Interfacial bonding mechanisms in ultrasonic-assisted soldered Si/Cu joint using Sn-3.5Ag-4Al solder

The low-temperature reliable joining of Si and Cu is vital to the service of power device. In this work, Sn-Ag-Al active filler layer was coated on Si surface by ultrasonic-assisted dipping at 250 °C, the joining of Cu and pre-metallized Si was achieved with ultrasonic assistance subsequently. The effects of ultrasonic dipping time on the bonding mechanism and mechanical property of the joints were analyzed in details. It was found that the main bonding medium at the Si/solder interface was the amorphous Al2O3 interphase, which had two types of growth mechanisms with the assistance of ultrasound. The epitaxial Al2O3 transition layer increased as ultrasonic dipping time was 50 s, which contributed to the high Si/solder strength (45.4 MPa). The microstructure and strength of Cu/solder interface were directly controlled by ultrasonic duration time. The brittle rupture of Si/Cu joint occurred inside CuAl2 layer and propagated into the solder when the time was 6 s, and the maximum strength with value of 31.2 MPa was obtained. Prolonging the time to 12 s, the strength decreased to 15.3 MPa, which was caused by the formation of voids in Cu6Sn5 layer due to the grain volume shrinkage induced by intense micro-jet and shock wave. This research presents a potential idea for the joining of ceramic/metal. •The Si/Cu joints were successfully fabricated with Sn-3.5Ag-4Al solder at 250 °C by ultrasonic-assisted dipping and soldering method.•The sound bonding of Sn-3.5Ag-4Al layer to Si was realized by the formation of amorphous Al2O3.•Two types of growth mechanisms of amorphous Al2O3 were proposed: displacement reaction and oxidation deposition reaction.•The maximum shear strength with 31.2 MPa of Si/Cu joint was obtained.