How Heat Treatment Affects Film Foil Capacitors

The effects of heat treatment between 74°C and 130°C on the dc breakdown voltage (VB) and the change of capacitance with temperature have been determined for capacitors with Mylar film insulation. Some effects of the temperature of heat treatment on discharge life and on the rate of water desorption from rolled capacitors were also determined. It was found that the breakdown voltage decreased from 11.5 kV dc for 0.4-/µF capacitors with 50.8 µm of insulation processed at 74°C to 9.9 kV for similar units processed at 130°C. The change in capacitance associated with a temperature change from -55°C to 74°C was 8.5 percent for the units processed at 74°C and 6.5 percent for units processed at 130°C. Evidence of burning at tabs and at random locations along the foils was found when capacitors heat-treated at 74°C were unrolled after failure in discharge tests. No similar evidence was seen in units processed at 100°C and subjected to the same tests. Finally, results from the water desorption studies show that drying of Mylar capacitors can be accomplished in 10 h or less at temperatures below 74°C. Hence the choice of temperature for processing must be based on a consideration of all requirements placed on a capacitor.