Highly thermostable joint of a Cu/Ni–P plating/Sn–0.7Cu solder added with Cu balls

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 °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)₆Sn₅ 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 °C, and in a high-temperature storage test at 200 °C for 1000 h.