Automatic breakdown voltage measurement of polymer films

Determining the breakdown strength of dielectric films is tedious, as the breakdown strength at 1% breakdown probability is more relevant to system insulation design than the Weibull characteristic breakdown strength, and obtaining reasonable confidence limits at 1% breakdown probability requires very large numbers of breakdowns. An automatic system for DC ramp breakdown strength measurements has been reported [1]. This system comprises a HV-probe and a movable arm equipped with an electrode holder that scans over the sample surface. One problem with this approach is that the metal electrodes are damaged by the breakdowns which, based on the author's experience, results in occasional low breakdowns that can distort the statistical distribution. To avoid this problem, a breakdown tester that can carry out breakdown strength measurements automatically utilizing metalized film electrodes has been developed. In this system, metalized films are used as electrodes and renewed after each breakdown. This is accomplished using a mechanical assembly under computer control through a LabView interface. By utilizing metalized films and renewing them after each breakdown, the system does not suffer from breakdown-induced electrode damage. To evaluate the system, we carried out 100 breakdowns with three active areas varying from 2 cm2 to 20 cm2. The breakdown data were analyzed using a Weibull distribution, agreed well with previous data obtained manually, and scaled properly as a function of area based on a Weibull distribution [2]. We have carried out over 600 breakdowns automatically, which is sufficient to provide very good data at low probability of breakdown. The development of the automatic breakdown strength tester provides a means to evaluate polymer films at low breakdown probability which is relevant to insulation design.