Effect of Process Parameters on the Insulation Resistance of Conformal Coating for PCBs

The insulation resistance of polyurethane and poly (para dichloroxylelene) conformal coating was characterised following exposure to hygrothermal environment. Three types of test specimens were used—the standard Y type pattern, the ‘comb’ pattern and the ‘twin’ conductor type. The PCB with the comb pattern has been found to be the best testing specimen for the evaluation and comparison of conformal coating materials and processes. Using this type of PCB, the effects of predrying and thickness in polyurethane coated PCBs have been studied. While predrying has proved to be beneficial for preventing degradation of insulation resistance upon exposure to hygrothermal conditions, only marginal improvement of resistance is obtained when the polyurethane coating thickness has been doubled from 50 to 100 µm. Tin‐plated copper exhibited enhanced insulation resistance compared with bare copper for both polyurethane and polyxylelene coatings. Finally, a thin polyxylelene coating has demonstrated improved moisture barrier properties compared with a thicker polyurethane coating as determined from the resistance degradation with exposure duration to heat and humidity.