Coating compositions for printed circuit boards
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to coating compositions for use in the production of printed circuit boards by the additive process. 
     2. Prior Art 
     There have heretofore been proposed printed circuit boards formed by disposing conductive circuit patterns on insulating substrates or base plates by the additive process. These known circuit boards find wide application to parts for use with electrical facilities and equipment. In the manufacture of the printed circuit boards, coating compositions consisting essentially of acrylonitrile/butadiene rubbers, phenolic resins and epoxy resins are applied on to insulating base plates made such as of an epoxy or imide resin or a ceramic material. After being cured or semi-cured, the coated plates on their surfaces are made hydrophilic or coarse with a mixture of chromic acid and sulfuric acid, followed by catalytic action with palladium chloride or the like. The plates are protected at portions where no circuit patterns are required, by photographic masking with use of a photosensitive lacquer, i.e. by photoresist formation with use of a screen printing technique. The plates so masked are electroless-plated at portions where printed circuitry is desired and subsequently dried. 
     Such prior art compositions, however, are encountered with blistering on the plated surfaces and also with reduced heat resistance at soldering. To cope with these problems, the compositions have been combined with silica as disclosed for instance in Japanese Patent Publication No. 52-31539 or with a filler such as clay. Silica has been found to be totally unsatisfactory in respect of heat resistance at soldering. In the meantime, clay causes, depending upon the conditions of etching, reduced blister resistance and insufficient heat resistance particularly in the case of glass-epoxy substrates. This has created an urgent need for improved coating compositions which are capable of giving printed circuit boards with good physical properties. 
     SUMMARY OF THE INVENTION 
     It has now been found that coating compositions blessed with excellent quality characteristics can be obtained by the use of a selected type of calcium sulfate combined with composite rubber materials. 
     It is the primary object of the present invention to provide a novel coating composition which exhibits excellent blister resistance by plating, high heat resistance at soldering and sufficient peel strength. 
     This and other objects and advantages of the invention can be achieved by the provision of a coating composition for use in the manufacture of printed circuit boards, which comprises a composite rubber material comprising a rubber and a phenolic resin, and calcium sulfate in an amount of 3-50 parts by weight of 100 parts of that rubber material. 
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Coating compositions of the present invention comprise composite rubber materials and calcium sulfate. 
     Composite rubber materials eligible for the purpose of the invention are composed of rubbers and phenolic resins. The rubbers include for example natural rubbers (NR), styrene/butadiene copolymer rubbers (SBR), acrylonitrile/butadiene copolymer rubbers (NBR) and the like. Where the NBR rubbers are used, there is no restriction imposed upon the nitriles contents; that is, such contents may be as low as 25% or smaller or as high as 43% or greater. Alternatively, the NBR rubbers may be of a carboxylated type. Specific examples of the phenolic resins include not only novolak phenol resins and resol phenol resins but also modified phenol resins in the form of alkyl cresols, cashews and the like. 
     Epoxy resins may be incorporated, where desired, in the coating compositions. Eligible examples of the epoxy resins include bisphenol A, novolak epoxy resins and the like. 
     Importantly, a certain type of calcium sulfate is combined with the composite rubber materials. Eligible types of calcium sulfate may be selected from those commercially available. Alpha-type CaSO 4 .1/2H 2  O and II-type CaSO 4  anhydride are particularly desired for their higher crystalline density than other types of calcium sulfate and hence are highly resistant to attack by plating solutions. 
     The amount of calicum sulfate to be added should be in the range of 3-50 parts by weight of 100 parts of the composite rubber material. Smaller than 3 parts by weight would invite insufficient heat resistance at soldering and reduced blister resistance by plating. Greater amounts than 50 parts by weight would make the plated surface too coarse, failing to give precise circuit pattern. 
     As regards the composite rubber materials, the weight ratio of rubber to phenolic resin should suitably be 70-40 to 30-60. Where an epoxy resin is employed, the weight ratio of these three components is 60-40 to 30-50 to 10. 
     There may be used other additives such as cross-linking agents, curing agents, leveling agents, solvents and the like. 
     The following formulation is typical of and practical for the invention. 
     
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Material       Part (by weight)                                           
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rubber         70-40                                                      
phenolic resin 30-50                                                      
epoxy resin     0-10                                                      
calcium sulfate                                                           
                3-50                                                      
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     The following examples are given to further illustrate the present invention, but it is to be understood that the invention is not limited thereto. In these examples all parts are by weight unless specifically noted. 
     EXAMPLE 1 
     A coating composition according to the invention was formulated and prepared as follows: 
     
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Material                Part                                              
______________________________________                                    
NBR (Nipol 1072, carboxylated NBR,                                        
                        60                                                
Nippon Zeon Co.)                                                          
phenolic resin (PR-175, ammonia-resol                                     
                        35                                                
phenol resin, Sumitomo Durez Co.)                                         
epoxy resin (Epikote-1001, BPA epoxy                                      
                        5                                                 
resin, Yuka-Shell Epoxy Co.)                                              
calcium sulfate (CaSO.sub.4.1/2H.sub.2 O,                                 
                        15                                                
type: alpha, particle size: 100-mesh                                      
pass 99.9%, 350-mesh pass 68%)                                            
t-butylperoxybenzoate   6                                                 
leveling agent          0.5                                               
defoaming agent         0.1                                               
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     A solution of the above composition in 25% methyl ethyl ketone (MEK) was applied on to a paper-phenol substrate (XPC-FR) and a glass-epoxy substrate (FR-4), respectively, on a bar coater in a dry thickness of 40 micron. The resulting substrates were cured at 150° C. for one hour, followed by etching with a chromic acid mixture (CrO 3  : 100 g/l, concentrated H 2  SO 4  : 300 ml/g) at 50° C. for 10 minutes. After being catalyzed with Cataposit 44 and Cataprip 404 (Shipley Inc.), the etched substrates were plated at 70° C. for 13 hours with use of an electroless plating solution (pretreated with Accelerator 19, Hitachi Co., Japanese Patent Publication No. 56-27594) and subsequently dried at 120° C. for 2 hours thereby obtaining a test piece. 
     COMPARATIVE EXAMPLE 1 
     The procedure of Example 1 was followed except that silica (SiO 2 , type: amorphous, synthetic, particle size: 25 micron) was used instead of calcium sulfate. 
     COMPARATIVE EXAMPLE 2 
     The procedure of Example 1 was followed except that clay (type: acidic, hydrous, SiO 2  : 45%, Al 2  O 3  : 38%, water of crystallization: 14%, particle size: 0.8 micron) was used instead of calcium sulfate. 
     Each of the test pieces provided in Example 1 and Comparative Examples 1 and 2 was tested for blister resistance by plating, peel strength and heat resistance at soldering under the conditions given below and with the results shown in Table 1. 
     Blister Resistance 
     Surface roughness (Rmax) was measured in accordance with JIS B0651 and JIS B0601. Below 15 micron is positive and above 25 micron negative. 
     Peel Strength 
     JIS C6481 was followed. Above 2 kg/cm is desirable. 
     Heat Resistance 
     Measurement was made in accordance with JIS C6481. Desired are above 20 seconds at 260° C. in the case of paper-phenol substrates and above 180 seconds at 260° C. in the case of glass-epoxy substrates. 
     EXAMPLE 2 
     Another coating composition according to the invention was prepared using the same formulation as in Example 1 except that the particle size of alpha-type CaSO 4 .1/2H 2  O was changed to 5 micron in place of 100-mesh pass 99.9% and 350-mesh pass 68%. 
     EXAMPLE 3 
     The procedure of Example 2 was followed except that II-type CaSO 4  anhydride (particle size: 8 micron) was substituted for alpha-type CaSO 4 .1/2H 2  O (particle size: 5 micron). 
     EXAMPLE 4 
     The procedure of Example 3 was followed except that the amount of calcium sulfate was changed to 30 parts in place of 15 parts. 
     EXAMPLE 5 
     The procedure of Example 2 was followed except that alpha-type CaSO 4 .2H 2  O was substituted for alpha-type CaSO 4 .1/2H 2  O. 
     EXAMPLE 6 
     The procedure of Example 2 was followed except that beta-type CaSO 4 .1/2H 2  O (particle size: 8 micron) was substituted for alpha-type CaSO 4 .1/2H 2  O (particle size: 5 micron). 
     COMPARATIVE EXAMPLE 3 
     The procedure of Example 2 was followed except that silica (type: amorphous, synthetic, particle size: 2.5 micron) was used instead of calcium sulfate. 
     COMPARATIVE EXAMPLE 4 
     The procedure of Example 2 was followed except that clay (type: acidic, hydrous, SiO 4  : 45%, Al 2  O 3  : 38%, water of crystallization: 14%, particle size: 0.8 micron) was used instead of calcium sulfate. 
     The same performance tests as in Example 1 were made for all the test pieces obtained in Examples 2 to 6 and Comparative Examples 3 and 4 with the results shown in Table 2. 
     It is to be noted as appears clear from Tables 1 and 2 that the coating compositions embodying Examples 1 to 4 are highly satisfactory in respect of all the performance characteristics tested. The controls in Comparative Examples 1 and 2 revealed blistering in the case of the glass-epoxy substrates, failing to give acceptable heat resistance and sufficient peel strength in the case of the paper-phenol substrates. The controls of Comparative Examples 3 and 4 were comparable with the controls of Comparative Examples 1 and 2 in the case of the glass-epoxy substrates. As is apparent from Examples 5 and 6, alpha-type CaSO 4 .2H 2  O and beta-type CaSO 4 .1/2H 2  O have been found to be less effective in blister resistance in the case of the glass-epoxy substrates than alpha-type CaSO 4 .1/2H 2  O and II-type CaSO 4  anhydride. 
     
                       TABLE 1                                                     
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                   Comparative                                            
                              Comparative                                 
          Example 1                                                       
                   Example 1  Example 2                                   
performance GE.sup.(1)                                                    
                    PP.sup.(2)                                            
                           GE    PP   GE    PP                            
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blister resistance.sup.(3)                                                
            o       o      x     o    x     o                             
peel strength                                                             
            2.5     2.6    1.5   2.0  1.7   2.1                           
(kg/cm)                                                                   
heat resistance at                                                        
            &lt;180    &lt;70    18    29   20    70                            
soldering (sec.)                                                          
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 .sup.(1) glassepoxy substrate                                            
 .sup.(2) paperphenol substrate                                           
 .sup.(3) surface roughness (Rmax)                                        
 o: positive (less than 15 micron)                                        
 x: negative (more than 25 micron)                                        
 
    
     
                                           TABLE 2                                 
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        Example                        Comparative Example                
        2      3      4      5    6    3     4                            
performance                                                               
        GE.sup.(1)                                                        
            PP.sup.(2)                                                    
               GE  PP GE  PP GE PP                                        
                                  GE PP                                   
                                       GE PP GE PP                        
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bliste  o   o  o   o  o   o  Δ                                      
                                o Δ                                 
                                     o x  o  x  o                         
resistance.sup.(3)                                                        
peel strength                                                             
        2.5 2.6                                                           
               2.6 2.8                                                    
                      2.4 2.6                                             
                             2.6                                          
                                1.2                                       
                                  2.0                                     
                                     1.1                                  
                                       1.5                                
                                          2.0                             
                                             1.7                          
                                                2.1                       
(kg/cm)                                                                   
heat resistance                                                           
        &lt;180                                                              
            70 &lt;180                                                       
                   60 &lt;180                                                
                          70 60 80                                        
                                  70 60                                   
                                       18 27 20 70                        
at soldering                                                              
(sec.)                                                                    
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 .sup.(1) glassepoxy substrate                                            
 .sup.(2) paperphenol substrate                                           
 .sup.(3) surface roughness (Rmax)                                        
 o: positive (less than 15 micron)                                        
 Δ: moderate (15-25 micron)                                         
 x: negative (more than 25 micron)