Current leakage failure of conformally coated electronic assemblies

Conformal coatings are widely used on circuit board assemblies as an attempt to improve reliability and to ensure high insulation impedances, which are for example demanded by low current consumption battery operated RF circuitry. However, components, such as small ceramic capacitors, have occasionally been found to fail in some applications, particularly when covered with a thick silicone conformal coating. This is thought to be due to the diffusion of water through the coating to the capacitor surface where it then combines with solder flux residue, or other organic or ionic contamination left on the components, thereby dramatically increasing the effective component leakage current. The primary objective of this experimental research is therefore to establish a clear understanding of the effects of moisture exposure on the surface insulation resistance (SIR) of conformally coated printed circuit board (PCB) assemblies. This has been achieved through leakage current measurements on multilayer ceramic capacitors during storage in an environmental chamber during testing similar to IPC standards for non-component loaded boards.