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Highly Reliable Pressure-Less Silver Sintering Joint
Keywords: silver sintering , die attach, pressure less
Development in the hybrid and electric cars, high speed train, aircraft/aviation and deep well oil/gas extraction demands more robust semiconductor power devices that can endure harsh operation conditions such as ≥ 200oC. In the meantime, recent availability of wide band gap silicon carbide devices makes it possible to operate theoretically at temperature above 500oC. High temperature die attach materials are thus under strong demand for these applications. Different materials have been explored for the high temperature die attach applications, including lead-free solder, silver and copper sintering, and transient liquid phase (TLP) materials. Lead-free solders which can operate under ≥ 200oC condition do exist, such as AuGe, AuSi, AuIn and ZnAl. However, these materials have drawbacks such as high price (Au-based alloys) or poor processability (Zn- Al). In contrast, Ag or Cu sintering and TLP materials can withstand a much higher operation temperature due to a much higher melting point. Also, these materials can be processed at a lower temperature comparing to that of solder materials, which will greatly help reduce the thermal stress during processing. Silver sintering materials are promising next generation lead- free die attach materials due to their capability to work under high temperature, their high thermal and electrical conductivities, and good thermomechanical properties. At present, through power cycling and temperature cycling tests, silver sintered joints obtained under pressure conditions have been reported to have much better reliability than that of solder materials. However, sintering under pressure conditions have some issues, for example, 1) the dies may crack due to the pressure, leading to a low yield; 2) it requires large financial investment for the equipment; 3) since the pressure sintering is a batch process, it may reduce the throughput compared to that of the reflow process. Therefore, there is a strong desire to have a pressure-less silver sintering paste to overcome the above problems. In this study, we report the development of pressure-less silver sintering materials with good reliability under temperature cycling test (TCT) and thermal aging test. The emphasis will be focused on finding the optimized processing condition and device combination to achieve best device reliability. The factors, including processing condition such as temperature and time, device structure such as die size and thickness, surface metallization and BLT will be studied in relation to the reliability of devices. Material characterization technique such as cross-section and SEM analysis will be used to understand the failure modes, and the relationship between processing condition and the microstructure evolution of the silver sintering materials under TCT condition.
Sihai Chen,
Indium Corporation of America
Clinton, NY
United States

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