Formation of intermetallic compound layers in Sn/Au/Sn diffusion couple during annealing at 433 K

The kinetics of the reactive diffusion between Au and Sn was experimentally studied in a metallographical manner. Sn/Au/Sn diffusion couples were prepared by a solid-state diffusion bonding technique and then annealed at 433 K for various times between 1 and 127 h (3.6 × 103 and 4.57 × 105 s) in an oil bath with silicone oil. Due to annealing, intermetallic compound layers consisting of AuSn4, AuSn2 and AuSn are produced at the Au/Sn interface in the diffusion couple. The thickness of the AuSn4 layer is four times greater than those of the AuSn2 and AuSn layers. The ratio of the thicknesses of the compound layers remains constant during annealing. The total thickness l of the compound layers increases with annealing time t. Such a relationship is mathematically described by the equation l = k(t/t0)n, where t0 is unity time, 1 s. From the experimental results, the proportionality coefficient and the exponent are determined to be k = 2.7 × 10−7 m and n = 0.42, respectively, by a least-squares method. The exponent n is slightly smaller than 0.5. This implies that the grain boundary diffusion partially contributes to the rate-controlling process for the growth of the compound layers.