Lead-free solder alloy

Abstract

A lead-free solder alloy includes a tin-copper alloy that also includes a minor amount of cobalt, which has been found to provide a shiny and reflective appearance to the solder alloy. Methods of soldering using such an alloy, as well as a solder joint including such an alloy also are disclosed.

What is claimed is: 
     
         1 . A method of hot air leveling comprising the steps of:
 a) providing in a molten state a lead-free solder alloy comprising, by weight, 0.2-2% copper, 0.02-0.3% cobalt, and at least 98% tin,   b) applying flux on a surface of a circuit board onto which conductive metal pads and/or traces have been previously deposited,   c) contacting said surface of said circuit board with said molten lead-free solder alloy,   d) removing said surface of said circuit board from contact with said molten solder alloy, said circuit board thereafter having molten solder alloy adhered to said exposed conductive metal surfaces of said pads and/or traces on said surface of said circuit board, and   e) permitting or causing said molten solder alloy adhered to said exposed conductive metal surfaces to solidify to thereby provide a protective lead-free solder alloy coating over said conductive metal pads and/or traces to protect the conductive metal from oxidation.   
     
     
         2 . A method according to  claim 1 , said surface of said circuit board being contacted with said molten solder alloy by dipping or immersing said surface in said alloy. 
     
     
         3 . A method according to  claim 2 , further comprising, on withdrawal of said surface from said molten solder alloy, directing a stream of hot air to impinge against the molten solder adhered to said conductive metal surfaces to level the molten solder. 
     
     
         4 . A method according to  claim 3 , said lead-free solder alloy coating having a coating thickness standard deviation of not more than 60 micro-inches. 
     
     
         5 . A method according to  claim 3 , said lead-free solder alloy coating having a coating thickness standard deviation of not more than 50 micro-inches. 
     
     
         6 . A method according to  claim 1 , said lead-free solder alloy consisting essentially of the cobalt, copper and tin constituents referred to in  claim 1 . 
     
     
         7 . A method according to  claim 1 , said lead-free solder alloy consisting of the cobalt, copper and tin constituents referred to in  claim 1  and impurities. 
     
     
         8 . A method according to  claim 1 , said conductive metal being copper. 
     
     
         9 . A method according to  claim 1 , said lead-free solder alloy comprising 0.02-0.1% cobalt. 
     
     
         10 . A method according to  claim 1 , said lead-free solder alloy comprising 0.04-0.1% cobalt. 
     
     
         11 . A method according to  claim 1 , said lead-free solder alloy comprising 0.05-0.1% cobalt. 
     
     
         12 . A method according to  claim 1 , said lead-free solder alloy comprising 0.06% cobalt. 
     
     
         13 . A method according to  claim 1 , said lead-free solder alloy comprising nominally 0.5% copper.