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New sintering joining technology

Si dies affixed to a lead frame using Ag films after treated at 250°C

A silver micron-particle sintering joining technology has been developed by scientists at The Institute of Scientific and Industrial Research, Osaka University.

The research, conducted as part of the Advanced Low CArbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), has developed a low-temperature pressureless die bonding, which can be used for all types of electrodes, including Cu and Au, as well as silver coating and will allow low-cost, reliable heat-resistant mounting technology for die attach and for printed wiring without changing conventional cheap electrode structures. Researchers suggest it could prove suitable for next-generation power semiconductors, such as GaN.

According to the team, led by Professor Suganuma, the new silver micron-particle sintering joining technology has enabled low-temperature and pressureless die bonding in an ambient environment at a low cost. Crucially, the technology has been able to show high reliability at high temperatures over 250°C and is proving suitable for next-generation main power semiconductor die bonding technology.

The team clarified the nano-level bonding mechanism last year, but the material of electrodes was limited to silver (Ag), because the key to that technology was based on interactions between Ag and oxygen (O).

Nickel/gold (Ni/Au) or copper (Cu) are used for electrodes for silicon (Si), silicon carbide (SiC) and gallium nitride (GaN) semiconductors as well as direct bonded copper (DBC) substrates. So, bonding to Ni/Au or Cu electrodes is in high demand, as a result the film bonding will expand the scope of its application significantly.

To solve these electrode-related challenges this group, in joint research with the Daicel Corporation, has developed a solvent that can promote interfacial activation of Ag, 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 two times of that of Ag, has been achieved, which can only be obtained with this new silver paste.

In conventional power semiconductor fabrication processes, films (or sheets) are often used instead of paste as a die attach material. This technology activates the surface of an Ag film by grinding it, forming hillocks on the surface of the Ag film at temperatures ranging from 200 to 250°C.

According to researchers this will enable not only high performance die bonding of next-generation power semiconductors like SiC and GaN, but also wiring according to surface roughness of a die with lower noise, by no load and low temperatures process. This will help achieve the reduction of energy loss during power conversion, which is characteristic of SiC and GaN power semiconductors.

Neil Tyler

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