Interfacial reaction and IMC growth between the undercooled liquid lead-free solder and Cu metallization

Based on the size effect on solidification behavior of lead-free solders, we propose a novel method to prepare solder joints with the metallurgical bonding between the undercooled liquid solder and Cu metallization under the temperature lower than the solder's melting point. The interfacial reaction and growth behavior of intermetallic compound (IMC) between the undercooled liquid Sn-3.0Ag-0.5Cu solder and Cu metallization have been investigated. The results show that for Sn-3.0Ag-0.5Cu solder as the undercooled melt or eutectic melt or normal melt in the joints, both of the interfacial IMC thickness and Cu6Sn5 grain size increase with prolonging time. The depth of the groove between scallop-like grains in the interfacial IMC layer increases with increasing either the dwelling time or temperature. However, under different liquid solder dwelling conditions, the change tendency of interfacial IMC thickness is different, which is directly related to the dynamic equilibrium between the deposition and dissolution of the interfacial Cu-Sn phase. Furthermore, when the joints are mainly formed at the undercooling temperature, the excessive growth of interface IMC and the consumption of UBM can be reduced. The proposed method has the potential to be a new technique for preparing solder joints with thin interfacial IMC layer.