Electro-chemical degradation of thin film X2 safety capacitors

Abstract

There is some field evidence that certain manufactured batches of thin film X2 capacitors are more susceptible to electro-chemical corrosion than others. Studies undertaken at the University of Leicester, City University London and the University of Southampton have investigated this degradation mechanism, developed underlying theory for this behaviour and validated the theory using data from damp heat testing. This paper details the anatomy of thin film X2 capacitors, details the principal mechanisms of degradation and breakdown before explaining the electrochemical corrosion mechanism and associated loss of capacitance. The effects of this degradation mechanism on other properties of the capacitor are shown to be minimal as evidenced by dielectric spectroscopy and other measurements. The ultimate conclusion is that unlike other types of capacitor, a pre-defined drop in initial capacitance does not signify end of useful life and for specific applications end of life of an X2 capacitor should be defined as the minimum value of X2 capacitance that will ensure reliable operation of a given circuit.