Growth behavior of compound layers in Sn/Cu/Sn diffusion couples during annealing at 433–473K

Abstract The kinetics of the reactive diffusion between Cu and Sn at solid-state temperatures was experimentally examined using Sn/Cu/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures between T  = 433 and 473 K for various times in an oil bath with silicone oil. Due to annealing, compound layers consisting of Cu 3 Sn and Cu 6 Sn 5 are formed at the Cu/Sn interface. In most of the annealed diffusion couples, grains of CuSnO exist in the Cu 6 Sn 5 layer. However, the CuSnO grain scarcely influences the growth behavior of the compound layers. The thickness of the Cu 6 Sn 5 layer is around twice greater than that of the Cu 3 Sn layer. The difference between their thicknesses slightly increases with increasing annealing temperature. At a constant temperature, the ratio of the thicknesses remains constant independent of the annealing time. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to equation l  =  k ( t / t 0 ) n , where t 0 is unit time, 1 s. The observations provide n  = 0.37, 0.43 and 0.50 at T  = 433, 453 and 473 K, respectively. The value n  = 0.5 indicates that the volume diffusion is the rate-controlling process for the reactive diffusion at T  = 473 K. As the annealing temperature decreases, the grain boundary diffusion will contribute to the rate-controlling process. Thus, the values of n smaller than 0.5 yield that there is the contribution of the grain boundary diffusion and the grain growth occurs at certain rates at T  = 433 and 453 K. Since the ratio between the thicknesses of the compound layers is kept constant during isothermal annealing, the same rate-controlling process is expected to work in both compound layers.