The fast formation of full Cu₃Sn solder joints in Cu/Sn/Cu system by thermal gradient bonding

To form full intermetallic compounds (IMCs) solder joints becomes widely available for the die bonding of the third generation semiconductor power devices. The fast formation of Cu₃Sn in Cu/Sn(10 µm)/Cu solder joints were investigated by thermal compression bonding in a few seconds and under a low pressure of 0.1 MPa at ambient temperature. The results show that the temperature gradient produced by thermal compression bonding contributes to enhancing the interfacial reaction at the liquid Sn/solid Cu metallization interface. The scallop-liked Cu₆Sn₅ is formed while planar-liked Cu₃Sn is formed between Cu and Cu₆Sn₅ at initial bonding stage. After that, the growth rate of scallop-liked Cu₆Sn₅ layer from cold end is faster than that from hot end and the thin planar-liked Cu₃Sn layer becomes thick. This abnormal growth behavior of Cu₆Sn₅ is due to the fact that Cu atoms migrate from hot end to cold end by temperature gradient. The middle Sn layer is completely consumed with increasing bonding time. Cu₆Sn₅ and Cu₃Sn are consisted of the solder joints and the planar-liked Cu₃Sn grows with a transition to scallop-liked morphology until the full Cu₃Sn solder joint is eventually formed, which indicates that the formation time of full IMCs solder joints can be narrowed by temperature gradient. This bonding process provides a new solution for the rapid acquisition of interconnect material for the third generation semiconductor power devices.