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 Cu3Sn layer grew faster on polycrystalline Cu than on single-crystal substrates. It was found that, on polycrystalline Cu, newly formed Cu3Sn grains with a smaller grain size nucleated and grew at both the Cu/Cu3Sn and Cu3Sn/Cu6Sn5 interfaces during reflow and solid-state aging. The consumption of Cu6Sn5 to form Cu3Sn was faster at the Cu3Sn/Cu6Sn5 interface. While on single-crystal Cu new Cu3Sn grains nucleated only at the Cu/Cu3Sn interface, the directional growth of the initial columnar Cu3Sn controlled the advance of the Cu3Sn/Cu6Sn5 interface.