Ultrasonic-assisted soldering of 7075 Al alloy joint using Ni mesh reinforced SAC305 composite solder: microstructure, bonding ratio, and mechanical properties.

Suppressing solder overflow has significant implications for promoting the application of ultrasonic-assisted soldering. In this work, an innovative strategy of adding metal mesh into Sn-based solder was utilized. 7075 Al alloy joints were ultrasonically soldered with Ni mesh reinforced SAC305 composite solders. The microstructure, bonding ratio, and shear properties of joints were systematically explored. Results showed that solder seams primarily consisted of Ni mesh, SAC305 solder, α-Al phase, AgAl phase, (Ni, Cu)Sn phase, Al(Ni, Cu) phase, and dispersed fine particles. The bonding interface between Ni mesh and Al substrate could be divided into contact and non-contact regions. A polycrystalline Al(Ni, Cu) phase and a Cu-Al-O amorphous layer were formed at the contact regions. The bonding ratio of joints was mainly affected by the cavitation effects within non-contact regions. Adding Ni meshes could enhance the acoustic pressure and accelerate the flow of local solder in the solder seams. The decrease in the bonding ratio was attributed to the excessive solder flow, which induced the formation of defects. Benefiting from the intrinsic strengthening of Ni mesh and metallurgical reaction strengthening, Al/250#Ni-SAC/Al joints exhibited a shear strength of 71.87 MPa.