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Advanced Performance Bulk Capacitors for Mission-Critical Applications in the Automotive and Aerospace Markets

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by Ron Demcko, AVX Fellow and Technical Sales Group Manager, AVX Corporation

The integration of smarter and more densely interconnected electronics into virtually all aspects of daily life has led to massive performance enhancements and increased efficiencies that are significantly improving user experiences and productivity, as well as actively pushing the envelope of technological capabilities and extending the bounds of current possibilities. With examples ranging from the rapid evolution of personal automobiles to out-of-this-world advances in aerospace technologies, the trend toward increased electrification, digitization, and automation highlights the growing need for complex control networks capable of performing at high speeds with absolute reliability.

Both the automotive and aerospace electronics industries experienced significant advancements over the last few decades, thanks in large part to extremely powerful field-programmable gate arrays (FPGAs) and data converters that enable direct RF conversion and leverage gallium nitride (GaN) high-electron mobility transistors (HEMTs) to achieve immense advantages in the linearity and efficiency of power amplifier (PA) designs. The main driver of these semiconductor advances has been the introduction of innovative and highly reliable integrated circuit (IC) material systems. These materials were successfully expanded through stringent, market-specific testing procedures and subsequent modifications and are now qualified for use in mission-critical automotive and spaceflight applications. In the automotive industry, these advanced semiconductor devices are enabling today’s advanced driver-assist systems (ADAS) and tomorrow’s autonomous driving applications, and some industry experts even predict that these rapidly evolving chipsets will eventually allow automobile manufacturers to adopt new design philosophies based on a range of reconfigurable and upgradeable features. In the aerospace industry, these advanced semiconductors are enabling equipment ranging from software-defined satellites and high performance CubeSats to large geosynchronous communications satellites to become powerful, reconfigurable flight systems capable of performing complex space missions both reliably and cost effectively.   

Figure 1: This diagram provides a visual comparison between a conventional tantalum capacitor (left) and a conductive polymer capacitor (right)

GaN and silicon carbide (SiC) have had a profound impact upon RF and power electronics. The evolution of Moore’s law combined with new electronics architectures has enabled the development of high-gate-density components optimized for customization, enhanced performance, and high reliability at a relatively low cost. These proven ICs can also act as reusable building blocks to help reduce costs associated with the development of electronic content in future automobiles and spacecraft, which is yet another reason that their popularity is growing.

As market demand for advanced semiconductors continues to increase, so do concurrent pressures for passive components to provide lower loss performance across a wider frequency spectrum within more complex signal environments and at higher rates of reliability than ever before. As a result, many completely new series of high performance capacitors optimized to address end circuit capacitor needs have been introduced to market in recent years, including:

  • High-Frequency Decoupling Multilayer Ceramic Capacitors (MLCCs)
  • High-C/V, Flight-Grade MLCCs
  • Harsh-Environment Capacitors
  • Pulse Withstanding Capacitors
  • Stacked Ceramic Capacitors 
  • Advanced Performance Bulk Capacitors
  • Advanced Performance Flight Bulk Capacitors

Advanced performance bulk capacitors have multiple performance designs and grades optimized for use in both automotive and aerospace environments. In general, devices deployed in these market sectors require large value, low loss, and high reliability bulk capacitors proven to provide the high quality performance required to power the most mission-critical, harsh environment applications, whether on the road or in outer space. Tantalum and conductive polymer capacitors (i.e., tantalum capacitors with conductive polymer electrodes) are two such solutions. 

Tantalum vs. Conductive Polymer Capacitors

Tantalum and conductive polymer capacitors are both solid electrolytic capacitors. Tantalum capacitors have a long history of use in mission-critical automotive-, military-, and space-qualified products (e.g., AEC-Q200, MIL-PRF-55365, and ESCC QPL 3012/004) and are available in a wide range of miniature, lightweight packages proven to provide large capacitance values across temperature, voltage bias, and time. Tantalum capacitors also share several design principles with and exhibit similar electrical performance characteristics to conductive polymer capacitors, and both exhibit high reliability performance relative to other bulk capacitor components. In addition, conductive polymer capacitors provide the added desired performance of very low ESR, high bulk capacitance, and improved capacitance retention at higher frequencies.

Figure 2: These graphs illustrate capacitance and ESR stability performance for various capacitor technologies and provide a high level overview of the benefits inherent to conductive polymer technologies. Detailed tantalum capacitor models and performance graphs, including simulation tools, are available on the AVX Corporation website.

Compared to traditional (MnO2 electrolyte) tantalum capacitors, conductive polymer capacitors provide: 

  • Significantly lower equivalent series resistance (ESR) and, therefore, higher ripple currents, typically on the order of roughly one-eighth the ESR and eight times the ripple current
  • Improved capacitance retention at high frequencies
  • Higher energy density (measured in Joules/cc)
  • A wider voltage range with ratings spanning 2.5V to 125V
  • A reduced voltage derating of 90% with 10% derating for products rated for up to 10V, or 80% with 20% derating for products rated for 16V and up
  • A benign failure mode

Typically, design engineers are free to choose which bulk capacitors to specify as long as they satisfy relevant corporate, agency, market, and/or application guidelines with regard to capacitance and ESR stability across temperature, voltage bias and time, and standards compliance. As such, options could include large-value MLCCs configured as either stacked single chips or discrete components designed to be arranged in parallel on a PCB, as well as aluminum electrolytic capacitors, tantalum capacitors, and conductive polymer devices. 

Automotive-Grade Electrical Performance

AVX has a long history of designing and developing ruggedized, ultra high reliability electronic components optimized for peak performance in mission-critical automotive applications including ADAS and autonomous driving systems. The most recent product release in this segment is the newly expanded and improved AEC-Q200-qualified TCQ Series Automotive Conductive Polymer Chip Capacitors, which have been field-proven in a wide range of demanding automotive, industrial, and telecommunications applications with limited board space and challenging operating environments since 2015. The updated series, officially released to market this March, now offers two new case sizes for a total of five, several new capacitance and voltage (C/V) ratings, 36 new part numbers for a total of 52, and twice the reliability specified in AEC-Q200. 

Figure 3: AVX Corporation’s TCQ series automotive conductive polymer chip capacitor

AVX’s TCQ Series conductive polymer capacitors have a compact and robust form factor that’s ideally suited for space-constrained, harsh environment applications including body electronics, cabin controls, comfort and infotainment systems, aftermarket automotive electronics, DC/DC converters, and coupling/decoupling, and are equipped with conductive polymer electrodes that enable a benign failure mode under recommended use conditions. Previously available in just three miniature, low profile case sizes — “B” (EIA Code 1210 or EIA Metric 3528-21), “D” (2917 or 7343-31), and “Y” (2917 or 7343-20) — the TCQ Series now offers “E” (2917 or 7343-43) and “U” (2924 or 7361-43) case sizes as well, for a total of five form factors. The series has also been expanded to include 36 new part numbers covering a newly extended range of capacitance and voltage ratings. It now offers 52 unique part numbers with C/V ratings spanning 10–470μF and 2.5–50V with a ±20% capacitance tolerance, which is a significant improvement over the previous range of 10–220μF (±20%) and 4–35V with just 16 unique part numbers. 

TCQ Series Automotive Conductive Polymer Chip Capacitors exhibit low DC leakage (0.1CV), high capacitance, and stable electrical performance in operating temperatures extending from -55°C to +125°C. They are also 3x reflow compatible at 260°C, lead-free compatible, and RoHS compliant, and are manufactured in an IATF 16949 certified facility and supplied with pure tin terminations on 7” or 13” reels for automated processing. However, their most impressive performance characteristics are their proven ability to satisfy AEC-Q200 humidity bias testing requirements (i.e., rated voltage [Ur] at 85°C and 85% relative humidity [RH] for 1,000 hours) due to materials, design, and manufacturing innovations AVX employed to overcome the inherent limitations of polymer materials — a feat that few other capacitor suppliers have accomplished — and deliver exceptional endurance and stability for 2,000 hours at 125°C, exceeding the already stringent AEC-Q200 operational life requirement by 100%. Contributing factors to said improvements include proprietary enhancements to the product design and polymerization process and additional humidity protection coats for the pellets within the package. 

In addition, TCQ Series conductive polymer capacitors exhibit basic reliability of 1% per 1,000 hours at 85°C and rated voltage with 60% confidence, which is so reliable — and especially so when compared to competing products available on the market when the series was introduced in 2015 — that AVX was able to further modify the TCQ Series in order to develop its space-grade TCS Series conductive polymer capacitors. Given the astronomical reliability required of the components, devices, and systems required to realize the life-critical autonomous vehicle technologies that engineers have been incrementally developing, testing, and honing in pursuit of achieving mass market, fail-safe autonomous vehicles, this type of automotive-to-aerospace innovation loop is becoming increasingly commonplace. 

Space-Grade Electrical Performance 

In addition to its industry-proven expertise in developing high reliability automotive electronics, AVX also has a long history of designing and developing ruggedized, ultra high reliability electronic components proven to deliver peak performance in mission-critical space applications. As such, the company was quick to leverage the massive database of R&D data it had generated by developing automotive-grade conductive polymer capacitors with increasingly impressive mechanical and electrical reliability characteristics to further advance the capabilities of conductive polymer technologies. Innovative new designs and, in some cases, even material systems were employed in the pursuit of achieving flight-grade conductive polymer components. Initial solutions based on the TCQ Series were then rigorously tested in AVX laboratories and high-reliability applications with similarly hazardous operating conditions, but without the same life-critical requirements  as flight-grade systems. The testing and performance data AVX generated through this process was then used to further hone and incrementally improve both the automotive-grade conductive polymer capacitors that inspired the pursuit of similar flight-grade components, as well as its new automotive-inspired designs intended to take to the skies. 

Figure 4: Examples of a finished surface-mount conductive polymer capacitor (left), along with a cross-section of the finished capacitor and an exploded, magnified diagram of the tantalum pellet makeup

Just last September, AVX proudly introduced the industry’s very first polymer electrolytic multianode tantalum capacitors approved for use in European Space Agency (ESA) programs including satellites, missiles, and rovers. These well-proven TCS Series COTS-Plus Solid Polymer Electrolytic Multianode Chip Capacitors, which are manufactured at the ESA-qualified AVX Czech Republic facility in Lanškroun, are now listed on the European Space Components Coordination (ESCC) Qualified Parts List (QPL), Detailed Specification 3012/006, as recommended by the Space Components Steering Board.

TCS Series solid polymer electrolytic multianode capacitors approved to the ESCC QPL have a ruggedized multianode construction featuring three parallel anodes within a single encapsulated chip. This design feature further reduces the ESR of the device, resulting in low ESR values ranging from 12–50mΩ depending on capacitance and voltage. These capacitors also feature an E-case (EIA Metric 7343-43) form factor that measures 7.30mm × 4.30mm × 4.10mm (L × W × H ±0.20) in order to satisfy the stringent space and weight requirements applicable to most aerospace equipment. Capacitance values extend from 22–470μF with a ±20% capacitance tolerance and rated voltage range span 6.3–35VDC in operating temperatures ranging from -55°C to +105°C. 

Figure 5: AVX Corporation’s COTS-Plus TCS ESCC conductive polymer capacitor

In addition, each lot is subjected to statistical screening, accelerated ageing, and comprehensive testing procedures to ensure long lifetime operation in extreme environments with improved base reliability of 0.5% per 1,000 hours. However, customers can also choose testing levels “B” or “C” and request one of three lot acceptance testing (LAT) procedures per ESCC regulations to ensure suitable reliability for various space, avionics, military, industrial, and telecommunications applications.

ESCC QPL approved TCS Series multianode conductive polymer capacitors are generally supplied in bulk packaging, but can be packaged on tape and reel packaging for automated processing upon request, and lead time for the series depends on customer test specification requirements.

Testing Qualifications 

In terms of reliability, all AVX series components are 100% surge current tested, 100% temperature/voltage aged, and 100% reflow preconditioned before being tested for electrical parameters including but not limited to capacitance, dissipation factor (DF), ESR, and DC leakage. Detailed testing, however, is end-sector dependent and performed in accordance with industry-specific standards’ bodies and specifications, such as the Automotive Electronics Council’s AEC-Q200 specification and the European Space Agency’s various specifications.

For instance, the ESA’s detailed ESCC3012 specification 3012/006 requires statistical screening along with accelerated testing and allows customers to choose between reliability testing levels “B” or “C” per the ESA document and to request one of three LAT procedures as defined by the ESCC. According to the ESCC, AVX is qualified to perform the following LAT procedures: 

  • LAT 3 – Based on a quantity of 10 pieces, including four “destructive samples” and six pieces that may be reserved for part of the order quantity or supplied in addition to the order quantity
  • LAT 2 – Based on a quantity of 26 pieces, including the 10 pieces from LAT 3. All 16 additional pieces are destructive samples.
  • LAT 1 – Based on a quantity of 34 pieces, including the 26 pieces from LAT 2. All eight additional pieces are destructive samples.

Regardless of the exact testing level, AVX’s ESA-approved TCS Series conductive polymer capacitors exhibit a basic reliability greater than or equal to 0.5% per 1,000 hours.

Additional Performance Considerations and Features

Although reduced ESR characteristics allow conductive polymer capacitors to operate at much higher currents than standard tantalum capacitors and other low-ESR capacitor technologies, there are still additional design and performance factors to consider. For example, higher-current conductive polymer technology may allow devices with smaller case sizes to be placed optimally around a PCB to provide bulk capacitance at the load for maximum efficiency, while lower-inductance miniature case sizes can further increase capacitors’ frequency range. The inductance values of AEC-Q200-qualified and ESSC-approved conductive polymer capacitors (Figure 4) compare favorably to alternative bulk capacitor technologies, such as parallel high-CV MLCCs and aluminum electrolytic capacitors.

Compared to traditional MnO2 electrolyte tantalum capacitors, conductive polymer capacitors have a moisture sensitivity level (MSL) 3 rating, which is typical among many traditional low-ESR tantalum capacitors and all molded conductive polymer capacitors. This rating means that the capacitors must be supplied in a dry pack and will either need to be processed within 168 hours after opening or dried at 40°C for 168 hours prior to use. However, it is possible for conductive polymer capacitors to achieve an MSL1 rating if the pellets are hermetically sealed inside the package.

Figure 6: These tables show the inductance values of various AEC-Q200-qualified and ESSC-approved conductive polymer capacitors

Conductive polymer capacitors are typically processed in either wave solder or reflow processes. Wave soldering is only allowed if the duration of temperature exposure is kept short and the part is not exposed to the maximum solder wave temperature for a slow dwell time. Since wave soldering is assumed to be more of an exception, greater emphasis is placed upon reflow soldering, which should follow JEDEC 020 recommendations for a maximum of three reflow cycles with a peak of 260°C for a total duration of 30 seconds. Further recommendations state that, to minimize the potential for defects, the number of reflows and peak temperature should be kept as low as is practical. It is also important to note that a critical reflow parameter is the maximum peak temperature gradient, which should be limited to approximately 2.5°C per second. In addition, the reflow profile should include a preheating period to allow moisture to slowly evaporate from the capacitor.

Another point to consider is the radiation performance of AVX’s ESA-approved TCS Series capacitors. These conductive polymer capacitors have been evaluated via a total ionization dose (TID) test using a Co-60 γ source with up to 200K radiation irradiation at a dose rate of 500rad/hour without impacting part performance, which allows the series to be deployed in a broad range of spaceflight applications.

Conclusion

Advanced performance bulk capacitors including tantalum and conductive polymer capacitors are ideal solutions for meeting the growing demands of today’s advanced FPGAs and GaN and SiC semiconductor devices. These devices are key enablers in the development of advanced technologies ranging from ADAS to autonomous driving systems and CubeSats to rovers destined to traverse the surface of distant planets. But in order to fulfill their potential, they require passive components capable of providing lower loss performance across a wider frequency spectrum within more complex signal environments and at higher rates of reliability than ever before. 

Progressive innovation in conductive polymer capacitor technology has enabled the development of low-ESR, high-CV bulk capacitors capable of providing high quality power consistent with the advanced semiconductors increasingly deployed in automotive and aerospace designs. Reduced ESR characteristics allow these capacitors to effectively reduce ripple voltage in a wide range of cutting-edge GaN and SiC IC applications, and the various case sizes they’re now available in allow these devices to be placed close to the load, which both simplifies board layouts and further reduces the inductance of the bulk decoupling capacitors. In addition, the inherently high reliability of these advanced conductive polymer capacitors coupled with their benign failure mode makes them an especially attractive option for safety- and life-critical automotive and aerospace electronics. As such, the integration of conductive polymer capacitors into ultra high reliability application in both these and other demanding market segments is expected to grow at a virtually exponential rate well into the foreseeable future.

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