Kinetics of solid-state reactive diffusion between Sn-Ni alloys and Pd

The kinetics of the solid-state reactive diffusion in the (Sn-Ni)/Pd system was experimentally studied to examine the influence of Ni on the growth behavior of compounds at the interconnection between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating. Diffusion couples composed of pure Pd and binary Sn-Ni alloys with y = 0.01, 0.03 and 0.05 were prepared by a diffusion bonding technique, and then isothermally annealed at temperatures of T = 453 K and 473 K for various times up to 325 h. Here, y is the mol fraction of Ni in the Sn-Ni alloy. Owing to annealing, compound layers of (Pd, Ni)Sn4, PdSn4, PdSn3 and PdSn2 are formed at the (Sn-Ni)/Pd interface in the diffusion couple. The thickness is much smaller for the (Pd, Ni)Sn4, PdSn3 and PdSn2 layers than for the PdSn4 layer. Therefore, the growth of the compound layers is governed by PdSn4. The square of the total thickness l of the compound layers is almost proportional to the annealing time t as follows: l2 = Kt, where K is the parabolic coefficient. This relationship is called the parabolic relationship. Since grain growth occurs at certain rates in the PdSn4 layer, the parabolic relationship guarantees that the growth of the compound layers is controlled by volume diffusion. At T = 473 K, K is insensitive to y. On the other hand, at T = 453 K, K remarkably decreases with increasing value of y, and reaches to the minimum at y = 0.03. Such dependencies of K on y and T are attributed to the formation of (Pd, Ni)Sn4. Hence, the addition of Ni with y = 0.03 into the Sn-base solder considerably suppresses the growth of compounds at the interconnection between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during solid-state energization heating unless the heating temperature exceeds 453 K.