Failure Mechanism of Solid Tantalum Capacitors

It has been suggested that failure of solid tantalum capacitors is due to thermal migration of impurities from the tantalum anode to flaws in the oxide layer. This implies, however, that leakage current gradually increases under normal operating conditions, an effect which has not been observed. An alternative hypothesis advanced here is that failure is due to crystallization of tantalum oxide under the influence of the electric field. Scanning electron microscopy of specially cleaned anodized tantalum sheet on which thin gold electrodes have been deposited clearly shows the occurrence of crystallization after 17 hours at an applied voltage of 75 V and a temperature of 65℃. Results of accelerated life tests on solid tantalum capacitors at temperatures of 85℃ and 125℃, and at up to 2,5 times rated voltage also accord better with a field crystallization hypothesis than with a thermal migration failure hypothesis.