Decomposition of Cu6Sn5 particles in solder for the growth of a ternary (Cu1−xNix)6Sn5 layer on a Ni substrate

The interaction between Cu6Sn5 particles in the bulk of a solder and a Ni substrate was examined during solid-state aging using Cu/Sn/Ni and Cu/Sn/Cu/Sn/Ni diffusion couples with initially thin Cu layers. The results clearly demonstrated that the (Cu,Ni)6Sn5 particles dispersed in the bulk solder decomposed in order for a ternary (Cu1−xNix)6Sn5 layer to grow at the solder/Ni interface during solid-state aging. The interaction between the (Cu,Ni)6Sn5 particles and the (Cu1−xNix)6Sn5 layer occurs owing to the driving force for the (Cu,Ni)6Sn5 compound to become saturated with Ni. A (Ni,Cu)3Sn4 layer forms at the (Cu1−xNix)6Sn5/Ni interface only after the Ni composition of the (Cu,Ni)6Sn5 phase in the bulk solder approaches that of the (Cu1−xNix)6Sn5 layer. Once the (Ni,Cu)3Sn4 layer has formed, it grows at an exceptionally rapid rate by consuming the (Cu1−xNix)6Sn5 and Sn layers, which can be problematic in solder joint reliability.