Tantalum Capacitor

Chapter 2 - Types

What are tantalum capacitors?

tantalum capacitorTantalum capacitors are a subtype of electrolytic capacitors. They are made of tantalum metal which acts as an anode, covered by a layer of oxide which acts as the dielectric, surrounded by a conductive cathode. The use of tantalum allows for a very thin dielectric layer. This results in a higher capacitance value per volume, superior frequency characteristics compared to many other types of capacitors and excellent stability over time. Tantalum capacitors are generally polarized, which means that they may only be connected to a DC supply observing the correct terminal polarity. The downside to using tantalum capacitors is their unfavorable failure mode which may lead to thermal runaway, fires and small explosions, but this can be prevented through the use of external failsafe devices such as current limiters or thermal fuses. Technology advances allow tantalum capacitors to be used in a wide variety of circuits, often found in laptops, automotive industry, cell phones and others, most often in the form of surface mounted devices (SMD). These surface mount tantalum capacitors claim much less space on the printed circuit board and allow for greater packing densities.

Tantalum capacitor definition

Tantalum capacitors are electrolytic capacitors which use tantalum metal for the anode. They are polarized capacitors with superior frequency and stability characteristics.

Characteristics

General characteristics

Tantalum capacitors are made with capacitance values ranging from 1nF all the way to 72mF and they are much smaller in size than aluminum electrolytic capacitors of the same capacitance. The voltage rating for tantalum capacitors varies from 2V to more than 500V.  They have an equivalent series resistance (ESR) ten times smaller than the ESR of aluminum electrolytic capacitors, which allows for larger currents to pass through the capacitor with less heat generated. Tantalum capacitors are very stable over time and their capacitance doesn’t change with age significantly, especially when compared to aluminum electrolytic capacitors. They are very reliable when handled properly and their shelf life is virtually unlimited.

Polarity

Tantalum electrolytic capacitors are exceptionally polarized devices. While aluminum electrolytic capacitors, which are polarized as well, might survive a briefly applied reverse voltage, tantalum capacitors are very sensitive to reverse polarization. If a reverse polarity voltage is applied, the dielectric oxide breaks down, sometimes forming a short circuit. This short circuit may later cause thermal runaway and destruction of the capacitor.

It should be noted that tantalum capacitors usually have their positive terminal marked, in contrast to aluminum electrolytic capacitors, which have their negative terminal marked on the casing.

Failure mode

Tantalum capacitors have a potentially dangerous failure mode. In case of voltage spikes, the tantalum anode may come in contact with the manganese dioxide cathode, and if the energy of the spike is sufficient it may start a chemical reaction. This chemical reaction produces heat and is self-sustaining and may produce smoke and flame. To prevent this thermal runaway from happening, external failsafe circuitry such as current limiters and thermal fuses should be used in conjunction with tantalum capacitors.

Construction and properties of tantalum capacitors

Tantalum electrolytic capacitors, just like other electrolytic capacitors, are consisted of an anode, some electrolyte and a cathode. The anode is isolated from the cathode so only a very small leakage DC current may flow through the capacitor. The anode is made of pure tantalum metal. The metal is ground into a fine powder, and sintered into a pellet at high temperatures. This forms a very porous anode with a high surface area. A high surface area directly translates to an increased capacitance value.

The anode is then covered with a layer of insulating oxide, which acts as a dielectric. This process is called anodization. This step must be precisely controlled to reduce tolerances and ensure correct capacitance values as the extent of oxide growth determines the dielectric thickness.

Electrolyte is added to the anode by means of pyrolysis in the case of solid tantalum capacitors. Solid tantalum capacitors are then dipped into a special solution and baked in an oven to produce a manganese dioxide coat. The process is repeated until a thick coating is present on all internal and external surfaces of the pellet. Finally, the pellet used in solid tantalum capacitors is dipped into graphite and silver to provide a good cathode connection. In contrast to solid tantalum capacitors, wet tantalum capacitors use a liquid electrolyte. After the anode is sintered and dielectric layer is grown, it is dipped into a liquid electrolyte within an enclosure. The enclosure and electrolyte together serve as the cathode in wet tantalum capacitors.

Applications for tantalum capacitors

Applications using tantalum capacitors take advantage of their low leakage current, high capacity and long term stability and reliability. For example, they are used in sample and hold circuits which rely on low leakage current to achieve long hold duration. They are also commonly used for power supply filtering on computer motherboards and cell phones due to their small size and long term stability, most often in surface mount form. Tantalum capacitors are also available in military specifications versions (MIL-SPEC), which offer tighter tolerances and a wider operating temperature range. They are a popular replacement for aluminum electrolytics in military applications because they do not tend to dry out and change capacitance over time. Medical electronics also rely on tantalum because of their high stability. Audio amplifiers sometimes use tantalum capacitors where stability is a critical factor.