Bench Talk

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Electronic systems are omnipresent in our society. We depend on them for everyday things like getting us to work or washing our clothes, as well as high-level endeavors like equipping the military or exploring the darkness of space. The overall reliability of any electronics-based system is determined by the reliability of every component used within it. Poor component reliability can lead to abnormal system behavior and, worse, failure. Your car failing on your daily commute is inconvenient compared to the consequences of a military vehicle failing in a conflict zone or a deep space research probe on its way to Mars. Reliability has far-reaching implications beyond the cost of failure, including safety consequences and the inability to repair systems. Unfortunately, understanding the importance of reliability and the impact of selecting the wrong type of capacitor grade is often overlooked.

Within any electronic system, you'll find hundreds and thousands of passive components, such as capacitors and resistors. However, just because they appear as commodity products does not lessen their functional importance and reliability characteristics. This blog highlights the differences between the grades so that engineers are more informed when making component selection and ordering decisions.

Capacitor Reliability

With capacitors, volumetric efficiency is often a critical parameter. It indicates the amount of capacitance provided within a given volume and is expressed as the product of capacitance and voltage (CV). A maximum CV may offer the lowest cost, but there is a trade-off: reliability improves as the CV lowers, resulting in a higher price point.

Consequently, a capacitor's cost reflects its reliability and the steps taken to improve it, such as material traceability, end-of-line testing, and burn-in voltage conditioning. Achieving compliance and qualification to relevant industry standards also requires extended testing.

Capacitor construction methods significantly influence its CV rating and, hence, reliability. Using thicker dielectric and film layers, tighter adherence to material procurement specifications, and more conservative design constraints improve a capacitor's reliability. However, this will reduce the total capacitance, and consequently the CV, within a given package size.

Capacitor Testing and Qualification

KEMET’s Multilayer ceramic capacitors (MLCCs) are graded by reliability according to their suitability for specific applications. In increasing order of reliability and reducing CV scale, these classifications are commercial, automotive, commercial-off-the-shelf (COTS), high reliability alternative (HRA), military, and space. Understanding these classifications' differences is paramount for engineers when selecting and ordering parts.

Some of the basic testing protocols include:

• Electrical parameters such as capacitance, dissipation factor (DF), insulation resistance (IR), and dielectric withstand voltage (DWV)
• Visual and physical inspections
• Solderability

More stringent testing introduces burn-in voltage conditioning and reporting the percentage of defects (PDA), destructive physical analysis (DPA), and environmental factors such as humidity and temperature.  The higher reliability grade classifications, formal qualification processes, and specifications, such as AEC-Q200 (automotive) and MIL-PRF-32535 (military and space), stipulate additional inspection routines and test methods.

Capacitor Grade Classifications

Classifications are crucial for selecting capacitors tailored to specific applications, with different grades ensuring reliability and performance in diverse industries.

Commercial

Commercial-grade capacitors target the highest production volumes and the lowest possible costs. They are characterized by the highest CV rating and aim for the smallest case sizes. These capacitors suit a broad range of general-purpose and consumer electronics applications. The manufacturer will decide on qualification parameters, typically stipulating its criteria.

Production testing is typically at a basic level of electrical testing, with capacitance value, dissipation factors, insulation resistance, and dielectric withstand voltage attributes. Solderability, in addition to physical and visual inspections, will be conducted by random sampling. The manufacturer will typically not follow any formal product change process apart from issuing change notifications.

Automotive

With the significant increase in electronic systems in modern vehicles and the trend towards software-defined vehicles, automotive-grade capacitors offer a more reliable alternative than their commercial counterparts. They still provide a CV rating close to commercial and small case sizes, albeit at a slightly increased price point. The qualification yardstick will be against the Automotive Electronics Council's AEC-Q200 criteria and a more formal approval-based product change notification and production part approval processes (PPAP). An enhanced version of commercial-grade testing coupled with 100 percent visual inspection and end-of-line testing will apply.

Automotive application examples include powertrains, sensors, sub-systems, electronic control units (ECUs), and infotainment. An example of an AEC-Q200-qualified part is the KEMET Commercial and Automotive SMD High-Voltage MLCC series.

Commercial-off-the-shelf

There is a common misconception that a KEMET commercial-off-the-shelf (COTS) part is commercial-grade and, subsequently, unsuitable for applications requiring higher reliability components. However, this perception is incorrect. While a COTS-grade part has a high CV, it suits higher-reliability applications with lower volume expectations than commercial- and automotive-grade parts.

KEMET COTS parts have a higher test protocol, including all those based on commercial and automotive parts, with the addition of burn-in voltage conditioning with PDA, electricals with PDA, and DPA. The price point is typically slightly lower than a military-grade part. A manufacturer will likely qualify a COTS part against a suitable industry standard, although this will vary by manufacturer, while some opt for AEC-Q200.

High Reliability Alternative

KEMET’s HRA capacitors provide a middle ground between automotive and MIL-SPEC components. HRA capacitors offer reliable performance in critical applications with higher CVs than comparable Military-grade capacitors. The rigorous testing protocol includes voltage conditioning, post-electrical testing per MIL-PRF-32535, and biased humidity testing per MIL-STD-202. Buyers also have access to full traceability to raw materials.

HRA capacitors meet demanding performance requirements for satellites with short life spans, launch vehicles, avionics, and ground systems.

Military

Military parts are manufactured and tested against defined military standards—like MIL-PRF-55681 and MIL-PRF-32535 M-level—and exhibit established reliability credentials. These parts will have a much lower CV than commercial- and automotive-grade components. The manufacturing process will include complete raw material traceability, and the manufacturer will also provide product data and test summaries.

Among the exemplary MIL-PRF-qualified parts is the KEMET LDD Mil X7R PRF39014 range of multilayer ceramic capacitors.

Space

With the highest reliability grade and the lowest CV characteristic, space-grade capacitors will be constructed and qualified against military standards MIL-PRF-123 and MIL-PRF-32535 T-Level. Every lot manufactured receives complete electrical, environmental, and mechanical testing and a single lot date code. As you might expect, applications are mission-critical, ranging from avionics to satellites and military missiles.

Conclusion

Understanding capacitor classifications is necessary for ensuring an electronic system’s reliability. Capacitor reliability is influenced by volumetric efficiency and rigorous testing protocols, which vary across classifications such as commercial, automotive, and space-grade. Each classification serves specific applications, with commercial-grade capacitors offering cost-effective solutions for general use, while automotive and space-grade capacitors meet higher reliability standards for more demanding environments.

These classifications must be considered when selecting capacitors to make sure they match the application’s requirements, ensuring optimal performance and minimizing the risk of failure. By thoroughly understanding the differences between capacitor grades, engineers can make informed decisions that enhance the reliability and efficiency of their designs, ultimately contributing to the overall success of electronic systems.

Author

Robert Huntley is an HND-qualified engineer and technical writer. Drawing on his background in telecommunications, navigation systems, and embedded applications engineering, he writes a variety of technical and practical articles on behalf of Mouser Electronics.