Accelerated Quality Testing for Coatings in High Voltage Applications

Abstract:
To protect against external influences such as harmful gases and moisture, electronic assemblies are often coated with polymer-based encapsulation materials, the so-called conformal coatings. Moreover, these coatings are now increasingly used as insulation material in power electronic devices to achieve isolation coordination in accordance with IEC 60664. Thus, they also should reduce the minimal possible distance between the potentials. In humid environments, especially cracks and penetration pathways in coating materials as well as adhesion weaknesses trigger failure mechanisms such as electrochemical migration (ECM) or – under high-voltage conditions – the anodic migration phenomenon (AMP). As reliable and compact power electronic devices are the key factors in several applications such as electromobility or renewable energies, the study focuses on the behavior of coatings under high voltage load in combination with humidity using accelerated quality test procedures. To get a fast statement on the quality of these mostly polymer-based protective systems, a high voltage modification of the Coating Reliability (CoRe) test according to IEC PAS 61191-10 was built. In this study, the quality of two poly urethanes, a silicone and a modified alkyd resin were evaluated. A direct comparison between low voltage and high voltage (500 V) behavior of these encapsulation materials was made. It could be shown that under low and high voltage load there is a high influence of the nature of hygroscopic residues under the coating on device reliability under humidity load. It could be demonstrated that coating systems might also be suitable for applications in power electronics. However, since fluctuating results were measured for different coating systems throughout the entire study, the focus shifts to the application process in the final analysis. Deviations in parameters during manual coating application can result in various coating qualities. Therefore, for consistently high coating quality, it is essential not only to choose the appropriate coating system but also to understand and control the individual coating process. If deeper information also about harmful gas resistivity of the coating are required, for testing multimodal stressors, accelerated coating quality testing can be performed by use of a highly accelerated stress test (HAST) based on iodine vapor.