Mitigation and elimination of tin whiskers

Abstract

A method of mitigating tin whisker formation on electronic assemblies includes exposing tin metal in the electronic assembly to a mitigating agent that interacts with the tin metal to produce a product that is resistant to forming tin whiskers.

1 . A method comprising:
 providing an electronic assembly, the electronic assembly having at least one electrical connection that includes at least a surface that is substantially pure tin metal, the pure tin metal having a thickness; the method comprising:   exposing the tin metal to at least one mitigating agent selected to interact with the tin metal to produce a product that is resistant to forming whiskers;   wherein the electronic assembly is exposed to the mitigating agent under appropriate conditions to promote the desired interaction.   
     
     
         2 . The method of  claim 1 , wherein the mitigating agent is a reagent, and the interaction with the tin metal is a chemical reaction. 
     
     
         3 . The method of  claim 2 , wherein the mitigating agent is oxygen, and the interaction with tin metal is oxidation of the tin metal to at least one of SnO and SnO 2 . 
     
     
         4 . The method of  claim 3 , wherein the appropriate conditions include heating the electronic assembly above room temperature. 
     
     
         5 . The method of  claim 4 , wherein the appropriate conditions include heating the electronic assembly to between about 25° C. and about 90° C. 
     
     
         6 . The method of  claim 4 , further comprising removing substantially all oxidized tin whiskers from the electronic assembly. 
     
     
         7 . The method of  claim 2 , wherein the appropriate conditions are sufficient to convert the tin metal surface of the at least one electrical connection to a product that is resistant to forming whiskers. 
     
     
         8 . The method of  claim 2 , wherein the appropriate conditions are sufficient to convert the surface and entire thickness of the at least one electrical connection to a product that is resistant to forming whiskers. 
     
     
         9 . The method of  claim 1 , wherein the mitigating agent is a metal, and the interaction with the tin metal is formation of a tin alloy that is resistant to forming whiskers. 
     
     
         10 . The method of  claim 9 , wherein the electronic assembly includes a mitigating agent that is a conducting layer of copper metal, and the tin alloy that is formed includes one or more of Cu 6 Sn 5  and CuSn 3 . 
     
     
         11 . The method of  claim 9 , wherein the electronic assembly includes a conducting layer of copper metal, and the mitigating agent is a metal film disposed between the copper metal and the tin metal. 
     
     
         12 . The method of  claim 9 , wherein the mitigating agent is at least one of gallium, indium, gold, copper, aluminum, silver, magnesium, nickel, zinc, and lead. 
     
     
         13 . The method of  claim 9 , wherein the formed tin alloy is less than about 97% tin. 
     
     
         14 . The method of  claim 1 , wherein the electronic assembly further includes a circuit board and at least one electronic component coupled by the at least one electrical connection. 
     
     
         15 . The method of  claim 14 , wherein the mitigating agent is applied to the electronic assembly prior to coupling the electronic component to the electrical connection. 
     
     
         16 . The method of  claim 14 , wherein the mitigating agent is applied to the electronic assembly after the electronic component is coupled to the electrical connection. 
     
     
         17 . The method of  claim 1 , wherein the mitigating agent is applied to the electronic assembly as a vapor or liquid 
     
     
         18 . The method of  claim 1 , wherein a first mitigating agent is a reagent that interacts with the tin metal in a chemical reaction at the surface of the tin metal; and a second mitigating agent is a metal film disposed between the tin metal and the at least one electrical connection that interacts with an underside of the tin metal to form a tin alloy that is resistant to forming whiskers. 
     
     
         19 . An electronic assembly, prepared by a process comprising:
 providing an electronic assembly having at least one electrical connection that includes substantially pure tin metal;   exposing the tin metal to a mitigating agent selected to interact with the tin metal to produce a product that is resistant to forming whiskers; and   curing the electronic assembly under conditions selected to promote the desired interaction.   
     
     
         20 . The electronic assembly of  claim 19 , wherein the mitigating agent is an oxidizing agent, curing the electronic assembly includes heating; and the product is a tin oxide. 
     
     
         21 . The electronic assembly of  claim 19 , wherein the mitigating agent is a metal capable of forming an alloy with tin that is resistant to forming whiskers. 
     
     
         22 . The electronic assembly of  claim 19 , wherein the electronic assembly includes a conducting layer of copper metal, and the mitigating agent is a metal film disposed between the conducting layer and the at least one electrical connection. 
     
     
         23 . An electronic assembly, comprising:
 a circuit board substrate;   a conducting trace comprising copper metal; and   an electronic component coupled to the conducting trace via tin metal;   wherein the conducting trace and the tin metal are coupled via a mitigating metal comprising at least one of gallium, indium, gold, copper, aluminum, silver, magnesium, nickel, zinc, and lead; the mitigating metal being present in sufficient quantity that upon heating the mitigating metal substantially converts the tin metal to an alloy that is resistant to whisker formation.   
     
     
         24 . The electronic assembly of  claim 23 , wherein heating the electronic assembly to between about 25° C. and about 90° C. converts the tin metal to an alloy that is resistant to whisker formation.