Power electronics reliability in a high humidity environment has recently been considered an important aspect of power converters. In this article boost stage power semiconductor modules from two vendors with varying device technologies were exposed to high humidity, high temperature and high voltage reverse bias direct current (H3TRB-HVDC) test conditions. The exposed modules were monitored with in situ leakage current measurement during the test. The in situ monitoring revealed that all samples from one vendor using a glass passivation technique failed while the samples from the second vendor with a polyimide passivation survived. After the test, electrical breakdown and associated thermal hotspots were consistently observed using lock-in thermography on top of the glass passivation covering the high voltage edge termination structure. It was concluded with microstructural and material analysis that the treelike structures that were observed right beneath the surface of the glass passivation were associated with the failure mechanism. The analysis revealed that the formed structures were amorphous and could not be caused by the electrochemical migration of metallic species. The failures were caused by the localised partial dissolution of lead from the glass passivation. The failure mechanism seems to be associated with water treeing.