Process for silver plating in printed circuit board manufacture

Abstract

In PCB manufacture, the pads and/or through-holes of a bare board are protected prior to mounting of other components by a silver plating process. The silver plate is applied from a plating composition by a replacement process in which silver ions oxidized copper and deposit a layer of silver at the surface. In the aqueous plating composition the silver ions are maintained in solution by the incorporation of a multidentate complexing agent. The compositions may contain a silver tarnish inhibitor and are free of reducing agent capable of reducing silver ions to silver metal, halide ions and substantially free of non-aqueous solvent. Surface mounted components can be soldered direct to the silver coated contact using any commercially available solder. Silver does not deposit onto solder mask, but only onto metal which is less electropositive than silver.

What is claimed is: 
     
       1. A process for providing a protective coating on metal conducting surfaces of bare boards in the manufacture of printed circuit boards, comprising the steps of contacting a substrate comprising the conducting surfaces with an aqueous displacement plating composition which comprises silver ions and a multidentate complexing agent in solution in an aqueous vehicle at a pH of from 2 to 12 to form a silver coating on the conducting surfaces, wherein said metal conducting surfaces are less electropositive than silver and comprise conductive metal pads or through-holes of a bare board, and the substrate includes non-metallic areas which remain uncoated in the process. 
     
     
       2. A process according to  claim 1  wherein the process further comprises forming a protective coating on a substrate wherein the metal conducting surfaces are less electropositive than silver and which comprise conductive metal pads or through-holes of a bare board, and in which the substrate includes non-metallic areas which remain uncoated in the process said multidentate complexing agent comprises a complexing agent for silver ions. 
     
     
       3. A process according to  claim 1  or  claim 2  in which the metal surface comprises copper. 
     
     
       4. A process according to  claim 1  in which the multidentate complexing agent is present in a higher molar amount than the silver ions. 
     
     
       5. A process according to  claim 1  in which the silver ions are present in the displacement plating composition at a concentration of from 0.06 to 32 g/l. 
     
     
       6. A process according to  claim 4  in which the multidentate complexing agent is present in the composition in an amount of from 0.1 to 250 g/l. 
     
     
       7. A process according to  claim 1  in which the multidentate complexing agent is selected from the group consisting of ethylenediamine tetra-acetic acid, diethylenetriamine penta-acetic acid and N,N,N′, N′-tetrakis(2-hydroxy propyl)ethylene diamine. 
     
     
       8. A process according to  claim 1  in which the displacement plating composition comprises surfactant, wetting agent, stabilizer, and wherein the displacement coating composition also includes a material selected from the group consisting of grain refiner or tarnish inhibitor. 
     
     
       9. A process according to  claim 8  in which surfactant is incorporated in the displacement plating composition at a concentration of from 1 to 15 g/l. 
     
     
       10. A process according to  claim 1  in which the displacement plating composition is free of reducing agent which reduces the silver ions to silver metal and free of halide ions. 
     
     
       11. A process according to  claim 1  in which the step of contacting the metal conducting surfaces with the aqueous displacement plating composition is by spraying or dip-coating in an immersion bath and in which excess composition is reused in the process. 
     
     
       12. A process according to  claim 1  in which the metal conducting surfaces are contacted with the aqueous displacement plating composition for from 10 seconds to 10 minutes, at a temperature of from 10° C. to 60° C. 
     
     
       13. A process according to  claim 1  in which the silver coating is less than 0.5 μm thick. 
     
     
       14. A process according to  claim 1  in which prior to the displacement plating composition step, the metal conducting surfaces are cleaned by contacting with an acidic cleaning solution in an acid cleaning step. 
     
     
       15. A process according to  claim 1  in which prior to contacting the metal surface elements with the displacement plating composition, the metal conducting surfaces undergo a micro-etching step, and wherein between the micro-etching step and the displacement plating composition step, there is an additional acid rinse step. 
     
     
       16. A process according to  claim 1  in which the silver coated metal surface is subsequently rinsed and, optionally, dried. 
     
     
       17. A process according to  claim 1  in which a component is subsequently soldered directly to the silver coating. 
     
     
       18. A multi-step process for producing a bare board of a printed circuit board including the steps of:
 i) providing, by a subtractive or additive process, exposed metal conductor traces and pads or through-holes on a substrate, said metal being less electropositive than silver; 
 ii) applying a mask to cover at least the said traces and leaving at least some of the pads or through-holes exposed, the mask being of an insulating composition; and 
 iii) forming a silver coating on the exposed pads or through-holes by contacting the metal surface with an aqueous displacement plating composition comprising silver ions and a multidentate complexing agent in solution in an aqueous vehicle and having a pH of from 2 to 12, to form a coating of silver on the metal surface. 
 
     
     
       19. A process according to  claim 2 , wherein the non-metallic areas are coated with solder-mask. 
     
     
       20. A process according to  claim 4 , wherein said aqueous displacement plating composition contains at least twice the molar amount of said multidentate complexing agent than of said silver ions. 
     
     
       21. A process according to  claim 1 , wherein said silver ions are present in said displacement plating composition at a concentration of from 0.1 to 25 g/l. 
     
     
       22. A process according to  claim 1 , wherein said silver ions are present in the displacement plating composition at a concentration of from 0.5 to 15 g/l. 
     
     
       23. A process according to  claim 1  in which the multidentate complexing agent is present in the composition in an amount of from 10 to 100 g/l. 
     
     
       24. A process according to  claim 1  in which the metal surface is contacted with said aqueous displacement plating composition for from 10 seconds to 10 minutes, at a temperature in the range from 15° C. to 50° C. 
     
     
       25. A process according to  claim 14  in which a rinse step is used in the process after said acid cleaning step and prior to said displacement plating composition step. 
     
     
       26. A process as set forth in claim 1 wherein said plating composition comprises silver nitrate.  
     
     
       27. A process as set forth in claim 26 wherein said multidentate complexing agent comprises an alkylene polyamine polyacetic acid.  
     
     
       28. A process as set forth in claim 27 wherein said complexing agent comprises N-hydroxyethylethylene diamine triacetic acid.  
     
     
       29. A process as set forth in claim 18 wherein said plating composition comprises silver nitrate.  
     
     
       30. A process as set forth in claim 29 wherein said multidentate complexing agent comprises an alkylene polyamine polyacetic acid.  
     
     
       31. A process as set forth in claim 30 wherein said complexing agent comprises N-hydroxyethylethylene diamine triacetic acid.