Effects of Electromigration(EM) on the Kirkendall Void Formation in Sn-3.5Ag/Cu Solder Joints

Kirkendall voids and the micro voids induced by electromigration result in the serious failure in the solder interconnect. Interfacial reactions between Sn-3.5Ag solder and the electroplated Cu using SPS additive in the bath were investigated under high current condition. During subsequent isothermal aging at 150°C, the formation of Kirkendall voids was accelerated by S segregation at the void surface and Cu/Cu3Sn interface, and then the voids were preferentially localized at the interface. And under high current flow (1X104A/cm2) at the cathode side, electromigration caused the micro voids at the Cu6Sn5/Sn interface. The growth of Kirkendall voids and the micro voids were both affected by aging time and current density. Effects of current flow on the growth of Kirkendall voids and the micro voids were examined by applying high electric current during isothermal aging at 150°C. From the measurements of the fraction of voids at the Cu/Cu3Sn and Cu6Sn5/Sn interfaces on SEM micrographs and analysis of the kinetics of void growth, which voids are more dominant was examined according to the time and the current density. Furthermore, under current flow the intermetallic compounds (IMC) thickening as well as voids growth was affected by polarity. Even though Cu was consumed remarkably, the growth of both Cu3Sn and Cu6Sn5 IMCs was suppressed at the cathode side. On the other hands, the thickness of Cu6Sn5 at the anode side was increased by reaction with Cu flux atoms which migrate towards the anode from the cathode Cu line.