Highly thermostable joint of Cu/Ni plating/composite Sn–0.7Cu solder with added Cu balls for die attachment in power modules

Abstract

Improving the reliability of solder joints for die attachment in power modules is one of the most important issues in creating environmentally friendly vehicles such as hybrid electric vehicles. Power modules must have highly reliable solder joints that must be thermostable at temperatures over 175 °C in the future. In die attachment, soldering surfaces are often finished with Ni plating, so for Cu/Ni plating/Sn–Cu solder joints it is necessary to suppress both Ni diffusion into the solder as well as growth of the (Ni,Cu)3Sn4 intermetallic compound (IMC). Ni diffusion in Ni plating can be suppressed by the presence of a continuous (Cu,Ni)6Sn5 IMC layer at the Ni plating/solder interface. To form this IMC, we investigated the interfacial reactions and growth behavior of IMC layers in the presence of composite Sn–0.7Cu solder with added Cu balls. Adding 2.5 mass% of Cu balls prompted the formation of a continuous (Cu,Ni)6Sn5 IMC layer at both the electroless Ni–P and the electrolytic Ni plating, and this IMC layer worked well as a Ni diffusion barrier during a high-temperature storage test at 200 °C for 1000 h.