Bonding and strengthening mechanism on ultrasonic-assisted soldering of sapphire using Sn-3.5Ag-4Al solder

The joining of sapphire with a Sn-3.5Ag-4Al solder was accomplished by a fast ultrasonic-assisted soldering (0.5 s) based on ultrasonic-assisted dipping in air at a low temperature of 250 degrees C. The sapphire joints were fabricated with different ultrasonic durations during the dipping. The uptrend of the shear strength of joints was composed of two processes: the formation of interfacial bonding and the interfacial strengthening. During the bonding process, the shear strength increased rapidly to 25 MPa on average as the dipping duration extended to 50 s. In the strengthening process, the shear strength of joints slowly increased and nearly doubled to 40 MPa as sapphires were dipped for 300 s. The interfacial microstructure was investigated by SEM and TEM to study the bonding and strengthening mechanisms of the sapphire/ Sn-3.5Ag-4Al joints. A transition layer of amorphous Al2O3 was found at the sapphire/ solder alloys interface and it is supposed to determine the bonding process of sapphire and the solder alloys. The Al enrichment layer and Ag3Al2 phases formed on top of the amorphous Al2O3 layer. These structures led to the formation of strong interfacial bonding between sapphire and the solder alloys, and thus improved the shear strength of the joints. The present results demonstrate the positive effects of Ag3Al2 and Al enrichment layer at the interface on the sapphire joints.