Kinetics and thermodynamics of compound growth due to reactive diffusion between solid Cu and binary Bi-Sn alloys

Abstract

The growth behavior of compounds for the reactive diffusion between solid Cu and molten binary Bi-Sn alloys was experimentally examined using semi-infinite diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of (Biy-Sn1-y)/Cu diffusion couples with Bi mol fraction of y = 0.27–0.46 was conducted in the temperature range of 453–603 K (180–330 °C) for various times up to 345.6 ks (96 h). In the diffusion couple with y = 0.44, an intermetallic layer composed of Cu6Sn5 with irregular scallop shapes and Cu3Sn with relatively uniform thickness forms at the (Bi-Sn)/Cu interface at 453–523 K (180–250 °C). In contrast, at 563–603 K (290–330 °C), the intermetallic layer consists of only Cu3Sn. Isothermal sections of the equilibrium phase diagram in the ternary Bi-Cu--Sn system were calculated at 453–603 K (180–330 °C) by a CALPHAD (calculation of phase diagrams) method. For the diffusion couple with y = 0.44, the diffusion path passes through the three-phase tie-triangles containing Cu6Sn5 and/or Cu3Sn in the isothermal section at 523 K (250 °C), but through only that containing Cu3Sn in the isothermal section at 603 K (330 °C). As a result, for y = 0.44, the intermetallic layer is composed of Cu6Sn5 and Cu3Sn at 453–523 K (180–250 °C), whereas it consists of only Cu3Sn at 563–603 K (290–330 °C). The mean thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is mostly smaller than 0.5. Such smaller values of the exponent mean that the layer growth is controlled by boundary diffusion across the intermetallic layer. Adopting the common value of 0.30 as the exponent of the power function for the Cu3Sn layer at 563–603 K (290–330 °C), we obtain a value of 17.9 kJ/mol for the activation enthalpy of the proportionality coefficient of the power function.