Reactive diffusion between Ag–Au alloys and Sn at solid-state temperatures

The kinetics of the solid-state reactive diffusion between binary Ag–Au alloys and Sn was experimentally studied using Sn/Ag0.87Au0.13/Sn, Sn/Ag0.75Au0.25/Sn and Sn/Ag0.63Au0.37/Sn diffusion couples. The diffusion couples were prepared by a diffusion bonding technique and then isothermally annealed at temperatures of 433, 453 and 473 K for various times up to 888 h in an oil bath with silicone oil. During annealing, a layer of AuSn4 distributed with particles of Ag3Sn is produced at each interface in the Sn/Ag0.63Au0.37/Sn diffusion couple. In contrast, AuSn4 and Ag3Sn layers are formed in the Sn/Ag0.75Au0.25/Sn diffusion couple, but only the Ag3Sn layer is produced in the Sn/Ag0.87Au0.13/Sn diffusion couple. For these compound layers, the total thickness is expressed as a power function of the annealing time. The exponent of the power function is close to 0.5 for the Sn/Ag0.87Au0.13/Sn and Sn/Ag0.75Au0.25/Sn diffusion couples. Thus, the growth of the compound layer is controlled by volume diffusion in each phase for these diffusion couples. On the other hand, the exponent takes values around 0.4 for the Sn/Ag0.63Au0.37/Sn diffusion couple. For this diffusion couple, the mean interdistance of the Ag3Sn particles was experimentally determined as a function of the annealing time. The annealing time dependence of the interdistance implies that grain growth occurs at certain rates in the compound layer and the growth of the compound layer is controlled by the grain boundary diffusion across the compound layer.