Growth behavior of compounds during reactive diffusion in the Solid-Cu/Liquid-Sn system

Abstract

Semi-infinite Cu/Sn diffusion couples prepared by an isothermal bonding technique were used to examine experimentally the kinetics of reactive diffusion in the solid-Cu/liquid-Sn system. Isothermal annealing of the diffusion couple was conducted in the temperature range of T= 753-793 K for various periods up to t= 144 ks (40 h). Owing to annealing, an intermetallic layer composed of ϵ-Cu3Sn with scallop morphology and δ-Cu4Sn with rather uniform thickness is formed at the original Cu/Sn interface in the diffusion couple. The total thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is close to unity at all the annealing temperatures. Such a power relationship holds also for the ϵ-Cu3Sn scallop and the δ-Cu4Sn layer. This means that volume diffusion controls the growth of the ϵ-Cu3Sn scallop and the morphology of the Cu3Sn/Sn interface infiuences the rate-controlling process. In contrast, the growth of the δ-Cu4Sn layer is governed by the interface reaction at the moving Cu4Sn/Cu interface. Adopting a mean value of 0.87 for the exponent, we obtain a value of 129 kJ/mol for the activation enthalpy of the intermetallic growth.