Die attachment materials for high-temperature power module packaging applications

Conventional Sn-Pb solders cannot be used in high-temperature power module packaging applications due to prohibitions from the restriction of hazardous substances (RoSH) directive. Alternatively, eutectic or near-eutectic Sn-Ag and Sn-Ag-Cu solders are unreliable because they are prone to cracking at elevated temperatures. In these aspects, the transient liquid phase sintering (TLPS), Ag and Cu sintering technologies have emerged as the most promising Pb-free interconnection solutions for high-temperature power module packaging applications. In this work, the large-area die (up to 193.6 mm2) attachment using the TLPS paste, Ag sinter nano-foil and Cu sinter paste is investigated. The bond quality was assessed using a scanning electron microscope equipped with energy dispersive X-ray spectroscope, a scanning acoustic microscope, qualitative bend testing, power cycling and thermal resistance measurements. TLPS process required a combination of N2 and formic acid reflow environments to achieve reliable joints, reaching 146,000 cycles during power cycling. The initial thermal resistance (Rth) of the TLPS die attach joint is 0.06±0.03 K/W. With a similar die size, theRthof Cu sinter die attachment is 0.0256 K/W, which is about 57.3% lower than TLPS. The Cu sintering joint exhibits superior heat dissipation because of its higher thermal conductivity and lower percentage of sintered porosity, which is about 11±3.4%. The Cu sintered die successfully withstood 123,100 cycles of active power cycling without showing any die sinter degradation. The Ag sinter joint formed on the molybdenum (moly) surface and the AMB Si3N4 substrate was confirmed to be robust, void-free and of consistent bond-line thickness.