Film Capacitor Failure Analysis

Film capacitors can fail due to a number of causes including:

Film capacitors use stacked metallized polymer films to form the capacitor structure. The metal plates are usually evaporated aluminum. The polymers can be Polyethylene-terephthalate (PET), polycarbonate (PC), polypropylene (PP) and others. The internal structure of these capacitors is illustrated below.

Film Capacitors

The metallization on the film is made thicker to support termination to the metal contact layer (see below). The relative peak heights for aluminum (Al-Ka1) and carbon (C-Ka1) are a measure of the aluminum thickness.

Film Capacitors
BSE SEM image on left, and EDS spectra on right.

The image below shows a capacitor roll prior to application of the metal contact layer. The distortion in the roll can lead to unpredictable results with the application of the metal contact layer and contribute to thermal-mechanical instability.

Film Capacitor
Secondary Electron SEM image of the end of a film capacitor roll.