An effective method for full solder intermetallic compound formation and Kirkendall void control in Sn-base solder micro-joints

Kirkendall voids are defects that significantly reduce the mechanical properties as well as the electromigration (EM) resistance of Sn base solder joints. In this paper, an effective method was utilized in the micro-joints to fully convert Sn-Ag solder into intermetallics (IMC), to suppress the formation of Kirkendall voids. Silicon chip samples with Sn-based micro-joints (30um diameter) and Cu pillar structures were fabricated with different joint layer metallurgy to investigate the impact of the Cu pillar thicknesses, solder amount and with or without Ni layer on the IMC formation behavior. Both IMC growth and Kirkendall evolution was studied under high temperature storage (HTS) at 150C for 500 hours and 1000 hours. A process called short term 2^nd reflow was used to enhance the Sn-Ag solder to IMC conversion. For comparison, Si chip samples with more typical joint diameter (82um) were also fabricated. We found the 2^nd reflow process fully converted Sn-Ag solder into IMC in the micro-joint samples. However, the 2^nd reflow process was not practical for samples with larger solder (82um diameter) volume, since this process consumed the Ni layer under bump metallurgy on the bottom side.