High-Temperature Encapsulation Materials for Power Modules: Technology and Future Development Trends

The applications of wide bandgap (WBG) semiconductors represented by silicon carbide (SiC) and gallium nitride (GaN) in power modules are currently limited by the thermal stability of encapsulation materials, especially when operating temperatures raising to 200 °C or above. According to their glass transition temperatures (Tgs), the typical encapsulation materials are commonly divided into two categories: hard encapsulation materials (Tg > highest operating temperature) represented by epoxy (EP) resins and soft encapsulation materials (Tg < lowest operating temperature) represented by silicone gels (SGs). However, both materials face reliability challenges under high working temperatures (over 200 °C). In this work, recent progress for EPs and SGs was first discussed with Tgs and coefficients of thermal expansion (CTEs), respectively. When considering EPs, their Tgs are usually around 150 °C, which is not sufficient enough for applications above 200 °C, while, for SGs, since they employ over Tgs, their high CTEs are not comparable with chips and other packaging materials, which leads to CTE mismatches for power modules' packaging. Therefore, a list of materials with high Tgs and proper CTEs was concluded for both academia research interests and industrial applications. High Tg polymers were introduced, including the EP-cyanate ester blend system, phthalonitrile, and bio-based polymers. Furthermore, innovative materials, including cement, glass, and insulating liquids, were also discussed. This article is established as a guideline for developing high-temperature encapsulation materials that offer a wide range of choices of materials and related selection criteria.