Solid-tantalum capacitors have evolved with better design, construction, and testing methods to handle emerging requirements for harsh, high-temperature applications that put a premium on reliability.
Chris Reynolds, AVX Corp.
Wet tantalum capacitors are a proven, mature technology. They’ve long been employed in demanding, high-temperature applications because they have a variety of useful qualities. These include a non-solid electrolyte capable of
controlled self-healing, high bulk capacitance at high-voltages (e.g., up to 5,600 µF at 125 Vdc), high volumetric efficiency, excellent stability, good high-temperature performance, high-reliability and a long lifetime. In recent years, this technology has been further developed to handle harsh 200°C industrial applications.
However, the trend in consumer, communications, medical markets has long been toward digital applications with low operating voltages (e.g., 6 –25 V) that could be handled by surface-mount capacitors. So, solid-tantalum chip capacitors up to 220 µF and rated to 200°C have come to be preferred over hermetically-sealed, axial-leaded wet tantalum capacitors. The reasons: Solid-tantalum chip capacitors are smaller, cost less, and deliver the low-ESR and high-frequency response necessary for high-speed digital applications.
Additionally, in recent years, solid-tantalum capacitors with hermetic, surface-mount device (SMD) packaging became available. This packaging allows the internal element of a solid tantalum chip capacitor to operate in an inert gas. This environment helps resist moisture ingress and enables operation at temperatures up to 230°C with higher capacitance and voltage ranges (up to 330 µF and 63 Vdc). Hermetically sealed, high-temperature,
solid-tantalum SMD capacitors also stand up to harsh mechanical shock and vibration. They are available with a range of termination finishes designed for compatibility with high-temperature PCB and hybrid circuit assembly, high-melting-point (HMP) solder, epoxy, and wire bonding processes.
Tantalum capacitors are one of just a few capacitor technologies that can operate reliably at temperatures above 175°C.
Basic capacitors consist of a pair of conductive or semi-conductive plates separated by an insulating dielectric. The dielectric stores charge when voltage is applied, effectively blocking dc and enabling the transmission of any ac signal. Tantalum capacitors are a subset of electrolytic capacitors. These are polar devices in which one plate is maintained at a positive potential and the other at a negative potential.
Solid tantalum capacitors have a tantalum positive plate with an insulating film of tantalum pentoxide, which acts as the dielectric, on the surface. A negative plate (i.e., a counter electrode or cathode) is made of manganese dioxide or a conductive polymer. In wet tantalum capacitors, the negative plate is a high-surface-area tantalum cathode that includes an acid electrolyte.
The top two tantalum technologies for high-temperature applications are solid tantalum capacitors with a manganese dioxide cathode in a molded SMD or hermetic ceramic package, and wet tantalum capacitors with a hermetic metal can construction and an axial lead. The cathode type has the greatest impact on a capacitor’s frequency qualities and determines the applications to which it is best suited. For example, solid tantalum exhibits a better frequency response for filtering applications, and wet tantalum exhibits higher bulk dc capacitance and higher voltage ratings.
Tantalum capacitor performance
Tantalum capacitors are known for their reliability, ruggedness, high volumetric efficiency, and parametric stability. Standard tantalum chip capacitors are rated for operating temperatures spanning -55 to +125°C. This range easily handles most consumer and in-cabin automotive electronics.
Enhanced, professional-series tantalum chip capacitors can be made to comply with AEC-Q200 automotive industry specifications and to withstand continuous operation at temperatures up to 175°C with a base reliability of 0.5% per 1,000 hours. Advanced, high-temperature, molded tantalum-chip capacitors capable of operating continuously at up to 200°C are also available. Beyond this are two recently introduced series: an extension to existing high-temperature axial-leaded wet electrolytics and a hermetically sealed SMD version. Both can operate as high as 230°C.
A look at the temperature coefficients of tantalum capacitors reveals they gain in capacitance value at higher temperatures. Wet tantalum capacitors can realize even higher capacitance at high temperatures. The capacitance tolerance limit at the 25°C specification for solid, hermetically sealed, high-temperature tantalum capacitors is ±20%, while wet electrolytic capacitors are made with tolerance bounds of ±10% and ±20%.
Tantalum and niobium-oxide ceramic capacitor technologies also have different temperature limits for capacitance. Solid, hermetically sealed tantalum chip capacitors exhibit narrow capacitance over their operating range, dropping up to 20% at -55°C, rising by 20% at 85°C, and holding to a maximum 30% increase from 150°C to 230°C. Wet tantalum capacitors display a much wider variation over their operating temperature range. The level of variation depends on their size and capacitance/voltage (CV) rating. The capacitance of wet tantalum capacitors can drop by 20% to 85% at -55°C and rise from 12% to 80% at 125°C.
In addition, tantalum capacitors exhibit an applied voltage vs. temperature relationship. At high temperatures, the voltage ratings specified at ambient temperatures must be derated to avoid diminished reliability. The category voltage — the maximum voltage at which the part can operate up to 230°C — for solid, hermetically sealed, high-temperature capacitors is high for solid tantalum technology: 50% of its rated voltage. This property can be advantageous at high temperatures. The higher reliability resulting from operating at a reduced voltage can more than compensate for the lower reliability resulting from operating at high temperature.
Tantalum capacitor direct current leakage (DCL) also depends on temperature, rising by a factor of 10 between 25 and 85°C. DCL arises from parallel resistance paths in the dielectric and results in the slow discharge of the capacitor. In tantalum capacitors, leakage current rises linearly with temperature, which increases energy loss. The DCL specification limit for 125°C operation for solid, hermetically sealed, high-temperature tantalum capacitors is 12.5 times the initial limit.
For about 50 years, wet tantalum capacitors packaged in a hermetically sealed, (typically) tantalum metal housing have been available in high-temperature designs suitable for operating up to 200°C with a category voltage equaling 60% of the rated voltage at room temperature (0.6Vr). More recently, material advances in powder, silver, molding resin, and other materials have also allowed solid tantalum chip capacitors in molded bodies to build up a track record in 200°C applications.
It has been challenging to boost the operating temperature for tantalum chip capacitors beyond 200°C because this temperature exceeds the glass transient temperature of the epoxy materials used in the capacitor body construction. Design engineers finally devised a reliable, SMD capacitor rated for 230°C by housing solid tantalum capacitor elements in hermetically sealed ceramic packages.
These high-temperature SMD tantalum capacitors enclose the capacitor element in a hermetically sealed ceramic housing filled with inert nitrogen gas to create an inner atmosphere that inhibits the oxidation of the solid tantalum electrolyte. This design has performed well during both high-temperature operating life testing (2,000 hours at 230°C and 0.5Vr and 10,000 hours at 200°C and 0.5Vr) and moisture resistance testing (1,000 hours at 85°C and 85%RH, Vr). Solid, hermetically sealed tantalum chip capacitors are also rugged, satisfying mechanical shock and vibration specifications of 100g shock and 20g vibration.
Available ceramic housing sizes include CTC-21D (“9”) and “I” cases with L-shaped or flexible J-lead terminations for applications subject to extreme shock and vibration or undertab (facedown) terminations for applications in which minimal footprints take precedence. The smallest case size measures 11(±0.2)x6(±0.2)x2.5 mm (LxWxH), while the largest measures 12.10×12.5×6.5 mm (LxWxH).
The capacitor elements in these packages provide high capacitance ratings (up to 100 µF) over a wide voltage range (16–63 Vdc). Their capacitance is stable over the range of rated operating temperatures (-55 to +230°C). In that the capacitance of tantalum capacitors actually rises with temperature, these capacitors can lead to significant size reductions and lower component counts in a variety of critical, high-temperature, high-reliability electronic systems.
New axial-leaded, hermetically sealed wet tantalum electrolytic capacitors are also capable of 230°C maximum operating temperatures. They are currently available in the standard T4 case size, measuring 26.97-mm long and 10.31 mm in diameter with an insulating sleeve. This series represents the largest standard case size for axial-leaded wet tantalum capacitors and provides high CV ratings up to 330 µF/125 V.
There are numerous emerging, short-lifetime applications that demand high energy density after a period of latency (e.g., a remote trigger). Such applications require that capacitors rest in a charged state for many months prior to activation. These situations may be able to use parts rated at 125°C, even if the environment periodically exceeds this temperature. In this regard, a new capacitor technology targets low-frequency pulse applications. The capacitors have high CV ratings (up to 50 mF/6 V) that overlap with supercapacitor technology but can operate continually at 125°C and exhibit extended lifetimes of up to 10,000 hours.
Termination material options
Tantalum capacitors are available with a variety of termination and metallization materials. High-temperature capacitor designs incorporate metals with melting points that are well above the temperature rating of the capacitor, such as tin (Sn), palladium/silver (PdAg), and gold (Au).
With regard to mounting, many solders used in commercial systems have low liquidus points that prevent their use at high temperatures. These applications require high-melting-point (HMP) solder and/or high-temperature epoxies with a liquidus phase around 265°C. Above the liquidus phase temperatures, the solder enters a phase of plasticity that can degrade PCB mounting. As such, it is important that the HMP solder material functions well above the operational temperature. Ditto for high-temperature epoxy used for mounting as well as mechanical reliability via its elastic properties.
Molded SMD, hermetic SMD, and axial-leaded, hermetically sealed wet tantalum capacitors all have different capacitance and voltage range capabilities. Each are better suited to certain applications than others. When their performance capabilities overlap, the cathode type, which also greatly affects frequency performance, will help determine the best applications for each technology.
In downhole electronics, high-temperature is usually classified as 150°C and above. Temperatures of 150°C to 175°C were long considered the maximum typical for drilling operations, but the need to drill deeper has significantly boosted this range in recent years. As such, the components employed in today’s wells must withstand extreme temperatures often exceeding 200°C at pressures greater than 25 kpsi. These components must also handle both continuous vibration up to 20 g and extreme shock spanning 100–2,000 g.
The aerospace and defense markets also increasingly demand extreme-temperature passive components. For instance, avionics continue to incorporate more sophisticated electronics that continuously provide power, diagnostics, and communications at temperatures exceeding those of traditional automotive underhood applications. Unmanned aerial vehicles increasingly work in environments too dangerous for manned flight. These critical systems demand the utmost reliability where components must be capable of withstanding temperatures exceeding the current military and aerospace standard of -55 to +125°C.
Designers now have a range of reliable, high-temperature capacitor solutions to choose from, and can more easily and effectively match the unique performance characteristics of their individual applications to the appropriate capacitor technology to achieve better performance than ever before.