Electromechanical Coupling in Sn-Rich Solder Interconnects

Electromechanical coupling in Sn-based solder interconnects was investigated by examining the effects of the electromigration on the mechanical behavior of microsized Sn-3.5Ag-0.7Cu, pure Sn and single-crystal Sn solder interconnects. Prior to mechanical loading, electromigration induced grain-boundary grooving, Sn hillock formation, Cu6Sn5 formation, and the wave-like surface relief on the solder surface. The tensile tests following electromigration showed that the strength of the pure Sn and Sn-3.5Ag-0.7Cu interconnects rapidly decreased with the electromigration time, while the strength of the Sn single-crystal interconnects remained relatively stable. From the stress-relaxation tests, it is shown that the stress-relaxation rate of the Sn-Ag-Cu and pure Sn solder interconnects were enhanced by the electromigration, as the stress-relaxation rate increased with the electromigration time. In the polycrystalline pure Sn, the reduced resistance to deformation was related to the Sn grain tilting or sliding as evident from the grain-boundary grooving. Such a grainboundary effect disappeared in the single-crystal Sn sample. For the Sn-3.5Ag-0.7Cu solder interconnect, the current-induced softening and the enhanced stress-relaxation rate is explained in terms of the dislocation interaction with excess vacancies produced by electromigration.