NIPPON CHEMI-CON CORPORATION

When designing devices using aluminum electrolytic capacitors, it is necessary to consider failure rate and useful life from a reliability perspective.
The failure rate of aluminum electrolytic capacitors is approximated by the bathtub curve shown in Fig-17.

1. Early failure period
   At the comparatively early periods of use, devices/components fail due to deficiencies in design or manufacturing process or incompatibility with operation conditions. For aluminum electrolytic capacitors, these defectives are removed by debugging at one of manufacturing processes before shipments.
2. Random failure period
   The failure occurrence is low and stable, appearing unrelated to operating time. Aluminum electrolytic capacitors are characterized by low catastrophic failures in this period compared with semiconductors and solid tantalum capacitors.
3. Wear-out failure period
   In this period, the failure rate increases with operating time. For aluminum electrolytic capacitors, since they were completed in manufacturing, the electrolyte impregnated has gradually evaporated and diffused out of the capacitors through the rubber seal materials with time, which leads to decrease in the capacitance and/or increase in tanδ. When any of these values changes beyond the allowable range of specifications, the capacitors are defined as having “reached the wear-out failure”. The operating time until the capacitors reach the wear-out failure period is called the useful life.

Aluminum electrolytic capacitors have two categories of failures: catastrophic failure and wear-out failure.

• 【Catastrophic failure】
  This is a failure mode that completely destroys the function of the capacitor such as short circuit and open circuit failure.
• 【Wear-out failure】
  This is a failure mode where the electrical parameters of the capacitor gradually deteriorate and fail. The criteria for determining if this failure has occurred depend on the purpose of a device. For each series of capacitors, the following electrical parameters have been defined as criteria in the specifications of Endurance.

• Change in capacitance
• tanδ
• Leakage current

Failure rates are often measured in units of % per 1000 hours (10^-5/hour). For higher reliability devices designed with a smaller failure rate, units of Failure In Time (FIT) (10^-9/hour) is used.

Aluminum electrolytic capacitors are considered as components of wear-out failure mode, the electrical characteristics of which gradually deteriorate and their failure rate increases with time. In general, the failure rate in FIT is determined by total component-hours (product of the number of tested components and test hours).

Due to the definition of FIT, the same FIT rate can be calculated in both cases of testing on a large number of tested components and also testing for long test periods of time. However, these cases have different meanings for aluminum electrolytic capacitors. Using the failure rate is not suited to express the reliability of aluminum electrolytic capacitors, but the electrical characteristics-based lifetime in hours should be considered to express the reliability.

Also, there are MTBF (Mean Time Between Failures) and MTTF (Mean Time To Failure) to express reliability. The latter is applicable for aluminum electrolytic capacitors because they are categorized into a group of non-repairable systems, equipment and devices for which MTTF is applicable, and is expressed as the average operating time to failure “MTTF [xxx] hours”.