Observations of breakdown through printed circuit board polymer coatings via a surface pollution layer

Evidence is presented of breakdown through a conductive surface pollution layer on silicone conformal coated boards at much lower voltages than would be expected between tracks through the bulk polymer coating. The preferential breakdown path is governed by the ratio between track separation and the thickness of coating. Finite element analysis has shown that the electric field strength can increase by a factor of ten above a 100 μm silicone coated energised track as a result of the presence of a conductive pollution layer above the coating. Experimental observations also revealed that the conductivity of the pollution does not affect breakdown voltage (when discharging through the pollution layer) for pollution conductivities of 2500 μ8 cm^-1 and greater. These observations are relevant to applications where the surface of coated power electronics boards can become contaminated and are expected to operate at high voltages.