Degradation of conformal coatings on printed circuit boards due to partial discharge

A series of experiments has characterised the nature of damage to the surface of silicone-coated printed circuit boards resulting from partial discharge. A potential difference was applied between tracks and observations indicate that partial discharge activity occurred above the coating surface. Analysis of damage development indicates that coatings 70 μm or less in thickness are more severely degraded and thus offer reduced protection from active partial discharge. Coating degradation is reduced with increasing thickness, with coatings of 180 μm and greater showing the least degradation. Finite element analysis indicated surface electric field strengths of 7.5 to 8.0 kV mm^-1 could be achieved before partial discharge damage was observed in the 180 μm coatings. Inconsistent and fluctuating partial discharge inception voltages may be a consequence of at least two competing factors that alter surface electric fields, namely the accumulation of surface pollution of a finite conductivity which reduces surface fields, and the surface field-strengthening effects of developing surface defects.