Directional Solidification Control in Joining-Casting of Pure Al Baseplate Integrated with AlN Substrate for Power Modules

Pure Al baseplates integrated with AlN substrates for power modules were tried to produce by joining-casting process, and applicability of the directional solidification technique using chills was investigated. Although no fetal casting defects were observed in obtained integrated baseplates, string-like surface defects, called 'wrinkles', were observed on them. On the basis of these generation status and the calculated solidification pattern, the wrinkles were considered to be appeared by the deformation of the front of the initially solidified shells, as in the case of low melting point metals in past studies. In the calculated surface temperature gradient distribution of the castings, a temperature gradient area smaller than that of surroundings ('minimal zone') appeared at a position similar to that of the wrinkle. These results suggest that the generation of wrinkles in the integrated baseplates can be predicted by checking the existence of minimal zones. In fact, the availability of this criterion for investigation of casting conditions was confirmed. Depending on optimization of casting design, it was found that the integrated baseplates which have no wrinkles were able to produce by joining-casting using double-layerd molds. In obtained Al-AlN interface, there were remarkably less joining defects. It was also found that the interface also have practically enough thermal-cycle reliability.