TANTALUM ELECTROLYTIC CAPACITORS

Tantalum electrolytic capacitors are made in three styles: (1) wet foil, (2) wet anode, and (3) solid anode. Tantalum capacitors typically have higher CV ratings than aluminum electrolytic capacitors with the same capacitance values. The dielectric formed, tantalum oxide (Ta2O5), has nearly twice the dielectric constant of aluminum oxide. All tantalum capacitors are inherently polarized. As a group, they offer long shelf life, stable operating characteristics, high operating temperature ranges, and higher CV ratios than aluminum electrolytic capacitors. However, they are more expensive than comparably rated aluminum capacitors and have lower voltage ratings.

WET-FOIL TANTALUM CAPACITORS

A wet-foil tantalum capacitor is made by a process similar to that used in making an aluminum electrolytic capacitor. These capacitors can withstand voltages of up to 300 VDC. Packaged in tantalum cases, they are primarily specified for military/aerospace and highreliability applications.


Figure 1-16 Wet-slug tantalum electrolytic capacitor.

WET-ANODE TANTALUM CAPACITORS

A wet-anode tantalum capacitor, as shown in Fig. 1-16, is made from a porous tantalum pellet that is formed by pressing finely ground tantalum powder and a binder in a mold and firing it in a vacuum furnace at about 2000°C. Heat welds or sinters the powder into a solid spongelike pellet with a large effective surface area. A thin film of tantalum oxide is grown electrochemically on the pellet and electrolyte is added. Packaged in silver or tantalum cases, their CV ratios are about 3 times those of wet-foil tantalum capacitors.


Figure 1-17 Epoxy-dipped solid-slug tantalum capacitor.

SOLID-ANODE TANTALUM CAPACITORS

A solid-anode tantalum capacitor, as shown in Fig. 1-17, is also made from a porous pellet anode. A thin film of manganese dioxide that is chemically deposited on the tantalum oxide dielectric serves as a solid electrolyte and cathode. Then a layer of carbon and conductive paint is applied to complete the cathode connection. The most popular and lowest-cost tantalum capacitors, they are available with either radial or axial leads. They are dipped or molded in plastic resin to form protective jackets. Some are also enclosed in tantalum cases for further environmental protection. These capacitors have the longest lives and lowest leakage current of any tantalum capacitors. They can have capacitive values of 0.10 to 680 μF, capacitive tolerances of +- 10 to 20 percent, and maximum voltages of 50 V. The popular ratings are 1 to 10 μF.



Figure 1-18 Tantalum chip capacitor.

SOLID-ANODE CHIP TANTALUM CAPACITORS

A solid-anode chip tantalum capacitor, as shown in Fig. 1-18, is made by the same methods as the radial-leaded version, but it is packaged in a leadless molded epoxy case for bonding to surface-mount cards or hybrid circuits. They can have capacitive values of 100 pF to 100 μF, capacitive tolerances of +- 5 to 20 percent, and maximum voltages of 50 V.