ELNA AMERICA, INC.

Failure Rates of Tantalum Chip Capacitors

Tantalum solid state electrolytic capacitors have tantalum and tantalum oxide layers in the structure of the anode and the dielectric. These layers are extremely stable chemically, but also the structure is essentially robust even at high temperatures (because the magnesium dioxide that is the solid electrolyte is a stable inorganic solid). Also, these layers have stable electrical characteristics and have long life expectancy. Since the tantalum oxide film that serves as the dielectric is an extremely thin solid element, it is susceptible to sudden failure. Consequently, the failure rate curve does not assume the same bathtub shape as for aluminum electrolytic capacitors. Rather, the tantalum failure rate curve can be thought of as showing a gradual declining tendency over time. Consequently, one should not think of the failure mechanism as being in the realm of wear-out failures, but rather as having a half-life distribution.

More than 90% of the failures in tantalum solid-state electrolytic capacitors are caused by shorts or increased leakage current due to deterioration of the dielectric thin film layer. A variety of factors affect the reliability (failure rates) of the tantalum solid-state electrolytic capacitors. The factors having the largest impact on reliability are the temperature, applied voltage, and series resistance.
The failure rate for the tantalum solid state electrolytic capacitors has been set at <1% per 1000 hours (60% reliability level) at 85°C, the rated voltage, and a circuit resistance of 3 per volt. (See the Elna catalog and technical notes.) The mean time between failures (MTBF) for this logarithmic reliability function shows the same failure rate regardless of the time. Also, the MTBF is the inverse of the failure rate.
With the above settings, the MTBF = 1/(1x10-5) = 1 x 105 Hours = more than 11.4 years

In actual application, factors such as the voltage and temperature are not as severe as the assumption above. Thus, the failure rate can be expected to be even lower. The change in reliability as a function of the temperature and the applied voltage, otherwise known as, the impact on the failure rate, follows the Arrhenius chemical reaction velocity theory. This theory is based on the dependence on the deterioration of the oxide thin film layer due to voltage and temperature.
As is convenient when calculating using MIL-HDBK-217, the correction coefficients for the basic failure rate due to the respective cause factors can be multiplied together and used to generally estimate failure rates.