Christopher Emersic

The effects of pressure and temperature on partial discharge degradation of silicone conformai coatings

The partial discharge damage rates for silicone-coated printed circuit boards have been quantified in a series of experiments at pressures and temperatures relevant to the aerospace industry (down to 116 mbar, −55°C to +70°C) and up to 6 kV. Surface cracking was observed, and damage magnitude was found to be non-linear with coating thickness, with thinner coatings experiencing relatively greater damage rates. This is attributed to higher surface electric fields for a given energisation voltage. Increasing temperature or reducing pressure increased the rate of damage. For coating thicknesses less than 100 μm, reducing pressure to 116 mbar (1 mbar = 100 Pa) increased the relative crack growth rate by nearly an order of magnitude. Temperature change had the most profound influence on damage; low temperatures were observed to substantially reduce damage rate, with very little or no damage observed, whereas higher temperatures substantially increased damage rate, with the resulting magnitude of surface damage too large to quantify. Silicone coatings of thickness greater than 250 μm showed no appreciable damage from partial discharge when aged at either low pressure or high temperature at voltages up to 6 kV. Corresponding damage-free surface electric fields are computed. No samples were observed to fail, indicating the robustness of high quality silicone coatings. Possible causes of crack formation in silicone are discussed.

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.

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.

Thermal stresses of conformal coatings on printed circuit boards

As part of the drive towards lighter aircraft and reduced fuel consumption, power electronics are expected to replace mechanical and hydraulic systems on aircraft necessitating a move to higher operation voltages. The combination of higher operating voltages. The aerospace environment introduces the potential for a number of failure mechanisms including regular temperature cycles, low atmospheric pressure, chemical contamination, a higher likelihood of partial discharge and condensing humidity that could reduce the performance or cause failure of the power electronics. Conformal coatings have previously been used to protect power electronics but the long term performance of the coatings on circuit boards in an aerospace environment is unclear. This paper will describe how the thermal cycles experienced by aircraft and the rapid temperature rise due to the switch-on of the electronics affects the development of mechanical stresses in the components of a printed circuit board. With the aid of thermogravimetric analysis, an assessment of whether the thermal cycles and switch-on of the electronics is likely to have an effect on the ageing of the conformal coating is made.