The reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures using Sn/Au/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were annealed at 393 and 473K for various times in an oil bath with silicone oil. After annealing, compound layers composed of AuSn4, AuSn2 and AuSn were recognized to form at the Au/Sn interface. The thickness of the AuSn4 layer was about 6 and 4 times greater than those of the AuSn2 and AuSn layers at 393 and 473K, respectively. The ratio of the thicknesses of the compound layers was kept constant independently of the annealing time. The total thickness l of the compound layers was described as a function of the annealing time t by the equation l = k(t/t0)n, where t0 was unit time, 1s. The exponent n was nearly equal to 1/2 at 393K but takes a value between 1/4 and 1/2 at 473K. Such an intermediate value of n at 473K indicates that the grain boundary diffusion contributes to the reactive diffusion and the grain growth occurred at certain rates. As the annealing temperature decreased, the contribution of the grain boundary diffusion should become more remarkable, but the grain growth will slow down. Consequently, n became close to 1/2 at 393K. According to the constancy of the ratio of the thicknesses, it was concluded that the same rate-controlling process works in the AuSn4, AuSn2 and AuSn layers at a constant annealing temperature.

Kinetics of Reactive Diffusion between Au and Sn during Annealing at Solid-State Temperatures. T.Yamada, K.Miura, M.Kajihara N.Kurokawa, K.Sakamoto: Materials Science and Engineering A, 2005, 390[1-2], 118-26