Transient liquid phase (TLP) bonding using Sn/Ag multilayers for high temperature applications

With the growing demand for electronic devices operating in harsh conditions, the interconnect technology for packaging these devices remains an important area of focus to ensure reliable and stable functioning capabilities. Because these devices may experience temperatures that are higher than those encountered by consumer electronic products (e.g. for down-well tools in oil and gas drilling up to 200°C, automotive application up to 400°C), the die attach material has to withstand high temperatures generated during the device functioning, as well as external conditions without affecting the device performance. Conventional solders used in consumer electronic products have melting points lower than the maximum operating temperatures of harsh environment applications, and they are thus not appropriate for use. In contrast, metallurgical systems that can be bonded using the transient liquid phase (TLP) bonding process is a promising solution for high temperature electronic devices due to the following benefits, namely: (1) high quality bond can be formed, (2) a bond can be formed at a temperature much lower than the melting point of the resulting joint. In this paper, the feasibility to bond silver-tin (Ag-Sn) system using the TLP bonding process and use it as a die attach solution for high temperature electronic systems is reported. The advantages Ag-Sn system provides are low bonding temperature, low cost, high de-bonding temperature, and it is also one of the fast inter-diffusion couples. The influence of bonding parameters, such as bonding temperature, bonding force, on the quality of the TLP bond is discussed in order to develop a Ag-rich bond. The joints developed were inspected using X-ray and SEM/EDX, coupled with shear tests performed at room temperature to determine the mechanical strength of the joint. From the bonding studies, a Ag-Sn TLP bond was successfully developed. No occurrence of voids was found in the bonded area, and from the SEM/EDX analysis the joint was comprised of Ag and Ag₃Sn intermetallic only, thereby achieving the desired Ag-rich joint. On the other hand, from the mechanical characterisation tests, the shear strength of the Ag-Sn bonded samples was observed to be higher than the minimum strength requirement for high temperature applications. The shear strength also increased with bonding pressure and temperature.