Evaluation of failure modes in film capacitors after accelerated lifetime testing compared to field returns

Abstract:
DC film capacitors were investigated regarding their reliability. After highly accelerated lifetime testing (HALT) at high temperatures and high humidity, the results showed advanced failure modes in comparison to parts from field applications which indicates an over-stressing of the materials. SEM and FTIR analyses revealed that during the HALT the polyurethan (PU) potting of the film capacitor degraded chemically due to hydrolyses, which subsequently affected the microstructure of the underlying schoopage and the metalized foils. FEM analysis on moisture permeation within the capacitor - including experimental determination of diffusion coefficients - concluded with an elevated diffusion rate in the foils due to the exceedance of the glass transition temperature for the polypropylene foils under HALT. In summary, the work presented will give a deeper understanding of potential failure modes and possible over-stressing of the materials used in common film capacitors for DC applications.