Power modules are used to operate three-phase alternative motors in hybrid vehicles and electric vehicles. The good fuel efficiency and high power density are requested in the field of hybrid vehicles. To achieve this goal, the miniaturization of power module will be necessary. This trend may make current density, which is operated 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 which exchanges momentum from electrons to metallic atoms .This phenomenon may cause delamination of the joint area and void formation. 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. But a current density of the double-sided cooling power modules with solder joint in 2007 is lower than 0.4 kA/cm2. 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 not only the current density but the temperature has 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 in Ni plating was different between a cathode side and an anode side. The diffusion ratio of a cathode side was higher than that of an anode side. In addition, the thickness of intermetallic compound (IMC) was thicker at an anode side than that at a cathode side. The electromigration mechanism of Ni plating/Sn-0.7Cu based joint system with a current density lower than 10 kA/cm2 was clarified.