Interface reaction between copper and molten tin–lead solders

The formation and growth of Cu–Sn intermetallic film at the interface between molten Sn–Pb solders and Cu were studied at different temperature and exposure times. The η-phase (Cu6Sn5) was observed to form at all conditions except at the lowest Sn level of 27 wt% and at the two highest temperatures of 290 and 310°C. The var epsilon-phase (Cu3Sn) was then obtained. At high Sn contents and short times, a cellular film with a rugged interface was obtained which evolved into a compact film with a scalloped interface as the Sn content decreased and exposure time increased. The var epsilon-phase film always formed with a relatively planar interface. The intermetallic grains showed preferred crystallographic orientations. Thickness measurements showed that the net growth rate depends not only on diffusion through the film but also on the film dissolution. Precipitation of the η-phase whiskers was detected during cooling, particularly when the temperature was significantly high.