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Description  |
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TABLE OF CONTENTS
1. Field of the Invention
2. Background of the Invention
3. Summary of the Invention
4. Brief Description of the Figure
5. Detailed Description of the Invention
5.1. Rosin Flux Cleaning and Residue Removal
5.2. Screen Ink Cleaning and Residue Removal
6. Examples
6.1. Rosin Flux Cleaning
6.2. Screen Ink Cleaning
1. FIELD OF THE INVENTION
The present invention relates to non-toxic, environmentally safe
compositions for use in removing organic residues from contaminated
surfaces. Dibasic ester compounds are utilized with or without dibasic
ester emulsifying surfactants to achieve a variety of objectives, among
which are the removal of solder flux, oils, waxes and greasy substances
from metal surfaces, as well as removing uncured inks and resists from
printing screens and stencils.
2. BACKGROUND OF THE INVENTION
The use of fluxes, particularly rosin fluxes, is a virtually inescapable
aspect of processes requiring soldering, i.e., the making of mechanical,
electromechanical or electronic connections with metal. Fluxes may be
applied to the metal surface separately or as part of the solder paste.
Several functions are achieved by flux application: the flux chemically
and/or physically combines with undesirable "debris" on the metal surface,
such as oxides and sulfides, and prevents oxide formation during the
solder heating process. It also aids in reducing surface tension of the
molten solder. After soldering is complete, however, the flux residues
must be removed, since they may be corrosive, or interfere with the normal
functioning of the apparatus to which they have been applied. The removal
of these compounds is frequently not a routine matter, and usually cannot
be achieved by a simple water wash. For example, 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 fluxes and non-rosin, or water soluble, fluxes. The
rosin fluxes, which are generally less 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.
Fabrication of circuit boards is just one example of the use of fluxes.
Other processes in which flux residues are produced, and in which removal
is necessitated, include circuit board assembly, component packaging, and
board clean-up after in-service maintenance. The joining of interconnect
devices, e.g. computer connections with cables, also may require the use
of solder and solder flux. More traditional uses of course include
pipe-fitting, or any other procedure which requires the joining of metal
surfaces. The most common flux used in both electronic and
electromechanical joining are rosin fluxes, and in such uses, the removal
of the flux is strongly preferred, if not absolutely necessary.
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,-trichloroethane, trichloromonofluoromethane, methylene chloride,
trichlorotrifluoroethane, or mixtures or azeotropes of these solvents.
These solvents are undesirable, however, because they are toxic or
otherwise environmentally objectionable. Thus, their use is subject to
close scrutiny by the Occupational Safety and Health Administration (OSHA)
or other regulatory bodies, 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 halogenated 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 of 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.
Another alternative to the use of halogenated hydrocarbons has also been
described in U.S. Pat. No. 4,640,719 to Hayes et al. Described therein is
a method for cleaning circuit boards which employs terpenes as cleaning
agents. Due to the above noted environmental conditions, there is an
increasing commercial demand for highly effective circuit board cleaners
having a variety of desirable characteristics, such as non-toxicity, high
flash point and low environmental impact which enhance their usefulness
under a variety of circumstances. The compositions of the present
invention, comprising dibasic esters, provide the desired characteristics
in their low toxicity relative to halogenated hydrocarbons, as well as
having a relatively high flash point. Moreover, the present compositions
do not have a harmful effect on the earth's stratospheric ozone layer;
depletion of the ozone layer is another problem which is encountered with
the use of halogenated hydrocarbons. Further, esters of the present
invention generally have a relatively low vapor pressure, and are
therefore less volatile, providing the advantage of lower volatile organic
compound (VOC) pollutant emissions, than prior art compositions. These
compositions are also useful in removing other difficult organic residues,
such as printing inks and resists. Thus, the present method and
compositions provide cleaning ability equivalent to those known in the
art, while also providing advantageous characteristics which have not
generally been found in most prior art methods and compositions.
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 organic residues from
contaminated surfaces without otherwise adversely affecting the surfaces
being cleaned.
This invention provides cleaning methods and compositions for the removal
of solder fluxes, and is particularly useful for rosin flux removal, which
cannot be achieved by water rinse. By use of the present method, the
possibility of premature circuit failure that might occur on electronic
circuit boards in the absence of such cleaning is eliminated or greatly
reduced. The compositions can also be used to clean other surfaces in
which solder is used, in conjunction with flux, to form metal-to-metal
contacts.
Additionally, the compositions can be effectively employed in the removal
of uncured or partially cured printing inks from screens and stencils, as
well as uncured resists used in etching, plating or soldering, from
soldermasks and photomasks. As used in the present specification and
claims, the phrase "printing inks" is intended to encompass thermally
curable inks (e.g., one- or two-part epoxy inks) and U-V curable inks
(e.g., acrylate-based inks). The term "resist" is intended to encompass
alkaline soluble and other thermally curable resists, and UV curable
resists.
The compositions used in the present method of the invention are
characterized by low toxicity, biodegradability and non-corrosiveness,
unlike the halogenated hydrocarbon solvents and alkaline screen cleaners
that have heretofore been employed for solder flux and ink removal. These
compositions also have a relatively high flash point, providing greater
safety when used under conditions of high temperature, as well as in
transportation. As a result, the need for costly containment equipment is
eliminated.
More particularly, the present invention utilizes compositions comprising
dibasic ester compounds, including aliphatic dibasic acid esters, such as
dimethyl succinate, dimethyl adipate, or dimethyl glutarate, or aromatic
dibasic acid esters such as diethyl phthalate, or mixtures of any of
these. These compounds which have the ability to dissolve or otherwise
remove rosin solder fluxes, oil, waxes, greasy substances, printing inks
and resists could be used as such and removed, e.g., with clean absorbent
materials. Preferably, however, the dibasic ester compounds of the present
invention are combined with appropriate emulsifying surfactants to
facilitate removal by water.
4. BRIEF DESCRIPTION OF THE FIGURE
One embodiment of 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 aliphatic or aromatic
dibasic acid ester compounds that have the ability to dissolve soldering
flux, particularly rosin flux that is commonly used in the fabrication,
assembly and maintenance of printed circuits or wiring boards. In
addition, these compositions are also useful in cleaning surfaces which
have been exposed to printing inks and resists. The compounds of the
invention have the formula:
(R.sub.1)OOC(R.sub.3)COO(R.sub.2)
wherein R.sub.1 and R.sub.2 are C.sub.1 -C.sub.6 alkyl, and may be the same
or different, and R.sub.3 is --(CH.sub.2).sub.n --, wherein n is an
integer from 2-8, or a C.sub.6 -C.sub.10 aromatic ring.
Although longer chain compounds may be as effective as shorter chain
components, they are also more viscous; therefore, for convenience
purposes, the preferred compounds of the present invention are those of
the above formula in which n=2, 3, or 4. Particularly preferred in the
present compositions are the compounds dimethyl succinate, dimethyl
adipate and dimethyl glutarate, or any combination thereof. A particularly
useful combination is available from E. I. DuPont (Wilmington, Del.) under
the name DBE, and consists of a mixture by weight of about 15-25% dimethyl
succinate, about 55-75% dimethyl glutarate, and about 10-25% dimethyl
adipate. Other dibasic ester products, also available from DuPont,
containing one or more of the aforementioned esters in different
proportions, are DBE-2, DBE-3, DBE-4, DBE-5 and DBE-9. Alternately,
dibasic esters are easily synthesized by methods well known in the art.
The compositions of this invention are characterized by low toxicity and
environmental biodegradability, unlike the halogenated hydrocarbon
solvents that had been used prior to this invention for printed wiring
board cleaning.
The useful compounds of the present invention are insoluble, or very
slightly soluble in water, and are also slightly viscous. The dibasic
esters may be used alone to clean a contaminated surface, but are
relatively difficult to flush off a surface with water. Therefore, in
practice, the preferred compositions contain an emulsifying surfactant in
combination with the dibasic esters. A thoroughly cleaned circuit board,
from which substantially all traces of the cleaner are also removed, is
necessary to meet the stringent military specifications for, such as the
MilP28809A standard, for low conductivity after cleaning. Similarly,
screens, masks and stencils which have been cleaned to remove inks or
resists frequently are degraded by the types of solvents that are
currently in use for this purpose; therefore, use of a noncorrosive
cleaner, as well as thorough removal, can extend the useful life of these
products.
Among those emulsifying surfactants which are useful for the present
purposes are nonionic, anionic, cationic or amphoteric surfactants, and
preferably nonionic surfactants such as condensation products of alkylene
oxides, for example ethylene oxide, with any of a variety of hydrophobic
moieties, as are well known in the art. Examples of these products are
ethoxylated aliphatic alcohols, ethoxylated alkylphenols, ethoxylated
amines, amides, and the like. Sulfated, sulfonated, phosphated and
carboxylated hydrophobes, such as alcohols, can also be used, as can the
salts of each. Particularly preferred as surfactants are ethoxylated
aliphatic alcohols and their derivatives. The combination of the dibasic
esters with these surfactants facilitates the removal of the cleaning
composition from the printed wiring board by simple flushing with water.
The quantity of the combined emulsifying agent in the present composition
will vary, depending on the particular ester or combination of esters
used. Generally, the added emulsifying agent will range, on a weight
basis, from about 0-40% of the composition. In a preferred embodiment, the
composition comprises a mixture of dimethyl adipate, dimethyl succinate,
and dimethyl glutarate in a total amount of about 90-100%, and an
ethoxylated aliphatic alcohol, in an amount of about 0-10%.
The compositions of the present invention are characterized by low toxicity
and environmental biodegradability, unlike the halogenated hydrocarbon
solvents that have been used prior to this invention for printed wiring
board cleaning. The dibasic esters of the invention also provide the
desirable characteristic of a high flash point. In addition to their
rather low toxicity, biodegradability, and cleaning effectiveness, the
present dibasic esters also have a much higher flash point than many
currently available compounds, providing an added element of safety to the
use of the present compositions, as well as relatively low vapor
pressures. Any compound having a vapor pressure below 0.1 torr is
considered by most states to be a non-volatile organic compound, and thus
not subject to stringent regulation. Certain of the dibasic esters have
vapor pressures as low as 0.06 torr.
In another embodiment of the invention, both terpenes and dibasic esters
are combined to provide an effective cleaning composition. The terpene
compositions are quite effective cleaners, the flash point of which can be
readily increased, by the addition of a dibasic ester. Terpene compounds
useful for this purpose are described in U.S. Pat. No. 4,640,719, the
teachings of which are incorporated herein by reference. As used in the
present specification and in the claims, the term "terpene" is intended to
be understood as one terpene, or more than one terpene; when a specific
terpene compound is referred to, both the pure compound, as well as
mixtures in which the named compound predominates, are encompassed.
Preferred terpenes are limonene and dipentene. Enhancement of cleaning
effectiveness may also be achieved by the combination of esters and
terpenes.
The relative proportions of the combination will vary depending upon the
temperature at which the work is to be conducted. For example, at
temperatures of about 140.degree. F., a combination containing about 80%
dibasic ester, and up to about 20% terpene will provide both the
effectiveness and safety factor desired. Adjustment of the ratios in
accordance with the preferred temperature is a routine modification well
within the ability of one skilled in the art, as is the selection of an
appropriate emulsifying surfactant, should such be desired.
The utility of the compositions of the invention in 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 manual or automatic means.
Interspersed with the soldering operations are cleaning procedures and
inspections to ensure that contaminants such as solder flux residues that
could lead to premature circuit failure do not remain. Certain points in
the fabrication process to which the compositions of the invention apply
will be identified below.
In a second embodiment, the compositions are useful in cleaning various
surfaces which are utilized in electronic printing of circuit boards,
particularly screen process printing and photomechanical methods. The
screen or stencil process involves the application of a stencil, which
represents the non-printing areas, to a silk, nylon, stainless steel,
polyester, or metallized polyester fine mesh screen, to which ink is
applied and transferred to the surface to be printed. The stencils and
screens used in this process accumulate a substantial amount of ink which
must be periodically removed. The photomechanical processes rely on
exposure of photooptic images on light-sensitive coatings that, on
exposure to light, undergo changes in physical characteristics so that
they can be developed to produce images that serve either as printing
images or as resists for printing images. When the hardened coatings are
used as resists, for example, in deep etch or metal plates, they must
subsequently be removed after the images have been produced. There are
many compositions which are currently used for removal of such compounds,
such as xylene, toluene, acetone, methyl ethyl ketone, lacquer thinner,
N-methyl pyrrolidone (NMP), chlorinated solvents, or caustic solutions.
However, materials such as halogenated or caustic compounds present
substantial safety concerns. Some of these materials, such as NMP, are not
readily biodegradable, and also chelate metals, so that contaminating
metals are also washed into the waste water with which the solvents are
removed. Additionally, certain of these materials also attack the screens
or stencils they are used to clean, and are therefore only marginally
acceptable for cleaning purposes. The present method, utilizing dibasic
esters, avoids the environmental problems, while providing an equivalent
or better level of cleaning of these organic contaminants.
1. ROSIN FLUX CLEANING AND RESIDUE REMOVAL
For the removal of rosin soldering flux deposits 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. A dibasic ester-emulsifier composition may be
used full strength or, in certain applications, 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 the 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. Removal of the compositions 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 dibasic ester emulsifying surfactants, the boards may simply
be flushed with water for a period of up to about 2 minutes. Deionized
water is preferred. In the absence of an emulsifying surfactant in a
composition, a suitable solvent, such as an alcohol, could be used for
flushing. The optimal rinsing time varies according to the kinds of
surfactants and their concentrations in the ester 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.
Variations of the foregoing method for use with removal of flux from other
surfaces, such as pipe joints, will be readily apparent to one skilled in
the art.
5.2. SCREEN INK CLEANING AND RESIDUE REMOVAL
In removing ink or resists from screens, excess ink is usually scraped from
the screen by hand. The screen may be optionally washed in the DBE screen
wash product to loosen any partially cured material. The screen wash
product is then high pressure-sprayed (e.g., 50-80 psi) onto the entire
screen, and allowed to stand for 30 to 60 seconds. The screen is then
reversed, and high pressure sprayed again over its entire area, and let
stand for 30 to 60 seconds. The screen is then rinsed in a high pressure
water rinse for a short time (e.g., 60-120 seconds). If the screens have
not been presoaked, and partially cured screen inks remain, this step may
be repeated. The stencil or image is then ready for removal; the stencil
is removed chemically, usually with sodium periodate, which is
hand-applied to the screen. Finally, a high pressure spray water rinse is
applied and the screens air dried or air-blow dry.
6. EXAMPLES
6.1. ROSIN FLUX CLEANING
To test the cleaning ability of the present dibasic ester containing
compositions, a dibasic ester composition was prepared as follows:
______________________________________
DuPont DBE .TM. solvent
90.0%
Shell Neodol .TM. 91-6
5.0%
Shell Neodol .TM. 25-9
5.0%
______________________________________
A test coupon was cut out of the center of a large, unpopulated circuit
board. The board substrate chosen was ordinary laminate and already had
its circuit tracings and through-holes tin-plated. The coupon was
rectangular and about 3.5.times.6.5 inches. The board was coated with
Alpha 711 flux (Alpha Metals, Inc., Jersey City, N.J.), an RA type flux
with about 57% solids. The board was then allowed to air dry for about 15
minutes.
In an attempt to mimic the "burning" effect which occurs to the flux during
the soldering operation, the board was baked in an oven for 2 hours at
155.degree. C. The boards were then allowed to cool, and then weighed on a
3-place toploading balance.
About 600 ml of the cleaning formulation was placed in the bottom of
9.times.13 inch stainless steel baking pans. The formulation was heated to
110.degree. F. on a hotplate. The board to be cleaned was laid flat in the
bottom of the pan, which was then agitated gently by rocking it slowly
back and forth. After one minute of rocking, the board was turned over to
permit the other side to have equal contact with the cleaner. The cleaning
process proceeded for a total of about 2 minutes.
The board was removed from the tray and immediately rinsed under cold
flowing tap water. The board was rinsed briefly with deionized water, and
oven dried at 60.degree. C. for 15 minutes. The board was then weighed
again to determine the amount of flux removed by the cleaning process.
The board was then cleaned in an ultrasonic bath with methylene chloride to
remove all traces of residual flux. The board was rinsed with deionized
water, oven dried, and reweighed to determine the amount of flux residue
left on the board. The results of this test are presented in Table I.
TABLE I
______________________________________
(DBE)
______________________________________
Tare (g) 52.386
Flux coated (g) 53.418
Cleaned (g) 52.576
MeCl.sub.2 extracted (g)
52.383
Flux wt. after baking
1.032
Wt. of flux removed
0.842
by cleaning
Flux remaining 0.190
after cleaning
% flux removed 81.6%
% flux remaining 18.4%
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(b) A similar test was undertaken with a small, 31/4" square, populated
board to determine if the dibasic ester composition could effectively
clean under small spaces. THe procedure for the test is identical to the
one described in (a), with the exception that the board was baked for 17
hours at 120.degree. C. to harden the flux. The results of this test are
shown in Table II.
TABLE II
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Populated Board Cleaned with DBE
______________________________________
Flux coated (g) 25.868
Cleaned (g) 25.213
MeCl.sub.2 extracted (g)
25.147
Flux wt. after baking
0.721
Wt. of flux removed
0.655
by cleaning
Flux remaining after
0.066
cleaning
% flux removed 90.8%
% flux remaining 9.2%
______________________________________
The foregoing results plainly demonstrate that the cleaning compositions of
the present invention are extremely effective in removing flux and
cleaning printed circuit boards.
6.2. SCREEN INK CLEANING
Two screens used to apply Hysol SR 1000 (Hysol Electronic Chemicals
Division, Industry, Calif.) ink were cleaned, one with a commercial screen
cleaner product, comprising a mixture of NMP and surfactants (85% NMP, 15%
surfactants) and the other with a dibasic ester-surfactant mixture,
Bioact.RTM.EC-8, comprising 90% DBE (DuPont) and 10% Tergitol NP-10 (Union
Carbide). The respective cleaners were left on the screen for about 5
minutes, then both were water rinsed. Both products removed ink with
approximately equal efficiency; however, a visual and physical examination
showed that the DBE composition did not attack the screen material,
whereas NMP did attack the screen.
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 to the terms of
the appended claims.
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