Silicone gel is widely used to encapsulate the power modules. In modules, partial discharges make the cavity and it propagates on the substrate. This article reports fundamental propagation characteristics of such cavity under repetitive voltage impulses, which will be applied to actual modules under the operation of pulse-width modulated voltage. The influence of frequency, rise time, pulse width of the repetitive surge voltage on the cavity length were investigated. The results show that the cavity length increases with the frequency. And also, the results show that the cavity length is especially long when the rise time is short.