Wet Tantalum Capacitors Meet DSCC 93026

Jan 1, 2009 12:00 PM

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.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.