Assessment of dielectric encapsulation for high temperature high voltage modules

The reliability of the encapsulation materials used in power electronic modules is one of the key issues for high temperature packaging technologies. In this paper, characterizations of different types of encapsulation materials in a temperature range extended up to 300°C are presented. The aim is to determine the ability of five potential candidates to be used as encapsulating materials for power devices at high temperature and for long-period applications. The studied temperature range deals with the possible operating junction temperature of wide bandgap Silicon Carbide (SiC) and Gallium Nitride (GaN) devices and can satisfy large industrial applications. In a first stage, the intrinsic dielectric properties, at high temperatures are presented. From room temperature until 300°C, the dielectric permittivity, dielectric losses and DC conductivity show acceptable values for 4 encapsulation materials, while high losses, high permittivity and high conductivity values are obtained for the last one. The second step was to investigate the ability of the four initially validated materials to operate for long period in thermo-oxidative atmosphere. During the aging in air in the temperature range from 250°C to 280°C, no significant degradation of the dielectric properties was observed until the cracks apparition in the silicone based materials. Hence, mechanisms leading to the cracks initiation and propagation cannot be probed by the dielectric properties in the tested temperature range (from 25°C until 300°C). Therefore, the crack apparition is considered as the endpoint criterion and the lifetime of the four-aged encapsulation materials at different aging temperatures are presented. Finally, the degradation mechanisms of silicone materials are investigated based on the obtained results as well as on other published researches.