Publications Details

Failure mechanisms in high voltage Mylar capacitors

Herzberger, Jaemi L.; Tanner, Danelle M.

Biaxially oriented polyethylene terephthalate (BO-PET/Mylar®) polymer film is commonly used as the dielectric in high-voltage, pulse-discharge capacitors because of its high dielectric strength and insulation resistance over a wide temperature range [1]. This study focuses on the use of a systematic physics of failure (PoF) approach to assess possible design and fabrication problems in BO-PET capacitors. A destructive physical analysis (DPA) procedure, which is an essential technique in understanding the failure modes and mechanisms in capacitors, has been developed through this research. Short-term breakdown (STB) testing was performed on capacitors from two independent development builds and the results are compared. It was identified that the two primary failure mechanisms occurring in these capacitors under high voltage conditions were edge margin arc-over and dielectric punch-through. Evaluation of the electrical parameters after accelerated voltage testing revealed that the combination of lower than expected voltage breakdown values (near the voltage rating of 3.6 kV) and in-spec capacitance and dissipation factor (C/DF) values indicated an arc-over failure, while high voltage breakdown values (greater than 2.5 times the voltage rating) and out-of-spec C/DF values indicated a dielectric punch-through failure. Thick buried edges, creasing, high curvature, insufficient inactive wraps, arc spray, and inadequate edge margin were some of the modes that led to arc-over and punch-through failures. Many of these failure modes were traced back to unsuitably designed capacitors or issues with the process control during manufacturing.