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

Abstract

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)Sn 4, PdSn 4, PdSn 3 and PdSn 2 are formed at the (Sn–Ni)/Pd interface in the diffusion couple. The thickness is much smaller for the (Pd, Ni)Sn 4, PdSn 3 and PdSn 2 layers than for the PdSn 4 layer. Therefore, the growth of the compound layers is governed by PdSn 4. The square of the total thickness l of the compound layers is almost proportional to the annealing time t as follows: l 2 = Kt, where K is the parabolic coefficient. This relationship is called the parabolic relationship. Since grain growth occurs at certain rates in the PdSn 4 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)Sn 4. 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.