Ultrasonic-assisted soldering of sapphire through metallic transition layers of Al and Zn using Sn-xZn-2Al solder alloys

Ultrasonic-assisted soldering has potential in the electrical industry especially for the joining of ceramics. The Al-activated Sn-based alloys are promising ultrasonic solders due to simple preparation, while the current approaches required long ultrasonic action. In order to increase the efficiency and reduce ultrasonic-induced damage, this study investigated the soldering of sapphire (monocrystalline α-Al2O3) performed under an ultrasonic action for 0.5 s by using Sn-xZn-2Al(x = 9, 25, 45) solder alloys. Microstructures of interfacial transition layers between the sapphire and solders were focused on. It has been found that at the interfaces no interfacial reaction phases formed and the interfacial bonding was realized via metallic transition layers. Three kinds of interfacial structures existed, that is, sapphire/Al atomic layer/β-Sn, sapphire/Al atomic layer/(Zn enrichment layer)/β-Sn and sapphire/Al atomic layer/Zn nanocrystalline clusters/β-Sn. The elements of Al and Zn in the solder alloys underwent a selective and asynchronous adsorption process during the ultrasonic action. An Al atomic layer formed on the sapphire surface by the stronger chemical adsorption and acted as a transition layer between sapphire and β-Sn. The Zn enrichment layer was distributed locally along the interface and as the Zn content increased in the solder alloys, more localized Zn nanocrystalline clusters formed. These Zn transition structures strengthened the interfacial bonding by transforming the Al atomic layer/β-Sn interface into the Al atomic layer/Zn transition structures/β-Sn interfaces. The joints possessed a shear strength of up to 28 MPa when soldering with Sn–45Zn–2Al at 350 °C.