Failure modes and bonding strength of ultrasonically-soldered glass joints

While useful for optics and solar energy applications, solder-glass joints are difficult to fabricate due to poor wettability and adhesion, but reliable joints can be achieved with the use of ultrasonic-assisted soldering (UAS). In this study, an automated UAS system was used to investigate the effects of processing parameters on the solder joint quality so that the technology can be more suitably scaled for future widespread use in industry. Through mechanical shear strength testing of soldered joints and microstructure failure surface analysis with XRD, a more comprehensive understanding of the relationships between processing parameters and joint quality was developed. Two novel failure surface types with distinct failure behavior were observed which could be predicted based on soldering tip speeds. The differences between the two failure surfaces appear to be due to the growth of a thin surface oxide layer on the solder lines from prolonged sonication, as determined from EDS and XRD microstructure analysis. These observations and analysis highlight a complex relationship that determines the quality of the solder-glass joint and suggest further research is necessary to refine the UAS process specifically for glass substrates and their varied applications in industry to prevent premature failure.