Development of a Shifting Melting Point Ag-In Paste Via Transient Liquid Phase Sintering for High Temperature Environments
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The increasing demand for electronic devices capable of operating at temperatures above the traditional 125°C limit is driving major efforts in research and development. Devices based on wide band gap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging still remains a major hurdle for product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task as they prohibit the use of certain materials in electronic products such as lead (Pb), which has traditionally been used in high temperature solder attach. The successful development of new attach materials and manufacturing processes will enable the realization of next generation products capable of operating reliably at elevated temperatures. In this investigation a shifting melting point silver (Ag) - indium (In) solder paste that uses a Transient Liquid Phase Sintering (TLPS) process was developed. This novel material and manufacturing process constitutes a major advancement over the conventional soldering process temperature hierarchy, in which the maximum allowable application temperature is limited by the melting point of the attach material. By virtue of a shifting melting temperature, which results from isothermal solidification during the TLPS process, this attach material can be processed at a relatively low temperature while being capable of sustaining much higher temperatures in use, limited only by its new melting point. In order to develop an empirical kinetics model of the Ag-In TLPS process, a design of experiments (DOE) was used to study the effect of multiple factors on the solidification reaction. These factors include particle size, weight fraction of solute, heating rate, holding time, and processing temperature. The physical implications of the empirical model were confirmed by constructing a diffusion based mechanistic model. Pivotal microstructural information was obtained from metallographic analysis where a transition from an In-rich matrix to an Ag-rich solid solution was observed. The metallographic characteristics, mechanical strength, and electrical conductivity of the resulting Ag-In TLPS material were assessed. This study has resulted in the creation of a novel attach material and method that will enable future development of electronic packaging for high temperature environments. The quantitative description of the reaction kinetics during the TLPS process provided a valuable tool for future development and an optimization of this system.