Metallurgical AuSn Bonding of Piezoelectric Layers

This study presents a novel and simple approach to form metallurgical bonds between layers of the acoustic stack in an ultrasound transducer using a eutectic AuSn preform, which is a pre-shaped thin sheet of solder. The bonding takes 10 minutes and yields uniform bondlines of 10 μm with a melting temperature of 280 °C. The resulting bonds are electrically conductive, mechanically robust and have a characteristic acoustic impedance of 29 Mrayls. AuSn bonding was demonstrated by successfully bonding two equally thick layers of PZT. The bonded PZT-stacks were polarized and sub-diced into a 2-dimensional array. Characterization was performed by optical microscopy, scanning acoustic microscopy and electrical impedance measurements. Combining scanning acoustic microscopy and electrical impedance measurements proved to be a useful approach to experimentally study inter-element variations induced by varying concentrations of voids in a metallurgical bondline.