Self-healing Process in Metallized Capacitors

Even the best plastic films, like ceramic materials, are not free from pin-holes. However, in the case of metallized film capacitors it is possible to eliminate these faults by applying a much higher voltage than the rated voltage. This process is known as self-healing and practically makes a "zero defect dielectric" possible.

Figure 1: Schematic representation of the self-healing process		Figure 2: Isolated area after the self-healing process

The self-healing process is started by an electric breakdown, which takes about 10^-8 secs. In the breakdown channel, the dielectric is transformed into a highly compressed plasma which is pushed out of the channel and presses the dielectric layers apart (figure 1). In the spreading plasma, discharging continues over the metal electrodes. Temperatures of approximately 6000 K occur and insulated areas are formed around the original failure spot (figure 2). This self-healing process takes a few µsec and the discharging in the plasma has already ceased before a greater loss of voltage takes place. This quick extinction of the plasma is necessary to avoid further damage to the dielectric layer next to the point of failure. The pressure between the layers must not be too great, so that the plasma can spread out from the breakdown channel quickly. Large parts of the plasma get into areas of low field strength. The flawless course of the self-healing process depends on the thickness of the metallization, on the chemical composition and on the rate of the applied voltage; here, apart from the chemical composition, the production conditions have to provide the prerequisites for optimum self-healing. Self-healing behaviour as a quality standard.