Microstructure transformation and mechanical properties of Al alloy joints soldered with Ni-Cu foam/Sn-3.0Ag-0.5Cu (SAC305) composite solder

In this study, Ni-Cu/SAC composite solders with different Cu contents were used as an interlayer to solder Al alloy joints at 250 ℃ for different time. The results showed that the formation and evolution of the microstructure were affected by the Cu contents. In the Ni18Cu/SAC joint, the Sn solder was completely consumed to form a solid-phase joint in 5 min. And (Cu,Ni)6Sn5 was the initial phase formed from the reaction between Sn solder and the Ni18Cu skeleton. Due to the higher Ni concentration than (Cu,Ni)6Sn5 phase in the Ni18Cu skeleton, the Ni atoms diffused from Ni18Cu skeleton to the (Cu,Ni)6Sn5 phase and replaced the Cu atoms in it. As a result, the content of Ni atoms in (Cu,Ni)6Sn5 phase increased continuously, and transformed into (Ni,Cu)3Sn4 at 10 min. Phase transition also occurred in the Ni38Cu/SAC joints. However, the initial phase formed from the reaction between Sn solder and the Ni38Cu skeleton was (Ni,Cu)3Sn4. Therefore, the (Ni,Cu)3Sn4 phase was transformed into (Cu,Ni)6Sn5 at 10 min because the Cu atoms diffused from Ni38Cu skeleton to (Ni,Cu)3Sn4. On increasing the soldering time, the shear strength of the joints first increased and then decreased, the Ni18Cu/SAC joints soldered for 10 min exhibited the highest shear strength of 58.3 MPa, the shearing failure mainly happened in the composite solder layers.