Methods for Lead Free Solder Interconnections for Integrated Circuits

Abstract

Methods for forming lead free solder interconnections for integrated circuits. A copper column extends from an input/output terminal of an integrated circuit. A cap layer of material is formed on the input/output terminal of the integrated circuit. A lead free solder connector is formed on the cap layer. A substrate having a metal finish solder pad is aligned with the solder connector. An intermetallic compound is formed at the interface between the cap layer and the lead free solder connector. A solder connection is formed between input/output terminal of the integrated circuit and the metal finish pad that is less than 0.5 weight percent copper, and the intermetallic compound is substantially free of copper.

1 . A method, comprising:
 providing an integrated circuit with input/output terminals for external connectors formed on one surface;   providing a plurality of copper columns extending from the input/output terminals, each copper column having a bottom surface;   providing a cap layer overlying at least a bottom surface of each of the copper columns;   providing a lead free solder connector over each of the cap layers;   orienting the integrated circuit, the copper columns with the cap layers and the lead free solder connectors facing a top surface of a substrate, the substrate having a metal finish covering a plurality of solder ball pads in alignment with respective ones of the lead free solder connectors;   placing the lead free solder connectors in proximity to corresponding ones of the solder ball pads; and   performing a thermal reflow process to solder the solder ball pads on the substrate to the input/output terminals of the integrated circuit, the lead free solder connectors bonding to the solder bail pads and the cap layers.   
     
     
         2 . The method of  claim 1 , wherein the cap layer is one selected from the group consisting essentially of nickel, platinum, cobalt, palladium, gold, silver, and combinations thereof. 
     
     
         3 . The method of  claim 1 , wherein the metal finish comprises one selected from the group consisting essentially of electroless nickel and alloy gold (ENAG), electroless nickel electroless palladium and immersion gold (ENEPIG), electroless nickel and immersion gold (ENIG). 
     
     
         4 . The method of  claim 1 , wherein providing the cap layer further comprises providing a cap layer comprising nickel. 
     
     
         5 . The method of  claim 4 , wherein the metal finish comprises nickel. 
     
     
         6 . The method of  claim 5  wherein providing the substrate with the nickel based metal finish comprises providing a metal finish of electroless nickel, electroless palladium and immersion gold. 
     
     
         7 . The method of  claim 1 , wherein following the thermal reflow process, the lead free solder connectors have a copper content of less than 0.5 weight percentage. 
     
     
         8 . The method of  claim 1  wherein providing the lead free solder connectors comprises providing a tin (Sn) based solder. 
     
     
         9 . The method of  claim 8  wherein providing the lead free solder connectors further comprises providing a tin based solder including silver (Ag). 
     
     
         10 . A method, comprising:
 providing an integrated circuit with a plurality of input/output terminals for external connectors formed on one surface;   providing a plurality of copper columns extending from the input/output terminals, each copper column having a bottom surface;   providing a cap layer comprising nickel overlying at least the most exterior surface of each of the copper columns;   providing a lead free solder connector over each of the cap layers of the plurality of copper columns;   orienting the integrated circuit, the copper columns with the cap layers and the lead free solder connectors facing a top surface of a substrate, the substrate having a nickel based metal finish covering a plurality of solder ball pads in alignment with respective ones of the lead free solder connectors;   placing the lead free solder connectors in proximity to corresponding ones of the solder ball pads; and   performing a thermal reflow process to solder the solder ball pads on the substrate to the input/output terminals of the integrated circuit, the lead free solder connectors bonding to the solder ball pads and the cap layers.   
     
     
         11 . The method of  claim 10 , wherein the lead free solder connectors have a copper content of less than 0.5 weight percentage. 
     
     
         12 . The method of  claim 10  wherein providing the lead free solder connectors comprises providing a tin (Sn) based solder. 
     
     
         13 . The method of  claim 12  wherein providing the lead free solder connectors further comprises providing a tin based solder including silver. 
     
     
         14 . The method of  claim 10  wherein providing the substrate with the nickel used metal finish comprises providing a nickel based metal finish that comprises gold. 
     
     
         15 . The method of  claim 14  wherein providing the substrate with the nickel based metal finish comprises providing a metal finish of electroless nickel, electroless palladium and immersion gold. 
     
     
         16 . A method comprising:
 providing an integrated circuit die having terminals for flip chip connection to a substrate;   forming a plurality of copper connectors on the terminals of the integrated circuit die;   forming a cap layer on each of the plurality of copper connectors, the cap layer covering at least an exposed exterior surface of each copper connector;   disposing a lead free solder connector on the cap layer of each of the plurality of copper connectors;   orienting the integrated circuit die such that the copper connectors and the lead free solder connectors are aligned with corresponding solder ball pads on a substrate, the solder ball pads each having a metal finish free of copper; and   performing a thermal reflow of the lead free solder connectors to couple the integrated circuit die to the substrate;   wherein the copper content of the lead free solder connectors following the thermal reflow is less than 0.5 weight percentage.   
     
     
         17 . The method of  claim 16 , and further comprising forming an intermetallic compound substantially free of copper at an interface between the cap layer and the lead free solder connectors of each of the plurality of copper connectors. 
     
     
         18 . The method of  claim 16 , wherein the cap layer and the metal finish each comprise nickel. 
     
     
         19 . The method of  claim 18 , wherein a material used for the cap layer and a material used for the metal finish are substantially identical. 
     
     
         20 . The method of  claim 16 , wherein the lead free solder connectors comprise tin and silver and the metal finish and the cap layer each comprise nickel.