Failure modes in capacitors

Knowing where, when, and how these components fail can help you down the line

BY WALTER BONOMO, GEOFF HOOPER, DAVID RICHARDSON, DEAN ROBERTS, and THEO VAN DE STEEG
Vishay Intertechnology
Malvern, PA
http://www.vishay.com

Capacitors are at great risk for failure. While it is certain that over time some wear out and no longer adequately serve their purpose, capacitors can also fail prematurely.

This article will show the various points where capacitors can be damaged and are at the highest risk of failure. Designers that are aware of these points will be better suited to choose the right capacitors for their applications and possibly avoid failures.

A component fails when it no longer meets the requirements of the application. Through quality control and good manufacturing, component manufacturers can prevent these components from reaching the customer. But failures can also happen during processing, assembly, and use of the part.

Transit and storage

Capacitors are at risk of damage in transit or even in storage, well before they are implemented in a design. If a capacitor becomes damaged, either externally or internally, there is a good chance that it will fail.

When transporting components, rough handling can damage boxes. They can be accidentally dropped, damaged with a forklift, or simply stored incorrectly.

For the capacitors inside the boxes, this can result in bent leads and taping distances being out of specification, both of which are detrimental. And if the customer uses a lead-forming machine, an off-center component on the tape can jam the machine and destroy the component.

In ceramic capacitors, long storage times can result in a loss of capacitance. In aluminum capacitors, this induces more leakage current, due to the aluminum oxide layer slowly dissolving into the liquid electrolyte. As this happens, the leakage current of the capacitor can be high, especially when it is first energized. As a result, the part may heat up and, in extreme situations, may experience thermal runaway and failure. Aluminum capacitors that have been in storage for a long time need to be re-formed by the application of voltage. This should restore the oxide layer and reduce the leakage current to acceptable levels.

Assembly

Soldering can also be critical to aluminum capacitors. Aluminum SMTs are on the topside of the board and are exposed to heat during the soldering process.

This heat, which will probably be the highest temperature the capacitor will be exposed to during its lifetime, can induce failures. This is especially challenging in RoHS-compliant (Pb-free) solder processes, as the reflow soldering temperatures are higher and the standard aluminum capacitors may not withstand them. Some manufacturers have aluminum SMT capacitors that suit the higher solder temperatures required by Pb-free processes.

Incorrect mounting or handling of capacitors may also damage them. Large capacitors can be improperly used as handles for the board, which can cause internal damage.

Special care must be taken with respect to the correct mounting of polarized capacitors, such as tantalum and aluminum electrolytics. For example, aluminum capacitors are dc only, and if ac voltage is applied to them it can result in catastrophic failures, including open or short circuits, leakage of electrolyte, or venting of the capacitor. For most aluminum capacitors, component manufacturers can provide terminations, such as J-leads or three-terminal snap-ins, to help prevent incorrect mounting.

In soldering, washing, and board-separation, the most problematic issues are temperature and board flexure. For ceramic capacitors, high temperatures and thermal shock can cause cracking.

Also, board flexure can cause mechanical stress, resulting in cracks. Heat produced in the washing/dryer or mounting processes can cause the electrolyte in aluminum capacitors to reach its boiling point, which might destroy the part. In radial aluminum electrolytics, the sleeve may also shrink when heated, causing at least a visual defect.

Failures during use

Randomly occurring failures in capacitors during their use are the most important source of failures in capacitors. But if capacitors are properly selected, they are also the least common.

Often the useful life of capacitors is longer than the application itself. However, it is very important in a critical application such as an air bag or automotive braking system that the components do in fact fulfill their useful life.

Several factors can prevent them from doing so. When capacitors are in use, energy surges and high temperatures cause different kinds of failure. In any design, it is important to know how a capacitor will react to a surge or high temperatures to determine the most suitable component for the application.

High energy

Capacitors react to energy surges in various ways. In most cases, the effects of a low-energy surge aren’t severe. Conversely, high-energy surges in most capacitors can be catastrophic.

In metallized film capacitors, a low-energy surge can cause a reduction of isolation. However, these devices are also self-healing, significantly limiting any damage.

If the energy is extremely high, however, a complete failure can occur. High currents can cause this failure by evaporating the connection between the metallization and the end contact. To avoid this, film/foil capacitors with infinite dU/dT, or complex series construction metallized capacitors with high dU/dT, should be used in high-current applications.

Electrolytic capacitors also have a self-healing ability, although to a lesser extent than film capacitors. In electrolytic capacitors, the dielectric can crack in both low- and high-energy surges.

When the electrolyte touches the aluminum through the crack in the dielectric, a reaction occurs that rebuilds the dielectric. The leakage current will increase to drive this self-healing effect. If, in reaction to the surge, the foil is punctured, venting may occur and the capacitor will dry out.

In ceramic capacitors, surges with low energy and high voltage can increase current leakage. Thermal stress can crack the dielectric and may also result in increased leakage or shorts. A high-energy surge may crack the ceramic and let in moisture, providing a conductive path.

In electrochemical double-layer capacitors, electrolysis decomposes the electrolyte if the voltage rating is exceeded. This generates gas, which increases internal pressure. If the pressure gets too high, the case will vent.

Improper voltage derating can damage tantalum capacitors; most tantalum manufacturers recommend derating the voltage down to 50% to 66% of rated voltage. Reverse voltage will also damage a tantalum, as will extreme thermal shock from out-of-control mounting profiles or heating from excess ripple current.

High temperature

Temperature is of great concern to any capacitor. On a circuit board, capacitors should not be mounted close to heat sources.

This applies to most capacitors, but especially to aluminum. A radiation shield between the cap and the hot component prevents the hot component from accelerating failure mechanisms, which can be simply a shorter lifetime (or faster parameter drift), or the opening of the pressure relief vent in extreme cases. To avoid failures in high-temperature applications, the designer should use capacitors with lower losses, a larger size, or a higher temperature rating.

Wear-out

At the beginning of a component’s life, failures per hour are very low, but they are random. During a wear-out period, failures increase per hour and become more predictable. The wear-out mechanism reaches a limit where devices will fall out of specification for the application.

Drifting parameters vary by technology and by conditions in the application. It is important for designers to know how capacitors react during wear-out, as it may be a factor, depending on how long their application is designed to last.

There is no wear-out mechanism for solid aluminum or tantalum capacitors, which is a major advantage over wet aluminum capacitors. Ceramics will have capacitance loss due to oxide vacancy migration.

Film capacitors will have some oxidation of the metal conductors, increasing the dissipation factor. For aluminum polymers, ESR impedance increases due to polymer degradation. For electrolytic and double-layer capacitors, there will be impedance and ESR increase due to electrolyte loss. ■

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