Influence of Interdiffusion in Sn Solution on Growth of Cu3Sn and Cu6Sn5 Formed in Semi-infinite and Finite Cu-Sn Diffusion Couples

Reactive diffusion in the Cu-Sn binary system has been studied by using pure Cu/electrically plated Sn with 0.1-0.2 mm thicknesses diffusion couples (EP-couples) at 473 K. The interdiffusion coefficients, \(\tilde{D}\), of the Cu3Sn and Cu6Sn5 diffusion phase layers were determined at the center of these layers by supposing linear concentration (\(C_{\text{i}}\))-distance (X) curves in these layers and by neglecting the interdiffusion in the Sn terminal solution (IDS) as the previous researchers have neglected it. By using \(\tilde{D}\) thus determined, the phase boundary concentrations for the layers obtained in this work and these parameters for the Cu terminal solution chosen appropriately, \(C_{\text{i}}\)-X curves were determined numerically for various values of interdiffusion coefficient, \(\tilde{D}_{\text{in Sn}}\), and the solubility limit of Cu mole fractions, \(N_{\text{Cu}}^{\text{in Sn}}\), in the Sn terminal solution by our method reported previously taking the molar volume change effect into account. The \(C_{\text{i}}\)-X curves obtained experimentally could be reproduced numerically well by neglecting IDS. This result, on the other hand, suggests a large influence of IDS in the semi-infinite diffusion couples (S-couples) or the diffusion couples used by the previous researchers. The quantitative evaluation of the influence in S-couples revealed that it makes the widths of the diffusion layers thinner than those in the present EP-couples in which the influence on the widths is negligibly small. The evaluation of the influence in the diffusion couples used by the previous researchers indicates larger values of \(N_{\text{Cu}}^{\text{in Sn}}\) than those reported as the value of the equilibrium phase diagram.