Pressureless sintering multi-scale Ag paste by a commercial vacuum reflowing furnace for massive production of power modules

Air is critical to Ag sintering. The Cu surface of the non-metallized direct-bonding-copper (DBC) substrate can be oxidized seriously for bonding power devices using Ag paste in air. In this paper, we presented a pressureless sintering approach for a multi-scale Ag paste in the formic acid vapor by a commercial vacuum reflowing furnace. The study found that the activation temperature of the formic acid vapor had an obvious effect on the sintering behavior of the multi-scale Ag paste. The multi-scale Ag paste could obtain much denser sintered Ag when the formic acid vapor was rapidly injected at 180 °C by the vacuum reflowing furnace. The sintered Ag necks larger than 0.4 μm and the porosity lower than 11.3% were achieved in formic acid vapor, which had better sintering behavior than Ag nanoparticles (NPs) paste. The thermal resistance of 1200 V/50 A half-bridge IGBT modules by sintering the multi-scale Ag paste in formic acid vapor was 0.41 °C/W, which was ~ 12% lower than those of the commercial IGBT modules using Pb_92.5Sn₅Ag_2.5. This work could overcome the contradiction between the Ag sintering and the Cu oxidation of the non-metallized DBC substrate by the formic acid vapor in a commercial vacuum reflowing furnace. The method is extremely useful for the traditional manufacturer, who do not have to invest any new facilities for the sintering.