When choosing capacitors, properties such as volumetric efficiency, frequency stability, temperature rating or equivalent series resistance are often the primary factors that govern technology selection. In these cases, understanding factors affecting lifetime can help engineers ensure the product will deliver the required reliability.

On the other hand, a long operational lifetime may be a key requirement of the end product and ultimately may determine device selection.

Capacitor manufacturing processes such as screening, or processes to control the purity of materials or components, can provide a higher assurance of reliability that allows engineers to reduce the number of capacitors in-circuit and hence reduce solution size and cost without compromising reliability.

Capacitor Properties

Capacitors made with metallised polyester or polypropylene film, for example, are known to have a long operational life. High-voltage or high-temperature properties make these devices ideal for applications such as automotive electronics or lamp ballasts, while self-healing helps to overcome the effects of small impurities in the dielectric that can lead to short-circuit failures.
On the other hand, as these weaknesses heal the total available capacitance begins to drop and the equivalent series resistance (ESR) starts to rise. This ultimately governs the lifetime of the device. Using high-quality materials and dielectric-manufacturing processes can minimise defects leading to a slower rate of self-healing.

In alternative-energy applications, where low ESR is particularly desirable to minimise energy losses, it is possible to verify operational lifetimes of several decades, even when capacitors are operated at temperatures of 70°C or above.

Aluminium capacitors cover a number of different types of construction, each of which has very different lifetime performance. Wet-electrolyte capacitors, for example, have a well-defined and understood wearout mechanism. The electrolyte is mildly acidic, and will therefore degrade the dielectric over time.

On the other hand, the electrolyte also provides the oxygen necessary to re-grow the dielectric. This is why it is important to consider the “shelf-life” of a wet aluminium electrolytic capacitor that has not been powered—whether on a shelf or on a board.

Figure 1: X5R and X7R MLCCs combine nickel-based BME and doped barium titanate dielectric.