SOLDER MASK COMPOSITIONS

Abstract: A process for producing a solder mask comprises: 1) coating a printed wiring board with a solder mask composition comprising an aqueous solution of (1) an aryl cyclic sulfonium zwitterion monomer or (2) an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups; 2) exposing the coating to UV light in selected areas to polymerize the zwitterion monomer on the exposed areas; 3) developing the exposed coating with water or alkaline aqueous solution, and 4) curing the developed coating.

Description: SOLDER MASK COMPOSITIONS This invention relates to solder masks, more particularly, to liquid- photoimageable solder masks, and printed wiring boards containing such solder masks A feature of a preferred photoimageable solder mask coating is an ability to be developed with an aqueous, slightly alkaline solution, thereby avoiding the use of organic solvents in the developing step. The desired solder mask coatings should also exhibit flexibility, heat and chemical resistance, surface hardness, abrasion resistance and a rapid curing rate These stringent requirements imply that the UV curable compound and compositions must be carefully formulated in order that the solder mask can achieve commercial acceptability A solder is typically an alloy of about 60 percent tin and 40 percent lead and used to attach electrical components to a printed wiring board (PWB). A solder mask is a coating which covers or masks off selected areas of a PWB and prevents those areas from accepting any solder during vapor phase or wave solder processing In wave solder processing, the PWB is passed over a standing wave of flux and, after preheating, is passed over a standing wave of molten solder The term "wave" relates to a standing wave which may be either a non-aerated liquid or may be an aerated or foaming liquid which is formed by pumping up the liquid from beneath its surface The prime function of a solder mask is to restrict the molten solder pick-up to those areas of the PWB (holes, pads, and conductor lines) that are not covered by the solder mask Other functions of a solder mask are to reduce solder bridging and eliminate electrical shorts, reduce the volume of solder pick-up to obtain cost and weight savings, reduce solder pot contamination (copper and gold), protect PWB circuitry from handling damage, provide an environmental barrier, fill space between conductor lines and pads with material of known dielectric characteristics and provide an insulation or dielectric barrier between electrical components and conductor lines or via interconnections when components are mounted directly on the conductor lines It is welt known that any surface, no matter how chemically and physically clean it is initially, will receive at least a slight oxide layer or film thereon when exposed to atmospheric conditions This is particularly true of such surfaces as, for example, copper, which will oxidize instantly when exposed to air Accordingly, when it is desired to apply solder to a metal surface or component lead, it is necessary, in order to obtain a good bond between the solder and the surface to be joined, to thoroughly clean the surface of the material to be soldered before applying the solder thereto For this purpose, fluxes, which are acidic compounds, are used to clean the metal surfaces in advance of the application of solder thereto In the soldering of PWB, the solder mask must be able to withstand the chemical attack of the flux It must also be able to withstand the high temperature of the solder bath Usually, solder baths are maintained at the temperature of 400°F to 600°F, which is too high a temperature for the employment of conventional photoresists The solder mask must be able to maintain good adhesion to the PWB in spite of the chemicals in the fluxes and the high temperature of the solder bath Liquid-photoimageable solder masks are known, and are described, for example, in U.S. Patents 3,660,088, 3,753,720, 4,199,163, 4,361 ,640, 4,436,806, 4,508,916 and 5,006,436 However, all of the solder masks described in these patents are developed with organic solvents Although there are several commercial solder mask materials that are aqueous developable, such as Ciba-Geigy's Probimer® 65M, Probimer® 52M, and Probimer® 71 H, W R Grace's Accumask® CM 2001 , and LeaRonal's Roπascreen® OPSR 5600, all of them contain organic solvents in their formulations. It would be desirable to provide a liquid-photoimageable solder mask composition which is aqueous-developable, does not contain an organic solvent, can withstand the chemical attack of fluxes and the high temperature of solder baths and maintain good adhesion to the circuit board in spite of the chemicals in the fluxes and the high temperature of solder baths. In a first aspect, this invention is a solder mask composition comprising an aqueous solution of (1 ) an aryl cyclic sulfonium zwitterion or (2) an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups In a second aspect, this invention is a printed wiring board containing a solder mask comprising a coating of a cured polymer prepared from (1 ) an aryl cyclic sulfonium zwitterion or (2) a mixture of an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups In a third aspect, this invention is a process for producing a solder mask which comprises: (1) coating a printed wiring board with an aqueous solution of (1 ) an aryl cyclic sulfonium zwitterion or (2) an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups; (2) exposing the coating to UV light in selected areas to polymerize the zwitterion monomer on the exposed areas; (3) developing the exposed coating with water or alkaline aqueous solution; and (4) curing the developed coating. Generally, the sulfonium zwitteπons which can be employed in the present invention are polyfunctional monomers represented by the general formula: wherein Z represents the bridging groups -O-, -S-, (CH_) c and -CR 2 ", where R" is an alkyl having from 1 to about 4 carbon atoms, and c is an integer from 1 to 10; each sulfonium group is meta or preferably ortho or para to the phenoxide group; R is hydroxyl, chlorine, bromine, an alkyl having from 1 to 16 carbon atoms (preferably from 1 to 4), or an alkoxy having from 1 to 12 carbon atoms (preferably from 1 to 4), said alkyl and alkoxy being optionally substituted with a hydroxyl group; n is an integer from 0 to 2; A' and B' independently are -CH,- or -CH(R" , wherein R'" is hydroxyl or an alkyl or hydroxyalkyl having from 1 to 8 carbon atoms; m is 0, 1 , 2 or 3; and k is 1 or 2. R" can also be CF 3 O- (CF 2 ) a CF 3 , or (CF 2 ) a CF 3 ; where a is an integer from 1 to 8 and Z can be Representative polyfunctional monomers within the scope of the above general formula include 1 ,1 '-((1 -methylethyiιdene)-bιs(6-hydroxy-3,1 -phenylene)) bιs(tetrahydrothιophenιum hydroxιde)bιs(ιnner salt), which is also referred to herein as p-bisphenol A zwitterion, 1 ,1 '-(dιmethylene-bιs(oxy-4-hydroxy-2,1 - phenylene))bιs(tetrahydrothιophenιum hydroxιde)bιs(ιnner salt); 1 ,1 '-(methylene- bιs(4-hydroxy-3,1 -phenylene))bιs(tetrahydrothιophenιum hydroxιde)bιs(ιnner salt), 1 ,1 '-((1 -methylethylιdene)-bιs(6-hydroxy-3,1-phenyfene))bιs(3- hydroxytetrahydrothiophenium hydroxιde)bιs(ιnner salt); and 1 ,1 '-((2,3,4,5- tetrahydroxyhexamethyiene)bιs(oxy-4-hydroxy-2, 1 - phenylene))bιs(tetrahydrothιophenιumhydroxιde)bιs(ιnner salt), which is also referred to herein as mannitol diphenolic zwitterion The preparation of polyfunctional zwitterion monomers corresponding to the above general formula wherein the sulfonium group is ortho or para to the phenoxide group is described in U.S Patents 3,636,052, 3,723,386, 4,089,877, and Journal Paint Technology, Vol 46, No. 588, January 1974, p. 41 Preferred zwitteπons which can be employed in the practice of the present invention for preparing a solder mask composition are those represented by the formulas- bisphenol A zwitterion 6- fluoro bisphenol A Twitteπon bisphenol zwitterion fluorene bisphenol zwitteπon The more preferred zwitterions which can be employed in the practice of the present invention for preparing a solder mask composition are bisphenol A zwitterion and 6-fluoro bisphenol A zwitterion, with the most preferred being bisphenol A zwitterion. In general, the solder mask composition of the present invention can be prepared by dissolving the zwitterion in water. The amount of zwitterion employed depends on a variety of factors including its solubility in water, the desired viscosity of the composition, and its desired end-use, but, in general, the zwitterion can be present in an amount of from 1 to 70 weight percent, preferably from 20 to 65 weight percent, more preferably from 30 to 60 weight percent and most preferably from 40 to 55 weight percent, based on the weight of the composition. The solder mask composition can include a polymer containing one or more carboxyl groups for increasing the viscosity of the composition. Advantageously, the polymer containing carboxyl groups is neutralized before adding it to the sulfonium zwitterion solution to avoid gelation under acidic conditions. While any neutralizing agent is suitable for this neutralization, ammonium hydroxide is the most preferred because it regenerates the acid during the drying of the coating prior to the curing reaction. The polymers containing carboxyl groups which can be employed in the practice of the present invention for preparing the solder mask composition include the polymers and copolymers of acrylic acid and methacrylic acid. Examples of these polymers are the hydrophilic. water-swellable, cross-linked acrylic acid polymers sold as Carbopol™ resins by B. F. Goodrich Company of Cleveland, Ohio. While the amount of the polymer containing carboxyl groups most advantageously employed depends on a variety of factors, such as the specific polymer, in general, the polymer containing carboxyl groups is used in an amount of from 0.05 to 10 weight percent, preferably from 0.1 to 5 weight percent, and most preferably from about 0.5 to 1.0 weight percent, based on the weight of the composition. in general, solder masks of the present invention are prepared by using as the starting substrate a partially completed printed circuit board. In the first step, the printed circuit board is coated with an aqueous solution of an aryl cyclic sulfonium zwitterion and the coated board is exposed to a UV light source in selected areas through a photomask to polymerize the zwitterion monomer on the exposed areas. The coating is developed with water or alkaline aqueous solution and the unexposed, and hence unpolymerized, sulfonium zwitterion is removed, exposing the underlying metal. The exposed metal areas are contacted with solder which remains on the exposed metal. Wave soldering methods can be conveniently used. The solder in these selected areas is then used to connect various electronic components and wires. The following working examples and comparative examples are given to illustrate the invention and should not be construed to limit its scope. Unless otherwise indicated, all parts and percentages are by weight. Example 1 A 20 percent aqueous solution of bisphenol A zwitterion was poured on to the center of a 4" wafer coated with copper. Some water was allowed to evaporate. The material on the wafer was spread at 500 rpm for 10 seconds and then at 2000 rpm for 30 seconds. A non-sticky film was formed. The film was exposed to UV light through a mask for 100 seconds. After exposure, the wafer is developed in a water bath. Unexposed areas of the film are removed. The wafer is placed in an oven at 100°C for 20 minutes. A hard coating which resisted the scratch of a 6H pencil is obtained. The coating sustained solder material at 260°C. Thickness of the coating is 15 μm. Example 2 To a 2 by 2.5 square inch FR4 (fiber-reinforced epoxy resin) panel with printed copper circuit is sprayed a 20 weight percent aqueous solution of bisphenol A zwitterion Water was allowed to evaporate to form a non-sticky coating Part of the printed copper circuit was covered with 0.125 inch OD by 0.31 inch ID and 0.02 inch wide printed circuit drafting aids (masks which contact or touch the film, also known as "contact masks"). The panel was exposed to UV light in an American UV model PC1421 equipped with a 5" arc at 200 watts per inch medium pressure mercury lamp for 100 seconds. Exposure dose was 0 8 J/cm 2 calibrated with a Uvicure Model 254 sensor. After exposure, the printed circuit drafting aids were removed The panel was placed in water and then in an oven at 90°C for 15 minutes A hard surface which sustained the scratch of a 6H pencil was obtained on areas not covered with the drafting aids For areas protected with the drafting aids, copper surface was visible. Solder was placed on copper which was not coated with the polymer. The polymer coating was not damaged by the temperature of the solder at 370°C. Examples 4 and 5 The procedure described in Example 3 was repeated, except that a 54 percent and a 40.3 percent aqueous solution of bisphenol A zwitterion are used. As in Example 3, a hard surface which sustained the scratch of a 6H pencil was obtained on areas not covered with the drafting aids In areas protected with printed circuit drafting aids, copper surface was visible in both examples Solder was placed on copper which was not coated with the polymer. In both examples, the polymer coating was not damaged by the temperature of the solder at 370°C. Example 6 To a jar is added 91.32 g of water. A Cowles-type mixing blade-rod was attached to an air stirrer and mixing of the water was started at about 2000 rpm To the water was slowly added 0.50 g of Carbopol™ 674 (a cross-linked acrylic acid polymer from B F Goodrich) The solution was left to stir for about a half hour Two drops of 28 weight percent ammonium hydroxide solution was added to the stirring solution. The solution became more viscous and the pH of the solution was 7.8 Next, 7 32 g of a 19 weight percent aqueous solution of bisphenol A zwitterion was added slowly to the Carbopol solution while it was stirring After all bisphenol A zwitterion is added, stirring was continued for an additional 15 minutes The resulting solution was clear and viscous. A 10 mil draw-down bar was used to cast this solution onto a glass plate. After 3 hours of drying the film under ambient conditions, the films were ready for UV curing. Half of the cast film was covered with aluminum foil to prevent exposure to UV radiation. The glass plate was placed under the UV lamp. The lamp used for the curing of the coatings was made by the American Ultraviolet Company. It was comprised of a Model PC10-1 -H2 200 WPI Lamp Intensity power supply and a lamp model IR 1000F. The samples were exposed to the lamp for 3 or 5 minutes A flux of 0 85 J/cm 2 was measured by a UVICURE UV Curing Radiometer (Electronics Instrument & Technology, Sterling, VA) for 3 minute exposure at the position of the coating to be cured. The term "position of the coating" refers to the distance of the coating from the lamp. This distance was adjusted until a flux of 0.85 J / cm 2 was registered by the UVICURE UV Curing Radiometer After 3 and 5 minute exposure times to the UV lamp, patterning was seen after rinsing with a 5 percent ammonium hydroxide solution with a pH of 12 The exposed area stayed on the glass plate and the unexposed area (that covered with the aluminum foil) rinsed off. Both the 3 and 5 minute glass plates were put in an oven at 120°C for 1 hour. After cooling, a solution of 0 1 /V NaOH is placed on the surface of the coating This was to simulate the exposure the coating must withstand after the patterning for a printed wire circuit board No change in the coating could be seen after 3 minutes of exposure to the 0 1 Λ/ NaOH solution.

Claims: CLAIMS. 1 A solder mask composition comprising an aqueous solution of (1 ) an aryl cyclic sulfonium zwitterion or (2) an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups 2 The composition of Claim 1 , wherein the sulfonium zwitterion is a polyfunctional monomer represented by the general formula wherein Z is a bridging group which is -0-, -S-, -CH 2 -, or -CR 2 ", where R" is an alkyl having from 1 to 4 carbon atoms, or -OfC a H 2a b (OH) b )0-, where "a" is an integer from 1 to 6 and "b" is an integer from 0 to 4; each sulfonium group is meta, ortho or para to the phenoxide group; R is hydroxyl, chlorine, bromine, an alkyl having from 1 to 16 carbon atoms, or an alkoxy having from 1 to 12 carbon atoms, said alkyl and alkoxy being optionally substituted with a hydroxyl group, n is an integer from 0 to 2, A' and B' independently are -CH 2 - or -CH(R'")-, wherein R'" is hydroxyl or an alkyl or hydroxyalkyl having from 1 e to θcarbon atoms, m is 0, 1 , 2 or 3, and k is 1 or 2. 3. The composition of Claim 2, wherein the polyfunctional monomer is 1 ,1 '-((1 -methylethylιdene)-bιs(6-hydroxy-3,1-phenylene))bιs(tetrahydrothιophenιum hydroxιde)bιs (inner salt), 1 ,1 '-(dιmethylene-bιs(oxy-4-hydroxy-2,1- phenylene)bιs(tetrahydrothιophenιum hydroxιde)bιs (inner salt), 1 ,1 '-(methylene- bιs(4-hydroxy-3,1-phenylene)bιs(tetrahydrothιophenιum hydroxιde)bιs (inner salt), 1 ,1 '-((1 -methylethylιdene)-bιs(6-hydroxy-3,1 -phenylene))bιs(3- hydroxytetrahydrothiophenium hydroxιde)bιs (inner salt), or 1 , 1 '-({2,3,4, 5- tetrahydroxyhexamethylene)bιs(oxy-4-hydroxy-2,1 - phenyiene))bιs(tetrahydrothιophenιum hydroxιde)bιs (inner salt) 4. The composition of Claim 2, wherein the sulfonium zwitterion is bisphenol A zwitterion, biphenol zwitterion, 6-fluoro bisphenol A zwitterion or fluorene bisphenol zwitterion. 5. The composition of Claim 4, wherein the sulfonium zwitterion is bisphenol A zwitterion or 6-fluoro bisphenol A zwitterion. 6. The composition of Claim 1 , wherein the polymer containing two or more carboxyl groups is a polymer or copolymer of acrylic acid or methacrylic acid. 7. A process for producing a solder mask which comprises: (1) coating a printed wiring board with an aqueous solution of (1 ) an aryl cyciic sulfonium zwitterion or (2) an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups; (2) exposing the coating to UV light in selected areas to polymerize the zwitterion monomer on the exposed areas; (3) developing the exposed coating with water or alkaline aqueous solution; and (4) curing the developed coating. 8. The process of Claim 7, wherein the sulfonium zwitterion is bisphenol A zwitterion, biphenol zwitterion, 6-fluoro bisphenol A zwitterion or fluorene bisphenol zwitterion. 9. The process of Claim 8 wherein the sulfonium zwitterion is bisphenol A zwitterion or fluorene bisphenol zwitterion. 10. A printed wiring board containing a solder mask comprising a coating of a cured polymer prepared from (1 ) an aryl cyclic sulfonium zwitterion or (2) a mixture of an aryl cyclic sulfonium zwitterion and a polymer containing two or more carboxyl groups. 1 1. The printed wiring board of Claim 10, wherein the sulfonium zwitterion is bisphenol A zwitterion, biphenol zwitterion, 6-fluoro bisphenol A zwitterion or fluorene bisphenol zwitterion. 12. The printed wiring board of Claim 1 1 wherein the sulfonium zwitterion is bisphenol A zwitterion or fluorene bisphenol zwitterion. 13. The printed wiring board of Claim 12 wherein the sulfonium zwitterion is bisphenol A zwitterion.

Inventors: DARBY NICHOLAS, ROSE GENE D, SCHMIDT DONALD L, SO YING H, WARREN MALCOLM W, DARBY, NICHOLAS, ROSE, GENE, D., SCHMIDT, DONALD, L., SO, YING, H., WARREN, MALCOLM, W.
Applicants: DARBY NICHOLAS, DOW CHEMICAL CO, ROSE GENE D, SCHMIDT DONALD L, SO YING H, WARREN MALCOLM W, DARBY, NICHOLAS, ROSE, GENE, D., SCHMIDT, DONALD, L., SO, YING, H., THE DOW CHEMICAL COMPANY, WARREN, MALCOLM, W.
Application year: 1996
Main classification: G03F7
IPC codes: G03F7, H05K3