Mechanism of the interfacial reaction between sapphire and Sn-3.5Ag-4Ti solder at a low temperature in air by ultrasound

The joining of sapphire at a low temperature is necessary for the packaging of temperature-sensitive components. In current processes, the sapphire was bonded to Sn-based Ti-activated solders by mechanical scraping and stirring. To improve the efficiency and the bonding strength, the joining of sapphire by ultrasonic-assisted hot dipping and soldering at 250 °C in air was investigated. The relationship between the interfacial structure and shear strength of the soldered joints was revealed. The joints dipped for different durations and soldered for 0.5 s had similar interface morphologies, while the shear strength of the joints was controlled by the dipping duration. The shear strength of the joints dipped for 100 s reached 33 MPa. An uneven reaction layer of Ti oxides was found at the interface by TEM and EDS. A physical model was established to explain the formation of the reaction layer and the evolution of the interface microstructure. The mechanism of the interfacial reaction was discussed based on thermodynamics. The results of the thermodynamic calculations show that the replacement reaction between Ti and sapphire could not occur at the joining temperature of 250 °C. The increasing temperature induced by ultrasound at the liquid/solid interface was calculated by bubble dynamics and thermal conduction methods. It could be inferred that the reaction may be dominated by a local high temperature at the interface. Our finding demonstrates that the ultrasonic effects promote and facilitate the interfacial reaction of sapphire and Sn-3.5Ag-4Ti.