Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates
To simulate the growth of Ni3Sn4 phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu1−xNix)6Sn5/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)3Sn4 and Ni3Sn4 layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)3Sn4 layer was formed at the (Cu,Ni)6Sn5/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)3Sn4 layer growing predominantly at the (Ni,Cu)3Sn4/Ni interface by fast diffusion of Sn across the (Ni,Cu)3Sn4 layer. It is proposed that the accelerated growth of the (Ni,Cu)3Sn4 and Ni3Sn4 layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.