Whisker-free lead frames

Abstract

The electrical and mechanical properties of structures such as lead frames and other electrical/electronic devices containing, during processing, copper/tin interfaces are improved by introduction of nickel to such interface. Typically, a weight percentage of nickel to tin in the range 1 to 12 weight percent yields upon melting of the tin, an intermetallic compound with essentially no occluded, unbound tin. Thus undesirable anomalous structures such as tin needles and substantially non-planar interface compositions are avoided. Advantageously a nickel/tin/copper intermetallic interface that is substantially planar is formed in the substantial absence of needle-like tin structures.

1 . A process of fabricating an interconnection between a first region comprising copper and a second conducting region wherein tin is present at an interface with said first region during said process characterized in that nickel is present with said tin wherein the weight percentage of said nickel relative to said tin at said interface is in the range 1 to 12 weight percent and the tin material is melted to induce formation of a nickel/tin/copper intermetallic composition at said interface with said copper.  
   
   
       2 . The process of  claim 1  wherein said melting is induced by subjecting said connection to a temperature in the range 232° C. to 270° C.  
   
   
       3 . The process of  claim 2  wherein said subjecting is continued for a period of 5 to 120 seconds.  
   
   
       4 . The process of  claim 1  wherein said weight percentage is in the range 3 to 7 weight percent.  
   
   
       5 . The process of  claim 4  wherein said weight percentage is in the range 4 to 6 percent.  
   
   
       6 . The process of  claim 1  wherein said interconnection comprises a lead frame.  
   
   
       7 . The process of  claim 6  wherein said lead frame includes a flip chip bump.  
   
   
       8 . The process of  claim 1  wherein said interconnection includes a region of copper as part of an electrical pad.  
   
   
       9 . The process of  claim 1  wherein said interconnection comprises an electrical interconnection.  
   
   
       10 . A structure comprising an interconnection between a first region comprising copper and a second electrically conducting region characterized in that a region of an intermetallic composition comprising copper, nickel, and tin is present between said first region and said second region and wherein the surface of said region of intermetallic composition includes less than about 15 weight percent occluded tin relative to the total free tin content in said interconnect.  
   
   
       11 . The structure of  claim 10  wherein said structure comprises an interconnection between a lead frame and a circuit board.  
   
   
       12 . The structure of  claim 10  wherein said structure comprises an interconnection between a flip chip and an electrical pad.  
   
   
       13 . The structure of  claim 10  wherein said intermetallic compound is essentially devised of occluded tin.  
   
   
       14 . The structure of  claim 10  wherein said interconnect comprises an electrical interconnect.