Evaluation of Metallurgical Bonding for Ultrasound Transducers

Abstract

Common ultrasound transducers use a piezoelectric element to convert between electrical and mechanical energy. The piezoelectric layer is sandwiched between acoustic matching and backing layers to maximize energy transfer and bandwidth. The bonding layers joining these materials are conventionally made of polymers, and decades of experience verify that it functions excellently in many ultrasound transducer applications. These bonding layers should be very thin compared to the acoustic wavelength to avoid reverberations and reduced bandwidth. Transducers for operation in harsh environments rely on careful selection of materials, and polymer materials are known to degrade at high temperatures. Metallurgical bonding is proposed as an alternative to polymeric adhesives. Metals form strong, electrically- and thermally conductive bonds with a high characteristic acoustic impedance. Solid-liquid interdiffusion (SLID) bonding is a technique which relies on the formation of intermetallic compounds, which are stable at temperatures above the processing temperature. SLID bonds are not reflowable, as compared to bonds formed through soldering. This thesis explores the binary metal system of gold (Au) and Tin (Sn) for bonding essential layers of ultrasound transducers, such as piezoelectric ceramic materials. Metallurgical bonding techniques, Au-Sn SLID and Au-Sn soldering, are evaluated and compared to conventional polymeric adhesives. Testing of bonding performance during and after exposure to high temperature and high pressure indicated high-temperature stability with high mechanical strength. A common challenge related to metallurgical bonding is the formation of voids within the layer, which are gas or vacuum-filled pockets. Reliable estimation of the influence from such voids on the acoustic performance is important if metallurgical bonding is to be used in acoustic transducers. A finite element method modelling technique is presented in this thesis, which can be applied to accurately estimate effective medium parameters of layers containing voids of arbitrary size, concentration, and distribution.