Tantalum capacitor
Encyclopedia
The tantalum capacitor is a highly reliable type of electrolytic capacitor
, available in both solid-bodied and separately encased forms. The encased "wet" variant is not used often in modern designs. Surface mount tantalum capacitors are widely used in circuit designs because of their volumetric efficiency, basic reliability and process compatibility.
to form a non-conductive protective oxide surface layer. They are made using tantalum powder pressed into a pellet shape, as one "plate" of the capacitor, with the oxide as a dielectric, and an electrolytic solution or conductive solid as the other "plate". Because the dielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a high capacitance can be achieved in a small volume. Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, personal computers, and automotive electronics.
The steady-state and dynamic reliability of a tantalum capacitor are influenced by several factors under the control of the circuit design engineer. These factors are voltage derating, ripple current and voltage conditions, maximum operating temperature
and circuit impedance. Tantalum capacitors possess very low electrical leakage (high leakage resistance), and will retain a charge for a long duration. They are tolerant of hot operating environments up to 125 °C, unlike standard aluminum electrolytic capacitors.
Tantalum capacitors are not particularly tolerant of heavy charge and discharge currents, particularly those of a repetitive nature, and their effective series resistance (ESR) can be high when compared to ceramics or aluminum electrolytics. Most tantalum capacitors are polarized devices; correct polarity must be observed, otherwise the capacitor will depolarize and the dielectric oxide layer will break down, reducing the resistance of the device and causing it to fail. Tantalum capacitors are relatively expensive, particularly the military grades.
Although tantalum capacitors are extremely reliable, when they do fail, they tend to short-circuit. Careful consideration of the effects of this failure mode is essential for good design practice. There is also a possibility of catastrophic thermal runaway
failure (see below).
Because of the solid nature of most tantalum capacitors being made today, there is no known wear-out mechanism. Solid-bodied tantalum electrolytic capacitors are less prone to "drying out" than aluminum capacitors, which tend to decrease in capacitance particularly when used in hot environments. When operated within their design limits, tantalum capacitors can maintain their designed capacitance under such conditions over decades.
. The capacitor typically consists of a sintered
tantalum
sponge acting as the anode
, a manganese dioxide cathode
, and a dielectric
layer of tantalum pentoxide created on the tantalum sponge surface by anodizing. The tantalum oxide layer may have weak spots that undergo dielectric breakdown during a voltage spike
. The tantalum then comes to direct contact with manganese dioxide and the leakage current causes localized heating; a chemical reaction then produces manganese(III) oxide
and regenerates (self-heals
) the tantalum oxide layer.
However, if the energy dissipated at the failure point is high enough, a new self-sustaining exothermic
reaction may initiate, similar to the thermite
reaction, with tantalum as fuel and manganese dioxide as oxidizer. This can destroy the capacitor and occasionally produces smoke
and possibly flame
. To prevent catastrophic thermal runaway failure, auxiliary protective devices (e.g. thermal fuses, circuit breakers) may be used to limit fault currents.
circuits to achieve long hold duration, and some long duration timing circuits where precise timing is not critical. They are also often used for power supply rail decoupling in parallel with film or ceramic capacitors
which provide low ESR
and low reactance at high frequency. Tantalum capacitors can replace aluminum electrolytic capacitors in situations where the external environment or dense component packing results in a sustained hot internal environment and where high reliability is important. Equipment such as medical electronics and space equipment that require high quality and reliability make use of tantalum capacitors.
Low-voltage tantalum capacitors are commonly used in large numbers for power supply filtering
on computer motherboards and in peripherals due to their small size and long-term reliability.
When applying tantalum capacitors, the possibility of thermal runaway (see above) or spontaneous short-circuiting is usually considered. In many cases, a failed capacitor will cause termination of normal circuit operation, but no other ill effects. However, if sufficient power is available, catastrophic thermal runaway may cause a fire or small explosion. If this situation is a possibility, external current limiting (e.g. thermal fuse, circuit breaker) is often used to prevent catastrophic failure.
Tantalum capacitors have their positive terminal marked, while aluminium electrolytics are often marked on the negative terminal. This can cause dangerous confusion, since reversed polarity can cause catastrophic failure of either type.
metal. A common figure of merit for comparing powders is expressed in capacitance (C) in microfarads (µF) times voltage (V) in volts (V) per gram (CV/g).
The typical particle size is between 2 and 10 μm. Figure 1 shows powders of successively finer grain, resulting in greater surface area per unit volume.
Note the very great difference in particle size between the powders.
The powder is compressed around a tantalum wire (known as the riser wire) to form a “pellet”. The riser wire ultimately becomes the anode connection to the capacitor. This pellet/wire combination is subsequently vacuum sintered
at high temperature (typically 1200 to 1800 °C) which produces a mechanically strong pellet and drives off many impurities within the powder. During sintering, the powder becomes a sponge-like structure with all the particles interconnected into a monolithic spatial lattice. This structure is of predictable mechanical strength and density, but is also highly porous, producing a large internal surface area (see Figure 2).
Larger surface area produces higher capacitance; thus high CV/g powders, which have a lower average particle size, are used for low voltage, high capacitance parts. By choosing the correct powder type and sinter temperature, a specific capacitance/voltage rating can be designed. For example, a 220μF 6V capacitor will have a surface area close to 346 cm2, or 80% of the size of a sheet of paper (US Letter, 8.5×11 inch paper has area ~413 cm2), although the total volume of the pellet is only about 0.0016 cm3.
is then formed over all the tantalum particle surfaces by the electrochemical process of anodization. To achieve this, the “pellet” is submerged into a very weak solution of acid and DC voltage is applied. The total dielectric thickness is determined by the final voltage applied during the forming process. Initially the power supply is kept in a constant current mode until the correct voltage (i.e. dielectric thickness) has been reached, it then holds this voltage and the current decays to close to zero to provide a uniform thickness throughout the device and production lot.
The chemical equations describing the dielectric formation process are as follows:
The oxide forms on the surface of the tantalum but it also grows into the material. For each unit of oxide two thirds grows out and one third grows in. It is for this reason that there is a limit on the maximum voltage rating of tantalum oxide for each of the presently available tantalum powders (see Figure 3).
This process is repeated several times through varying specific gravities of nitrate solution, to build up a thick coat over all internal and external surfaces of the “pellet”, as shown in Figure 4.
In traditional construction, the “pellet” is successively dipped into graphite
and then silver
to provide a good connection to the manganese dioxide cathode plate. Electrical contact is first established by deposition of graphitic carbon
onto the surface of the cathode. The carbon is then coated with a conductive material to facilitate connection to the external cathode termination (see Figure 5).
In the newer tantalum polymer capacitor process, conductive polymer
s are used as cathode material http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/Product%20Release%20-%20T521/$file/T521%20High%20Voltage%20Polymer%20CARTS%20Paper.pdf.
Most used cases: A, B, C and D.
Electrolytic capacitor
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics"...
, available in both solid-bodied and separately encased forms. The encased "wet" variant is not used often in modern designs. Surface mount tantalum capacitors are widely used in circuit designs because of their volumetric efficiency, basic reliability and process compatibility.
Characteristics
Tantalum electrolytic capacitors exploit the tendency of tantalumTantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
to form a non-conductive protective oxide surface layer. They are made using tantalum powder pressed into a pellet shape, as one "plate" of the capacitor, with the oxide as a dielectric, and an electrolytic solution or conductive solid as the other "plate". Because the dielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a high capacitance can be achieved in a small volume. Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, personal computers, and automotive electronics.
The steady-state and dynamic reliability of a tantalum capacitor are influenced by several factors under the control of the circuit design engineer. These factors are voltage derating, ripple current and voltage conditions, maximum operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
and circuit impedance. Tantalum capacitors possess very low electrical leakage (high leakage resistance), and will retain a charge for a long duration. They are tolerant of hot operating environments up to 125 °C, unlike standard aluminum electrolytic capacitors.
Tantalum capacitors are not particularly tolerant of heavy charge and discharge currents, particularly those of a repetitive nature, and their effective series resistance (ESR) can be high when compared to ceramics or aluminum electrolytics. Most tantalum capacitors are polarized devices; correct polarity must be observed, otherwise the capacitor will depolarize and the dielectric oxide layer will break down, reducing the resistance of the device and causing it to fail. Tantalum capacitors are relatively expensive, particularly the military grades.
Although tantalum capacitors are extremely reliable, when they do fail, they tend to short-circuit. Careful consideration of the effects of this failure mode is essential for good design practice. There is also a possibility of catastrophic thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
failure (see below).
Because of the solid nature of most tantalum capacitors being made today, there is no known wear-out mechanism. Solid-bodied tantalum electrolytic capacitors are less prone to "drying out" than aluminum capacitors, which tend to decrease in capacitance particularly when used in hot environments. When operated within their design limits, tantalum capacitors can maintain their designed capacitance under such conditions over decades.
Self-destruction and thermal runaway
Tantalum capacitors are under some conditions prone to self-destruction by thermal runawayThermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
. The capacitor typically consists of a sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
sponge acting as the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, a manganese dioxide cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
, and a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
layer of tantalum pentoxide created on the tantalum sponge surface by anodizing. The tantalum oxide layer may have weak spots that undergo dielectric breakdown during a voltage spike
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
. The tantalum then comes to direct contact with manganese dioxide and the leakage current causes localized heating; a chemical reaction then produces manganese(III) oxide
Manganese(III) oxide
Manganese oxide is the chemical compound of formula Mn2O3.-Preparation and chemistry:Heating MnO2 in air at below 800°C α-Mn2O3 is produced . γ-Mn2O3 can be produced by oxidation followed by dehydration of manganese hydroxide...
and regenerates (self-heals
Self-healing
Self-healing is a phrase applied to the process of recovery , motivated by and directed by the patient, guided often only by instinct. Such a process encounters mixed fortunes due to its amateur nature, although self-motivation is a major asset...
) the tantalum oxide layer.
However, if the energy dissipated at the failure point is high enough, a new self-sustaining exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
reaction may initiate, similar to the thermite
Thermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...
reaction, with tantalum as fuel and manganese dioxide as oxidizer. This can destroy the capacitor and occasionally produces smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
and possibly flame
Flame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
. To prevent catastrophic thermal runaway failure, auxiliary protective devices (e.g. thermal fuses, circuit breakers) may be used to limit fault currents.
Uses
The low leakage and high capacity of tantalum capacitors favors their use in sample and holdSample and hold
In electronics, a sample and hold circuit is an analog device that samples the voltage of a continuously varying analog signal and holds its value at a constant level for a specified minimal period of time. Sample and hold circuits and related peak detectors are the elementary analog memory...
circuits to achieve long hold duration, and some long duration timing circuits where precise timing is not critical. They are also often used for power supply rail decoupling in parallel with film or ceramic capacitors
Ceramic capacitor
In electronics, a ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. The temperature coefficient depends on whether the dielectric is Class 1 or Class 2...
which provide low ESR
Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated to a very good approximation as ideal capacitors and inductors in series with a resistance; this resistance is defined to be the equivalent...
and low reactance at high frequency. Tantalum capacitors can replace aluminum electrolytic capacitors in situations where the external environment or dense component packing results in a sustained hot internal environment and where high reliability is important. Equipment such as medical electronics and space equipment that require high quality and reliability make use of tantalum capacitors.
Low-voltage tantalum capacitors are commonly used in large numbers for power supply filtering
Filter capacitor
Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:* Glitch removal on Direct current power rails...
on computer motherboards and in peripherals due to their small size and long-term reliability.
When applying tantalum capacitors, the possibility of thermal runaway (see above) or spontaneous short-circuiting is usually considered. In many cases, a failed capacitor will cause termination of normal circuit operation, but no other ill effects. However, if sufficient power is available, catastrophic thermal runaway may cause a fire or small explosion. If this situation is a possibility, external current limiting (e.g. thermal fuse, circuit breaker) is often used to prevent catastrophic failure.
Tantalum capacitors have their positive terminal marked, while aluminium electrolytics are often marked on the negative terminal. This can cause dangerous confusion, since reversed polarity can cause catastrophic failure of either type.
Anode
Tantalum capacitors are manufactured from a powder of relatively pure tantalumTantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
metal. A common figure of merit for comparing powders is expressed in capacitance (C) in microfarads (µF) times voltage (V) in volts (V) per gram (CV/g).
The typical particle size is between 2 and 10 μm. Figure 1 shows powders of successively finer grain, resulting in greater surface area per unit volume.
Note the very great difference in particle size between the powders.
The powder is compressed around a tantalum wire (known as the riser wire) to form a “pellet”. The riser wire ultimately becomes the anode connection to the capacitor. This pellet/wire combination is subsequently vacuum sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
at high temperature (typically 1200 to 1800 °C) which produces a mechanically strong pellet and drives off many impurities within the powder. During sintering, the powder becomes a sponge-like structure with all the particles interconnected into a monolithic spatial lattice. This structure is of predictable mechanical strength and density, but is also highly porous, producing a large internal surface area (see Figure 2).
Larger surface area produces higher capacitance; thus high CV/g powders, which have a lower average particle size, are used for low voltage, high capacitance parts. By choosing the correct powder type and sinter temperature, a specific capacitance/voltage rating can be designed. For example, a 220μF 6V capacitor will have a surface area close to 346 cm2, or 80% of the size of a sheet of paper (US Letter, 8.5×11 inch paper has area ~413 cm2), although the total volume of the pellet is only about 0.0016 cm3.
Dielectric
The dielectricDielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
is then formed over all the tantalum particle surfaces by the electrochemical process of anodization. To achieve this, the “pellet” is submerged into a very weak solution of acid and DC voltage is applied. The total dielectric thickness is determined by the final voltage applied during the forming process. Initially the power supply is kept in a constant current mode until the correct voltage (i.e. dielectric thickness) has been reached, it then holds this voltage and the current decays to close to zero to provide a uniform thickness throughout the device and production lot.
The chemical equations describing the dielectric formation process are as follows:
- 2 Ta → 2 Ta5+ + 10 e−
- 2 Ta5+ + 10 OH− → Ta2O5 + 5 H2O
The oxide forms on the surface of the tantalum but it also grows into the material. For each unit of oxide two thirds grows out and one third grows in. It is for this reason that there is a limit on the maximum voltage rating of tantalum oxide for each of the presently available tantalum powders (see Figure 3).
Cathode
The next stage for solid tantalum capacitors is the application of the cathode plate (wet tantalum capacitors use a liquid electrolyte as a cathode in conjunction with their casing). This is achieved by pyrolysis of manganese nitrate into manganese dioxide. The “pellet” is dipped into an aqueous solution of nitrate and then baked in an oven at approximately 250 °C to produce the dioxide coat. The chemical equation is:- Mn(NO3)2 → MnO2 + 2 NO2
This process is repeated several times through varying specific gravities of nitrate solution, to build up a thick coat over all internal and external surfaces of the “pellet”, as shown in Figure 4.
In traditional construction, the “pellet” is successively dipped into graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
and then silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
to provide a good connection to the manganese dioxide cathode plate. Electrical contact is first established by deposition of graphitic carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
onto the surface of the cathode. The carbon is then coated with a conductive material to facilitate connection to the external cathode termination (see Figure 5).
In the newer tantalum polymer capacitor process, conductive polymer
Conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...
s are used as cathode material http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/Product%20Release%20-%20T521/$file/T521%20High%20Voltage%20Polymer%20CARTS%20Paper.pdf.
Packaging
Once fabricated, finished capacitor pellets are packaged into four basic forms: chip, resin-dipped, molded. or axial-leaded.Surface mount tantalum capacitor dimensions (EIA standard package sizes)
L: length, W: width, H: height, W1: width of contact plate, A: length of contact plate.Most used cases: A, B, C and D.
