Growth behavior and formation mechanism of porous Cu3Sn in Cu/Sn solder system

It is the first time that Cu/Sn films were used to replace micro-bumps to investigate the growth behavior and formation mechanism of porous Cu3Sn in Cu/Sn solder system under high temperature storage. 10 mm × 10 mm Cu/Sn films and Cu/Sn/Cu bonded films were prepared by electroplating and non-flux reflow, which effectively avoids the influence of fluxes, water vapor, size effect and surface diffusion at the sidewall. Scanning electron microscopy (SEM) results show that intermetallic compounds (IMCs) evolution at the interface between Cu substrate and tin solder was scallop-liked Cu6Sn5 to layered Cu3Sn to porous Cu3Sn. The critical thickness of layered Cu3Sn depends on the temperature, structure of Cu/Sn system. Pores in Cu3Sn were caused by Molar volume difference in the decomposition of Cu6Sn5. For different aging temperatures, two possible lattice transformation models from η'-Cu6Sn5 to Cu3Sn and η-Cu6Sn5 to Cu3Sn were established. Based on the results, growth model of porous Cu3Sn in Cu/Sn solder system was proposed.