Power Electronics

Wet Tantalum Capacitors Meet DSCC 93026

Wet tantalum capacitors now use the intrinsic capability of a proprietary cathode system to provide high capacitance/voltage characteristics, allowing them to be qualified to DSCC 93026.

Wet tantalum capacitors now use the intrinsic capability of a proprietary cathode system to provide high capacitance/voltage characteristics, allowing them to be qualified to DSCC 93026.

The Next generation of wet tantalum capacitor technology enables both higher-efficiency and higher-reliability capacitor designs, extending their application capabilities. Traditional wet tantalum capacitors use a sleeve of pressed and sintered tantalum powder for the cathode system. Devices manufactured in this manner are qualified to the military specification 39006.

The key to higher-efficiency wet tantalum capacitor technology, such as those manufactured to the military drawing 93026, is the use of a more volumetrically efficient cathode system, which in turn enables a larger internal capacitor element (referred to as “anode”) to be used. Traditionally, for the cathode system, a metal oxide, such as ruthenium oxide, or a carbon cathode, such as a carbon palladium system, has been used. Now, the introduction of a new proprietary cathode system provides higher energy densities.

Basic wet tantalum construction starts with the electrochemical manufacture of the capacitor element itself. Extremely fine particle size, high-purity tantalum powder is pressed into a cylindrical pellet, at the same time embedding a tantalum riser wire into the center of the pellet. The pellet is then sintered, causing neighboring tantalum particles to fuse together into a continuous matrix with very high internal surface area. A tantalum pentoxide dielectric is formed over this surface by immersing the body of the pellet in acid, making electrical contact via the riser wire and applying current and voltage. So far, the process to make the formed pellet (often called the anode, as the riser wire forms the positive contact) is identical to that of solid tantalum anode construction.

The remaining stages of the process are to contact the dielectric surface with an electrolyte (that forms the negative contact) and then establish an external electrical contact layer. For solid tantalums, the formed anode is impregnated with manganese dioxide (solid electrolyte), which then has an external carbon/silver coat for external epoxy or solder contact. As the name implies, wet tantalums use a wet electrolyte system, typically sulfuric acid. To establish an external negative contact, this anode is placed into a cylindrical case that holds the electrolyte solution. The housing typically is made of either tantalum, and itself becomes part of the cathode of the capacitor. To increase the effective area of the cathode, thereby increasing the capacitance, additional cathode material is set inside the case surrounding the anode of the can.

To complete the assembly of the device, an insulated mount is inserted into the case, providing internal support for the anode. The anode is inserted and the electrolyte solution dispensed, and then a hermetic-insulated seal is applied to the top of the case — which allows the positive riser to exit — and a lead is attached to the other end to make the negative lead. Once this assembly is complete, the top of the case is welded to provide a hermetic seal.

WET TANTALUM HISTORY

The first wet tantalum capacitors were developed 40 years ago, and comprised a tantalum anode surrounded by an electrolyte inside a silver case with an epoxy end seal. This design was problematic because it could be prone to leakage of the electrolyte through the epoxy seal. It also had a limited ability to withstand any reverse voltage. The silver case material was later replaced with tantalum, which proved more stable over a range of applications. The use of a tantalum case made it easier to construct a tantalum base-to-metal end seal that could be laser-welded to the tantalum can, thus making a hermetic capacitor. This addressed the risk of fluid leakage from the part and improved overall reliability. The process also included the use of a porous tantalum sleeve inside the case to increase the area of the cathode system.

Military specification MIL-STD-39006 was generated to define qualification testing (based on MIL-STD-202 tests) for the various families of wet tantalum that were developed. Fig. 1 shows the construction details of the conventional wet tantalum capacitor.

Because the bulk of the capacitance attainable is strongly dependant on the area of the cathode, alternative cathode systems using metal oxides were developed that significantly increased available capacitance. The ratings achieved with this design surpassed those available within the existing MIL-STD-39006, and a new DSCC drawing was created to define the available range DSCC 93026.

The current series of wet tantalums has taken the expansion of the cathode system one step further. Using new proprietary cathode material, the cathode system has been sintered directly to the interior of the tantalum can. This system not only increases the area of the cathode, but also increases the internal volume available for the anode, thus significantly increasing the potential capacitance/voltage (CV) ratings available in any given case size. Fig. 2 shows this construction.

ELECTRICAL PROPERTIES

The resulting system gives very high capacitance (7000 mF/cc), and there is the potential to increase capacitance to 30,000 mF/cc by reducing the thickness of the cathode layer.

Current-voltage ratings range from 25 V to 125 V and only require an 80% derating below 85°C. Because the new cathode system also gives improved ESR performance, it is capable of operating under high ripple currents and is currently qualified for use up to 125°C, as well as able to withstand reverse voltages up to 1.5 V.

In terms of CV ratings available, previously the largest capacitor value in the largest T4 case size at 25 V was 1800 uF; 2200 uF is now attainable. Another example is in the 60-V range, where the available capacitance has almost been doubled from 560 uF to 1000 uF.

The wet tantalums are fully qualified to DSCC 93026, the current revision (Rev. P) of which has been updated to include the additional ratings described previously. The DSCC drawing references several qualification tests taken from MIL-STD-202 to establish component reliability, including but not limited to the tests listed in the table.

In addition, the new cathode system adds extra mechanical reliability. The proprietary material mitigates oxygen migration to the can wall, which can cause embrittlement. This is critical for the crimping operation.

Also, the sintered cathode on the can wall, along with a larger anode, offers improved resistance against mechanical strain during extreme vibration. This previously may have been an issue in some space applications that may now find additional applications for the wet tantalum.

APPLICATIONS

Tantalum dielectric has excellent temperature characteristics and reliability (no wear-out mechanism), and the wet electrolyte system has many advantages over solid technology. Because it features a more efficient, continual self-healing mechanism, it can support higher voltage ratings (up to 125 V compared to 50-V maximum typical for most solid capacitors) and lower leakage current. This extends the application suitability far beyond the 28-V bus. Wet tantalum capacitors also are an ideal solution, specifically where size and weight are concerns. Qualification to DSCC 93026 makes the wet tantalums a suitable fit for commercial avionics. Currently, these components are widely used in avionic power and engine control, including FADEC (full authority digital engine control) applications.

From avionics to industrial applications, the most common use is for power-supply input and output filtering. The major benefit of wet tantalum is its high bulk capacitance and extended voltage range for use in high-power designs where solid tantalum capacitors are not always suitable.

The combination of high bulk capacitance and low ESR provides high joule discharge capability, suitable for pulse power applications ranging from radar to jet engine (or ejector seat) ignition and high-power burst-mode transmission. In dc applications, they also can be used for a wide range of voltage holdup and timing applications.

New application opportunities also exist for axial modules. Wet tantalums in a modular format can help reduce required layout size, component count, component weight and even add extra reliability. By using a module, the ESR of the individual components can be matched, and the unit can be tested as a whole to ensure optimum performance and reliability.

SOLID TANTALUM CAPACITORS

SOLID ELECTROLYTE CAPACITORS contain manganese dioxide on a tantalum pentoxide dielectric layer. Next, this tantalum pellet is coated with graphite, followed by a layer of metallic silver — providing a solderable surface between the pellet and the can in which it will be enclosed. The pellet, with lead wire and header attached, is inserted into the can where the pellet is held in place by solder. After assembly, the capacitors are tested and inspected for reliability.

Another variation of the solid electrolyte tantalum capacitor encases the element in epoxy resins. It offers excellent reliability and high stability for consumer and commercial electronics with the added feature of low cost. Surface-mount designs of solid tantalum capacitors use lead frames or lead frameless designs.

Solid electrolyte designs are used in applications where their very small size for a given unit of capacitance is important. They typically will withstand up to about 10% of the rated dc working voltage in a reverse direction. Also important are their good low-temperature performance characteristics and freedom from corrosive electrolytes.

Solid tantalum capacitors have no limitation on shelf life. The dielectric is stable and no reformation is required. The only factors that affect future performance of the capacitors are high humidity conditions and extreme storage temperatures. Solderability of solder-coated surfaces may be affected by storage in excess of one year under temperatures greater than 40°C or humidities greater than 80% relative humidity. Terminations should be checked for solderability in the event an oxidation develops on the solder plating.

Wet electrolyte types have the following advantages over the solid system:

  • Supports higher voltage dielectrics — voltage ratings to 125 V are available for wet tantalum, but 50 V is the maximum for dry tantalum

  • Better self-healing characteristics requiring less voltage derating — recommended voltage derating is typically 50% for solid tantalum but only 20% for wet tantalum, enabling higher application capacitance use.

  • Construction supports dielectric formation for larger anodes than available for tantalum chips, allowing CV combinations to 2200 uF/25 V or 1000 uF/60 V — typically 10× the CV available in discrete tantalum chip.

Fig. 3 compares the wet and solid tantalum capacitors in terms of capacitance/voltage.

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