Growth Behavior of Au–Sn and Ag–Sn Compounds during Solid-state Reactive Diffusion between Au–Ag Alloys and Sn

Abstract

The kinetics of the solid-state reactive diffusion between binary Au–Ag alloys and Sn was experimentally examined using Sn/Au0.75Ag0.25/Sn and Sn/Au0.5Ag0.5/Sn diffusion couples. The diffusion couples were prepared by a diffusion bonding technique and then isothermally annealed at temperatures of T=393, 433 and 473 K for various times up to 1272 h in an oil bath with silicone oil. Under the present experimental conditions, AuSn4 and AuSn2 compound layers were observed after annealing. Furthermore, fine particles of Ag3Sn were rather uniformly distributed in the Au–Sn compound layers. The total thickness l of the Au–Sn compound layers is expressed as a power function of the annealing time t as follows: l=k(t⁄t0)n, where t0 is unit time, 1 s. Here, the exponent takes values of n=0.34–0.40. The mean interdistance r of the Ag3Sn particles is also described as a power function of t: r=kr(t⁄t0)p, where p=0.28–0.43. Assuming that the interdistance r varies in proportion to the grain size of the Au–Ag compound during annealing, the rate-controlling process of the reactive diffusion was estimated. If the grain boundary diffusion across the Au–Sn compound layers is the only rate-controlling process, the values of n calculated from the equation n=(1−p)⁄2 become smaller than the experimental values of n=0.34–0.40. Consequently, both the volume diffusion and the grain boundary diffusion should contribute to the rate-controlling process of the reactive diffusion.