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. REEXAMINATION RESULTS The questions raised in reexamination proceedings Nos. 90/012,336 and 90/009,934, filed Jun. 4, 2012 and Aug. 3, 2011 respectively, have been considered, and the results thereof are reflected in this reissue patent which constitutes the reexamination certificate required by 35 U.S.C. 307 as provided in 37 CFR 1.570(e) for ex parte reexaminations, and/or the reexamination certificate required by 35 U.S.C. 316 as provided in 37 CFR 1.997(e) for inter partes reexaminations.

What is claimed is: 
     
       1. A process for forming a silver coating on a surface of a first metal having a first electropositivity less than an electropositivity of silver, the process comprising a step of:
 (a) contacting the surface of the first metal with an aqueous composition comprising silver ions and a complexing agent that is a multidentate ligand, the aqueous composition having a pH of from 2 to 12, to form a coating of silver on the surface of the first metal, wherein the aqueous composition is free of ammonium ions, thiosulphate ions and combinations thereof, said surface of said first metal having conductive metal pads, conductive metal through-holes or a combination thereof of a bare board, and in which the substrate includes non-metallic areas which remain uncoated in the process. 
 
     
     
       2. A process according to  claim 1 , wherein said complexing agent in step (a) includes contacting the aqueous composition with a surface of the first metal having conductive metal pads, conductive metal through-holes or a combination thereof of a bare board, and in which the substrate includes non-metallic areas which remain uncoated in the process comprises a complexing agent for silver ions. 
     
     
       3. A process according to claim  1  or  claim 2  37, wherein the surface of the first metal is a copper surface and step (a) includes contacting the copper surface with the aqueous composition. 
     
     
       4. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the complexing agent has a first molarity and the silver ions have a second molarity, the first molarity being greater than the second molarity. 
     
     
       5. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the silver ions are present at a concentration of from 0.06 g/l to 32 g/l. 
     
     
       6. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the complexing agent is present in in an amount of from 0.1 g/l to 250 g/l. 
     
     
       7. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the complexing agent is selected from the group consisting of ethylenediamine tetra-acetic acid, diethylenetriamine penta-acetic acid, N,N,N′,N′-tetrakis(2-hydroxy propyl)ethylene diamine and mixtures thereof. 
     
     
       8. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition which further includes an ingredient selected from the group consisting of a surfactant, wetting agent, stabilizer, grain refiner, tarnish inhibitor and mixtures thereof. 
     
     
       9. A process according to  claim 8 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which a surfactant has a concentration of from 1 g/l to 15 g/l. 
     
     
       10. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition free of any reducing agent capable of reducing the silver ions to silver metal. 
     
     
       11. A process according to  claim 1 , wherein step (a) includes a step of spraying the aqueous composition or dip-coating the aqueous composition in an immersion bath. 
     
     
       12. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous 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 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition so that the the silver coating on the first metal is less-than 0.5 μm thick. 
     
     
       14. A process according to  claim 1 , further comprising, prior to step (a), a step of contacting the surface of the first metal with an acidic cleaning solution to clean the surface of the first metal. 
     
     
       15. A process according to  claim 1 , further comprising, prior to step (a), a step of micro-etching the surface of the first metal. 
     
     
       16. A process according to  claim 1 , further comprising steps of:
 rinsing the coating of silver; and 
 drying the coating of silver. 
 
     
     
       17. A process according to  claim 1 , further comprising a step of soldering a conductor component to the coating of silver. 
     
     
       18. A process for producing a bare printed circuit board comprising steps of:
 i) providing a substrate having exposed metal conductor traces and pads through-holes or a combination thereof, the metal having an electropositivity less than an electropositivity of silver, 
 ii) applying a mask to cover at least the traces and to leave at least some of the pads, the through-holes or the combination thereof exposed, the mask being an insulator, and 
 iii) contacting the metal surface with an aqueous composition comprising silver ions and a complexing agent that is a multidentate ligand, the aqueous composition having a pH of from 2 to 12, to form a coating of silver on the metal surface, wherein the aqueous composition is free of ammonium ions, thiosulphate ions and combinations thereof. 
 
     
     
       19. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition including a buffering agent. 
     
     
       20. A process according to  claim 4 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the first molarity is twice as large as the second molarity. 
     
     
       21. A process according to  claim 5 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the silver ions are present at a concentration of from 0.1 g/l to 25 g/l. 
     
     
       22. A process according to  claim 5 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the silver ions are present at a concentration of from 0.5 g/l to 15 g/l. 
     
     
       23. A process according to  claim 6 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition in which the complexing agent is present in an amount of from 10 g/l to 100 g/l. 
     
     
       24. A process according to  claim 10 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition free of halide ions. 
     
     
       25. A process according to  claim 12 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition at a temperature of from 15° C. to 50° C. 
     
     
       26. A process according to  claim 1 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition that is free of ingredients selected from the group consisting of cyanide ions, formaldehyde, reducing sugars and combinations thereof. 
     
     
       27. A process according to  claim 1 , wherein step (a) includes contacting the aqueous solution with a surface of the first metal having conductive metal pads, conductive metal through-holes or a combination thereof of a bare board, and in which the substrate includes solder-mask coated areas. 
     
     
       28. A process according to  claim 18 , wherein step (a) includes contacting the surface of the first metal with an aqueous composition that is free of ingredients selected from the group consisting of cyanide ions, formaldehyde, reducing sugars and combinations thereof. 
     
     
       29. A process according to  claim 18 , wherein the exposed metal conductor is an exposed copper conductor. 
     
     
       30. A process according to  claim 18 , wherein the silver ions are present in the aqueous composition in a concentration of from 0.06 g/l to 32 gl. 
     
     
       31. A process according to  claim 18 , wherein the complexing agent of the aqueous solution has a first molarity and the silver ions of the aqueous solution have a second molarity, the first molarity being greater that the second molarity. 
     
     
       32. A process according to  claim 18 , wherein the complexing agent of the aqueous composition is present in an amount from 0.1 g/l to 250 g/l. 
     
     
       33. A process according to  claim 18 , wherein the complexing agent of the aqueous composition is selected from the group consisting of ethylenediamine tetra-acetic acid, diethylenetriamine penta-acetic acid, N,N,N′,N′-tetrakis (2-hydroxy propyl) ethylene diamine and mixtures thereof. 
     
     
       34. A process according to  claim 18 , wherein the aqueous composition further includes a compound selected from the group consisting of a surfactant, a wetting agent, a stabilizer, a grain refiner, a tarnish inhibitor and mixtures thereof. 
     
     
       35. A process according to  claim 34 , wherein the complexing agent is a surfactant that has a concentration of from 1 g/l to 15 g/l in the aqueous solution. 
     
     
       36. A process, comprising the steps of:
 applying a mask to cover a metal surface of a substrate, the metal surface having an electropositivity less than an electropositivity of silver, the mask being an insulator; and 
 contacting the metal surface with an aqueous composition comprising silver ions and a complexing agent that is a multidentate ligand, the aqueous composition having a pH of from 2 to 12, to form a coating of silver on the metal surface, wherein the aqueous composition is free of ammonium ions, thiosulfate ions and combinations thereof. 
 
     
     
       37. A process as set forth in claim 2 wherein said aqueous composition comprises silver nitrate. 
     
     
       38. A process as set forth in claim 18 wherein said complexing agent comprises a complexing agent for silver ions, and said aqueous composition comprises silver nitrate and is free of any reducing agent capable of reducing the silver ions to silver metal. 
     
     
       39. A process as set forth in claim 38 wherein wherein said complexing agent comprises an alkylene polyamine polyacetic acid. 
     
     
       40. A process as set forth in claim 38 wherein said complexing agent is selected from the group consisting of ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentaacetic acid, N-hydroxyethyl ethylene diamine triacetic acid, 1,3-diamino-2-propanol-N,N,N′,N′-tetra-acetic acid, bishydroxyphenylethylene diamine diacetic acid, diamino cyclohexane tetra-acetic acid or ethylene glycol-bis-[(β-aminoethylether)-N,N′-tetraacetic acid)] and N,N,N′,N′-tetrakis-(2-hydroxypropyl) ethylene diamine. 
     
     
       41. A process as set forth in claim 39 wherein said aqueous composition further comprises a citrate or tartrate of said alkylene polyamine polyacetic acid. 
     
     
       42. A process as set forth in claim 39 wherein said aqueous composition further comprises a non-ionic surfactant. 
     
     
       43. A process as set forth in claim 39 wherein said aqueous composition further comprises nitric acid. 
     
     
       44. A process as set forth in claim 39 wherein said aqueous composition further comprises a tarnish inhibitor. 
     
     
       45. A process as set forth in claim 44 wherein said tarnish inhibitor comprises an ethoxylated alkylamine or an alkyl-substituted triazole. 
     
     
       46. A process as set forth in claim 1 wherein said complexing agent comprises a complexing agent for silver ions, and said aqueous composition comprises silver nitrate and is free of any reducing agent capable of reducing the silver ions to silver metal. 
     
     
       47. A process for forming a silver coating on a surface of a first metal having a first electropositivity less than an electropositivity of silver, the process comprising a step of:
 (a) contacting the surface of the first metal with an aqueous composition comprising silver ions and a complexing agent that is a multidentate ligand, the aqueous composition having a pH of from 2 to 12, to form a coating of silver on the surface of the first metal, wherein the aqueous composition is free of ammonium ions, thiosulphate ions and combinations thereof; and   (b) soldering a conductor component to the coating of silver.   
     
     
       48. A process as set forth in claim 47 wherein said complexing agent is a complexing agent for silver ions. 
     
     
       49. A process as set forth in claim 48 wherein said aqueous composition contains silver nitrate. 
     
     
       50. A process as set forth in claim 49 wherein the surface of the first metal is copper and step (a) includes contacting the copper surface with the aqueous composition. 
     
     
       51. A process as set forth in claim 50 wherein said complexing agent comprises an alkylene polyamine polyacetic acid. 
     
     
       52. A process as set forth in claim 51 wherein said complexing agent is selected from the group consisting of ethylene diamine tetra-acetic acid (EDTA), diethylene triamine penta-acetic acid, N-hydroxyethyl ethylene diamine triacetic acid, 1,3-diamino-2-propanol-N,N,N′,N′-tetra-acetic acid, bishydroxyphenylethylene diamine diacetic acid, diamino cyclohexane tetra-acetic acid or ethylene glycol-bis-[(β-aminoethylether)-N,N′-tetra-acetic acid)] and N,N,N′,N′-tetrakis-(2-hydroxypropyl) ethylene diamine. 
     
     
       53. A process as set forth in claim 51 wherein said aqueous composition further comprises a citrate or tartrate of said alkylene polyamine polyacetic acid. 
     
     
       54. A process as set forth in claim 51 further comprising a non-ionic surfactant. 
     
     
       55. A process as set forth in claim 51 further comprising nitric acid.