Changes in the dielectric breakdown field of polyimide (PI) films have been studied from 25 to 400°C under dc ramps. Both the area (from 0.0707 to 19.635 mm²) and thickness (from 1.4 to 6.7 ¿m) dependences of the dielectric breakdown field have been carried out using the Weibull distribution function. The 63%-breakdown field value (i.e. the ¿-scale parameter) of PI shows a decrease with increasing area, thickness and temperature but always remains above 2 MV/cm. The ß-scale parameter of the distribution shows a typical decrease with increasing area, however, it exhibits an increase with increasing thickness. This 'curious' behavior is discussed on the basis of the percolation theory. No temperature-dependence is clearly observed. Moreover, physical interpretations are carried out using the pre-breakdown current analysis.

High-temperature power electronics represent an increasing demand. Higher power density or severe ambient temperature applications become the trend, while silicon carbide components with 250-300 °C Tjmax are emerging. Among materials used in high-voltage power module, soft encapsulants play a significant role in improving both semiconductor die and module package voltage ratings, especially under enhanced electrical and thermal constraints. In operation close to their upper temperature limit, two silicone materials were selected among the most thermally stable soft insulators available today. Up to 300 °C, dielectric properties and their stability under isothermal aging in air ambient tests were characterized. The gel, tested using sandwich structures, exhibits cracking of its exposed-to-air face, at an aging temperature as low as 250 °C after less than 100 h. The elastomer, tested as free films, presents no cracking, no degraded electrical characteristics, and a 6 % relative mass loss, after 500 h aging. Moreover, the elastomer insulating properties, at low and high electric field, remains stable up to 300 °C (short-term tests), contrary to the gel which shows a strong increase in dc electrical conductivity. So the elastomer shows promising properties for improved encapsulation performance at 250 °C, to be further investigated in package configurations.

Applications of polymeric materials in electrical engineering increasingly require improvements in operating voltages, performance, reliability, and size reduction. However, the resulting increase on the electric field in electrical systems can prevent achieving these goals. Polymer composites, functionalized with conductive or semiconductive particles, can allow us to reduce the electric field, thus grading the field within the system. In this paper, a comprehensive review of field-grading materials, their properties, and recent developments and applications is provided to realize high-performance high-voltage engineering applications.

Space-charge-limited currents have been identified in thin polyimide film capacitor structures as the main conduction process in the very high temperature range from 320 °C to 400 °C before the breakdown. The transition field between the trap-filled-limit conduction and the trap-free conduction is reported versus temperature. Assuming an exponential distribution of the traps in the forbidden gap, both the characteristic temperature and trap energy are obtained at 446 °C and 62 meV, respectively. The total trap density is accurately estimated at 1.5 × 10¹⁷ cm⁻³ using the Kumar approximation [Kumar et al., J. Appl. Phys. 94, 1283 (2003)]. Finally, the mobility temperature dependence of free charges is reported between 1.6 × 10⁻⁶ and 2.3 × 10⁻⁶ cm² V⁻¹ s⁻¹ in the range from 340 °C to 400 °C.