Cu6Sn5 and Cu3Sn lntermetallics Study in the Sn-40Pb/Cu System during Long-term Aging

Abstract

Replacing Sn-Pb solder with lead-free solder is a great challenge in the electronics industry. The presented lead-free solder is Sn based and forms two intermetallic species upon reaction with the Cu substrate, namely Cu₆Sn₅ and Cu₃Sn. The growth of Cu₆Sn₅ and Cu₃Sn intermetallics have been investigated with respect to Sn-40Pb/Cu solder joints. The joints were aged under long-term thermal exposure using single shear lap joints and the intermetallics were observed using scanning electron microscopy. As-soldered solder joints exhibit a single Cu₆Sn₅ phase, however after aging a Cu₃Sn layer below the Cu₆Sn₅ is observed to manifest. The Cu₆Sn₅ layer develops with a scalloped morphology, whereas the Cu in layer always develops an undulating planar shape in phase with the Cu₆Sn₅. The Cu₆Sn₅ layer begins to transform from a scalloped- to a planar-shape as aging progresses in order to minimize the interfacial energy. The intermetallic layers exhibit a linear dependence on the square root of aging time, which corresponds to diffusion-controlled growth. The activation energy for the growth of the Cu₆Sn₅ intermetallic layer has been determined to be 56.16 kJ/mol.