In-situ-X-ray investigation on vacuum soldering processes for conventional and diffusion soldering

In power electronics there is currently a development to higher operating temperatures of up to 300 °C. This was made possible by an improved availability of semiconductor materials such as silicon carbide. At these temperatures conventional solders can no longer be used for Die-Attachment. Therefore new technologies are necessary. One possible solution is to use diffusion soldering. Khaja et al. [1] presented an approach for diffusion soldering by printing of a conventional solder paste with a 20 μm thin stencil. In power the void content is typically required to be less than 10 %. Khaja et al. showed that the application of a vacuum vapour phase soldering process can ensure a very low void content of the resulting solder joints. In a previous paper we showed that vacuum soldering processes are in principle better suited for this purpose than overpressure processes [2]. In this work we found the same effects, which we observed in our previous work, also for conventional SAC305 solder paste on DCB substrates. Furthermore we investigated different vacuum soldering processes. We could not observe any apparent difference between vacuum soldering processes with several separate vacuum steps and processes with a single vacuum step. However the strength of the vacuum step influences the final void content significantly. Furthermore we found that void content over time curves of soldering processes without pressure changes exhibit a minimum. When a solder joint is kept at peak temperature for a longer period of time the void content steadily increases again. We showed that this can be suppressed with overpressure. Therefore we conclude that for diffusion soldering with long peak times of several minutes a combined soldering process of vacuum and overpressure steps is required.