Hybrid sintering paste for chip bonding (elektronik industrie 12/2024)
"elektronik industrie 12/2024"
Power electronics using SiC or GaN place extreme demands on the bonding between semiconductor and substrate. Hybrid sinter pastes with high thermal conductivity and improved mechanical properties enable demanding applications, for example, in electromobility and mobile communications.
Using materials like silicon carbide (SiC) and gallium nitride (GaN), wafers are getting thinner and thinner, resulting in higher power densities. Manufacturers of semiconductors for electromobility, cellular base stations, and many other applications involving high-power switching are pushing the boundaries of what's possible.
The bottleneck here is the paste that ensures contact between the semiconductor chip and the substrate when bonding it. The substrate can be a circuit board or ceramic, but in power electronics it is usually a metal plate. The paste ensures a mechanically secure and permanent bond between the two components and at the same time ensures the dissipation of heat from the chip to the outside.
Bonding is done using a variety of methods using different pastes. The paste is spread or printed onto a substrate, and the semiconductor chip is placed on top. This sandwich is heated, during which different chemical and physical processes occur depending on the paste. Electrical contacts are then made by wire bonding, and the chip is encapsulated in a housing.
Chip bonding methods
There are two conventional methods for chip bonding: adhesive bonding and sintering.
Adhesion
In the case of pastes based on epoxy resin or other synthetic resins, bonding is achieved by the adhesive resin. However, their thermal conductivity is low, only a few W/m･K. By adding metal particles such as silver to the paste, thermal conductivity can be increased to 50 W/m･K. However, it is very difficult to achieve a higher thermal conductivity than this. Heat is transferred through the contact surfaces of the silver particles, but the contact surfaces are not firmly bonded metallically. Adhesion is a very common method today and is suitable for logic chips that do not generate much heat.
Sintering
The paste contains metal powder and organic components, such as solvents, which are removed during sintering. The paste is heated to 200°C to 250°C, which causes the silver particles to bond with each other and to the substrate and chip metallically. Sintering: The paste contains metal powder and organic components, such as solvents, which are removed during sintering. The paste is heated to 200°C to 250°C, which causes the silver particles to bond with each other and to the substrate and chip metallically. During sintering, the solvent evaporates, leaving pores in the silver. The thermal conductivity is very high, exceeding 200 W/m·K. This method is suitable for power electronics applications, for example, with semiconductor materials such as SiC and GaN, which are used in high-voltage inverters in electric vehicles and mobile communication base stations.
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