I. Introduction
Partial discharge (PD) is a key degradation mechanism of insulating materials in high voltage power modules. PD is caused by high electric field strengths in the vicinity of the edge structures of the installed ceramic circuit boards such as DBC (Direct Copper Bonded) or AMB (Active Metal Brazed) substrates. Especially for upcoming high-blocking (HV) semiconductors beyond 6.5 kV novel substrate technologies must be developed to withstand the electrical strains over the lifetime of power modules. A simple scaling of the ceramic thickness has two drawbacks. The specific insulating properties of the ceramic material decrease disproportionately with elevated thickness. Furthermore, the increased ceramic thickness leads to an increase in thermal resistance. Moreover, the commonly used organic encapsulation materials, applied to prevent electrical discharges in air and to protect from contamination, also have to withstand these electric field strains. Already small voids inside these coatings or wetting imperfections at interfaces provoke local electrical field enhancements, where PD may take place and damage the insulation materials.
Cross sections of coated DBC trenches, left: Coating with low dilution rate (left), right: Coating with high dilution rate