Electromigration Analysis of Solder Joints for Power Modules Using an Electrical-Thermal-Stress Coupled Model

Abstract

Power modules are being developed with the aim of increasing power output. Achieving this aim requires increased current density in power modules. However, at high current densities, power modules can degrade as a result of electromigration, which is a phenomenon where atoms move due to momentum transfer between conducting electrons and metal atoms. In addition, atoms are also moved by mechanical stress gradients and temperature gradients, so it is necessary to consider the combined effects of electrical, thermal, and mechanical stress. This report describes an electromigration analysis of solder joints for power modules. First, we validated our numerical implementation and showed that it could reproduce the distributions of vacancy concentrations and hydrostatic stress that were almost the same as those in previous studies. We then describe the effects of electromigration in a single solder joint. Due to the appearance of plastic and creep strains, the rate of increase in vacancy concentration was very slow and inelastic strain grew at an increasing rate. This result indicates that inelastic properties may strongly affect electromigration-induced degradation. Next, we present results for the solder joint with a SiC device and substrate. A current crowding appeared at the edge of the solder joint, and a vacancy concentration gradient was generated in not only the thickness direction but also the longitudinal direction. The absolute value of vacancy concentration increased significantly at the edge and did not reach a steady state even after a long time. These results indicate that peripheral components may strongly affect the electromigration-induced degradation. In addition, we modeled the behavior of metal atoms passing through the interface of the solder joint and simulated the growth of the intermetallic layer by electromigration.