Evaluation of Metallurgical Bonding for Ultrasound Transducers
Doctoral thesis
Published version
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https://hdl.handle.net/11250/3126467Utgivelsesdato
2024Metadata
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Sammendrag
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.
Består av
Article 1 P. K. Bolstad, M. E. Frijlink, T. Manh and L. Hoff, "Estimating Effective Material Parameters of Inhomogeneous Layers Using Finite Element Method," in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 69, no. 12, pp. 3402-3410, Dec. 2022.Article 2 P. K. Bolstad, M. Frijlink, and L. Hoff, “Evaluation of bonding techniques for ultrasound transducers,” in Microelectronics Reliability, vol. 151, p. 115272, Dec. 2023.
Article 3 P. K. Bolstad, D. Le-Anh, L. Hoff and T. Manh, "Intermetallic Bonding as an Alternative to Polymeric Adhesives in Ultrasound Transducers," in 2019 IEEE International Ultrasonics Symposium (IUS), Glasgow, UK, 2019.
Article 4 P. K. Bolstad, S. L. Kuziora, H. -V. Nguyen, T. Manh, K. E. Aasmundtveit and L. Hoff, "Impact of High Pressures on Au-Sn Solid Liquid Interdiffusion (SLID) Bonds," in 2020 IEEE 8th Electronics System-Integration Technology Conference (ESTC), Tønsberg, Norway, 2020.
Article 5 P. K. Bolstad, M. Frijlink and L. Hoff, "Metallurgical AuSn Bonding of Piezoelectric Layers," in 2022 IEEE International Ultrasonics Symposium (IUS), Venice, Italy, 2022.
Article 5 P. K. Bolstad, M. Frijlink and L. Hoff, "Metallurgical AuSn Bonding of Piezoelectric Layers," in 2022 IEEE International Ultrasonics Symposium (IUS), Venice, Italy, 2022.