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High thermal conductive die attach paste using polymer and micron size silver for power semiconductor package
Keywords: Die attach, Silver sintering, High thermal conductivity
Power semiconductor package manufactures and electronic device suppliers have been looking for Pb-free alternative to traditional high Pb solder die attach adhesive. Pb solders have high thermal conductivity, 30-50W/mk, and known process with some difficulties in high volume mass production such as void, bond line control, and inert gas(Forming gas) environment. Pb is now categorized as hazardous substance to human body and environment and its products are scheduled to be banned. Silver epoxy paste is another die attach adhesive but its thermal conductivity is not high enough for Power devices. Eutectic gold tin solder(80Au20Sn) has 57W/mK but it is high cost material. Currently silver sintering material becomes popular for electronic device because it has high thermal conductivity (150~250W/mk) by using nano silver sintering. But it requires high bonding temperature and pressure and only works with metalized die, not bare silicone die. It makes brittle bonding structure and has limitation in die size due to high stress. New silver sintering material in this paper is composed of micron size silver powder and organic polymer. This technology overcomes all the limitations of conventional silver epoxy, eutectic gold thin solder and silver sintering product by using the unique design of polymer composition. The unique polymer composition does the critical role to accelerate silver sintering with relatively low temperature compared with sintering silver and enables 250W/mk thermal conductivity without bonding force. The polymer has good adhesion to a variety of surfaces, bare silicon, gold and silver metalized die and silver and copper metal surfaces while Pb solder and sintering silver can’t have good adhesion to bare silicone surface which silver and solder cannot sinter into. The viscosity of this new silver sintering product is not much different from standard epoxy paste and its application process is as easy as conventional silver epoxy. This new silver sintering product is a drop in replacement for standard die attach adhesives. The moisture absorption is much lower than standard epoxy paste and capable to pass MSL1. Nano silver sintering material has high modulus and generates high stress during temperature cycling test, especially for large die. But the polymer part of this new product reduces the stress of final package and enable to pass temperature cycling(-65C to 150C) 3000cycles. This new silver sintering technology using polymer and micron size silver powder is a cost effective solution to replace Pb solder for power device and the thermal performance is almost same as nano silver sintering product. The application process is the same as standard silver epoxy and not requiring new equipment. It is a cost effective drop in solution.
Howard(Hwa Il) Jin, R&D Manager
Alpha Advanced Materials
Suwanee, GA

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