Abstract
The failure mechanism of solid tantalum and aluminium capacitors have been investigated using a combination of electrical measurements and electron microscopy. The capacitor dielectric was examined before and after life testing and changes correlated with electrical measurements.The basic mechanism of failure of solid tantalum capacitors is found to be field crystallisation of the essentially amorphous dielectric oxide. The growth of higher conductivity crystalline oxide during operation of the capacitors causes an increase in leakage current and may result in catastrophic failure. The effect of field crystallisation can be minimised by using high purity tantalum to reduce the number of crystallisation nucleation sites. Since crystalline growth is primarily dependant on applied voltage, high voltage capacitors are much more susceptible to failure than low voltage units.There appears to be no long term failure mechanism in solid aluminium capacitors. However, a particular problem with these units is that they are difficult to make. This is because the anodic layer is chemically less stable in the case of aluminium than in the case of tantalum. The attack is initiated during the deposition of manganese oxide by pyrolysis from manganese nitrate solution and developed by the reform process. Solid aluminium capacitors often have a lower capacitance and higher initial leakage current than comparable solid tantalum units; however, the leakage current decreases on life tests and their reliability is high.
Highlights
In solid electrolytic capacitors the liquid electrolyte of conventional liquid filled types is replaced by a solid conductor of manganese oxide. 1’5 The higher conductivity of manganese oxide imparts significant improvements in performance, especially under conditions of low temperature and high frequency
Failure processes in solid aluminium capacitors, the dielectric of which is known not to undergo field crystallisation, have been examined
It can be seen that low voltage capacitors and capacitors using purified foil have low and constant leakage currents, whereas high voltage capacitors exhibited a sharply rising leakage current which peaked early in the life test and slowly declines thereafter
Summary
In solid electrolytic capacitors the liquid electrolyte of conventional liquid filled types is replaced by a solid conductor of manganese oxide. 1’5 The higher conductivity of manganese oxide imparts significant improvements in performance, especially under conditions of low temperature and high frequency. When the capacitor is used in a low impedance circuit, failure rate may be high. Earlier studies in these laboratories of field crystallisation of anodised tantalum in aqueous electrolytes[3] prompted a belief that the same process might be the principal cause of failure in solid capacitors, it has been shown that at least for aqueous electrolytes, other failure mechanisms are possible.[4]. The present work attempts to determine whether field crystallisation is responsible for the observed failures in solid tantalum capacitors and the mechanism by which such failures occur. Failure processes in solid aluminium capacitors, the dielectric of which is known not to undergo field crystallisation, have been examined. The work has principally involved a study of the changes in the physical, chemical and electrical properties of the dielectric of experimental capacitors during storage at elevated temperature with voltage applied
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