Surface Discharges in Silicone Gel on AlN Substrate

AlN substrate is widely used along with silicone gel to encapsulate power electronic circuits. It is known that the weakness of the insulation system is surface discharges propagating at the gel-substrate interface. In this research, the nature of surface discharges in gel, on various substrates was investigated. The results are as follows: the maximum stopping length of cavities on AlN substrate was more than twice than that on other substrates, and light emission due to discharges in cavities on AlN substrate was different from that on other substrates; for AlN substrate, tip of the cavity emitted light while other part did not emit light, and for other substrates, all the path along the cavity emitted light. These results indicated that the surface of the AlN substrate is degraded by discharges and becomes conductive. We have confirmed this assumption by measuring the conductivity of the cavity path: 5 k Omega/100 mu m for that on AlN and above 1 M Omega/100 mu m for Al2O3 and glass substrates. The high electric field at the tip of conductive path elongates the cavity stopping length on AlN substrate compared with others. In order to investigate the degradation process of AlN substrate surface, energy dispersive X-ray spectroscopy analysis was carried out. Reduction of nitrogen component on the cavity path was seen, indicating that generation of Al increases the conductivity of cavity path. We have experimentally shown that oxidation treatment of the AlN substrate significantly reduces the cavity stopping length.