Electrical treeing in silicone gel under repetitive voltage impulses

Silicone gel is widely used to encapsulate power electronic modules. The weakness of the electrical insulation is surface discharges initiated at the gel-substrate-electrode triple junction and the subsequent formation of cavities; so called electrical trees. The propagation characteristics of cavities under high frequency and fast rise time voltages, which are typical waveforms formed in power electronic modules, have not been fully understood. Thus, in this research, the influence of frequency, rise time, and polarity on electrical treeing under repetitive voltage impulses are investigated. The results show that discharge tracks under positive impulses have filamentary shapes, and discharge tracks under negative impulses have spindle-shapes. The cavity length increases with the frequency and is especially long when the rise time is short under high frequency. This indicates the importance of understanding the cavity propagation characteristics under actual voltage waveforms in power electronics modules.

1 Introduction

Recently the power electronic modules has grown rapidly for efficient power conversion [1]. Figure 1 shows a cross section view of a power electronic module [2]. There are wire bonds in the modules for electrical connections between the chip and output pins [3]. The wire bonds can easily be pulled off because of thermomechanical stresses. Silicone gel is used to encapsulate the modules because of its good characteristics such as adherence, elasticity to absorb mechanical stress, non-combustibility and self-healing.