should obtain
    price quotations from local suppliers.

Investment Costs
  The approach to estimating investment
cost focused  on the cost of dispensers.

-------
There is  no such investment  exist for  R-
12. Costs for the alternative cooling mate-
rial dispensing equipment are discussed
below.
  For compressed air, investment cost is
expected to range widely because the con-
dition and capacity of existing compressed-
air supplies at test stations will vary widely.
Some sites may not have an  existing air
supply. Potential users will need to deter-
mine what, if any, investment is needed to
obtain  compressed air in the quantities
and  quality required.  Implementation  of
compressed air  requires, at a minimum,
investment in the air tools at approximately
$200/unit. The investment required to gen-
erate and deliver 15 scfm at 100 psi to the
tools at a work position will vary with each
potential  user.  If no compressed air  is
available in a shop, the minimum equip-
ment required to supply one air tool is a
5-hp compressor, oil-filter and desiccant
filters,  and nonrestrictive  air  lines,  con-
nectors, and valves. Purchase and instal-
lation costs also will vary for each potential
user.
  Implementation of liquid nitrogen would
require approximately $500 for each 1/2-L
Dewar flask. Heat exchangers or other
accessories would be  additional. Cylin-
ders  for bulk liquid nitrogen generally are
provided  by the suppliers at no charge. If
use rate  is low,  suppliers may  require a
leasing arrangement for the bulk contain-
ers.

Economics Assessment
  Data presented in the project report in-
dicate that a material cost savings of $5.287
circuit board can be projected if testing is
done with liquid nitrogen instead of R-12.
This  would result in  payback of a $500
dispenser  investment  after  95  circuit
boards have been tested.
  For a  shop that has an existing ad-
equate air  supply,  the average operating
cost  savings for compressed air is $5.267
board. This would pay back a  $200 air-
tool investment after 38 circuit boards have
been tested. The payback period would
be extended if additional investment were
required  to  compress and deliver air  to
the work positions.
  Table  2 summarizes investment and
payback  figures for each alternative tech-
nology.

Discussion
  The objective of this study was to char-
acterize and compare the use of aerosol
cans of refrigerant, compressed air, and
Table 2. Investment Cost and Payback
  Cooling Method
Investment
     Payback
(circuit boards tested)
  Compressed Air
  Liquid Nitrogen
  $200
  $500
        38
        95
liquid nitrogen as methods to  cool elec-
tronic circuits during troubleshooting. Data
obtained from testing were used to com-
pare the alternative  cooling methods in
terms of accuracy, electrostatic discharge
risk, cooling performance, technician safety
hazards, pollution prevention potential, and
economics. Interpretation of the results of
this study are:

  • The compressed-air tool  evaluated
    during the study was  unable to cool
    components to the temperature level
    that was obtained with either R-12 or
    liquid nitrogen.  The  results of  the
    accuracy test, however, indicate that
    during all but one test, temperatures
    achieved with the compressed-air tool
    were low enough to reproduce circuit
    failures.
  • Liquid nitrogen has the capability to
    readily cool components to below -
    175°C  if dispensed closely  enough.
    At  such temperatures,  components
    may fail from temporary changes in
    output signals or fail permanently from
    physical damage.  Two methods to
    control the temperature of components
    are to  maintain dispensing nozzle
    distance and to slow the cooling rate
    of  the  dispenser by  adding heat
    exchangers or smaller orifices. Both
    methods rely on technician skill to a
    greater extent than do  either
    compressed  air or R-12.  Further
    discussion of component temperature
    control with liquid nitrogen is provided
    in an appendix to the project report.
  • The  results  of  the  accuracy
    experiment do not support conclusions
    regarding the relative effectiveness of
    alternative  cooling  methods and
    aerosol  cans of refrigerant.
  • Neither  alternative is  expected to
    increase safety  risks  to technicians
    when compared with those of aerosol
    refrigerants. Noise levels are higher
    during compressed-air tool operation
    than with R-12 or liquid nitrogen, but
    they are not  high enough  to pose a
    health hazard to users. Handling of
            liquid nitrogen presents a safety risk
            in the  form  of  exposure to  low
            temperatures,  but technician training
            and  proper safety  procedures  and
            equipment are expected to reduce risk
            to acceptable  levels.  As  with  any
            aerosol, release of refrigerants under
            pressure presents a safety risk that is
            minimized through training.
          • Replacement  of  aerosol  refrigerant
            prevents emissions of substances that
            deplete the stratospheric ozone layer
            as well  as accumulation  of empty
            aerosol cans requiring landfill disposal.
            With liquid nitrogen, only  nitrogen is
            emitted and refillable bulk containers
            and    dispensers    are   used.
            Compressed  air  generates a small
            amount of pollution in the forms of
            waste  compressor oil  and  filter
            elements, but the incremental increase
            in these  wastestreams  that would
            follow adoption of the compressed-air
            method is not  expected  to  be
            significant.
          • Material costs of either  alternative are
            expected to be lower than those of R-
            12 or R-22 at current prices. Prices of
            R-12  and R-22 will  undoubtedly
            escalate and, eventually, these
            materials will be  unavailable due to
            regulatory prohibition.
          • Investment cost to  implement liquid
            nitrogen is expected to consist of the
            price of Dewar flask  dispensers at
            approximately  $500 each in the  1/2-L
            size.  Compressed-air  tools  cost
            approximately  $200 each. The  cost
            of equipment to deliver compressed
            air that is clean, dry, and near  room
            temperature  in  the  volume  and
            pressure  required   to  achieve
            maximum cooling  capability  will
            depend on  existing equipment  and
            the number of tools to be used.

          The full  report was submitted in fulfill-
        ment  of  Contract No. 68-CO-0003, Work
        Assignment No. 2-36, by Battelle  under
        the sponsorship of the U.S. Environmen-
        tal Protection Agency.

-------

-------
  Stephen Schmitt and Robert Olfenbuttel are with Batelle Memorial Institute
    Columbus, OH 43201-2693
  Johnny Springer, Jr., is the EPA Project Officer (see below).
  The complete report, entitled "Electronic Component Cooling Alternatives:
      Compressed Air and Liquid Nitrogen," (Order No. PB95-100087/AS;
      Cost: $27.00, subject to change) will be available only from:
          National Technical Information Service
          5285 Port Royal Road
          Springfield, VA 22161
          Telephone: 703-487-4650
  The EPA Project Officer can be contacted at:
          Risk Reduction Engineering Laboratory
          U.S. Environmental Protection Agency
          Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268

Official Business
Penalty for Private Use
$300
      BULK RATE
POSTAGE & FEES PAID
         EPA
   PERMIT No. G-35
EPA/600/SR-94/170

-------