Solid-State Reactive Diffusion between Sn and Electroless Ni–P at 473 K

Abstract

Nickel–phosphorus alloys are electrolessly deposited onto Cu-base conductors to suppress formation of Cu–Sn compounds during soldering using Sn-base solders. However, a Ni–Sn compound is produced during soldering, and continuously grows 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 473 K in the present study. For the experiment, pure Cu sheets were electrolessly deposited with Ni–P alloys containing 4.6 at%, 18.5 at% and 20.4 at% of P, and then sandwiched between pure Sn plates. Such Sn/(Ni–P)/Cu/(Ni–P)/Sn diffusion couples were isothermally annealed at 473 K for various periods up to 1307 h. During annealing, a layer of Ni3Sn4 is formed along the Sn/(Ni–P) interface in the diffusion couple. The annealing time dependence of the mean thickness of the Ni3Sn4 layer is expressed by a parabolic relationship. The parabolic coefficient slightly increases 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 is quantitatively explained by the dependence of the interdiffusion coefficient on the P concentration in the Ni–P. [doi:10.2320/matertrans.MRA2008266]