Failure Modes and Mechanisms in Solid Tantalum Capacitors

Both the catastrophic and leakage current increase failure modes are directly associated with the current flickering phenomenon peculiar to the solid tantalum capacitor. These current bursts are themselves associated with imperfections in the tantalum oxide film. Imperfections or flaws can arise from several sources such as impurities (both metallic and gaseous) in the base metal, surface geometry of the metal, physical damage to the oxide, or thermal cycling of the metaloxide during processing. Localized heating within microscopic flaws due to relatively high current flowing through such sites can account for leakage current rises and also subsequent flickers assuming an energy barrier has been exceeded which allows a sudden avalanche flow of current to occur. Surges which occur with no apparent gradual leakage current increase are difficult to explain, but one can consider such possibilities as slow chemical reactions at extremely small areas or field-induced diffusion of ions and/or vacancies in the oxide. Occurrence of either failure mode can be reduced or eliminated by various combinations of applied voltage, environmental temperature, or circuit resistance. Infrequently failures due to increases in capacitance and/or dissipation factor are found. A defective hermetic seal which allows moisture to enter the unit is the most common cause for such failures. Since more of the available tantalum oxide surface is contacted by the moisture than is by the MnO2, the capacitance shows an increase. The increase in dissipation factor results from an increase in the resistivity of the MNO2 due to the presence of moisture.