Fast concurrent growth of Ni ₃Sn ₄ and voids during solid-state reaction between Sn-rich solder and Ni substrates

To simulate the growth of Ni ₃Sn ₄ phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu _1-x Ni _x ) ₆Sn ₅/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu) ₃Sn ₄ and Ni ₃Sn ₄ layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu) ₃Sn ₄ layer was formed at the (Cu,Ni) ₆Sn ₅/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) ₃Sn ₄ layer growing predominantly at the (Ni,Cu) ₃Sn ₄/Ni interface by fast diffusion of Sn across the (Ni,Cu) ₃Sn ₄ layer. It is proposed that the accelerated growth of the (Ni,Cu) ₃Sn ₄ and Ni ₃Sn ₄ 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.