Although not all applications are safety critical or mission critical, reliability is still a vital consideration for many electronic products. Making informed choices at the component selection stage can help ensure the product will perform correctly over its intended lifetime, writes James Lewis.
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.
An interesting fact is that aluminium capacitors with a diameter of 30mm or more tend to have a more neutral electrolytic, rather than acidic, and so can have shelf-lives of two to four years at relatively moderate conditions. These figures, of course, vary by electrolyte used in each product family.
Solid “aluminium polymer” or “organic polymer” capacitors, on the other hand, have very different lifetime characteristics.
These devices have no electrolyte in the finished product. Instead the cathode is a solid conductive polymer material. This results in exceptionally long operational lifetime under rated conditions, which can be close to that of other solid capacitors.
Some datasheets describe the endurance of these types of devices in terms of properties such as capacitance change, ESR and appearance after 1,000 hours of operation.
It is important to note that the 1000 hours does not represent the capacitor’s operational life. Rather, this endurance testing is similar to the types of accelerated life testing that is typically used to qualify passive components.
As far as commercial-grade ceramic capacitors are concerned, the typical electrode system is a base metal electrode (BME) system, see figure 1, that primarily utilises nickel.
Compared to the earlier precious metal electrode (PME) systems, BME allows higher voltage stress capability. Popular X7R and X5R type dielectrics today are based on barium titanate, with additives such as manganese dioxide that coexist with the BME chemistry and prevent reduction of the dielectric by the firing processes applied to the capacitor during manufacture.
Improvements in dielectric composition have greatly increased the reliability and life of ceramics.
Tantalum Capacitor Reliability
Capacitors made with a tantalum dielectric have an exceptionally long operational life. Being a completely solid device, there is virtually no wear-out mechanism.
The most common failures for tantalum-based devices are so-called “turn on” failure. This can occur where a step-voltage is applied and the capacitor is able to draw a large initial current. This can activate a defect in the dielectric, which may cause device failure in the event that the dielectric cannot heal.
Polymer-tantalum devices benefit from a pronounced self-healing capability, and are known to be robust against this type of failure. Studies have shown that the lifetime of the capacitive elements may be in the hundreds or even thousands of years. This is likely to be considerably longer than the lifetime of other materials used in capacitor construction, such as epoxies.
Capacitor manufacturers tend to screen tantalum capacitors to identify potentially weaker devices, by applying tests such as voltage and current surge tests in a controlled sequence.
However, it is worth noting that the capacitors can be weakened by stresses introduced due to coefficient of thermal expansion (CTE) mismatches between constituent materials: hence reflow soldering conditions and the number of reflow cycles the capacitor is subjected to during final product assembly can affect the susceptibility to device failures.
On the other hand, the voltage rating of the device, relative to the applied voltage, can significantly influence capacitor lifetimes generally.
For this reason, recent development of polymer-tantalum capacitors has focused on realising higher voltage ratings such as 63V and higher for use with commonly used supply voltages such as 24V or the 28V avionics rail.
James C. Lewis is technical marketing director, Kemet.
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