Low Temperature Transient Liquid Phase (TLP) Bonding using Eutectic Sn-In Solder Anisotropic Condctive Films (ACFs) for Flexible Ultrasound Transducer

A TLP (Transient Liquid Phase) bonding of Ultrasonic transducers using low temperature eutectic Sn-In solder ACFs and Au pads, where the final solder joint consists of Au-In-Sn intermetallics with higher melting point, has been investigated. Sn52In solder ACFs were fabricated and thermo-compression bonded at 130°C for 30 seconds to bond ultrasonic transducer on Flex Printed Circuits (FPCs). Scanning electron microscopy (SEM) / energy dispersive x-ray spectroscopy (EDX) analysis of solder joint cross-sections show that the microstructure of intermetallic compounds (IMCs) were consisted of Au-In and Sn rich-In phases. Based upon EDX analysis, the Au-In-Sn intermetallic compounds are found. The aim of this study is to characterize eutectic Sn-In solder ACFs bonding method suitable for temperature limited bonding environment as well as for flexible ultrasonic transducer applications. Pressure Cooker Test (2 atm, 100% Relative Humidity at 121°C) and Dynamic bending test (R: 7mm) were also carried out to examine the bonding reliability. And these results were compared with conventional metal coated polymer particles ACFs. After a pressure cooker test, the final intermetallic compound was Au-In-Sn ternary phase, which is thermally stable at higher temperatures as high as 380°C. The TLP bonding with Sn-In solder ACFs offers very reliable metallurgical bonding and excellent compatibility for ferroelectric ultrasonic transducers with rough surfaces.