Endurance of Thin Insulation Polyimide Films for High-Temperature Power Module Applications

This paper deals with thermal endurance tests carried out on thin polyimide (PI) films widely used in high-temperature power electronics applications. Specimens deposited on stainless steel supports are aged in an oven in air at four different temperatures up to the glass transition temperature (between 250°C and 360°C). Dielectric strength is measured at different aging times as an indicator of PI films endurance. In this paper, in addition to this property, other properties are measured during aging, such as dc conductivity at low field, film thickness, and film morphology. The evolution of these parameters is correlated to the breakdown voltage of the films to establish more accurate endpoint criteria. The results show that the breakdown voltage decreased during aging but not likewise over the whole aging period, because of the effect of the initial surface degradation. This might lead to an underestimation of the lifetime of the films. The thickness degradation rate is constant over the rest of the aging period, making it a suitable endpoint criterion. However, the low field dielectric properties show an improvement during the aging making them an unsuitable aging marker.