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 mu m 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 mu m 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 mu m 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.