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/ Au 0 . 7 5 Ag 0 . 2 5 /Sn and Sn/Au 0 . 5 Ag 0 . 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, AuSn 4 and AuSn 2 compound layers were observed after annealing. Furthermore, fine particles of Ag 3 Sn were rather uniformly distributed in the Au-Sn compound layers. The total thickness I of the Au-Sn compound layers is expressed as a power function of the annealing time t as follows: l = k(t/t 0 ) n , where to is unit time, 1 s. Here, the exponent takes values of n = 0.34-0.40. The mean interdistance r of the Ag 3 Sn particles is also described as a power function of t: r = k r (t/t 0 ) 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.