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Description  |
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TABLE OF CONTENTS
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Page
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1. Field of Invention 1
2. Background of the Invention 2
3. Summary of the Invention 4
4. Brief Description of the Figure
6
5. Detailed Description of the Invention
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5.1. Rosin Flux Cleaning and Adhesive Tape Residue
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Removal
5.2. Quality Control Testing for Improperly
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Cured U.V. Soldermask
6. Example: Rosin Flux Cleaning 12
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1. FIELD OF THE INVENTION
The present invention relates to non-toxic, environmentally safe
compositions for use in cleaning during the fabrication of printed circuit
or printed wiring boards. Terpene compounds are utilized with or without
terpene emulsifying surfactants to achieve a variety of objectives, among
which are the removal of solder flux, oils, waxes and greasy substances or
adhesive tape residues. The compositions of the invention may also be used
to test the quality of curing of ultraviolet soldermask.
2. BACKGROUND OF THE INVENTION
In the fabrication of printed wiring boards and/or printed circuit boards,
soldering fluxes are first applied to the substrate board material to
ensure firm, uniform bonding of the solder. These soldering fluxes fall
into two broad categories: rosin and non-rosin, or water soluble, fluxes.
The rosin fluxes, which are generally non-corrosive and have a much longer
history of use, are still widely used throughout the electronics industry.
The water soluble fluxes, which are a more recent development, are being
used increasingly in consumer products applications. Because they contain
strong acids and/or amine hydrohalides and are thus corrosive, the water
soluble soldering fluxes can cause circuit failure if residual traces of
the material are not carefully removed. For that reason military
specifications require the use of rosin fluxes.
Even the use of rosin soldering fluxes, however, can lead to premature
circuit failure due to decreased board resistance if traces of residual
flux are not removed following soldering. While water soluble fluxes can
be easily removed with warm, soapy water, the removal of rosin flux from
printed wiring boards has traditionally been carried out with the use of
chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane,
trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane,
or mixtures or azeotropes of these solvents. These solvents are
undesirable, however, because they are toxic. Thus, their use is subject
to close scrutiny by the Occupational Safety and Health Administration
(OSHA), and stringent containment equipment must be used. Moreover, if
released into the environment these solvents are not readily biodegradable
and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanol amines, usually in the
form of monoethanolamine, have been used for rosin flux removal as an
alternative to the toxic chlorinated hydrocarbon solvents. These compounds
chemically react with rosin flux to form a rosin soap through the process
of saponification. Other organic substances such as surfactants or alcohol
derivatives may be added to these alkaline cleaning compounds to
facilitate the removal of such rosin soap. Unfortunately, these compounds
like the water soluble soldering fluxes have a tendency to cause corrosion
on the surfaces and interfaces of printed wiring boards if they are not
completely and rapidly removed during the fabrication process.
In another approach, Bakos et al. [U.S. Pat. No. 4,276,186] have used
mixtures of N-methyl-2-pyrrolidone and a water miscible alkanolamine to
remove solder flux and solder flux residue from integrated circuit
modules. These mixtures were also said to be useful for removing various
cured synthetic organic polymer compositions such as cured polyimide
coating compositions from integrated circuit chip modules.
During the manufacture of printed wiring boards, it is sometimes necessary
to temporarily protect certain portions of the board from processing steps
such as the process of creating corrosion resistant gold connecting tabs
at the board edges. This transient protection of portions of the circuit
board can be achieved by the application of special adhesive tape to
susceptible areas. Once such protection is no longer needed, the adhesive
tape must be removed. A residue of the tape adhesive generally remains
which, if not throughly removed, can cause premature board failure.
Removal of this tape residue has traditionally been carried out by the use
of chlorinated solvents which, as already described, are toxic and
environmentally undesirable.
A further procedure that is commonly carried out during printed circuit
board fabrication is the application of a soldermask. As the name implies,
a soldermask is a polymer or resin coating that is selectively applied to
a printed circuit board to shield areas where solder is not required or
desired. Where such masking is to be permanent, the soldermask must be
"cured," a process by which monomeric reactants are made to polymerize. If
curing of the soldermask is incomplete, solder will penetrate areas that
are to be shielded, resulting in destruction of the board.
One kind of soldermask is based on acrylate chemistry and is
photo-initiated and polymerized, and thus cured, using ultraviolet (U.V.)
light. The use of U.V.-cured soldermask is advantageous in that its curing
is rapid, but the thickness of such soldermask films must be uniform for
successful curing. The U.V. irradiation used cannot penetrate to the
bottom of deep soldermask deposits, and attempts to increase the intensity
or the duration of irradiation to cure thick sections can result in the
embrittlement of the thinner mask film regions.
3. SUMMARY OF THE INVENTION
It is an object of the present invention to provide compositions and
methods for the safe and effective removal of rosin soldering fluxes from
printed wiring boards without otherwise adversely affecting the boards. It
is a further objective of this invention to provide safe and effective
compositions and methods for the removal of adhesive tape residues from
printed wiring boards. It is a still further objective of the present
invention to provide methods and compositions to detect improperly cured
U.V. soldermask and to thus avoid printed wiring board failure on that
account.
This invention provides cleaning methods and compositions for the removal
of rosin solder fluxes and adhesive tape residues during the fabrication
of printed circuit or wiring boards. As a result, the possibility of
premature circuit failure that might occur in the absence of such cleaning
is eliminated or greatly reduced. The cleaning efficacy of the
compositions of the invention is such that printed wiring boards thus
treated meet stringent U.S. Department of Defense specifications.
The compositions of the invention can also be used to detect improperly
cured U.V. soldermask. This task is accomplished by exploiting an ability
of the compositions to selectively lift inadequately cured regions of such
soldermask from the board, while leaving properly cured regions intact.
The compositions of the invention are characterized by low toxity,
biodegradability and non-corrosiveness, unlike the chlorinated hydrocarbon
solvents and alkaline cleaners that have heretofore been employed for
printed wiring board and printed circuit board cleaning. As a result, the
need for costly containment equipment is eliminated.
More particularly, the present invention provides printed circuit/wiring
board treatment compositions comprising terpene compounds such as limonene
or dipentene. These terpene compounds, which have the ability to dissolve
or complex with and remove rosin solder fluxes, oils, waxes and greasy
substances or adhesive tape residues, could be used as such and removed,
e.g., with clean absorbent materials. Preferably, however, the terpene
compounds of the invention are combined with terpene emulsifying
surfactants to facilitate removal by water.
4. BRIEF DESCRIPTION OF THE FIGURE
The present invention may be more readily understood by reference to FIG.
1, wherein a flow chart shows a representative printed circuit/wiring
board fabrication process.
5 DETAILED DESCRIPTION OF THE INVENTION
The compositions of the present invention contain terpene compounds that
have the capability to dissolve rosin soldering flux or adhesive tape
residues that are commonly used in the fabrication of printed circuit or
wiring boards. Terpene compounds that are suitable for this purpose
include but are not limited to pinene, both alpha and beta isomers, gamma
terpinene, delta-3-carene, limonene and dipentene (the. racemic mixture of
the isomers of optically active limonene), with limonene and dipentene
being preferred.
These terpene compounds could be used alone and removed, after allowing
them to complex with and dissolve rosin flux or adhesive tape residues,
e.g., by flushing with excess terpenes or by absorption into paper or
cloth. Because they are almost completely insoluble in water, however, the
terpenes cannot be directly flushed away by water. Alternatively and
preferably, the terpene compounds are combined with one or more terpene
emulsifying surfactants. The addition of such surfactants facilitates
removal of the terpenes from printed wiring boards by rinsing with water,
whereby the terpenes are formed into oil-in-water emulsions.
Numerous surfactants capable of emulsifying the terpenes of the invention
may be used, including but not limited to the linear alkyl benzene
sulfonates, linear or branched chain alcoholic ethoxylates and
ethoxysulfates, polysorbate esters, ethoxylated alkylphenols and alkyl and
dialkyl succinate compounds. An example of the latter class of compounds
is sodium dioctyl sulfosuccinate. The ethoxylated alkylphenols contain
various alkyl side chains and various numbers of linked ethylene oxide
units. Useful compounds of this class contain from about 5 to about 20
ethylene oxide groups, with 7 or 8 being preferred.
The quantity of terpene emulsifying surfactants in the compositions of the
present invention may range on a weight basis from 0 to about 40%, with
the terpene compounds accounting for the balance of the compositions. In a
preferred embodiment designated EC-1, the composition comprises by weight
about 6.5% poly (7) ethoxy nonylphenol, 2.1% poly (10) ethoxy nonylphenol,
1.4% sodium dioctyl sulfosuccinate and 90% limonene.
The compositions of this invention are characterized by low toxicity and
environmental biodegradibility, unlike the chlorinated hydrocarbon
solvents that had been used prior to this invention for printed wiring
board cleaning. Limonene, for example, is a natural product in various
ethereal oils such as oils of lemon, orange, caraway, dill and bergamot
which possesses a pleasant, lemon-like odor. EC-1 has biological oxygen
demand and chemical oxygen demand values of 295 and 1,425 milligrams per
liter, respectively, at a dilution of 1,000:1, as determined by methods
described in Standard Methods for the Examination of Water and Wastewater,
16th Edition, American Public Health Association.
The applicability of the compositions of the invention to various aspects
of the printed circuit/wiring board fabrication process can best be
understood by reference to a representative process flow chart, which is
shown in FIG. 1.
The assembly manufacturing process involves the placement of components
such as integrated circuits, resistors, capacitors, diodes, etc. on the
surface of the board or their insertion through pre-drilled holes. The
components are then secured by soldering by mechanical or automatic means.
Interspersed with the soldering operations are cleaning procedures and
inspections to ensure that tape and solder flux residues that could lead
to premature circuit failure do not remain. Points in the fabrication
process to which the compositions of the invention apply will be
identified below.
5.1. ROSIN FLUX CLEANING AND ADHESIVE TAPE RESIDUE REMOVAL
For the removal of rosin soldering flux deposits or adhesive tape residues
during printed circuit/wiring board fabrication, the compositions of the
invention may be applied to the boards by immersion in dip tanks or by
hand or mechanical brushing. Alternatively, they may be applied by any of
the commercially available printed wiring board cleaning equipment.
Dishwasher size units may be employed, or much larger continuous belt
cleaning machinery such as Total Systems Concept's Dual Process Cleaning
System models 31-218, 31-418, 31-224 and 31-424 (Total Systems Concept,
St. Louis, Mo). Depending upon their design, these washers may apply the
compositions of the invention by spraying with mechanical nozzles or by
rolling contact with wetted roller surfaces. The temperature at which the
compositions may be applied can range from room, or ambient, temperature
(about 70.degree. F.) to about 150.degree. F. The compositions containing
terpene emulsifying surfactants may be used full strength or diluted with
water to as low as about a 2 volume percent concentration. The
compositions should contact the boards for about 1 to about 5 minutes.
The points at which the compositions of the invention would be applied for
solder flux removal in a typical printed wiring board fabrication process
are indicated in FIG. 1 by the term "post clean," referring to the post
solder cleaning process.
Once solder flux has been loosened and solubilized during a period of
contact which typically ranges from about 1 to about 5 minutes, the
compositions of the invention are removed Where the terpene compounds are
used alone, their removal can be carried out by flushing with a non-toxic,
miscible solvent or by absorption into appropriate materials. In the
preferred embodiments of the invention, which contain terpene emulsifying
surfactants, the boards may simply be flushed with water for a period of
up to about 2 minutes. Deionized water is preferred. The optimal rinsing
time varies according to the kinds of surfactants and the concentrations
of the terpene formulations used and can easily be determined by routine
experimentation.
The cleaned boards are then dried, preferably with forced air. Drying is
expedited if the air is warmed, preferably to above about 100.degree. F.
The efficacy of rosin soldering flux removal from printed wiring boards is
such that the boards meet stringent military specifications for low
resistivity after cleaning. For example, the boards meet the Mil-P-28809A
standard for low resistivity of solvent extracts by which contamination
has been removed from a cleaned board according to Mil-P-55110C. The
resistivity of solvent extracts of cleaned boards is most easily
determined with an Omega Meter. Omega Meter is the registered trademark of
Kenco Industries, Inc., Atlanta, GA, for a microprocessor-controlled
contamination test system that rapidly measures changes in resistivity due
to contamintating ions.
The result of Omega Meter measurements are expressed in equivalent units of
mg NaCl/in.sup.2 or its metric equivalent. According to MIL-P-28809A, the
acceptable resistivity value for a cleaned board is equivalent to 2.2 ug
NaCl/cm.sup.2 or 14 ug NaCl/in.sup.2, but far better results are routinely
obtained after solder flux has been removed with the compositions of the
present invention. A value of about 0.67 ug NaCl/cm.sup.2 or 4.3 ug
NaCl/in.sup.2 is typical.
During the process of gold plating connecting tabs to improve corrosion
resistance, tin-lead residues must first be removed from the unplated
tabs. Removal of these residues is carried out through the use of etching
chemicals that can damage other unprotected printed circuit/wiring board
components. To protect vulnerable components from the etching chemicals,
boards are wrapped on both sides with an adhesive plating tape which forms
a shield or splash guard for all but the exposed tab area. The etching
chemicals then remove the tin-lead residues on the tabs, a nickel plate is
applied as a base for the gold, and gold plating of the tabs is finally
carried out. The adhesive plating tape which was maintained in place
through all of these etching and plating steps, is then removed. The use
of adhesive tape is indicated in FIG. 1 at the point labeled "tape for
gold plating." The tape is removed following the "nickel and gold plate"
step, and it is at this point that the terpene compositions of the
invention may most advantageously be used.
Following removal of the tape, a silicone-based and/or rubber-based residue
may remain on the board. This residue may easily be removed by the
compositions of the invention under the same conditions described above
for solder flux removal. The exact operational parameters will be
determined by the nature of the tape residue and the tenacity with which
it adheres to the board, but the conditions described above are generally
effective. As in the case of solder flux removal, treatment of the board
with the terpene compounds of the invention is generally followed by water
flushing and air drying.
The efficiency of removal of adhesive tape residues from printed
circuit/wiring boards by the compositions of the invention is such that no
residues are visible after cleaning. A simple 5-10X stereomicroscope can
facilitate visual inspection for tape residues following cleaning.
5.2. QUALITY CONTROL TESTING FOR IMPROPERLY CURED U.V. SOLDERMASK
It has unexpectedly been found that the compositions of the invention will
cause a lifting or bubbling of uncured areas U.V. curable soldermask on
printed circuit/wiring boards, while leaving properly cured regions
undisturbed. The detection of improperly cured soldermask in this fashion
can be beneficial by eliminating the use of such defective boards before
valuable components are placed thereon. Such detection is carried out at
the "Board Compete and Ready for Assembly" point in FIG. 1.
Soldermask to which such detection is applicable is based upon methacrylate
polymer chemistry. Such polymers, upon exposure to ultraviolet light,
become "cured" through the photoinitiated cross-linking of the polymers to
produce a tough protective coating that resists subsequent solder
adherence. It is poorly crosslinked regions of the soldermask that are
susceptible to disruption by the terpene compounds of the invention.
The detection of such uncured soldermask regions can be carried out by
suspending printed circuit/wiring boards to which a U.V. soldermask has
been affixed and cured in a composition such as EC-1 for a period of time,
rinsing in water and then making a visual inspection. Submersion in the
cleaning solution or spray application etc. for from about 1 to about 5
minutes at a temperature of from about 70.degree. to about 150.degree. F.
is usually sufficient. The boards are then generally flushed with water
and dried as described above for solder flux and adhesive tape residue
removal. Where dip tanks are used, a second tank that is continually
flushed with fresh water is preferably used to rinse the boards.
Inspection can be facilitated by the use of a microscope such as a 5-10X
stereomicroscope, but the use of such an instrument is not essential.
Boards having improperly cured U.V. solder mask show evidence of blisters,
bubbles, discoloration, cracks and deformation in the defective regions.
6. EXAMPLE: ROSIN FLUX CLEANING
To illustrate the cleaning ability of the compositions of the invention,
printed wiring boards that had been wave soldered using rosin flux
conforming to MIL-F-14256 (type RMA) were cleaned in a mechanical cleaning
system.
The cleaning composition, designated EC-1, contained by weight 90%
d-limonene, 6.5% poly (7) ethoxy nonylphenol, 2.1% poly (10) ethoxy
nonylphenol and 1.4% sodium dioctyl sulfosuccinate. Concentrations of 25
to 100 volume percent of EC-1 diluted in water were used as indicated
below. The cleaning system was a Series 400, Model TRL-HSE Aqueous
Cleaning System manufactured by The John Treiber Company, Huntington
Beach, Calif.
The cleaning system has a conveyor driven modular design in which the
operations of loading, pre-wash rinsing, washing, air knife drying, first
rinsing, air knife drying, final rinsing and high speed drying are carried
out in succession. Rinse water at ambient temperature and EC-1 for washing
at 120.degree. F. were applied through spray nozzle manifolds as the
boards traversed the system at a conveyor speed of 8 feet/minute. In air
knife drying, turbine propelled air shears fluids from the board surfaces.
In the final high speed drying module, two sets of dual air knives above
and below the conveyor thoroughly dry the cleaned boards. The air in this
module was heated to 120.degree. F.
Cleaned and dried boards were evaluated for cleaning efficiency by Omega
Meter resistivity measurements. To make these measurements, cleaned and
dried boards were loaded into a test cell of the instrument and then
extracted with a circulating solution of isopropanol: water (25:75, v/v)
as specified by MIL-P-55110C and MIL-P-28809A. The resistivity of the
solution was measured at a rate of 24 times per minute over a period of
about 5-15 minutes until equilibrium was reached, indicating that
extraction of board surface contamination was essentially complete.
Equilibrium was defined as the point at which the change in measured
resistivity of the solution was less than or equal to 5% of any value
measured in the previous two minutes.
The equilibrium resistivity measurements for a number of cleaning tests are
shown in Table 1.
TABLE 1
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RESISTIVITY MEASUREMENTS OF EC-1 CLEANED
ROSIN FLUX SOLDERED PRINTED WIRING BOARDS
EC-1 Equivalent NaCl
Test Concentration
Contamination
Number (volume %) (ug/in.sup.2)
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1 25 4.7
2 25 4.1
3 50 4.1
4 50 4.7
5 75 3.9
6 75 4.0
7 100 4.6
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As shown in Table 1, EC-1 at all concentrations examined were effective in
producing levels of residual board surface contamination that were far
below the MIL-P-28809A requirement of 14 ug NaCl/in.sup.2 equivalent.
Many modifications and variations of this invention may be made without
departing from its spirit and scope, as will become apparent to those
skilled in the art. The specific embodiments described herein are offered
by way of example only, and the invention is limited only by the terms of
the appended claims.
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Description |
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