” celektronik industrie 12/2024 ”

Power electronics with silicon carbide (SiC) or gallium nitride (GaN) impose extreme requirements on the bonding of semiconductors and their support. Hybrid sinter paste with high thermal conductivity and improved mechanical properties enable demanding applications, for example in electromobility or mobile communication.

Materials such as SiC or GaN, ever thinner wafers, and ever higher power densities: Manufacturers of semiconductors for electromobility, mobile radio base stations, and other applications where high electric power is switched, are pushing the boundaries of what is possible. Here, a bottleneck is the paste that facilitates contact between the semiconductor and the substrate during bonding of semiconductor chips. The substrate can be printed or ceramic while in most power electronics applications, it is a metal plate. This paste ensures on the one hand that both components are bonded in a mechanically lasting manner, and on the other hand, works in such a manner that the heat from the chip is discharged outside.

Various processes using various pastes are available for bonding. Paste is spread over or printed onto the substrate, then a semiconductor chip is positioned over it. The resulting sandwich is heated in various chemical and physical processes depending on the paste. Finally, electric contact is established through wire bonding and the chip is inserted in a housing.

Processes for Chip Bonding

There are two existing processes for chip bonding: sticking and sintering.

Sticking: With paste of epoxy or other artificial resin, bonding is achieved by the sticking resin’s adhesion. However, the thermal conductivity (W/m·K) is low. It can be raised up to 50 W/m·K when metal particles such as silver particles are added to the paste. Raising the value higher is very difficult since the heat is conducted through the contact surfaces of the silver particles which are not bonded firmly as metal. The bonding process is very popular today and is suitable for logic chips with low heat generation.

Sintering: The paste contains metal powder as well as organic components such as solvent, which are removed during sintering. The paste is heated up to a temperature range between 200°C and 250°C, thereby metallically bonding the silver particles with each other and them with the substrate and chip. During sintering, the solvent is vaporized and pores are left in the silver. The heat conductivity is very high with over 200 W/m·K. This process is suitable for applications of power electronics and is predestined for semiconductor materials such as SiC and GaN which are used in high-voltage converters onboard E-vehicles or in mobile radio base stations.

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