Growth behavior of Ni3Sn4 layer during reactive diffusion between Ni and Sn at solid-state temperatures

The solid-state reactive diffusion between Ni and Sn was experimentally studied using Sn/Ni/Sn diffusion couples. The diffusion couples were prepared by a diffusion bonding technique and then annealed at temperatures between T = 433 and 473 K for various times in an oil bath with silicone oil. At the Ni/Sn interface in the annealed diffusion couple, a rather wavy layer of Ni3Sn4 was observed at T = 453 and 473 K, but granular particles of Ni3Sn4 were recognized at T = 433 K. The average thickness l of the Ni3Sn4 layer monotonically increases with increasing annealing time t according to the equation l = k(t/t0)n, where t0 is unit time, 1 s. The experimental results provide n = 0.46 and 0.41 at T = 453 and 473 K, respectively. The values of n smaller than 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. Under the present annealing conditions, the thickness of the Ni3Sn4 layer is more than one order of magnitude smaller than the total thickness of the AuSn4, AuSn2 and AuSn layers formed by the reactive diffusion between Au and Sn. This indicates that the Ni layer is an effective inhibitor against the reactive diffusion between the Sn-base solder and Au/Ni/Cu multilayer conductor alloys during annealing at solid-state temperatures after complete dissolution of the Au layer into the molten solder.