Electrolytic capacitors are used in various medical, military, aerospace, and commercial applications where it is critical that the capacitors are reliable and have extremely low failure rates. As such, various screening methods such as accelerated aging tests, reflow tests, surge current tests, and breakdown voltage tests have been developed for screening electrolytic capacitors to eliminate defective parts. However, many of these tests have failure criteria that only look at catastrophic failures (i.e., fuse failures), which can allow defective parts to be released into the good population, and these screening methods are not capable of detecting latent defects. For instance, although a fuse may not have failed under highly stressed conditions such as high voltage and temperature, the capacitor being tested may still be damaged during screening, which can result in long-term instability. Traditional methods for screening and delivery of high reliability electrolytic capacitors have involved Weibull calculations based on a lot by lot sampling where a small number of capacitors are subjected to highly accelerated voltage (e.g., 1.5 times the rated voltage (VR)), temperature (e.g., 85° C.), and time (e.g., 40 hours or more) conditions during burn in. However, traditional Weibull burn in allows parts that are statistically different pre-burn in to move into the normal population post-burn in because there is no pre-burn in screening to remove parts with early failures. Although a majority of these parts appear to be stable through long-term reliability testing (e.g., life testing), possibly due to self-healing during burn-in, a portion of the parts passing through to the normal population are unstable and may have long term reliability issues in the field. The Weibull statistical calculation promotes the practice of leaving these unstable parts in the population so that a Weibull distribution can be created for grading purposes, as described in MIL-PRF-55365H. As a result, screening using Weibull testing cannot insure the removal of unstable or defective capacitors from the population, which can result in a capacitor lot that has an unacceptable level of reliability. Thus, despite the benefits achieved, a need exists for an improved screening method for electrolytic capacitors that can detect and remove capacitors having latent defects and well as for a method of determining the predicted failure rate of the screened capacitors that does not take the removed capacitors into account, in contrast to the Weibull method.
USPTO Backgrounds
USPTO Backgrounds