Isothermal solidification of Cu/Sn diffusion couples to form thin-solder joints

Isothermal solidification of conventional Cu/Sn diffusional couples was performed to form thin (30 μm) joints consisting of Cu-Sn intermetallics. During initial stages of isothermal solidification, both Cu₆Sn₅ and Cu₃Sn phases grow, even though the former is the dominant. After consumption of all available Sn, the Cu₃Sn phase grows reactively at the expense of Cu and Cu₆Sn₅. Finally, we obtain solder joints that consist of only Cu₃Sn. Indentation fracture-toughness measurements show that Cu₃Sn is superior to Cu₆Sn₅. Furthermore, indentations of Cu₃Sn exhibit the presence of shear bands, which are not observed in Cu₆Sn₅, implying that the former is more ductile than the latter. Ductile intermetallic-based joints formed by isothermal solidification are promising candidates to form thin (as thin as 5–10 μm or less) solder joints, as they are thermally and thermodynamically stable compared to conventional solder joints. Excess copper in the interconnect provides ductility to the interconnect.