Compatibility study of no-clean flux residue and conformal coatings using two electrode electrochemical impedance method

Conformal coatings are widely used on electronic assemblies to protect electronic devices from ionic contaminants, humidity, and dust. This study focused on the influence of the process related no-clean flux residue on the protection performance of three conventionally used conformal coatings under cyclic humidity conditions. A test Printed Circuit Board (PCB) with comb-structure surface insulation resistance (SIR) pattern was used as test vehicle. Two-electrode electrochemical impedance spectroscopy (EIS) was used to monitor the water uptake behavior of the coatings under cyclic thermal-humidity exposure in the air, followed by chrono-amperometry to evaluate the effect of penetrated moisture reaching the PCB surface. Failure modes of the SIR include increased leakage current and degradation product such as electrochemical migration (ECM) between the comb-structure electrodes. The surface morphology and adhesion was inspected using scanning electron microscopy and corrosion product analysis was carried out using energy dispersive spectroscopy. Results suggest that the moisture barrier properties of the coating and adhesion of the coating are important parameters affecting the corrosion protection performance and the presence of no-clean flux residue has most significant influence on the adhesion of the coatings. Among the coatings investigated, elastomeric acrylate showed best performance due to unsurpassed adhesion even under aggressive solder flux contaminated conditions. Two electrode EIS method could differentiate the compatibility of flux residue and conformal coating within few hours.