Power modules are used to operate three-phase alternating current motors in hybrid vehicles and electric vehicles. Good fuel efficiency and high power density are required in the field of hybrid vehicles. To achieve this goal, the miniaturization of the power module will be necessary. This trend may make a current density, which is created by insulated gate bipolar transistors (IGBTs) and free wheel diodes (FWDs), higher in power modules. Solder is often used as the joint material of power modules. It is known that a current density larger than 10 kA/cm2 causes solder electromigration. This phenomenon may cause delamination of the joint area. In addition, the ambient temperature has an influence on electromigration. The temperature of an engine compartment is high, so it is likely to cause electromigration. However, the current density of the double-sided cooling power modules in 2007 with solder joint is lower than 0.4 kA/cm2, and this value is lower than 10 kA/cm2. This current density is not so severe to the solder joint system. Black’s equation shows that current density and the temperature have an effect on the mean time to failure (MTTF). We investigated mechanisms of electromigration when applying a current density lower than 10 kA/cm2. In this research, the solder joint system was composed of Ni plating/Sn-0.7Cu. The diffusion ratio of Ni was different between the cathode side and the anode side. The diffusion ratio of the cathode side was higher than that of the anode side. In addition, the thickness of the intermetallic compound (IMC) was larger at the anode side than that at the cathode side. The electromigration of Ni plating/Sn-0.7Cu based joint system with a current density lower than 10 kA/cm2 was clarified.