Partial discharges in ceramic substrates embedded in liquids and gels

In Insulated gate Bipolar Transistor (IGBT) modules, ceramic substrates are used as insulation between semiconductor chips and the ground potential. The substrate, semiconductor chips and connections are covered with silicone gel to prevent partial discharges (PDs) from occurring. In this work, partial discharges are investigated on laboratory samples made of metalized substrates embedded in silicone gel and various liquids. Substrates are made of aluminum nitride (AlN), alumina (Al₂O₃), or glass/epoxy composite. The objective is to determine the actual origin of PDs, by changing the nature of encapsulating material and substrate. The various liquids used are first characterized using a point-plane geometry. Experiments show that a rather large discrepancy on the PD behavior can be due to the ceramic substrate itself. "Good" and "bad" substrates can be identified on the basis of PD measurements. The influence of the sample preparation (degassing procedure) is also evidenced. The main conclusion obtained in this paper shows that partial discharges do not occur within the encapsulating material, contrary to most previous hypotheses, but within pores existing in sintered ceramic materials. Changing the substrate nature has a large influence on PD behavior, whereas a modification of the encapsulating material has almost no effect.

I. Introduction

The development of Insulated gate Bipolar Transistor (IGBT) modules operating under high voltage (up to 6.5 kV in railway applications) requires to study in details the degradation mechanisms of insulating materials, in order to ensure the long-term reliability of the devices. To evacuate the heat coming from semiconductors (transitors or diodes) towards heat-dissipating systems, these components are usually mounted on metallized ceramic substrates, choosen for both their electrical properties under high voltage, thermal properties (resistance to high temperatures, good thermal conductivity), and mechanical properties (dilatation coefficients as close as possible with the other materials). The substrate materials frequently used are Aluminium Nitride (AlN) and Alumina (Al₂O₃).