NON-TOXIC, NON-FLAMMABLE CLEANER FOR PRINTED BOARD CLEANING

Abstract: Cleaning solutions for printed circuit boards are disclosed, comprising dialkyl esters of C4 to C6 acids and from 0 to 40 percent of a non-ionic surfactant.

Description: -i- N0N-T0XIC, NON-FLAMMABLE CLEANER FOR PRINTED BOARD CLEANING 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. Organic esters are utilized with a non-ionic surfactant to achieve a variety of objectives, among which are the removal of solder flux, oils, waxes, greasy substances, and adhesive tape residues. 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 are not carefully removed. 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, ethylene chloride, trichlorotrifluoroethane, or mixtures or azeotropes of these solvents. One typical, disclosure of such azeotropic mixtures is U. S. Patent 3,733,218, of Begun. Such 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 methods and 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, such as the al anolamines, 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, l ke 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. In another method utilized for cleansing of printed wiring boards, Hayes et al (U.S. Patent 4,640,719), teach the use of terpene compounds, such as limonene or dipentene, in combination with terpene emulsifying surfactants to facilitate removal by rinse water. A major problem with such compositions, however, is their relatively low flash point (120°F), presenting flammability and safety hazards. 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. 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 the most stringent specifications. It is a further object of this invention to provide safe and effective compositions for the removal of oils, waxes, greasy substances, fingerprints, etc., from circuit boards and printed circuits. It is a still further object of the present invention to provide room temperature cleaning solutions for circuit boards and printed circuits which are effective on baked-on rosin, and safe for soldermasks and epoxy marking inks. The compositions of the present invention are characterized by low toxicity, biodegradability, non-corrosiveness, high flash point and non-flammability, and long shelf life, 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, safety, and handling equipment is greatly reduced. More particularly, the present invention provides printed circuit/wiring board cleaning compositions comprising high boiling esters. These esters, which have the ability to dissolve or complex with and remove rosin solder fluxes, oils, waxes, and greasy substances or adhesive tape residues, may be used by themselves and removed, e.g., with absorbent materials. Preferably, however, the esters of this invention are combined with non-ionic surfactants to facilitate removal by water. Detailed Description of the Invention The compositions of the present invention comprise solutions of esters having the capability to dissolve rosin soldering flux and adhesive tape residues that are commonly encountered in the fabrication of printed circuits, surface mounted chips, or wiring boards. Esters which are suitable for this purpose include, but are not limited to, the dialkyl esters of C 4 to Cg acids, with dimethyl adipate being preferred. Included among such esters are dimethyl, diethyl, and dipropyl succinates, glutarates, and adipates. Mixtures of two or more of such esters are to be considered within the scope of this invention, but have not been found to be more effective. The esters of the present cleaning compositions have relatively high flash points, e.g., 225-230T, thus providing a safety feature with respect to terpenes having a 120-140'F flash point. Such esters present no fire hazard and are thus non-flammable even at temperatures of about 140"F, a temperature at which such esters are particularly effective in the removal of baked-on rosin fluxes, which are extremely difficult to remove. Room temperature use is preferred, however, for simplicity, ease, and cost. These esters could be used alone and removed, after dissolving the rosin flux or adhesive tape residues, by flushing with excess esters or by absorption with paper or cloth. Because they are poorly soluble in water, however, the esters may not be flushed away well with water. Accordingly, the preferred embodiment of the present invention encompasses forming a solution of such esters with a non-ionic surfactant. The addition of such surfactants facilitates removal of the esters from the printed circuit boards or such by rinsing with water. Numerous surfactants may be used, including but not limited to the linear alky! benzene sulfonates, linear or branched chain alcoholic ethoxylates and ethoxysulfates, polysorbate esters, ethoxylated alkylphenols and alky! and dialkyl succinate compounds. An example of the latter class of compounds is sodium dioctyl sulfosuccinate. The ethoxylated alkylphenols contain various al yl side chains and various numbers of linked ethylene oxide units. The preferred surfactants comprise polyoxyethylated nonylphenols. The quantity of non-ionic surfactant in the compositions of the present invention may range from 0 to about 40 percent by weight, with the ester comprising the balance. A preferred ratio of ester to surfactant is from 99/1 to 70/30, with a more preferred range of from 90/10 to 75/25, and a most preferred ratio of 80/20. Combinations of esters and surfactants as described are true solutions, resulting in extended shelf life for the cleaning compositions of this invention. While such solutions, particularly those having relatively high surfactant concentration, may be diluted with water, dilution is not preferred, in that it reduces the effective strength of the ester constituent. The compositions of the present invention are characterized by non-toxicity, non-flammability, favorable environmental degradability, strong activity at temperatures ranging from about 60 β F to about 150"F, non-conductivity, relatively favorable cost considerations, capability of safe use on soldermasks and epoxy marking inks, and effectiveness against rosin flux, mildly activated rosin, activated rosin, organic fluxes, oils, waxes, greasy substances, fingerprints, and adhesive. For cleaning circuit boards or printed circuits and the like, application of the compositions of this invention may be by immersion in dip tanks, or by hand or mechanical brushing. Ultrasonic agitation during immersion is not recommended. Also, vapor (degreasing) techniques are inappropriate. Alternatively, they may be applied by any of the commercially available printed wiring board cleaning equipment. Such washers may apply the cleaning solutions by spraying or by rolling contact with wetted roller surfaces. Spray application methods are preferred. The compositions should be in contact with the boards for from about 1 to about 5 minutes. It is permissible to leave the cleaning solution in contact with the boards for longer periods, however, since tests have shown that no etching of the board occurs during contact for up to one week. Once the solution has had sufficient contact time to loosen or solubilize the flux or other substance to be cleaned, the cleaning compositions are removed. Where the esters are employed without a surfactant, their removal may be accomplished by flushing with a non-toxic, miscible solvent or by absorption into an appropriate material. In the preferred embodiment of the invention, wherein surfactants are present, the boards may be flushed with water, with or without a compatible surfactant, for a short period of time, such as up to about 2 minutes. Optimal rinse periods may be determined by routine experimentation. It is noted that there is no negative effect in the event that rinsing is incomplete and a dried residue of cleaning solution is left on the board, since the cleaning composition of this invention is non-conductive and non-ionic. The cleaned boards are then dried, preferably with forced air. Drying may be expedited by heating the drying air to above about 100°F. The following illustrative examples are not intended to limit the scope of this invention, but to illustrate its application and use. EXAMPLES To illustrate the cleaning ability of the compositions of the present invention, printed wiring boards having uniform amounts of solder rosin flux residue were prepared and baked at llO'C for 1/2 hour, then cooled to room temperature. Samples of the "standard" printed board were then immersed in various cleaning solutions, until clean, with occasional stirring of the solution. Various dialkyl esters of succinic, glutaric, and adipic acid were tested, in combination with a non-ionic surfactant in various concentrations It was found that the diethyl esters of adipic acid were preferred, with glutaric acid and succinic acid esters decreasingly effective. To determine the optimum ratio of ester to surfactant, the following solutions were tested, with the corresponding times required for cleaning. The surfactant utilized in each instance was IGEPAL C0-630, available from GAF Corporation, a nonionic surfactant comprising a nonylphenoxypoly(ethyleneoxy)ethanol, illustrated by the formula wherein n is 8. The ester utilized for this test was diethyladipate. Cleanin Sol t o Time A significant drop-off in cleaning performance, as reflected by time, is seen at 70 percent ester/30 percent surfactant. It is concluded that a ratio of 80/20 is the best for cleaning efficiency and rinsing ability. Example 1: Fifteen drops of Kester #185 RMA (rosin, mildly activated) flux were applied with a dropper to a standard 2" x 5" unetched Pb-Sn epoxy board. The board was then placed in an oven at 110°C for a given time interval and baked. After the time interval, the board was removed from the oven and let cool to room temperature. Various boards were cleaned with the commercial product Bioact EC-7 terpene based cleaner and a cleaner of the present invention (80% diethyl adipate/20% Igepal CO-630) by immersing in one liter of cleaning solution and swirling the board until clean. The results are as follows: Cleaning Time Baking Time EC-7 Invention 0 min. 40 sec. 60 sec. 30 min. 70 sec. 120 sec. 60 min. 120 sec. 150 sec. Example 2: Fifteen drops of Kester #197 RMA flux were applied with a dropper to a standard 2" x 5" unetched Pb-Sn epoxy board. The board was then placed in an oven at 100'C for 30 minutes and baked. After 30 minutes, the board was removed from the oven and let cool to room temperature. One board was cleaned with EC-7, another with the cleaner of Example 1, by immersion in 1 liter of solution and swirling the board until clean. The results are as follows: Cleaner Cleaning Time Invention 195 sec. EC-7 135 sec. While EC-7 was able to clean the RMA fluxes (#185, 197) more quickly than the invention, the present cleaner was able to clean other types of fluxes [RA (rosin activated) and halide activated] while EC-7 was unable to clean (dissolve) the flux from the boards in a reasonable amount of time, if at all. Example 3: Fifteen drops of Kester #RA 1587 mil. flux were applied with a dropper to a standard 2" x 5" unetched Pb-Sn epoxy board. The board was then placed in an oven at HO'C for 30 minutes and baked. After 30 minutes, the board was removed from the oven and allowed to dry to room temperature. Boards were cleaned with either EC-7 or the cleaner of Example 1, by immersing in one liter of cleaning solution and swirling the board until clean. The results are as fol1ows: Cleaner Cleaning Time Invention 210 sec. EC-7 *not clean after 300 sec. *The RA flux was removed from EC-7 cleaned board in "pieces and flakes" and not "dissolved," unlike the invention cleaner. After 300 seconds approximately 50% of the surface was still covered by flux. The cleanliness of each board was confirmed by analysis of the ionic contaminants as determinedf by the conductivity of alcohol/water rinsing mixtures (measured by the Omega Meter) and by analysis of the organic residue, determined spectrophotometrically by measuring the UV absorption at 242 nm. of an isopropanol rinse. The Omega Meter result for the cleaner of this invention was 1 jugm NaCl equivalent/in 2 , indicating a high degree of cleanliness. The spectrophotyometric analysis yielded a residue of 9-_ugm/in 2 , again indicating a high degree of cleanliness. The corresponding numbers for cleaning with Bioact EC-7 were 2 ιgm NaCl equivalent/in 2 for the Omega test and 20/ιgm/in 2 for the spectrophotometric test. Example 4: Fifteen drops of Kester #1588 halide activated flux were applied with a dropper to a standard 2" x 5" unetched Pb-Sn epoxy board. The board was then placed in an oven at 100°C for 30 minutes and baked. After 30 minutes, the board was removed from the oven and let cool to room temperature. Boards were cleaned with either EC-7 or the cleaner of Example 1 by immersing in one liter of cleaning solution and swirling the board until clean. The results are as follows: Cleaner Cleaning Time Invention 210 sec. EC-7 *not clean after 300 sec. *The halide activated flux was removed from the EC-7 cleaned board in "flakes and pieces" and not "dissolved," unlike the inventive cleaner. After 300 seconds, approximately 20% of the surface of the board remained uncleaned by the EC-7 solution. It is understood that the above description of the present invention is susceptible to various modifications, changes and adaptations by those skilled in the art, and that the same are to be considered within the scope of this invention, which is set forth by the claims which follow.

Claims: CLAIMS 1. A cleaning solution consisting of a dialkyl ester of an alkyl acid having from 4 to 6 carbons and from 0 to about 40 weigh percent non-ionic surfactant. 2. The cleaning solution of Claim 1 wherein said ester is selected from the group consisting of dimethyl glutarate, dimethyl succinate, dimethyl adipate, diethyl glutarate, diethyl succinate, diethyl adipate, and mixtures thereof. 3. The cleaning solution of Claim 2 wherein said ester comprises from about 70 to about 99 percent by weight. 4. The cleaning solution of Claim 2 wherein said ester comprises from about 75 to 90 percent by weight. 5. The cleaning solution of Claim 2 wherein said ester comprises about 80 percent by weight. 6. The cleaning solution of Claim 5 wherein said ester is dimethyl adipate. 7. The cleaning solution of Claim 2, wherein said surfactant selected from the group consisting of linear alkyl benzene sulfonates, linear or branched chain alcoholic ethoxylates and ethoxysulfates, polysorbate esters, alkyl and dialkyl succinate compounds, and ethoxylated alkylphenols. 8. The cleaning solution of Claim 7 wherein said ester is dimethyl adipate. 9. The cleaning solution of Claim 8 wherein the ratio of este to surfactant is from about 99/1 to about 70/30. 10. The cleaning solution of Claim 8 wherein the ratio of est to surfactant is from about 90/10 to about 75/25. 11. The cleaning solution of Claim 8 wherein the ratio of est to surfactant is from about 80/20. 5 12. The cleaning solution of Claim 11, wherein said surfactan is nonylphenoxypol (ethyleneoxy)ethanol . 13. A method for removing rosin soldering flux or adhesive ty residue from a printed wiring board, said method comprising: a. contacting said board with a solution comprising a " 10 dialkyl ester of an alkyl acid having from 4 to 6 carbons and from to about 40 percent by weight non-ionic surfactant; b. allowing the contact to continue for sufficient time solubilize the soldering flux or tape residue; and c. removing the solution and solubilized soldering flux 15 tape residue from the board. 14. The method of Claim 13 wherein the ester is selected from the group consisting of dimethyl glutarate, dimethyl succinate, dimethyl adipate, diethyl glutarate, diethylsuccinate, diethyl adipate, and mixtures thereof. 20 ' 15. The method of Claim 14 wherein the contact is carried out a temperature of from about room temperature to about 150'F. 16. The method of Claim 13 wherein the surfactant is selected from the group consisting of linear alkyl benzene sulfanates, line or branched chain alcoholic ethoxylates and ethoxysulfates, 25 polysorbate esters, alkyl and dialkyl succinate compounds, and ethoxylated alkylphenols. 17. The method of Claim 16 wherein said ester is dimethyl adipate. 18. The method of Claim 17 wherein the ratio of ester to surfactant is from about 99/1 to about 70/30. 19. The method of Claim 17 wherein the ratio of ester to surfactant is from about 90/10 to about 75/25. 20. The method of Claim 17 wherein the ratio of ester to surfactant is about 80/20. 21. The method of Claim 17 wherein the surfactant is a nonylphenoxypol (ethyleneoxy)ethanol . 22. The method of Claim 13 wherein the composition and solubilized soldering flux or tape residue are removed by rinsing with water.

Inventors: MATZDORF CRAIG ALLEN, SHAH TUSHAR KANAIYALAL, TADROS MAHER EBEID, MATZDORF, CRAIG, ALLEN, SHAH, TUSHAR, KANAIYALAL, TADROS, MAHER, EBEID
Applicants: MARTIN MARIETTA CORP, MARTIN MARIETTA CORPORATION
Application year: 1990
Main classification: C23G5
IPC codes: C11D3, C11D7, C23G5, H05K3