The larger current densities accompanying increased output of power modules are expected to degrade solder joints by electromigration. Although previous research has experimentally studied electromigration in solder, die-attach solder joints in Si-based power modules have not been studied because the average current density of the die-attach solder is much smaller than the threshold of electromigration degradation. However, in die-attach solder, the electromigration degradation may appear where current crowding occurs. This report describes electromigration analysis of die-attach solder joints for Si-based power modules using an electrical-thermal-stress-atomic coupled model. First, we validate our numerical implementation and show that it can reproduce the distributions of vacancy concentrations and hydrostatic stress almost the same as the analytical solutions even at current densities assuming current crowding. We then simulate the die-attach solder joint with an Si-based power device and a substrate. Due to current crowding, the current density at the edge of the solder exceeds the electromigration threshold. Unlike general electromigration phenomena, the vacancy concentration increases at the center and decreases at the edges of the solder joint, regardless of whether it is on the cathode side or anode side, due to the longitudinal driving force in the solder joint generated by the current crowding. Creep strain increased remarkably at the anode edge and the cathode center. The absolute vacancy concentration clearly increased with increasing current density and size ratio. Creep strain significantly increased with increasing current density, size ratio, and temperature.