Structure-induced metastable phase transformation in Cu6Sn5 intermetallics

Abstract

The retarded η → η′ phase transformation in Cu6Sn5 has become a potential risk to the service reliability of micro-solder joints. In this paper, both Cu6Sn5 crystal and powder samples were fabricated to explore the phase transformation behaviour of Cu6Sn5. Because of the difference in stress accumulation, η′ ↔ ηc ↔ η and η′ ↔ η phase transformations occurred in the Cu6Sn5 core layers and surface layers, respectively. Moreover, formation of the metastable intermediate ηc phase was confirmed to be both stress induced and structure dependent. In addition, by decreasing the crystal size or using a proper isothermal treatment, formation of the metastable phase in Cu6Sn5 can be significantly reduced. Our study can provide an understanding of the phase transformation behaviour of Cu6Sn5 and may provide visual data for solving the risk of retarded Cu6Sn5 phase transformation.