Degradation of printed circuit board coatings due to partial discharge

The use of power electronics associated with the drive toward higher voltages in more compact and lighter all-electric aircraft introduces new risks and failure modes. Boards are often coated for added protection; however, the performance and level of protection provided by various coatings in these environments, after some degree of degradation, is not well characterised. Partial discharge has been identified as a particularly significant and aggressive source of coating degradation which can lead to electrical failure. A series of experiments have attempted to characterise the magnitude and nature of damage from partial discharge on a silicone coating as a function of applied voltage above partial discharge inception. Simple coated boards designed around industry test standards (IPC2221A and IEC60664) were left to age and degrade while subjected to partial discharge of a given magnitude and were periodically observed under a microscope. The nature of the damage and its evolution was observed in a range of cases and these observations are presented.