Design and Reliability of Acoustic Wedge Transducer Assemblies for Outdoor Touch Panels

Abstract

Acoustic wedge transducer assemblies (AWTA) for generating and detecting surface acoustic waves (SAW) on non-piezoelectric substrates were designed, fabricated and evaluated for outdoor touch panel applications. An AWTA consists of a piezoelectric acoustic transducer plate bonded to an acoustic prism that is bonded to a glass panel. The acoustic prism converts a longitudinal acoustic wave into a SAW on the glass. This conversion is based on the acoustic analogy of optics Snell's law. Thus, the prism material must have low longitudinal acoustic velocity and low acoustic attenuation. For nearly all SAW-based touch panels on the market, the acoustic prisms are made of Lucite. An issue with Lucite is high coefficient of thermal expansion (CTE), much higher than that of glass panels and piezoelectric ceramics. In outdoor environments with large temperature variations, this large CTE mismatch causes the Lucite prism to break off the glass panel and the piezoelectric transducer to break apart from the Lucite prism. After extensive search, we identified liquid crystal polymer (LCP) and bismuth (Bi) as potential acoustic prism materials having low CTE. Wedge transducer assemblies with Lucite, LCP, and Bi prisms, were fabricated and evaluated. Following Telcordia GR-468-Core recommendation, the wedge assemblies went through thermal cycling between ${-}{\rm 45}^{\circ}{\rm C}$ and 85 $^{\circ}{\rm C}$ for 1000 cycles. The wedge assemblies with Lucite prism broke before 50 cycles. Wedge assemblies with LCP and Bi prisms survived 1000 cycles. In terms of acoustic performance, wedge assembly with LCP prisms is as good as wedge assembly with Lucite prism. Accordingly, LCP is the best prism material for SAW-based touch panels in outdoor applications.