Electromigration of solder balls for wafer-level packaging with different under bump metallurgy and redistribution layer thickness

Electromigration (EM) has been conducted on lead-free solder balls in wafer-level packages for different redistribution layer (RDL) thicknesses, under bump metallurgy (UBM) schemes, and lead-free solder alloys. Two different types of EM-induced voids were observed at the electron-source side: pancake void between the solder/RDL interface and through-thickness voids in the RDL. In both cases, voids formed at the interface of CuSn intermetallic compound and solder. A Ni-layer in the UBM was found to prolong EM lifetime by slowing the diffusion of Sn into the Cu RDL relative to a Cu-only UBM. The absence of UBM led to the shortest EM lifetime due to direct contact of solder to RDL. Also, a thicker RDL extended lifetime proportionately to the decrease in current density and Joule heating at the critical interface. On the other hand, adding Ni and Ge to the SAC alloy did not statistically impact lifetime.