Investigation of the diffusion behavior in Sn-xAg-yCu/Cu solid state diffusion couples

Abstract

The interfacial reactions and growth behavior of the Cu3Sn and Cu6Sn5 phases in Sn-3.0%Ag-0.5%Cu/Cu and Sn-3.5%Ag-0.7%Cu/Cu diffusion couples were investigated at 130 °C, 150 °C, 170 °C and 200 °C and compared with those in binary Sn/Cu diffusion couples using identical experimental conditions. The Ag solubility in the Cu3Sn and Cu6Sn5 phases was found to be quite small. The parabolic growth constants of Cu3Sn and Cu6Sn5 in Sn-xAg-yCu/Cu diffusion couples were calculated. A transient stage for the growth of Cu3Sn was observed. A pseudo-binary method was proposed and performed to obtain the integrated interdiffusion coefficients and activation energies for diffusion of the intermetallics with limited solubility in one of the elements. The growth of Cu3Sn in Sn-xAg-yCu/Cu diffusion couples was found to be a little slower than that in binary Sn/Cu diffusion couples. The growth rate of the Cu6Sn5 phase in ternary Sn-xAg-yCu/Cu diffusion couples is quite close to that in Sn/Cu diffusion couples. The Cu addition in the Sn-based end-member is expected to favor the growth of Cu6Sn5 providing instantly available Cu atoms for the formation of Cu6Sn5. The Ag addition is suggested to slightly decrease the thermodynamic activity of Sn and further decrease the diffusion of Sn. These interpretations were confirmed by a study of the diffusion behavior in Sn-3.5%Ag/Cu and Sn-1.0%Cu/Cu diffusion couples. The calculated activation energy for interdiffusion in the Cu3Sn and Cu6Sn5 phases in Sn-xAg-yCu/Cu diffusion couples are quite close to those obtained for binary Cu/Sn couples. The obtained conclusions are informative for the Ag-Cu-Sn solder design.