Kinetics of Solid-State Reactive Diffusion in the (Ni-Cr)/Sn System

The kinetics of solid-state reactive diffusion in the (Ni-Cr)/Sn system was experimentally observed to examine the influence of the addition of Cr into Ni on the growth behavior of compounds at the interconnection between Sn-based solder and multilayer Au/Ni/Cu conductor during energization heating. In the experiment, Sn/(Ni-Cr)/Sn diffusion couples with Cr mol fractions of y = 0.051 to 0.282 were isothermally annealed at temperatures of T = 433 K to 473 K for various times up to t = 3167 h. During annealing at T = 453 K to 473 K, a two-phase compound layer with rather uniform thickness consisting of Ni3Sn4 and CrSn2 is produced at the initial (Ni-Cr)/Sn interface in the diffusion couple, where the compound layer indicates a periodic lamellar microstructure parallel to the initial interface typically for y = 0.150. On the other hand, at T = 433 K, Ni3Sn4 and CrSn2 are formed also as a two-phase layer for y = 0.117 but as granular particles for y = 0.051 and y = 0.150 to 0.282. The mean thickness l of the compound layer is proportional to a power function of the annealing time t. Since the exponent n of the power function takes values of 0.58 to 0.85, interface reaction as well as diffusion contribute to the rate-controlling process for compound growth. The overall growth rate of the compound layer reaches a maximum value at y = 0.12 within the experimental annealing times. Thus, the compound growth at the interconnection is remarkably accelerated by the addition of Cr with y = 0.12 into Ni in the multilayer Au/Ni/Cu conductor, though the addition of Cr improves the heat resistance of Ni.