Partial discharge deterioration of epoxy resin for electronic parts

Recently many electronic parts are widely used for the power electric field, such as power transistor module, diode and solid state relay. These are encapsulated with solid insulating materials such as epoxy resin or silicon rubber. In these category of parts, majority of electrical failures has been caused by inadequate material selection or workmanship error for encapsulation systems. It is suspected that the cause of the electrical failures is due to partial discharge (PD) in voids of insulating materials or in cracks in the vicinity of conductors. Accordingly, the test on PD has been performed for these products. Although PD quantity (e.g. PD current, Qmax), decreases gradually during voltage application with ordinary electrical PD measurements, while solid insulating materials degrade along with time and finally result in electrical breakdown [1,2].