How can material selection improve capacitor reliability?

James Lewis

Tuesday, April 26th, 2016

ceramic cut away

Even in today’s throwaway society, reliability is an important issue for manufacturers wishing to protect their brand’s reputation. Even simple devices will receive negative reviews if they do not perform as expected. Selecting the right component set is critical to developing reliable products at the correct cost point.

Often relatively little attention is paid to passive devices when designing and simple electrical parameters tend to be the basis for selection. As engineers better understand factors affecting lifetime they can ensure the end product will deliver the required reliability.

Film Capacitor

Metallized polyester or polypropylene film capacitors are known to have a long operational life. Their self-healing helps prevent short-circuit failures, but causes the total capacitance to drop and the ESR to rise. These changes shorten the lifetime of the device. Using high-quality materials KEMET has been able to verify operational lifetimes of several decades.

Aluminum Capacitor

There are many types of aluminum capacitors each with very different lifetimes. Wet-electrolyte capacitors have a mildly acidic electrolyte that degrades the dielectric over time. The electrolyte also provides the oxygen to re-grow the dielectric. Engineers must consider the “shelf-life” of an unpowered wet aluminum electrolytic capacitor. Aluminum capacitors with a diameter greater than 30mm tend to have a neutral electrolyte, and can have shelf-lives of two to four years.

Solid ‘aluminum polymer’ or ‘organic polymer’ capacitors have very different lifetime characteristics. These devices, such as KEMET AO-CAP® capacitors, have no electrolyte, using a solid polymer cathode instead – giving a lifetime close to that of solid capacitors.

Ceramic Capacitor

In commercial-grade ceramic capacitors, the typical electrode system is a (primarily Nickel) Base Metal Electrode (BME) system. BME allows higher voltage stress capability. Popular X7R and X5R types use barium titanate, with additives such as manganese dioxide that prevent reduction of the dielectric. One KEMET study found that BME ceramics had lifetimes of several decades even under harsh conditions.

Polymer (Tantalum)

Tantalum capacitors are solid devices and have an exceptionally long operational life. Polymer-tantalum devices benefit from a pronounced self-healing capability and are robust against ‘Turn-On’ failure. KEMET studies have shown that the lifetime of the capacitive elements may be in the hundreds or even thousands of years.

Conclusion

Apart from the material itself, capacitor manufacturing processes such as screening, or processes to control the purity of materials, can also contribute to a higher level of reliability.

If you need help selecting a particular capacitor type for your application, see some suggested capacitors in our Application Guide.

Note: The full article originally appeared on Electronics Weekly.com titled “Capacitor reliability can be improved with the right materials.”