Influence of Pd on kinetics of solid-state reactive diffusion between Sn and Ni

In order to examine the growth behavior of compounds at the interconnection between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating, the influence of Pd on the kinetics of the reactive diffusion between Sn and Ni was experimentally observed at solid-state temperatures. In the experiment, diffusion couples consisting of pure Ni and binary Sn–Pd alloys with Pd concentrations of 1 at.%, 3 at.% and 5 at.% were prepared by a diffusion bonding technique, and then isothermally annealed at temperatures of 453 K and 473 K for various times up to 768 h. At these annealing temperatures, the Sn–Pd alloy is composed of the β–Sn matrix dispersed with coarse blocks of PdSn4. During annealing, compound layers of (Pd, Ni)Sn4 and Ni3Sn4 are formed at the (Sn–Pd)/Ni interface in the diffusion couple. The thickness is slightly greater for (Pd, Ni)Sn4 than for Ni3Sn4. The square of the total thickness of the compound layers is proportional to the annealing time. Hence, the growth of the compound layers is controlled by volume diffusion. The proportionality coefficient is one order of magnitude greater for 1–5 at.% Pd than for 0 at.% Pd. Consequently, PdSn4 in the Sn-base solder considerably accelerates the solid-state reactive diffusion between the solder and the Ni layer of the Pd/Ni/Cu conductor during energization heating.