Accelerating factors in capacitor life testing
Ed Jones, Product Manager and Sandy Campbell, Associate Product Manager KEMET Corporation, explain the need for commerical-off-the-shelf (COTS) components to be screened and qualified for use in military projects.
Applicable reliability standards have been established for various COTS component types such as MIL-PRF-55182 for metal-film resistors and MIL-PRF-55365 for electrolytic tantalum chip capacitors. These set out accelerated life test conditions and reliability targets for various levels, including Established Reliability (ER), non-ER and Space Level components.
Failure rate data for components is obtained by performing accelerated life tests under conditions specified in the MIL-PRF standard. Screening tantalum capacitors in accordance with MIL-PRF-55365 usually involves a combination of surge current testing and Weibull screening. The acceleration factors developed for MIL-PRF-55365 are applicable to MnO2 type (often referred to as ‘classic tantalum’) capacitors, but are found to be unsuitable for polymer-type tantalum devices. Currently there is no MIL-PRF document directly applicable to the polymer tantalum capacitors. Until recently, engineers wishing to use these devices in military projects have had to consider developing their own methods for screening devices.
In 2011, KEMET supported a team at Raytheon Aerospace and Space Systems to help develop accelerated life tests for T541 tantalum polymer capacitors as potential replacements for MnO2 capacitors originally specified in one of the company’s airborne radar systems. The project sought to establish whether polymer capacitors should undergo surge current testing and whether voltage conditioning is necessary, and to determine screens and qualification test methods for polymer devices that are equivalent to those documented in MIL-PRF-55365.
Ultimately, the work at Raytheon proved that the T541 capacitors could be screened adequately to allow replacement of the MnO2 capacitors in the radar application. The team recommended that a military specification or slash sheet should be generated for polymer tantalum capacitors and that the acceleration factors in MIL-PRF-55365 should be updated to include factors appropriate for polymer tantalum capacitors.
Following on from that work, KEMET has developed a set of assessment tests using acceleration factors developed for its T540 and T541 organic polymer tantalum (KO) capacitors. These tests now enable KEMET to supply assessed reliability levels for its T540/541 KO capacitors for use in military projects.
Classic tantalum capacitors containing a manganese dioxide (MnO2) cathode are the longest established surface mount tantalum capacitors. These devices have excellent characteristics for use in demanding applications such as aerospace, military and down-hole drilling equipment. Properties include high volumetric efficiency, self-healing capability and the ability to withstand high operating temperatures. They also maintain excellent capacitance stability with time and temperature. On the other hand, if the device fails when a high current is passing, a thermal event may occur that can result in emission of smoke or damage to the circuit board or surrounding components.
This failure mode has been found to result from imperfections in the dielectric, which cause mechanical stresses that eventually lead to disruption of the dielectric. The high oxygen content of the MnO2 cathode provides conditions for a thermal event. Some have suggested applying a 50% voltage de-rating to prevent these failures, but this requires specifying a larger size device. Moreover, the larger device has greater dielectric surface area and hence a higher probability of imperfections. KEMET has developed the F-Tech manufacturing process to tackle the problem at the source, by eliminating imperfections from the dielectric during the manufacturing process.
F-Tech provides an assurance of reliability for users of MnO2 capacitors. Even so, the benign failure mode of polymer tantalum capacitors makes these devices an attractive alternative to the classic tantalum capacitor, particularly in equipment for use in mission critical or manned situations. With the benefit of conducting polymer cathode technology, these capacitors also have very low Equivalent Series Resistance (ESR), which delivers advantages such as low self-heating (ripple current) and high energy efficiency. ESR can be as low as one-tenth that of a comparable MnO2 capacitor.
Initially, polymer devices were available in a limited range of voltage ratings, typically below 10V. More recently, however, new pre-polymerisation and doping techniques have enabled KEMET to produce capacitors with 25-63V ratings. With the advent of these devices, designers can take advantage of polymer tantalum capacitor benefits in a wider range of applications including pulse voltage applications such as the Raytheon airborne radar.
Meeting the needs of applications
The KEMET T540 and T541 series were developed to address these types of applications. Development for the T540 and T541 started with selecting the most robust designs of the commercial T525 and T530 series, which offered 125°C ratings. During qualification, a more severe accelerated life test was used to better simulate the extended operational life typical of high reliability applications. The designs chosen to be included in the T540 and T541 series are able to stand-up to additional MIL-PRF-55365 screenings popular in high reliability applications. These are high reliability devices that offer the same performance characteristics as other KEMET Organic (KO-CAP) capacitors, including low ESR, high capacitance and volumetric efficiency, as well as long operational life and high ripple current capability.
The T540 and T541 series are now available with failure rate options as defined by KEMET’s KO-CAP Reliability Assessment method, which assesses long term device reliability by applying accelerated voltage and temperature conditions to board mounted samples. The failure rates available are B (0.1% per 1,000 hours), C (0.01% per 1,000 hours) and D (0.001% per 1,000 hours). Current surge testing options are also available, although today’s high standards in capacitor manufacturing processes mean that tests performed at 25°C typically reveal a very low PPM failure rate. Testing at higher temperatures places greater stress on components, and hence exposes a higher number of marginal parts.