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.

1 . A lead-free solder alloy comprising, by weight, at least 90% tin, 0.005-5% copper, and 0.005-5% cobalt.  
   
   
       2 . A lead-free solder alloy according to  claim 1 , said alloy being shiny and reflective.  
   
   
       3 . A lead-free solder alloy according to  claim 2 , said alloy producing shinier and more reflective solder joints and surface coatings compared to plain Sn99.5/Cu0.5 and Sn99.7/Cu0.3 alloys without cobalt.  
   
   
       4 . A lead-free solder alloy according to  claim 1 , said alloy comprising nominally 0.6% cobalt by weight.  
   
   
       5 . A lead-free solder alloy according to  claim 1 , said alloy comprising nominally, by weight, 99.5% tin, 0.5% copper and 0.06% cobalt.  
   
   
       6 . A lead-free solder alloy according to  claim 1 , said alloy comprising nominally, by weight, 99.7% tin, 0.3% copper and 0.06% cobalt.  
   
   
       7 . A lead-free solder alloy according to  claim 1 , said alloy being effective to produce more uniform thickness coatings across pads and traces on printed circuit boards compared to tin/lead alloys when used in a hot air leveling process.  
   
   
       8 . A lead-free solder alloy according to  claim 7 , said alloy being effective to produce thinner coatings across pads and traces on printed circuit boards compared to tin/lead alloys when used in a hot air leveling process.  
   
   
       9 . A lead-free solder alloy according to  claim 1 , said alloy being in wire form.  
   
   
       10 . A lead-free solder alloy according to  claim 1 , said alloy being in bar or ingot form.  
   
   
       11 . A lead-free solder alloy according to  claim 1 , said alloy being provided in powder form and suspended in a solder flux to provide a solder paste.  
   
   
       12 . A method of soldering, comprising melting a solder alloy to produce a molten alloy, contacting the molten alloy with at least one solid metal piece, and causing or permitting the molten alloy to cool and harden to thereby provide a solder joint, said solder alloy being a lead-free solder alloy that comprises, by weight, at least 90% tin, 0.005-5% copper, and 0.005-5% cobalt.  
   
   
       13 . A method according to  claim 12 , said lead-free solder alloy comprising nominally 0.6% cobalt by weight.  
   
   
       14 . A method according to  claim 12 , said lead-free solder alloy comprising nominally, by weight, 99.5% tin, 0.5% copper and 0.06% cobalt.  
   
   
       15 . A method according to  claim 12 , said lead-free solder alloy comprising nominally, by weight, 99.7% tin, 0.3% copper and 0.06% cobalt.  
   
   
       16 . A method according to  claim 12 , said solid metal piece being an electronic component lead in contact with a printed circuit board, wherein said solder joint is provided between said component lead and said printed circuit board.  
   
   
       17 . A solder joint comprising a solidified alloy, said solidified alloy being a lead-free solder alloy that comprises, by weight, at least 90% tin, 0.005-5% copper, and 0.005-5% cobalt.  
   
   
       18 . A solder joint according to  claim 17 , said lead-free solder alloy comprising nominally 0.6% cobalt by weight.  
   
   
       19 . A solder joint according to  claim 17 , said lead-free solder alloy comprising nominally, by weight, 99.5% tin, 0.5% copper and 0.06% cobalt.  
   
   
       20 . A solder joint according to  claim 17 , said lead-free solder alloy comprising nominally, by weight, 99.7% tin, 0.3% copper and 0.06% cobalt.  
   
   
       21 . A method of hot air leveling comprising the steps of: 
 a) providing a lead-free solder alloy according to  claim 1  in a molten state,    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.    
   
   
       22 . A method according to  claim 21 , said surface of said circuit board being contacted with said molten solder alloy by dipping or immersing said surface in said alloy.  
   
   
       23 . A method according to  claim 22 , 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, thereby producing a more uniform protective solder coating over the conductive metal pads and traces on solidification of the molten solder compared to the coating that would be produced absent the hot air impingement step.  
   
   
       24 . A method according to  claim 23 , said lead-free solder alloy coating having a maximum thickness of 350 micro-inches, and a standard deviation of not more than 60 micro-inches.  
   
   
       25 . A method according to  claim 21 , said conductive metal being copper.  
   
   
       26 . A method of wave soldering comprising the steps of: 
 a) providing a circuit board having a series of predrilled holes therethrough and a conductive coating on the inner surface of said predrilled holes,    b) placing an electronic component so that metallic electrical leads thereof extend from a first surface of said circuit board, through said predrilled holes, toward a second surface thereof,    c) applying a layer of flux over said second surface of said circuit board, and    d) contacting said second surface of said circuit board with a wave of molten lead-free solder alloy according to  claim 1  so that molten solder alloy from said wave contacts said second surface of said circuit board, activating the flux previously coated thereon and causing lead-free molted solder alloy from said wave to wick up the exposed electrical leads from said second surface into said predrilled holes, to provide a solder joint between said leads and the conductive coating on the interior surfaces of said predrilled holes.    
   
   
       27 . A method according to  claim 26 , said metallic electrical leads extending out from said predrilled holes slightly beyond said second surface of said circuit board.  
   
   
       28 . A method according to  claim 26 , said conductive coating being copper.