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Description  |
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FIELD OF THE INVENTION
The present invention relates to environmentally safe aqueous flux removing
compositions for cleaning electronic circuit assemblies, such as printed
circuit or printed wiring boards, during their fabrication. Alkali metal
carbonate and bicarbonate salts are utilized preferably with various
adjuvants, including a specified anti-corrosion and brightening agent, to
achieve a variety of objectives, among which are the removal of solder
flux, oils, waxes and greasy substances and adhesive and other residues.
BACKGROUND OF THE INVENTION
The cleanliness of electronic circuit assemblies (ECA), such as printed
circuit boards (PCB) or printed wiring boards (PWB), is generally regarded
as being critical to their functional reliability. Ionic and nonionic
contamination on circuit assemblies is believed to contribute to premature
failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic
contamination can accumulate after one or more steps of the process.
Circuit assembly materials are plated, etched, handled by operators in
assembly, coated with corrosive or potentially corrosive fluxes and
finally soldered.
In the fabrication of electronic circuit assemblies, e.g., 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 only moderately 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 water soluble fluxes contain strong acids and/or
amine hydrohalides, such fluxes are very corrosive. Unfortunately,
residues of any flux can cause circuit failure if residual traces of the
material are not carefully removed following soldering and thus remain on
an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water,
the removal of rosin flux from printed circuit boards is more difficult
and has therefore traditionally been carried out with the use of
chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane,
trichloroethylene, trichloromonofluoromethane, methylene chloride,
trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or
mixtures or azeotropes of these and/or other solvents. These solvents are
undesirable, however, because they are toxic and when released into the
environment deplete the ozone layer and/or contribute to the greenhouse
global warming effect. Thus, use of such solvents is subject to close
scrutiny by the Occupational Safety and Health Administration (OSHA) and
the Environmental Protection Agency (EPA), 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 alkanolamines, usually in the form
of monoethanolamine, have been used for rosin flux removal as an
alternative to the toxic chlorinated hydrocarbon solvents. These high pH
compounds (e.g., about 12 pH), 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, as well as the water soluble soldering
fluxes, have a tendency to cause corrosion on the surfaces and interfaces
of printed wiring boards if such compounds and fluxes are not completely
and rapidly removed during the fabrication process.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly
caustic solution having a pH of 13 in a batch cleaning process. This
method severely oxidizes the solder applied to the circuit board. In Hayes
et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and
other residues are removed from electronic assemblies by means of terpene
compounds in combination with terpene emulsifying surfactants by rinsing
in water.
The complete removal of adhesive and other residues also poses a problem.
During the manufacture of electronic circuit assemblies the components are
mounted on the upper surface of the board with leads protruding downwardly
through holes in the board and are secured to the bottom surface of the
board by means of an adhesive. Further, 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. In both instances, a residue of adhesive generally
remains which, if not thoroughly removed, can cause premature board
failure. Removal of this adhesive residue has traditionally been carried
out by the use of chlorinated solvents which, as already described, are
toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic
circuit assemblies and which can be removed by the compositions and method
of the present invention include, but are not limited to, for example,
rosin flux, photoresist, solder masks, adhesives, machine oils, greases,
silicones, lanolin, mold release, polyglycols and plasticizers.
In copending, commonly assigned U.S. Ser. No. 731,512, filed Jul. 17, 1991,
improved cleaning compositions characterized by non-corrosiveness and low
environmental impact, unlike the prior art chlorinated hydrocarbon
solvents and alkaline cleaners, are employed for printed wiring board and
printed circuit board cleaning. As disclosed therein, printed
circuit/wiring board cleaning compositions are provided comprising alkali
metal carbonate and bicarbonate salts so combined that they have, when
used in concentrations of about 1 to 15 percent by weight, a Ph of from
about 10, or less, to 12 and an adequate reserve of titratable alkalinity,
at least equivalent to from about 0.2 to 4.5 percent caustic potash
(potassium hydroxide), when titrated to the colorless phenolphthalein end
point. At least about 50 percent and, preferably, at least about 65
percent by weight of the carbonate salts comprise potassium carbonate. The
aqueous cleaning solutions generally contain from about 1 to 15 percent or
even more depending on the particular conditions and, preferably, from
about 2 to 8 percent by weight of the salts comprising the cleaning
composition. In addition, the cleaning solutions usually contain a small
amount, e.g., from about 50 to 5000 ppm of a water soluble reducing agent
(oxygen scavenger). Preferably, the cleaning solutions also contain at use
a small amount, e.g., up to about 0.1 percent by weight of an antifoam
agent. These, as well as other adjuvants, e.g., wetting agents,
surfactants, etc., can be included with the salts per se or in any
solution thereof no matter what the concentration of salts therein. When
used according to the above, the compositions do not leave an undesirable
residual film.
While the cleaning compositions of the above-mentioned copending
application advantageously achieve the objectives stated therein such as
providing a method for the safe and effective removal of rosin soldering
fluxes from electronic circuit assemblies such as printed circuit boards
without otherwise adversely affecting the boards, further improvement with
respect to providing anti-corrosion protection to metal components
including the solder joints as well as the connecting tabs along the edges
of the boards and for providing brightening of all solder joints and metal
connectors is still needed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide compositions and
soldering fluxes from electronic circuit assemblies, e.g., printed circuit
boards, without t assemblies, e.g., printed circuit 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 other residual contaminants from printed circuit
assemblies. Still further, another objective of the invention is to
provide anti-corrosion protection to the printed circuit assemblies and to
brighten solder joints, metalized connecting tabs at the board edges and
other metal or metalized features of the assemblies.
This invention provides cleaning compositions and methods for the removal
of rosin solder fluxes and other 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 are characterized by non-corrosiveness
and anti-corrosive protection as well as low environmental impact, unlike
the chlorinated hydrocarbon solvents and highly alkaline cleaners that
have heretofore been employed for printed wiring board and printed circuit
board cleaning. Advantageously, the flux removing compositions, as used
herein, exhibit lower biological oxygen demands (BOD) and chemical oxygen
demands (COD) than formulations currently available. For example, BODs and
CODs below 20,000 ppm in the wash water and considerably lower, e.g. less
than 300 ppm in the rinse water result upon using the cleaning
compositions of this invention. Accordingly, the rinse water can be
sewered without further treatment and minimal, if any, treatment is needed
to remove the organics from the wash water before sewering, thus
eliminating the need for costly water treatment.
The present invention provides printed circuit/wiring board cleaning
compositions essentially formed from the aqueous cleaning compositions as
set forth in aforementioned U.S. Ser. No. 731,512 and comprising alkali
metal carbonate salts or a mixture of alkali metal carbonate and
bicarbonate salts. At least about 50 percent and, preferably, at least
about 65 percent by weight of the carbonate salts comprise potassium
carbonate. The aqueous cleaning solutions generally contain from about 0.1
to about 15 percent by weight of the salts or more depending on the
particular conditions. In addition and in accordance with the improvements
found with respect to anti-corrosion protection and metal brightening, the
aqueous cleaning solutions of this invention further contain from about
0.01 to 2 percent by weight of an alkali metal silicate. Other adjuvants,
e.g., wetting agents, anti-foam agents, surfactants, etc., can be included
with the salts per se or in any solution thereof no matter what the
concentration of salts therein.
BRIEF DESCRIPTION OF THE FIGURES
The efficacy of this invention will be better understood by reference to
FIGS. 1-7 herein wherein the test results of certain embodiments of the
cleaning solutions of this invention are illustrated.
FIGS. 1, 2, 4, and 6 represent typical curves showing the cleaning
efficiencies of various concentrations of cleaning solutions resulting
from visual testing as described herein.
FIGS. 3, 5 and 7 represent typical curves showing the cleaning efficiencies
of various concentrations of cleaning solutions resulting from equilibrium
resistivity measurements as described herein.
DETAILED DESCRIPTION OF THE INVENTION
The objects and advantages mentioned above as well as other objects and
advantages may be achieved by the compositions and methods hereinafter
described.
Essentially, the flux removing compositions of this invention comprise
certain alkali metal salts and alkali metal salt mixtures as specifically
set forth in afore-mentioned U.S. Ser. No. 731,512 with the addition of an
alkali metal silicate. Accordingly, the term "flux removing compositions"
as used herein is intended to define the mixture of active ingredients
comprised of the alkali metal salts including the added alkali silicate
and, if desired, any added adjuvants as hereinlater described.
As hereinlater set forth, the flux removing compositions may be formulated
into concentrated solutions. The terms "flux removing concentrated
solutions" or "concentrates" as used herein define aqueous mixtures
containing from about 5 to 45 or more percent by weight of the flux
removing compositions with the balance being essentially water.
As used herein the terms "flux removing solutions" or "flux removing
solutions in use" is meant to define aqueous mixtures containing from
about 0.1 to 15 percent by weight of the flux removing composition with
the balance comprised essentially of water and which are the solutions
employed in the cleaning methods of the invention. Also, as used herein,
"flux removing composition" and "cleaning composition" have the same
meaning since as stated previously, the electronic circuit assemblies
including printed circuit boards and printed wiring boards often contain
residues other than fluxes which the compositions of this invention are
able to remove and thus "flux removing composition" is intended as an
all-purpose cleaner.
In accordance with the invention, additives, adjuvants, or the like, may be
included with the flux removing compositions, flux removing concentrates,
or the flux removing solutions in use.
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