TEM observations of the growth of intermetallic compounds at the SnBi/Cu interface

The microstructure of the eutectic SnBi/Cu interface was investigated by transmission electron microscopy to study the growth mechanisms of the intermetallic compounds (IMCs). Although the growth kinetics of the total IMC layer were similar, the individual Cu ₃Sn layer grew faster on polycrystalline Cu than on single-crystal substrates. It was found that, on polycrystalline Cu, newly formed Cu ₃Sn grains with a smaller grain size nucleated and grew at both the Cu/Cu ₃Sn and Cu ₃Sn/Cu ₆Sn ₅ interfaces during reflow and solid-state aging. The consumption of Cu ₆Sn ₅ to form Cu ₃Sn was faster at the Cu ₃Sn/Cu ₆Sn ₅ interface. While on single-crystal Cu new Cu ₃Sn grains nucleated only at the Cu/Cu ₃Sn interface, the directional growth of the initial columnar Cu ₃Sn controlled the advance of the Cu ₃Sn/ Cu ₆Sn ₅ interface.