By improving silver paste, Prof Katsuaki Suganuma at the Institute of Scientific and Industrial Research at Osaka University has developed low-temperature pressureless die bonding, which can be used for all types of electrodes, including Cu and Au, as well as silver coating. These achievements have enabled low-cost, reliable heat-resistant mounting technology for die attach and for printed wiring without changing a conventional cheap electrode structure.
The technology, developed with Daicel, showed high reliability at high temperatures over 250°C, and the use is spreading throughout the world as a next-generation main power semiconductor die bonding technology. Nickel/gold (Ni/Au) or copper (Cu) are often used for electrodes for silicon, silicon carbide (SiC) and GaN semiconductors as well as direct bonded copper (DBC) substrates.
In conventional power semiconductor fabrication processes, films (or sheets) are often used instead of paste as a die attach material. The group developed technology to activate the surface of a silver film by grinding it. The introduction of this processing formed abundant hillocks on the surface of the Ag film at temperatures ranging from 200 to 250°C, demonstrating that this would lead to the development of new film bonding technology. The group also developed a solvent to promote the activation of the silver particles, achieving pressureless sintering technology of joining various electrodes even at 200°C, lower than that of conventional technology. With this new type of solvent (paste), a low electrical resistivity of 4×10-6Ωcm, about twice that of silver was achieved.
The results also apply to die wiring to provide lower noise with a no load and low temperatures process. This will achieve the reduction of energy loss during power conversion, which is characteristic of SiC and GaN power semiconductors. This also reduces the size of power converters, contributing to energy savings.
Details of the work are at resou.osaka-u.ac.jp/en/research/2017/20170830_1