Formation of bulk Cu6Sn5 intermetallic compounds in Sn–Cu lead-free solders during solidification

The influence of cooling rate and Cu content on the microstructures of the solidified Sn–Cu alloys was investigated in terms of microstructural observation, thermal analysis and theoretical calculation. It was found that bulk Cu6Sn5 intermetalic compounds (IMCs) formed only in the Sn–1.0Cu alloy with lower cooling rate. At higher cooling rate, the actual eutectic point of Sn–Cu solder alloy shifts to the direction of higher Cu concentration. Thus the eutectic and hypereutectic alloys experience a metastable pseudoeutectic solidification route instead. The volume fraction of the bulk Cu6Sn5 IMCs formed in the Sn–1.0Cu alloy, measured by thermal analysis, is larger than that predicted by the equilibrium phase diagram. This could be attributed to eutectic Cu6Sn5 phases, which cling to the primary Cu6Sn5 crystal during the eutectic reaction due to their matching crystalline orientation relationship.