Solder joint reliability in power modules is one of the most important issues for hybrid, electric, and fuel cell vehicles; these modules must have highly reliable solder joints, i.e., they must be highly thermostable at temperatures over 175 A degrees C in the future. The soldering surfaces in power modules are often finished with electroless Ni-P plating. Thus, for Cu/Ni-P plating/Sn-0.7Cu joints, it is necessary to suppress Ni diffusion into the solder. Ni diffusion can be suppressed in the presence of a continuous Cu6Sn5 intermetallic compound (IMC) layer at a Ni-P plating/solder interface. To form this IMC, we investigated the composite Sn-0.7Cu solder added with Cu balls. It was confirmed that the addition of 2.5 wt% Cu balls formed a continuous (Cu, Ni)(6)Sn-5 IMC layer between the solder and the Ni-P plating. It is concluded that the IMC layer works well as a Ni diffusion barrier in multiple reflow tests, of which the peak temperature was 330 A degrees C, and in a high-temperature storage test at 200 A degrees C for 1000 h.