Transfer molding for power semiconductor modules

As driven by potential improvements in cost and reliability, transfer molding recently emerged as an attractive alternative to conventional silicone gel encapsulation in power semiconductor modules. In this study a large-area transfer-molded test module was used to assess the reliability performance of transfer-molded power modules for high-temperature applications up to junction temperatures of Tjmax = 225 °C. Tests comprised high-temperature reverse bias (HTRB), temperature cycling, high-humidity, high-voltage reverse bias (H3TRB) as well as power cycling. As a result, mold compound adhesion has been identified as a key issue for large-area transfer molding. To cope with the problem, it is recommended to focus on materials with low values of flexural modulus and coefficient of thermal expansion. The data also indicates that mold compounds are suitable for high-temperature encapsulation even when operated above their glass transition tem-perature: Overmolded SiC MOSFETs successfully passed HTRB, temperature cycling and power cycling in the temperature range >200 °C. In addition, also H3TRB testing at 960V blocking voltage was passed.