Decomposition of Cu₆Sn₅ particles in solder for the growth of a ternary (Cu_1-xNi_x)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 (Cu_1-xNi_x)₆Sn₅ layer to grow at the solder/Ni interface during solid-state aging. The interaction between the (Cu,Ni)₆Sn₅ particles and the (Cu_1-xNi _x)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 (Cu_1-xNi _x)₆Sn₅/Ni interface only after the Ni composition of the (Cu,Ni)₆Sn₅ phase in the bulk solder approaches that of the (Cu_1-xNi_x)₆Sn ₅ layer. Once the (Ni,Cu)₃Sn₄ layer has formed, it grows at an exceptionally rapid rate by consuming the (Cu _1-xNi_x)₆Sn₅ and Sn layers, which can be problematic in solder joint reliability.