Whisker-free electronic structures

Abstract

It has been found that composition containing copper, tin, and silver prevents tin whisker formation on an electronic structure while allowing solders to wet such structures during soldering processes. It has further been found that conventional techniques, such as electrolytic plating, electroless plating, wet dipping and vapor deposition, for forming such materials have undesirable limitations and/or characteristics. However, by forming a Ag/Sn precursor on a copper containing electronic structure and inducing a self-limiting reaction between the precursor and the copper of the structure, the advantageous Ag/Sn/Cu material is formed without the undesirable limitations and characteristics associated with conventional techniques.

1 . A method of forming a material on an electronic structure containing copper such that said material comprises silver, tin, and copper constituents with a composition among said constituents of about 0.1 to 10.0 weight percentage Cu, 0.1 to 20.0 weight percent silver and the remaining weight percentage tin, said process comprising the steps of forming a Ag/Sn precursor on said structure and inducing with energy a reaction between said precursor and said copper of said structure such that said material is formed.  
   
   
       2 . The method of  claim 1  wherein said energy comprises heating said structure to a temperature above about 232° C.  
   
   
       3 . The method of  claim 1  wherein said structure comprises a device lead frame.  
   
   
       4 . The method of  claim 3  wherein said device is attached to said lead frame and encapsulated before said precursor is formed.  
   
   
       5 . The method of  claim 3  wherein said device is attached to said lead frame and then encapsulated after said precursor is formed.  
   
   
       6 . The method of  claim 1  wherein said precursor is formed by first forming an area of silver and then forming tin on said silver.  
   
   
       7 . The method of  claim 6  wherein said area of silver comprises discontinuous regions.  
   
   
       8 . The method of  claim 1  wherein said precursor is formed by first forming a tin region and then a silver region.  
   
   
       9 . The method of  claim 8  wherein said region of silver is discontinuous.  
   
   
       10 . The process of  claim 1  wherein said weight percentage of silver is about 0.1 to 10.  
   
   
       11 . The process of  claim 10  wherein said weight percent of silver is about 1.0 to 4.0.  
   
   
       12 . The process of  claim 11  wherein said weight percent of silver is about 2.0 to 4.0.  
   
   
       13 . The process of  claim 1  wherein said weight percent of silver is about 3.4 to 3.8.  
   
   
       14 . The method of  claim 1  wherein inducing comprises heating said precursor to a temperature in the range of about 232° C. to 260° C.  
   
   
       15 . The method of  claim 1  wherein said precursor comprises a binary Sn/Ag composition.  
   
   
       16 . The method of  claim 15  wherein said binary composition is formed by co-electrolytic deposition on said structure.  
   
   
       17 . The method of  claim 1  wherein said precursor comprises a portion of tin overlying a portion of silver on said structure such that the ratio of the thickness of said portion of tin to the thickness of said portion of silver is less than about 10.  
   
   
       18 . The process of  claim 1  wherein said structure comprises an alloy of copper.  
   
   
       19 . The method of  claim 1  wherein said material includes less than 15 weight percent of a species other than Ag, Sn, or Cu.  
   
   
       20 . The method of  claim 1  wherein said electronic structure comprises an integrated circuit.