A variety of environmental, safety, and health concerns exist over use of chlorinated and fluorinated cleaning solvents. Sandia National Laboratories, Lawrence Livermore National Laboratories, and the Kansas City Division of AlliedSignal have combined efforts to focus on finding alternative cleaning solvents and processes which are effective, environmentally safe, and compliant with local, state, and federal regulations. An alternative solvent has been identified, qualified, and implemented into production of complex electronic assemblies, where aqueous and semi-aqueous cleaning processes are not allowed. Extensive compatibility studies were performed with components, piece-parts, and materials. Electrical testing and accelerated aging were used to screen for detrimental, long-term effects. A terpene, d-limonene, was selected as the solvent of choice, and it was found to be compatible with the components and materials tested. A brief history of the overall project will be presented, along with representative cleaning efficiency results, compatibility results, and residual solvent data. The electronics industry is constantly searching for proven methods and environmentally-safe materials to use in manufacturing processes. The information in this presentation will provide another option to consider on future projects for applications requiring high levels of quality, reliability, and cleanliness from non-aqueous cleaning processes

Meier, Gerald J.

English

NASA Technical Reports Server

N94-30466
NON-AQUEOUS CLEANING SOLVENT SUBSTITUTION
Gerald J. Meier, Senior Engineer
AlliedSignal Inc.
Kansas City Division"
P.O. Box 419159 D/343 MY40
Kansas City, Missouri 64141-6195
ABSTRACT
A variety of environmental, safety, and health concerns exist over use of chlorinated and
fluorinated cleaning solvents. Sandia National Laboratories, Lawrence Livermore National
Laboratories, and the Kansas City Division of AlliedSignal have combined efforts to focus on
finding alternative cleaning solvents and processes which are effective, environmentally safe, and
compliant with local, state, and federal regulations. An alternative solvent has been identified,
qualified, and implemented into production of complex electronic assemblies, where aqueous and
semi-aqueous cleaning processes are not allowed. Extensive compatibility studies were
performed with components, piece-parts, and materials. Electrical testing and accelerated aging
were used to screen for detrimental, long-term effects. A terpene, d-limonene, has been selected
as the solvent of choice, and has been found to be compatible with the components and materials
tested. A brief history of the overall project will be presented, along with representative
cleaning efficiency results, compatibility results, and residual solvent data. The electronics
industry is constantly searching for proven methods and environmentally-safe materials to use
in manufacturing processes. The information in this presentation will provide another option to
consider on future projects for applications requiring high levels of quality, reliability, and
cleanliness from non-aqueous cleaning processes.
INTRODUCTION
For many years, complex electronic assemblies have been successfully cleaned with
trichloroethylene (TCE), rinsed with isopropyl alcohol (IPA), and dried with
trichlorotrifluoroethane (FREON). Spray cleaning by hand and vacuum baking have been used
in production at the Kansas City Division for over twenty years, and are the preferred methods
for cleaning and drying electronic assemblies. This cleaning process has been extremely
effective in removing the intentional surface contaminants (solder fluxes and mold releases) and
any unintentional contaminants (silicones, greases, lotions, and oils) from electronic assemblies
prior to encapsulation.
* Operated for the U.S. Department of Energy under contract number DE-ACO4-76-DP00613.
© Copyright 1993, AlliedSignal Inc.
223
https://ntrs.nasa.gov/search.jsp?R=19940025961 2020-06-16T14:47:54+00:00Z
However, a variety of environmental, safety, and health concerns exist over use of these
chlorinated and fluorinated solvents. Sandia National Laboratories, Lawrence Livermore
National Laboratories, and the Kansas City Division of AlliedSignal combined efforts to find
alternative cleaning solvents and processes which are effective, environmentally safe, and
compliant with local, state, and federal regulations. Work was performed to identify, qualify,
and implement the alternatives to the chlorinated and fluorinated cleaning solvents. Several
aqueous, semi-aqueous, and non-aqueous cleaning solvents and processes were evaluated, d-
Limonene, a terpene, has been selected as the solvent of choice to replace TCE and FREON in
electronic assembly cleaning.
STATEMENT OF THE PROBLEM
In January 1989, the future availability of halogenated cleaning solvents was in jeopardy. There
was growing concern over the environmental impacts of FREON usage and an increased
awareness of TCE toxicity. New development programs were reluctant to select baseline
cleaning processes because of the uncertainty of halogenated solvents.
N0n-Aqueous Requirements
Systems Engineers are concerned with the material compatibility problems caused over time by
water (i.e. corrosion). As a result, system requirements prohibited the use of water in processes
where organic materials are present, to limit the amount of water absorbed by the organic
materials and control the overall water content of a final assembly. Since most of the
mechanical parts and assemblies that require solvent cleaning are inorganic, aqueous and semi-
aqueous cleaning solvents and processes could be used as replacements for TCE and FREON.
However, most electrical parts and assemblies contain organic materials that absorb water (which
sometimes can not be easily removed), and new non-aqueous cleaning solvents and processes
had to be found. The majority of the work performed on this project went into identifying and
evaluating suitable non-aqueous cleaning processes for use on the electrical parts and assemblies.
Eliminating CFC and CHC Usage
The search for alternative cleaning solvents and processes began at the Kansas City Division,
and was primarily driven by draft requirements to reduce and eliminate use of
chlorofluorocarbons (CFC's) and chlorinated hydrocarbons (CHC's) in traditional cleaning
processes. Since then, increasing numbers of regulations have been issued by federal, state, and
local regulatory agencies to ban, or to place more stringent controls on the production, use, and
disposal of these halogenated solvents. In addition, company policy also dictated that usage of
CFC and CHC materials at the Kansas City Division be eliminated.
224
CLEANING EVALUATIONS
A new development project, consisting of a final unit with twelve subassemblies, was selected
as the pilot project to evaluate new cleaning solvents and processes. The subassemblies were
primarily printed wiring boards containing resistors, capacitors, diodes, transformers, and hybrid
micro-circuits; special design components bonded together with epoxy adhesives and electrically
connected with wires or fiat cables; and multi-pin connectors in wiring harnesses. These
modules were assembled in a stainless steel housing to become the final unit.
Solvents and Processes Evaluated
Seven different cleaning solvents and processes (two aqueous, three terpenes, a hydrocarbon,
and an alcohol) were evaluated. They are listed below:
1) An aqueous mixture of ethanol amines (5% solution in deionized water), rinsed with
deionized water and then isopropyl alcohol.
2) An aqueous solvent based on n,n-dimethylacetamide, rinsed with deionized water and
then isopropyl alcohol.
3) A terpene (d-limonene) with emulsifiers, rinsed with deionized water and then isopropyl
alcohol.
4) A terpene (d-limonene) with phase separation agents, rinsed with isopropyl alcohol only.
5) d-Limonene (Food Grade--97% pure), rinsed with isopropyl alcohol only.
6) A hydrocarbon solvent based on octadecyl acetate, rinsed with isopropyl alcohol only.
7) Isopropyl alcohol (solvent and rinse).
Musts and Wants
Four "must" requirements were defined for the solvents:
• not compromise quality or reliability,
• not require a design change,
• be acceptable to the Environmental, Safety, and Health Departments, and
• have cleaning comparable to TCE.
Only after meeting these four must requirements would the new solvents be judged against other
requirements, including:
• compatibility,
• corrosion-resistance,
• cleaning effectiveness,
• environmental, safety, and health conformance,
• manufacturing efficiency, and
• low implementation and operating costs.
225
Solvent Screening
A solvent evaluation matrix was defined to screen each solvent. The matrix contained four
general categories:
1) Cleaning Evaluations, consisting of solvent cleaning, ionic contamination testing, organic
contamination testing, AES/XPS surface analysis, and process development;
2) Solvent Analysis, consisting of composition, removal, and residuals;
3) Material Compatibility, consisting of screening tests, individual evaluations, and physical
properties; and
4) Hardware Evaluations, consisting of accelerated aging and electrical performance testing.
There were three categories of contaminants that each replacement solvent had to remove from
various substrates. The categories were: solder flux, mold release, and general contaminants
consisting of resins, curing agents, cover coats, waxes, greases, oils, lubricants, plasticizers, and
other contaminants found in a typical manufacturing environment. Other solvent evaluation
methods used to screen the candidate cleaners were adhesion strength testing, high voltage
testing, corrosion testing, and thermal characteristics testing.
Cleaning Results
For solder flux and mold release removal, the hydrocarbon solvent and the three terpenes were
comparable to TCE. For removing the general contaminants, only the hydrocarbon solvent and
the terpene containing emulsifiers were comparable to TCE for all of the general contaminants.
d-Limonene and the terpene containing phase separation agents were able to remove most of the
general contaminants, but more time and solvent were necessary. Generally speaking, d-
limonene was not quite as effective as the terpene containing phase separation agents, which was
not quite as effective as the terpene containing emulsifiers. Table 1 highlights the cleaning
results for the general contaminants that were the most difficult to remove.
Test results from adhesion strength testing, high voltage testing, and corrosion testing agreed
with the solvent cleaning results mentioned above: the hydrocarbon solvent and the terpene
containing emulsifiers are comparable to TCE; d-limonene was not as effective as the terpene
containing phase separation agent, which was not as effective as the terpene containing
emulsifiers. However, d-limonene had the best thermal characteristics of the three terpenes.
FREON was used primarily as the drying agent in the TCE cleaning process, and was not used
for removing contaminants from the assemblies. Adequate drying was being obtained from the
existing nitrogen drying and vacuum baking processes, and the FREON drying process was
simply halted.
226
Epoxy Curing Acrylic Mold
Solvent and Rank Resin Agent Cover Co_t Relea_
1) Baseline Clean Clean Clean Clean
(TCE) 15 sec 30 sec 15 sec 60 see
2) Terpene With Clean Clean Clean Clean
Emulsifiers 15 sec 90 sec 4 min 2 rain
3) Hydrocarbon Clean Clean Clean Clean
Solvent 15 sec 3 min 2.5 min 4 min
4) Terpene With Clean Clean "Gross Clean
Separation Agent 45 sec 4 min 4 rain 4 min
5) d-Limonene Dirty Dirty "Gross Dirty
(97% Pure) 1 min 4 min 4 rain 4 min
* Note: Although the coupons were still grossly contaminated, there was no concern.
This acrylic material was used as a cover coat for ink marking, and there was no
desire to remove it. In actual practice, it was discovered that both solvents easily
remove normal amounts of the acrylic cover coat.
Table 1: General Contaminant Removal From Aluminum Coupons (grossly contaminated
and allowed to air cure for 3 days).
CLEANING SOLVENT SELECTION
The hydrocarbon solvent and the terpene containing emulsifiers were chosen for further
evaluation. However, the terpene containing emulsifiers was used with only an isopropyl alcohol
rinse, to make its cleaning process similar to the hydrocarbon solvent's, and to avoid anticipated
compatibility problems from cleaning with water. Both solvents were eventually eliminated,
though, because solvent residue from either solvent could not be removed from the actual
assemblies with the isopropyl alcohol rinse. Next the terpene containing phase separation agents
was evaluated. It did not require any deionized water rinse, which eliminated the compatibility
concerns from using water. However, the terpene containing phase separation agents was
eliminated. Once again, the isopropyl alcohol rinse could not remove the residual solvent.
227
Selection of d-Limonene
During the cleaning evaluation of the terpene containing phase separation agents, two extreme
formulations were detected. While most of the formulations contained 90% d-limonene, one
formulation contained nearly 100% d-limonene. The key discovery was not finding the two
extreme formulations but in the observed cleaning results. There were no differences in the
cleaning efficiencies of the two extreme formulations. The presence of the additives (which
remained on the assemblies after the cleaning process) did not affect the cleaning ability of the
d-limonene. When this fact was determined, the terpene containing phase separation agents was
replaced with food grade d-limonene (97% pure). The existing cleaning processes had little
difficulty removing the residual d-limonene, and since the additives were no longer present, the
amount of residual solvent remaining in each assembly was substantially decreased.
COMPATIBILITY EVALUATIONS
Compatibility studies involving accelerated aging, functional testing, solvent absorption, and
residual solvent removal were performed on all of the components, piece-parts, and materials
in the assemblies. The materials were divided into two categories: organic and inorganic. The
inorganic materials compatibility testing consisted of corrosion studies on copper and solder
dipped copper. The organic materials were further divided into adhesives, encapsulation foams,
and a general category consisting of polyurethanes, acrylics, polycarbonates, silicones,
polyolefins, epoxies, solder masks, cables, inks, and others.
Material Compatibility
No adverse affects were observed in the inorganic material corrosion studies or in the organic
material encapsulation foam studies. As for the remaining organic materials, only one known
material was determined to be completely incompatible with d-limonene: polystyrene. Other
organic materials that are known to absorb d-limonene are: polyurethane elastomers, polyolefin
sleevings, some nylon tie wraps, acrylics, silicones, and rubbers (EPDM). Note that these
conclusions were based upon worst-case d-limonene exposure testing--soaking and/or saturated
vapor. Vacuum baking processes are effective in minimizing actual amounts of absorbed solvent
in these materials. All other organic materials, and all of the components and piece-parts tested
were judged to be compatible with d-limonene. Table 2 contains selected information on
material compatibility with TCE and with terpenes containing at least 90% d-limonene.
228
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229
Hardware Compatibility
Ten complete sets of modules and five complete final units were built for the accelerated aging
testing. The units were separated into three groups of five. Within each group, one set was
cleaned with TCE and aged in a nitrogen environment, while four sets were cleaned with d-
limonene and aged in a saturated d-limonene environment. Each unit completed 99 thermal
cycles from -54 to 71°C (-65 to 160°F) and six months of isothermal aging at 71"C (160*F).
After the accelerated aging testing was complete, all ten module sets and all five final units were
judged to be compatible with d-limonene (no visual or electrical failures caused by d-limonene).
RESIDUAL SOLVENT ANALYSIS
A gas analysis technique was developed and optimized to quantitatively determine the amount
of d-limonene remaining in a final unit. The d-limonene concentrations were measured as a
function of time. Aging canisters containing units cleaned with d-limonene were evacuated
through a liquid nitrogen cold trap. The trap was washed with methylene chloride, and the wash
solution was analyzed using gas chromatography (flame ionization detection). Known quantities
of d-limonene were used to characterize and optimize the process. After six generations of
improvement, recovery efficiencies from 92% (for a 50 mg sample) to 99.5% (for a 2000 mg
sample) were achieved. Earlier in the project, it was estimated that 1000 to 2000 mg would
remain in the final unit after the subassemblies and the final unit were cleaned with d-limonene.
Results
Three d-limonene units were tested three repetitive times using this gas analysis process (one
unencapsulated unit, one encapsulated unit, and one alternate encapsulated unit). Test results
concluded that negligible amounts of d-limonene were detected in these assemblies. The results
are tabulated in Table 3.
Unencapsulated Unit:
Encapsulated Unit:
Alternate
Encapsulated Unit:
Sample 1 _ Sample 3
1.26 mg 1.70 mg 1.08 mg
2.75 mg 0.31 mg < .01 mg
< .01 mg < .01 mg < .01 mg
Table 3: Residual Solvent Remaining in a Final Unit
Given this actual data and the uncertainty at these detection levels, it was decided (with at least
92 % confidence!) that less than 50 mg of solvent remained in the final units cleaned with d-
limonene. These measured values were acceptable and quite pleasing, especially when 1000 to
2000 mg was expected!
23O
CONCLUSIONS
Halogenated cleaning solvents used to clean complex electronic assemblies have been replaced
with d-limonene, an environmentally safe, terpene cleaning solvent. This enhances the hazard
elimination efforts at the Kansas City Division by following company policy to eliminate CFC
and CHC usage, and complies with environmental regulations by eliminating the emission of
CFC's (known ozone depleters) and the exposure to CHC's (suspect carcinogens).
All materials tested in this evaluation were judged to be compatible with d-limonene under
routine, everyday operating conditions. Through accelerated aging testing, the long-term
compatibility of the modules and final assemblies d-limonene was verified. Functional testing
of complex electronic assemblies was completed. No electrical failures were caused by the use
of d-limonene. New analytical techniques were developed and used to measure and quantify
residual solvent levels in the assemblies. Only trace amounts were detected in each of three
consecutive tests.
231


Non-aqueous cleaning solvent substitution

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