A Coupling between Electromigration(EM) and Kirkendall Effect in Sn-3.5Ag/Cu Solder Joint

Sn-3.5Ag/Cu solder joints were prepared by reflowing the eutectic solder on the PCB Cu opening which was electroplated using SPS additive in the bath. Effects of electromigration on the Kirkendall void growth were examined by applying electric current during isothermal aging at 150℃,and analyzing SEM micrographs. Without current, Cu3Sn and Cu6Sn5 intermetallic compounds (IMC) formed at the Sn-3.5Ag/Cu interface with a diffusion controlled mechanism. With electricity, (1×104 A/cm2), kinetics of Kirkendall void and IMC growth were affected, which depend on the direction of the electric current flow. At the anode side, IMC thickness, δCu6Sn5 increased remarkably, and the same was true of Kirkendall voids localized at the Cu3Sn/Cu interface. On the other hand, δCu3Sn and δCu6Sn5 were both substantially suppressed at the cathode side. Cu6Sn5 showed a strong susceptibility to the polarity effect, while Cu3Sn did not. Electromigration force induced additional tensile (or compressive) stress at the anode (or cathode), which accelerated (or decelerated) the void growth.