Abstract
This paper presents a bonding technique based on gold-tin (Au-Sn) intermetallics to bond an active layer to a backing material in a piezoelectric ultrasonic transducer stack. The bonding process was performed at 310°C. The temperature ramping rate was found to play an important role to the bond quality. Three different bonding temperature profiles P1, P2 and P3 corresponding to a ramping rate 120°C/min., 45°C/min. and 20°C/min., respectively, were investigated. The bonded samples were characterized by electrical impedance measurements in air and cross-sectional microscopy inspection. The most reproducible bondlines were found to be with the heating rate 120°C/min. The resulted bondlines show a layered structure of Au/Au-Sn intermetallic/Au with a corresponding average thickness of 5 μm/17 μm/5 μm, respectively. Based upon energy dispersive spectroscopy (EDS) analysis, the obtained Au-Sn intermetallic compounds are found in good agreement with those from literature. A 64-element one dimensional array formed by Au-Sn SLID method was fabricated and electrical impedance across elements was measured. The results show a good uniformity across elements in the array, and good accordance to Finite Element simulations from COMSOL. This shows the feasibility of using SLID bonding technology to assembly stacks of piezoelectric ultrasonic transducers
