Electromigration behaviors in Sn–58Bi solder joints under different current densities and temperatures

Abstract

Sn–58Bi eutectic solder is attracted much attention to replace Sn–Ag–Cu Pb-free solder due to its lower melting temperature in recent years. However, its low melting temperature also raised serious electromigration (EM) reliability due to the accelerated atomic migration from the high local temperature induced by Joule heating. This paper mainly studied the EM behaviors occurred in Sn–58Bi solder joints with consideration the effect of different current densities and temperature. Variation on the atomic migration and phase coarsening were observed in Sn–58Bi solder joint, and the threshold current densities to trigger EM damage were estimated. The results showed that Bi-rich layer was produced at anode side during stressing with low current density while caused Cu cathode to dissolve into Sn–Bi solder and formed Cu6Sn5 intermetallic compounds near anode side during stressing with high current density. Accordingly, the corresponding dominant diffusing species in Sn–58Bi solder were Bi atoms at low temperature and Cu and Sn atoms at high current density, respectively. High temperature applied with EM accelerated the atomic migration. The threshold current densities at different temperature were then calculated from the thickness of Bi-rich layer at anode side. There existed a lower threshold current density at higher temperature due to the active atomic mobility. Finally, Bi phase coarsening was observed in Sn–Bi solder under different current densities and temperatures. Higher current density or higher temperature induced faster grain growth, but the applied current had a greater effect on Bi phases coarsening in Sn–Bi solder matrix than the applied temperature. The controlling kinetics on the growth of Bi phases were also suggested for the effect of current densities and temperatures.