Reliability of Substrate Embedded Rectifiers for High Voltage Applications

Abstract

As of today, various solutions to handle the dissipating heat of power electronics devices are available. These include the application of heatsinks, overmolding, embedding of components into substrates, use of substrates with embedded metal or ceramic heatsinks or liquid cooling approaches. When it comes to power electronics for high voltages and fast switching the parasitic capacity has to be considered. This parasitic capacity affects the electrical performance and may finally even lead to damage of the device. Embedding of metal heatsinks or mounting a substrate to a metal heatsink can even increase the parasitic capacity and hence, worsen the scenario. In this project a rectifier had to be built suitable for voltages of up to 20 kV and switching frequencies of 100 kHz while achieving a low parasitic capacity of max. 3 pF. High voltage diodes were selected to meet the electrical requirements. To fullfil both the thermal and capacitance demands the diodes were embedded into a substrate made from a highly thermal conductive FR4 material. In addition, the substrate is mounted to a ceramic heatsink to enable a superior cooling but to limit the parasitic capacity at the same time. This setup was characterised for its thermal management behaviour in the as build state. Though the lamination of the substrate to the ceramic heatsink showed some challenges its cooling performance could be assessed. Subsequently, the system without the ceramic heatsink was exposed to temperature shock cycles at −40/+125°C for up to 2,000 cycles to analyse the long term stability of the system behaviour. For the repeated investigation of the thermal behaviour and the structural integrity of the system a novel analysis approach using an infrared camera was applied. Cross sections were done in addition to verify the results from the novel thermal analysis approach. As of now no thermo-mechanical damage of the rectifier could be observed proving the ability of the embedding approach and the validity of the results gained with the novel non-destructive analysis approach.