Intermetallic Bonding as an Alternative to Polymeric Adhesives in Ultrasound Transducers

Abstract

Common ultrasound transducers utilize epoxy for bonding the active piezoelectric to the rest of the acoustic stack. This paper explores some of the potential benefits of replacing this soft epoxy layer with a harder metallurgical bond. The influence of varying the bond layer thickness was evaluated for epoxy-and intermetallic bonding layers in an ultrasound transducer, as was the effect of voids and delamination that may occur during fabrication of intermetallic bonds. It was found that the intermetallic bond could be made much thicker than an epoxy bond without degrading the transducer performance, and the influence by thickness variations was less. Small voids in the intermetallic where shown to cause a downshift in resonance frequency, whereas regions with delamination will cause reflections that degrade the performance of the transducer.

Ultrasound Transducers for High Pressure Environments Up to 1000 Bar

“…The SLID bondline thickness was approximately 27 µm and contained small voids. As shown in our previous work [8], intermetallic Au-Sn bondlines of these dimensions has negligible influence on the electro-acoustic transfer function of the transducer due to the increased characteristic acoustic impedance. Our measurements found no change in the voids contained in the intermetallic bondline after exposure to pressure.…”

“…One challenge related to SLID bonding for ultrasound transducers is the formation of voids in IMCs. Our previous work [8] reported insignificant acoustic effects of voids occupying up to 20% of the Au-Sn bondline for ultrasound transducers up to 10 MHz, whereas 15% delamination of the width of the bondline would introduce noticeable effects for frequencies above 5 MHz. In HPHT environments, voids may have considerable impact on the mechanical strength of the bond, hence the performance of the ultrasound transducer.…”

Impact of High Pressures on Au-Sn Solid Liquid Interdiffusion (SLID) Bonds

“…In high frequency ultrasound applications, single crystal ferroelectric materials, such as Lead Magnesium Niobate-Lead Titanate (PMN-PT), are preferred where the surface roughness can be machined comparable to that of Si. Our previous work [9] reported insignificant acoustic effects of voids occupying up to 20% of the Au-Sn bondline for ultrasound transducers up to 10 MHz, whereas 15% delamination of the width of the bondline would introduce noticeable effects for frequencies above 5 MHz. In highpressure environments, voids may have considerable impact on the mechanical strength of the bond, hence the performance of the ultrasound transducer.…”

“…Based on previous studies [9], the acoustic performance of an ultrasound transducer would not be significantly impacted for low void concentrations of 5 µm in diameter. The void size and concentration found in the PZT-Si samples are not of critical concern for the acoustic performance of ultrasound applications up to 10 MHz.…”

Ag-(In-Bi) Solid–Liquid Interdiffusion Bonding

“…The bonds provide better acoustic coupling than the traditionally used epoxy solutions. 3 The material cost of gold, however, is a deterrent for the final application of the Au-(In-Bi) system.…”