Role of active element Al in the interfacial bonding of sapphire by ultrasonic-assisted soldering using Sn-based alloys

Ultrasonic-assisted soldering using Al-activated Sn-based alloys holds great potential for low-temperature joining of sapphire, but its widespread application is limited by reliability issues associated with excessive Al content. In this study, the sapphire was joined with Sn–Al solder coatings and the commercial Sn–3Ag–0.5Cu filler at 250°C. The Sn–2Al coating was prepared by ultrasonic-assisted dipping, followed by rinsing in Sn–1Al, Sn–0.5Al, and Sn to reduce Al. Joints with Sn–2Al, Sn–1Al, and Sn–0.5Al coatings were successfully formed with the Sn–3Ag–0.5Cu filler, whereas those with Sn-rinsed coatings failed. Al-related precipitates, including α-Al, Ag–Al, and Cu–Al intermetallic compounds (IMCs), were observed in the bond metal layer. As Al content decreased, α-Al and Ag–Al phases progressively diminished, whereas Cu–Al IMCs remained. The Sn–2Al coating yielded joints with a shear strength of 43.1 MPa with primarily ductile fracture, whereas Sn–0.5Al gave only 12.8 MPa with brittle fracture. The performance degradation was primarily driven by the reduced interfacial Al segregation, which resulted from the consumption of active Al through reactions with Cu and Ag at the soldering temperature. These results demonstrated that Al content could be optimized while preserving joint strength, through the fine-tuned formation of Al segregation and strengthening phases of Ag–Al and Cu–Al IMCs.