When the Mission Just Can’t Fail
Maintaining technological superiority remains critical for national defense forces. Many face informal adversaries that are adept at harnessing today’s sophisticated civilian technologies, like mobiles, M2M communication and social media, to launch attacks unexpectedly. Defense operations are intelligence-led; responses must be surgical and precision-guided to avoid harming civilians; and every measure must be taken to avoid injury to soldiers or loss of equipment. At the same time, government funding of defense forces, particularly in the West, is under pressure.
As a result, defense today, more than ever, is about prevention and deterrence, based on intelligence, surveillance information, and seamless coordination between planners and the teams in the field. At the same time, lives are at stake, and equipment reliability is of paramount importance. Every component must be trusted to perform faultlessly over its lifetime, from powerful, high-value ICs like graphics processors and DSPs to discrete devices like capacitors that have numerous vital roles ranging from stabilizing power supplies to blocking noise and conditioning analog signals. Procurement authorities impose exacting requirements on capacitors in terms of quality assurances and screening.
Making the Grade
As the reliability grades of components progresses, more screening and testing is done to ensure that only the most robust parts make it into end users’ hands. Figure 1 illustrates the notion. Table 1 illustrates the manufacturing and screening capabilities that are applied to components from commercial grade parts, where basic part level and process testing is done to ensure a high standard of starting quality, all the way through to space-grade and custom parts.
Multilayer ceramic capacitors (MLCCs) are found in many high reliability and space applications. Figure 2 illustrates some of the most important approval documents for this class of device.
When it comes to the options available to items based on DLA drawings, various MIL specifications are applicable. Some examples include M55681 for surface-mount parts, M39014 for leaded components, and M49470 for stacked capacitors. Various sub-groups of device types may also be defined within these categories. From a qualification and lot release standpoint, extended accelerated life testing and LVH testing is applied. Additional screening such as thermal shock tests, shear tests and biased humidity may also be required, and the supplier must be able to comply. Single-lot date code (SLDC) and BR/BX/BP dielectric options are also available. In addition, KEMET is able to perform in-process inspection and A/B grouping per M123 as well as SLDC and BR/BX/BP options in a variety of termination options including 70/30 lead/tin, gold, and 100% tin.
When a device needs additional amounts of energy storage capabilities, stacked capacitors provide a proven and effective solution for those cases where much higher CVs are needed. Stacked devices are often placed in the input and output filters of power supplies and in capacitor banks. The number of capacitors in a stack can be customized and fit to a number of leaded configurations, as Figure 3 illustrates, including surface mount and through-hole.
Space-grade MLCCs must perform extremely highly in terms of their volumetric efficiency and lightness, as well as reliability. Efficient and robust Base Metal Electrode (BME) technology, combined with X7R and C0G dielectrics allow high capacitance values in extremely small 0402 and 0603 case sizes.
Polymer and Tantalum
Polymer and tantalum capacitors are also extremely important in the defense/high-reliability market. Their high volumetric efficiency makes them ideal for applications where a large amount of capacitance is needed in a small package. Device manufacturers must follow the strictest reliability standards such as those outlined in MIL-PRF-55365 for both MnO2 and polymer-cathode tantalum devices targeted for defense and high-reliability applications. This supports a robust assurance of the highest levels of reliability for applications where failure is simply not an option.
In addition to following the military standards set out to produce highly reliable parts, extra methods have been developed to assess the reliability for COTS polymer electrolytic devices. As an example, KEMET’s T540 and T541 series have undergone enhanced reliability assessment. The process is one that ensures reliability levels corresponding to 0.1 percent per 1,000 Hours, 0.01 percent per 1,000 hours and 0.001 percent per 1,000 hours. The T540 and T541 Series are the first polymer electrolytic capacitors available with failure rate options defined by the KO-CAP® assessment method.
The basic principle is to test a significant representative sample of each manufacturing lot ordered with this feature under accelerated voltage and/or temperature conditions to obtain the necessary unit hours with an Accept/Reject (A/R) Number of 1/2 to demonstrate the claimed reliability level. As an example, to achieve a reliability level of 0.001 percent per 1,000 hours, one failure is allowed in 108 accumulated part hours. For the lower reliability levels, fewer part hours are required. The acceleration factors are predetermined for each design based on testing that is conducted under multiple conditions of temperature and voltage. The assessed reliability is equivalent to steady-state operation at +85°C and full rated voltage, as are the MIL-PRF-55365 Weibull Failure Rate Estimates.
The rules surrounding high-reliability electronic equipment cover every aspect of component selection, to ensure the best possible reliability and long-term safety. Understanding the diverse requirements placed on capacitors demands familiarity with a wide range of test specification that are applicable to various device technologies, construction types and functional roles in enduser equipment. Knowing the right documents to reference can be challenging without guidance.
This article was written by Wilmer Companioni, Technical Marketing Manager, KEMET Corporation (Simpsonville, SC). For more information, Click Here .
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