Microelectronic assemblies with composite conductive elements

Abstract

A microelectronic assembly includes composite conductive elements, each incorporating a core and a coating of a low-melting conductive material. The composite conductive elements interconnect microelectronic elements. At the normal operating temperature of the assembly, the low-melting conductive material melts, allowing the cores and microelectronic elements to move relative to one another and relieve thermally-induced stress.

1 . An assembly comprising: 
 (a) a first microelectronic element having contacts thereon;    (b) a second microelectronic element including a flexible sheet having lateral directions along the sheet and having vertical directions transverse to the sheet, said sheet having first contacts facing toward said first microelectronic element and having second contacts facing away from said microelectronic element, said second contacts being offset from said first contacts in one or more of said lateral directions, at least some of said second contacts being electrically connected to at least some of said first contacts; and    (c) first electrically conductive elements connecting said first contacts to said first microelectronic element, said sheet being vertically spaced from said first microelectronic element so that said sheet is free to flex in said vertical directions, whereby flexural movement of said sheet can accommodate movement of said second contacts relative to said first microelectronic element.    
   
   
       2 . An assembly as claimed in  claim 1  wherein said first electrically conductive elements support said sheet away from said first microelectronic element.  
   
   
       3 . An assembly as claimed in  claim 2  further comprising a compliant filler disposed between said sheet and said first microelectronic element.  
   
   
       4 . An assembly as claimed in  claim 2  further comprising a third microelectronic element, said sheet being disposed between said first and third microelectronic elements, the assembly further comprising second conductive elements electrically connecting said second contacts and said third microelectronic element.  
   
   
       5 . A conductive element including a spherical core and a layer of a conductive material on said core, said conductive material having a melting temperature less than about 150° C., said core having a melting temperature above the melting temperature of said conductive material.  
   
   
       6 . A conductive element as claimed in  claim 5  wherein said layer of conductive material is less than about 50 microns thick.  
   
   
       7 . A conductive element as claimed in  claim 5  wherein said core is hollow.  
   
   
       8 . A conductive element as claimed in  claim 5  wherein said core is solid throughout.  
   
   
       9 . A conductive element as claimed in  claim 5  wherein said core and said conductive material are metallic.