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 influences 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.