Nickel–phosphorus alloys were electrolessly deposited onto Cu-base conductors to suppress formation of Cu–Sn compounds during soldering using Sn-base solders. However, a Ni–Sn compound was produced during soldering, and continuously grew during energization heating at solid-state temperatures. To examine influence of P on the growth behavior of the Ni–Sn compound during energization heating, the kinetics of the solid-state reactive diffusion between Sn and electroless Ni–P alloys was experimentally determined at 473K in the present study. For the experiment, pure Cu sheets were electrolessly deposited with Ni–P alloys containing 4.6, 18.5 and 20.4at%P, and then sandwiched between pure Sn plates. Such Sn/(Ni–P)/Cu/(Ni–P)/Sn diffusion couples were isothermally annealed at 473K for various periods up to 1307h. During annealing, a layer of Ni3Sn4 was formed along the Sn/(Ni–P) interface in the diffusion couple. The annealing time dependence of the mean thickness of the Ni3Sn4 layer was expressed by a parabolic relationship. The parabolic coefficient slightly increased with increasing P concentration in the Ni–P. Thus, P in the Ni–P lightly accelerates the growth of Ni3Sn4 at the interconnection between the Ni–P and the Sn-base solder. Using the experimentally determined values of the parabolic coefficient, the interdiffusion coefficient in Ni3Sn4 was analytically evaluated by a mathematical model. The acceleration effect of P on the growth of Ni3Sn4 was quantitatively explained by the dependence of the interdiffusion coefficient on the P concentration in the Ni–P.

Solid-State Reactive Diffusion between Sn and Electroless Ni–P at 473K. M.Yamakami, M.Kajihara: Materials Transactions, 2009, 50[1], 130-7