30. March 2017

Minimum shelf-life for storing electronic devices

Some important aspects

From a technical point of view, the "shelf-life" of an electronic device is significantly influenced by its aluminum electrolytic capacitors ("E capacitor"). This is what the analysis of shelf-life with its usual calculations of longevity and reliability initially has in common. If, in a conventional reliability calculation for continuous operation, the operating temperatures, which gradually lead to a drying out of the electrolytic capacitor and to an irreversible loss of capacity, are crucial, completely different effects must be taken into account for passive storage to determine the shelf-life of the device. Some important aspects are listed below:

  • Storage temperature and humidity
  • b) Number of climate fluctuations and intensity
  • c) Quality of the capacitor seals at the lower end of the cup (stability or porosity of the rubber mixture of seals)
  • d) Nature and chemical composition of the capacitor's electrolytes
  • e) Dealing with formation effects of the capacitors

A particularly noteworthy effect is the degeneration of the inner oxide films on the anodic surface which form the actual dielectric of the electrolytic capacitors. With some specific electrolytes, a reversible chemical effect occurs due to the intercalation of ions into the oxide structure, and a corresponding increase in DC cutoff currents caused by the capacitor. Due to the different electrolytes used, high-voltage electrolytes (for example glycol-based electrolytes) present a greater risk than low-voltage electrolytic capacitors. Electrolytic capacitors for smaller voltages are generally considered stable and are not significantly affected by these cutoff effects.

By applying voltage and the associated current flow, the oxide film of a long-stored capacitor can be reconditioned due to the flowing direct current, and the capacitors can be reused after a short time. Only the initial cutoff current (leakage current) can reach relatively high values and permanently damage the electrolytic capacitor, which is why a limited gradient of the voltage increase or a direct limitation of the leakage current by a series resistor is recommended. Depending on the size of the chosen series resistor, the period for a reconditioning of the oxide film may take more or less time. A decreasing cutoff current indicates such a process.
In their data sheets, the manufacturers also describe these effects. The following example is an original quote taken off a major supplier's electrolytic capacitor data sheet for long-term storage of this component:

" Long term storage
Leakage current of a capacitor increases with long storage times. The aluminum oxide film deteriorates as a function of temperature and time.
If used without reconditioning, an abnormally high current will be required to restore the oxide film. This current surge could cause the circuit or the capacitor to fail. After one year, a capacitor should be reconditioned by applying rated voltage in series with a 1000 Ω, current limiting resistor for a time period of 30 minutes. If the expired date of products date code is over eighteen months, the products should be returned to confirmation."

The required measures are, of course, extremely conservative. It is hardly possible to implement them for both slowly changing capacitor stocks and complete devices which also explains why nobody wants to grant warranty.

In addition to the electrolytic capacitors in a device there are also many other components with their own dependencies in shelf-life. These are, however, much more long-term compared to the observed capacitor's effects and can therefore be neglected, provided that the devices are not to be stored over decades.

The problem with the electrolytic capacitors does not arise with the storage of finished new products: new assemblies are usually started power-limited which fortunately allows the capacitors to recondition the potentially lost oxide film.
Typical minimum storage times, while keeping the nominal cutoff currents at T<40°C, are barely mentioned by capacitor manufacturers. A constant lowering of the storage temperature at 25°C, however, considerably extends the possible storage periods.

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