The amorphous origin and the nucleation of intermetallic compounds formed at the interface during the soldering of Sn-3.0Ag-0.5Cu on a Cu substrate

A short time reflow of 15 s at 250 °C, followed by a liquid nitrogen quench, of the Sn-3Ag-0.5Cu solder on a Cu substrate gave rise to a three-layer interfacial structure. In addition to the commonly known ≠-Cu ₆Sn₅ ε-Cu₃Sn layers, an amorphous layer with a thickness of less than 50 nm exists between the Cu substrate and the thin ε- Cu₃Sn layer. Nanocrystalline ε-Cu₃Sn cells, revealed by transmission electron microscopy (TEM), as fine as 10 nm were detected within the amorphous region. The results of TEM analysis suggest that the nucleation of interfacial intermetallic compound ε-Cu₃Sn occurs within the amorphous layer.