Characterization of the reaction process in diffusion-soldered Cu/In–48 at.% Sn/Cu joints

This work describes a Pb-free solder alternative for the interconnection technology and its implementation in a diffusion soldering technique: In–48 at.% Sn solder (eutectic alloy), with a melting point of 120 °C. The system proposed has the advantages of both traditional soldering and diffusion bonding, i.e., good joint filling, high service temperature, and good mechanical properties. The diffusion reaction processes in Cu/In–48 at.% Sn/Cu joints were investigated between 180 and 400 °C. Electron microprobe analysis revealed the presence of one or two intermetallic layers in the interconnection zone: a layer of the η phase below 200 °C, and layers of the η and ζ phases above 200 °C. The η and ζ phases form through a solid–liquid and a solid–solid diffusion reaction, respectively. Below 200 °C the η phase exhibits two different morphologies: large coarse grains at the η/(originally liquid)In–48 at.% Sn interface and a fine-grained region at the Cu/η interface. The thickness of the ζ layer shows a constant growth rate (linear growth) at constant temperature. The temperature dependence of the growth rate constant of the ζ layer is described by an Arrhenius relationship with an activation energy equal to 121 kJ/mol and a pre-exponential factor of about 57 m/s.