Polymer capacitor

Rectangular-shaped polymer aluminum (black) and tantalum (brown) electrolytic chip capacitors

Cylindrical (wound) polymer Al-caps

A polymer capacitor, or more accurately a polymer electrolytic capacitor, is an electrolytic capacitor (e-cap) with a solid electrolyte made of a conductive polymer. The three different types are:

• Polymer Ta-cap
• Polymer Al-cap
• Hybrid polymer/liquid e-cap

A fourth type, polymer niobium e-caps, are not in production.

Polymer e-caps are available in rectangular surface-mounted device (SMD) chip style or in cylindrical SMDs (V-chips) style or as radial leaded versions (single-ended).

Polymer capacitors are characterized by low internal equivalent series resistances (ESR) and high ripple current ratings. Their electrical parameters have similar temperature dependence, reliability and service life compared to solid Ta-caps, but much better temperature independence and a longer service life than Al-caps with liquid electrolytes. In general polymer e-caps have a higher leakage current rating than others.

Polymer e-caps are mainly used as power supplies of integrated electronic circuits as buffer, bypass and decoupling capacitors, especially in devices with flat or compact design. Thus they compete with multi-layer ceramic chip (MLCC) capacitors, with higher capacitance values. They display no microphonic effect.

History

Aluminum capacitors (Al-caps) with liquid electrolytes were invented in 1896 by Charles Pollak.

Tantalum e-caps (Ta-caps) with solid manganese dioxide (MnO₂) electrolytes were invented by Bell Laboratories in the early 1950s, as a miniaturized and more reliable low-voltage support capacitor to complement the newly invented transistor.^[1][2] The first Ta-caps with solid MnO2 electrolytes had 10 times better conductivity and a higher ripple current load than earlier types of liquid e-caps. Additionally, unlike standard e-caps, the equivalent series resistance (ESR) of Ta-caps is stable in varying temperatures.

Conductivities of some electrolytes

During the 1970s the increasing digitization of electronic circuits came with decreasing operating voltages and increasing switching frequencies and ripple current loads. This had consequences for power supplies and their e-caps. Capacitors with lower ESR and lower equivalent series inductance (ESL) for bypass and decoupling capacitors used in power supply lines were needed.^[3]

A breakthrough came in 1973, with the discovery by Heeger and Wudl of an organic conductor, the charge-transfer salt TCNQ.^[4] TCNQ (7,7,8,8-tetracyanoquinodimethane or N-n-butyl isoquinolinium in combination with TTF (Tetrathiafulvalene)) is a chain molecule of almost perfect one-dimensional structure that has 10-fold better conductivity along the chains than does MnO2 and has 100-fold better conductivity than liquid electrolytes.

OS-CON capacitors with solid TCNQ electrolyte had a typical lilac insulation sleeve

The first Al-cap to use TTF-TCNQ was the OS-CON series offered in 1983 by Sanyo. These were wound, cylindrical capacitors with 10x increased electrolyte conductivity compared with MnO2.^[5][6][7] These capacitors were used in devices for applications that required the lowest possible ESR or highest possible ripple current. One OS-CON e-cap could replace three more bulky "wet" e-caps or two Ta-caps. By 1995, the Sanyo OS-CON became the preferred decoupling capacitor for Pentium processor-based personal computers.

The Sanyo OS-CON e-cap product line was sold in 2010 to Panasonic. Panasonic then replaced the TCNQ salt with a conducting polymer under the same brand.

Conducting polymers were invented by Heeger, MacDiarmid and Shirakawa in 1975,^[8] including polypyrrole (PPy) ^[9] or PEDOT.^[10] These lowered ESR by a factor of 100 to 500 versus TCNQ, approaching the conductivity of metals.

In 1988 the first polymer electrolyte e-cap, "APYCAP" with PPy polymer electrolyte, was launched by Nitsuko.^[11] The product was not successful, in part because it was not available in SMD configurations.

In 1991 Panasonic launched its "SP-Cap",^[12] a polymer Al-cap. These used polymer electrolytes to achieve ESR values that were directly comparable to ceramic multilayer capacitors (MLCCs). They were less expensive than Ta-caps and with their flat design were useful in compact devices such as laptops and cell phones.

Ta-caps with PPy polymer electrolyte followed three years later. In 1993 NEC introduced its SMD devices, called "NeoCap". In 1997 Sanyo followed with its "POSCAP" polymer Ta-caps.

Kemet presented a new conductive polymer for polymer Ta-caps at the "1999 Carts" conference.^[13] This capacitor used the conductive polymer PEDT (Poly(3,4-ethylenedioxythiophene)), also known as PEDOT (trade name Baytron®).^[14]

Two years later at the 2001 APEC Conference, Kemet introduced PEDOT polymer Al-caps.^[15] Its AO-Cap series included SMD capacitors with stacked anode in "D" size with heights from 1.0 to 4.0 mm, competing with Panasonic.

Around the millennium hybrid polymer capacitors were developed, which add a liquid electrolyte to the polymer electrolyte.^[16][17] The liquid electrolyte provides oxygen that allows self-healing processes to reduce the leakage current in damaged devices. In 2001, NIC launched a hybrid polymer e-cap at a lower price and with lower leakage current. As of 2015 hybrid polymer capacitors were available from multiple manufacturers.

Application basics

Role of ESR, ESL and capacitance

The predominant application set for e-caps and polymer capacitors is power supplies. They cause behind the rectifying smoothing of the rectified AC voltage or interference suppression and buffer or stabilize the DC voltage at a sudden power demand of the subsequent circuit. They are called backup-, bypass- or decoupling capacitors.^[18] In addition to the size, the capacitance, the impedance Z, the ESR and the inductance ESL offer important electrical characteristics.

For a sudden power demand of a subsequent circuit, the supply voltage drops by ESL, ESR and capacitance charge loss

The change to digital electronic equipment led to the development of switching power supplies with higher frequencies and "on-board" DC/DC converter, lower supply voltages and higher supply currents. Decoupling capacitors needed lower ESR values, which at that time could only be realized with larger case sizes or much more expensive solid Ta-caps.

ESR's influence on integrated circuit function is that under a sudden power demand, the supply voltage drops:

ΔU = ESR • I

For example:^[3]

Given a supply voltage of 3 V, with a tolerance of 10% (200 mV) and supply current of a maximum of 10 A, a sudden power demand drops the voltage by:

ESR = U / I = 0.3 V / 10 A = 30 milliohms.

This means that the ESR in a CPU power supply must be less than 30 mΩ, otherwise the circuit malfunctions.

Electrolytic capacitors

Anodic oxidation

Basic principle of anodic oxidation (forming), in which, by applying a voltage with a current source, an oxide layer is formed on a metallic anode

Electrolytic capacitors use a chemical feature of some special metals, earlier called "valve metals" that by anodic oxidation form an insulating oxide layer. By applying a positive voltage to the anode, an oxide barrier layer with a thickness corresponding to the applied voltage forms. This oxide layer acts as the dielectric in an e-cap. The cathode must conform to the oxide surface. This is accomplished by the electrolyte, which acts as the cathode.

The main difference between the polymer capacitor families is the anode material and its oxide:

• Polymer Ta-caps use high purity sintered tantalum powder as an anode with tantalum pentoxide (Ta₂O₅) as the dielectric.
• Polymer Al-caps use a high purity and electrochemically roughened aluminum foil as an anode with aluminum oxide (Al₂O₃) as the dielectric.

Characteristics of the different oxide layers in aluminum and tantalum e-caps^[19]

Anode- material	Dielectric	Relative permittivity	Oxide structure	Breakdown voltage (V/µm)	Dielectric layer thickness (nm/V)
Tantalum	Tantalum pentoxide Ta2O5	27	amorphous	625	1.6
Aluminum	Aluminum oxide Al2O3	9.6	amorphous	710	1.4
			crystalline	1000	1.0

A dielectric material is placed between two conducting plates (electrodes), each of area A and with a separation of d.

Every e-cap in principle forms a "plate capacitor" whose capacitance is an increasing function of the electrode area A, the permittivity ε and the thinner the dielectric (d).

$C = \varepsilon \cdot \frac{A}{d}$

Capacitance is proportional to the product of the area of one plate multiplied by the permittivity and divided by the dielectric thickness.

This thickness is in the range of nanometers per volt. Etched or sintered anodes have a higher surface area compared to a smooth surface of the same areal dimension. The capacitance value, depending on the rated voltage, increases by a factor of up to 200 for liquid Al-caps and solid Ta-caps.^[20][21][22]

Because the forming voltage defines the oxide thickness, the voltage proof can be produced simply for the desired rated value. Therefore, the volume of a capacitor is defined by the product of capacitance and voltage, the so-called "CV product".

Comparing the dielectric constants of tantalum and aluminum oxides, Ta₂O₅ has permittivity approximately 3-fold higher than Al₂O₃. Ta-caps therefore theoretically can be smaller than Al-caps with the same capacitance and rated voltage. Ta-cap oxide layers are much thicker than the rated voltage requires. This is done for safety reasons to avoid shorts from field crystallization,^[23] but reduces the size advantage.

Electrolytes

The most important electrical property of an electrolyte is its electrical conductivity. The electrolyte forms the counter electrode of the e-cap, the cathode. The roughened structures of the anode surface continue in the structure of the oxide layer, the dielectric. The cathode must adapt precisely to the roughened structure. With a liquid, as in the conventional "wet" e-caps this is easy to achieve. In polymer e-caps in which a solid conductive polymer forms the electrolyte, this is much more difficult to achieve, because its conductivity comes by a chemical process of polymerization. However, the benefits of a solid polymer electrolyte, the significantly lower ESR and the low temperature dependence of the electrical parameters, in many cases justify the additional production steps and higher costs.

Conducting salt TCNQ electrolyte

Structural formula of TCNQ

The original Samsung TCNQ e-caps with TCNQ as electrolyte were not polymer capacitors, unlike the modified Panasonic devices marketed under the same name^[24] that use a conductive polymer electrolyte (PPy).^[25]

Polymer electrolyte

Polymers are formed by a chemical reaction, polymerization. In this reaction monomers are continuously attached to a growing polymer strand.^[26] Usually polymers are electrical insulators or semiconductors. In e-caps, conductive polymers are employed. Conductivity is provided by conjugated double bonds that permit free movement of charge carriers in the doped state. The charge carriers are electron holes. Conducting polymer conductivity is nearly comparable with metallic conductors. The polymers must be oxidatively or reductively doped.

A polymer electrolyte must be able to penetrate the anode's finest crevices to form a complete, homogeneous layer, because only anode oxide sections covered by the electrolyte contribute capacitance. The precursors of the polymer must consist of small base materials that can penetrate the smallest pores. The size of the precursors implicitly limit the size of the pores in the aluminum anode foils or tantalum powder. The rate of polymerization must be controlled for capacitor manufacturing. Too rapid polymerization does not lead to complete anode coverage, while too slow polymerization increases production costs. The oxide must not chemically or mechanically attack either the precursors, the polymer or its residues. The electrolyte must have high stability over a wide temperature range and a long interval. The polymer film is the capacitor's counter electrode and protects the dielectric against external influences such as direct contact with graphite in the cathode.

Polymer e-caps employ either polypyrrole (PPy)^[27] or polythiophene (PEDOTor PEDT).^[28][29]

Polypyrrole PPy

Structural formula of polypyrrole, doped with p-Toluenesulfonic acid

Pyrrole can be polymerized electrochemically to control the rate of polymerizion.^[30]

Polypyrrole (PPy) is a conducting polymer formed by oxidative polymerization of pyrrole. A suitable oxidizing agent is iron (III) chloride (FeCl3). Water, methanol, ethanol, acetonitrile and other polar solvents may be used for PPy synthesis.^[31] As a solid conducting polymer electrolyte it achieves conductivity up to 100 S/m. Polypyrrole was the first conductive polymer used in polymer e-caps and the first in polymer Al-caps, followed by polymer Ta-caps.

In situ polymerization of PPy features a slow rate of polymerization. When pyrrole is mixed with the desired oxidizing agents at room temperature, polymerization begins immediately. Thus polypyrrole begins to form before the chemical solution enters the anode's pores. The polymerization rate can be controlled by cryogenic cooling or electrochemical polymerization. The cooling method is delicate and is unfavorable for mass production. In electrochemical polymerization an auxiliary electrode layer has to be applied on the dielectric and connected to the anode.^[29] For this purpose, ionic dopants are added to the polymer, forming a conductive surface layer during the first impregnation. During subsequent impregnations, the in-situ polymerization can be time-controlled by the current flow after applying a voltage between the anode and cathode.^[32] Both methods are complex and require repetitive polymerization steps that increase manufacturing costs.

The polypyrrole electrolyte has two fundamental disadvantages. It is toxic and becomes unstable at the temperatures required for lead-free soldering.^[29]

Polythiopene PEDOT and PEDOT:PSS

Structural formula of PEDOT

Structural formula of PEDOT:PSS

Poly(3,4-ethylenedioxythiophene), abbreviated PEDOT or PEDT^[28] is a conducting polymer based on 3,4-ethylenedioxythiophene or EDOT monomer. PEDOT is polarized by the oxidation of EDOT with catalytic amounts of iron (III) sulfate. The re-oxidation of iron is given by Sodium persulfate.^[33] Its advantages are optical transparency in its conducting state, non-toxicity, stability up to 280 °C and conductivity up to 500 S/m.^[29] Its heat resistance allows polymer capacitors to be manufactured that withstand the higher temperatures required for lead-free soldering. These capacitors also have better ESR values.^[29]

Pre-polymerized dispersions of PEDOT allow the anodes to be dipped and dried at room temperature. Sodium polystyrene sulfonate (PSS) is dissolved in water with PEDOT precursors.^[34] The complete polymer layer is then composed of pre-polymerized particles from the dispersion. These dispersions are known as PEDOT: PSS (trade names Baytron P^[35] and Clevius),^[36] protecting PEDOT's valuable properties.^[37][38]

PEDOT:PSS dispersions are available in different variants. High capacitance capacitors with roughened aluminum anode foils or fine-grained tantalum powders can use small particle sizes. The average size of these particles is about 30 nm, small enough to penetrate the finest anode capillaries. Another variant offers larger particles leading to a relatively thick polymer layer to envelop and protect rectangular Ta and Al polymer capacitors against mechanical and electrical stress.^[29][36]

PEDOT:PSS polymer Al-caps reach voltages of 200 V^[39] and 250 V.^[40] Leakage current values are significantly lower than for polymer capacitors having in-situ polymerized layers. This approach offers better ESR values, higher temperature stability, lower leakage current and ease of manufacture, requiring only three immersions,^[34] significantly reducing costs.

Hybrid electrolyte

Hybrid polymer Al-caps coat the anode with a conductive polymer and add a liquid electrolyte. The liquid connects the polymer layers covering the dielectric and the cathode. The liquid electrolyte supplies oxygen for self-healing processes, which restores the oxide layer and reduces the leakage current, so that values common to conventional "wet" e-caps can be achieved. The safety margin for the oxide thickness for a desired rated voltage can be reduced.

The detrimental effects of the liquid electrolyte on ESR and temperature characteristics are relatively minor. Appropriate organic electrolytes and good sealing allow a long service life.^[17][41]

Types

Based on the used anode metal and the combination of a polymer electrolyte together with a liquid electrolyte, the three different types are:

• Polymer Ta-cap
• Polymer Al-cap
• Hybrid polymer Al-cap

These types or families are produced in two different styles:

• Rectangular SMD chip, usually molded with a plastic case, available with sintered tantalum anode or with stacked aluminum anode foils and
• SMD cylinder with a wound cell in a metal case, either V-chips style or radial leaded versions (single-ended)

• Styles of polymer electrolytic capacitors
• 

  Rectangular

• 

  Cylindrical

Packaging

Rectangular style

In the early 1990s polymer Ta-caps coincided with the emergence of flat devices such as mobile phones and laptops using SMD assembly technology. The rectangular base surface achieves the maximum mounting space, which is not possible with round base surfaces. The sintered cell can be manufactured so that the finished component has a desired height, typically the height of other components. Typical heights range from about 0.8 to 4 mm.

Ta-caps

Polymer Ta-caps are Ta-caps in which the electrolyte is a conductive polymer instead of MnO₂. Ta-caps are manufactured from a powder of relatively pure elemental tantalum metal.^[42][43][44]

The powder is compressed around a tantalum wire, the anode connection, to form a "pellet". This pellet/wire combination is vacuum sintered at 1200 to 1800 °C, making it mechanically strong. During sintering, the powder takes on a sponge-like structure, with all the particles connecting as a monolithic spatial lattice. The result is highly porous, offering a large surface area.

The dielectric layer is formed covering the tantalum particle surfaces via anodization or forming. The pellet is submerged into a weak solution of acid and DC voltage is applied, creating the oxide layer. After the oxide layer is impregnated with the polymer precursors, they are polymerized. This polymerized pellet now is successively dipped into conducting graphite and then silver to provide a good connection to the conducting polymer. These layers form the cathode connection. The capacitive cell then is generally molded by a synthetic resin.

• Basic construction of a polymer tantalum capacitor
• 

  Layer structure of a polymer tantalum capacitor with graphit/silver cathode connection

• 

  Basic cross-section of a rectangular polymer tantalum chip capacitor

• 

  Rectangular polymer tantalum chip capacitor

Next multiple anode blocks are connected in parallel in one case, to further reduce the ESR value and lower ESL. Polymer Ta-caps have ESR values approximately 1/10 that of MnO₂ Ta-caps. Three parallel capacitors with an ESR of 60 mΩ each have a resulting ESR of 20 mΩ.^[45][46] In this construction up to six anodes in one device are connected. Multi-anode polymer Ta-caps have ESR values in the single-digit milliohm range.

The disadvantage of polymer Ta-caps is the higher leakage current, higher by a factor of 10 higher compared to MnO₂ Ta-caps. Polymer SMD Ta-caps are available up to a size of 7.3 x 4.3 x 4.3 mm (length x width x height) with a capacity of 1000 µF at 2.5 V. They cover temperature ranges from −55 °C to +125 °C and are available in rated voltage values from 2.5 to 63 V.

Lowering ESR and ESL

Multi-anode construction: sintered tantalum anodes are connected in parallel, reducing both ESR and ESL.

Lowering ESR and ESL remains a major research and development objective. Directions include low ohmic polymer electrolytes and parallel connection of conventional capacitor cells in one case.

ESL can be reduced by shortening the internal leads, by asymmetric sintering of the anode lead since ESL is a positive function of the lead length. This technique is called "face-down" construction. The lower ESL shifts the resonance to higher frequencies, which handle the faster load changes of digital circuits with higher switching frequencies.^[47]

Face-down construction: the internal current path is shortened, which reduces parasitic impedance, shifting the resonance to higher frequencies.

.

These enhancements bring Ta-caps ever closer to MLCC capacitors.

Al-caps

Rectangular polymer Al-caps have one or more layered aluminum anode foils and a conductive polymer electrolyte. The layered anode foils are at one side contact each other. After the dielectric is created and polymerized, it is successively dipped into conducting graphite and then silver to connect to the conducting polymer and then to the cathode. The capacitive cell then generally is molded by a synthetic resin.

• Basic construction of a polymer aluminum capacitor with layered anode stripes
• 

  Layer structure

• 

  Cross-section

• 

  Assembled device

The layered anode foils are parallel-connected single capacitors, reducing ESR and ESL and allowing them to operate at higher frequencies.

These Al-caps are available in the "D"-case form factor with 7,3x4,3 mm and heights of 2–4 mm. They provide a competitive alternative to Ta-caps.^[48]

Comparing the two chip capacitor types shows that the different permittivities of aluminum oxide and tantalum pentoxide have little impact on specific capacity due to different safety margins in oxide layers. Ta-caps use an oxide layer thickness that corresponds to approximately four times the rated voltage, while the polymer Al-caps have about twice the rated voltage.

Cylindrical (radial) style

Cylindrical polymer Al-caps use liquid electrolytes. They are available only with aluminum as the anode material.

They are intended for larger capacitance values compared to rectangular polymer capacitors. Due to their design, they may vary in height on a given surface mounting area so that larger capacitance values can be achieved by a taller case without increasing the mounting surface. This is primarily useful for printed circuit boards without a height limit.

Cylindrical capacitors are made of two rolled up aluminum foils, an etched and formed anode and a cathode foil that are mechanically separated by a separator and wound together. The winding is impregnated with the polymer precursors, which are then polymerized to form the conductive polymer as a layer between the dielectric and the cathode foil, electrically connecting both layers. The winding is built into an aluminum case and sealed with rubber. For the SMD version (Vertical chip= V-chip) the case is provided with a bottom plate.

Polymer aluminum

These capacitors use a solid polymer electrolyte as the dielectric. They are less expensive than polymer Ta-caps for a given CV. They are available up to a size of 10x13 mm (diameter x height) with a CV value of 3900 µF/2.5 V^[49] They can cover temperature ranges from -55 °C to +125 °C and are available in nominal voltage values from 2.5 to 200 V.^[39]

Unlike "wet" Al e-caps the cases of polymer Al capacitors don’t have a vent (notch) in the bottom of the case, since a short circuit does not form gas, which would increase pressure in the case.

• Design principles of cylindrical polymer aluminum capacitors
• 

  Winding of an aluminum electrolytic capacitor

• 

  Cross-sectional view of a wound polymer aluminum capacitor

• 

  Cylindrical polymer aluminum capacitors with wound cell in cylindrical metal case, in radial leaded (single-ended) and SMD style (V-chip)

Hybrid polymer Al-caps

Cross-sectional view

Hybrid polymer capacitors are available only in the cylindrical style. The anode and cathode foils are separated by a spacer, leaded in the radial (single-ended) design or with a base plate in the SMD version (V-chip). The separator is impregnated with a liquid electrolyte as in a conventional wet Al-cap. The liquid electrolyte delivers the oxygen that is necessary for defect self-healing.

The current that flows through a defect results in selective heating, which normally destroys the overlying polymer film, isolating, but not healing, the defect. In hybrid polymer capacitors liquid can flow to the defect, delivering oxygen and healing the dielectric by generating new oxides, decreasing the leakage current. Hybrid polymer Al capacitors have a much lower leakage current than non-hybrids.

Comparison

Benchmarks

The polymer electrolyte, the anode materials, together with design differences led to multiple polymer e-cap families with different specifications.

Comparison of benchmark values of the different polymer capacitor families

Anode material	Electrolyte	Style	Capacitance range (µF)	Rated- voltage (V)	Max. operation- temperature (°C)
Tantalum	Manganese dioxide	rectangular	0.1…1,500	2.5…63	105/125/150/175
	Polymer	rectangular	0.47…3,300	2.5…125	105/125
Aluminum	Polymer	rectangular	2.2…560	2.0…16	105/125
	Polymer	cylindrical (SMD and radial)	3.3…3,900	2.0…200	105/125/135
	Hybrid, Polymer and liquid	cylindrical (SMD and radial)	6.8…1,000	6.3…125	105/125

(As of April 2015)

Electrical parameters

Electrical properties of polymer capacitors can best be compared, using consistent capacitance, rated voltage and dimensions. The leakage current is significant, because it is higher than that of e-caps with non-polymer electrolytes. The respective values of Ta-caps with MnO₂ electrolyte and wet Al e-caps are included.

Comparison of the main electrical parameters of different e-cap families for types with the same size

E-cap family Electrolyte	Type^1	Dimensions WxLxH ^2 DxL ^3 (mm)	Max. ESR 100 kHz, 20 °C (mΩ)	Max. Ripple current 85/105 °C (mA)	Max. Leakage current for 100µF/10V after 2 min.^4 (µA)
MnO2-Ta-caps MnO2-Electrolyte	Kemet, T494 330/10	7.3x4.3x4.0	100	1,285	10 (0.01CV)
MnO2-Ta-caps Multianode, MnO2-Electrolyte	Kemet, T510 330/10	7.3x4.3x4.0	35	2,500	10 (0.01CV)
Polymer Ta-caps Polymer electrolyte	Kemet, T543 330/10	7.3x4.3x4.0	10	4,900	100 (0.1CV)
Polymer Ta-caps Multianode, Polymer electrolyte	Kemet, T530 150/10	7.3x4.3x4.0	5	4,970	100 (0.1CV)
Polymer Al-caps Polymer electrolyte	Panasonic, SP-UE 180/6.3	7.3x4.3x4.2	7	3,700	40 (0.04CV)
Polymer Al-caps Polymer elecrolyte	Kemet, A700 220/6.3	7.3x4.3x4.3	10	4,700	40 (0.04CV)
"Wet" Al-caps, SMD Ethylene glycol/Borax-electrolyte	NIC, NACY, 220/10	6.3x8	300	300	10 (0.01CV)
"Wet" Al-caps, SMD Water-based electrolyte	NIC, NAZJ, 220/16	6.3x8	160	600	10 (0.01CV)
Polymer Al-caps Polymer electrolyte	Panasonic, SVP 120/6.3	6,3x6	17	2,780	200 (0.2CV)
Hybrid polymer Al-caps Polymer + liquid electrolyte	Panasonic, ZA 100/25	6.3x7.7	30	2,000	10 (0.01CV)

1) Manufacturer, Series, Capacitance/Rated voltage, 2) rectangular style (Chip), 3) cylindrical style, 4) Leakage current, calculated for a capacitor with 100 µF/10 V,

(As of June 2015)

Advantages and disadvantages

Advantages against wet e-caps:

• articulately lower ESR values.
• articulately higher ripple current capability
• articulately lower temperature depending characteristics
• no evaporation of electrolyte, longer service life
• no burning or exploding in case of shorts

Disadvantages against wet e-caps:

• more expensive
• higher leakage current
• damageable by transients and higher voltages spikes

Advantages of hybrid polymer Al-caps:

• less expensive
• lower leakage current
• impassible against transients

Disadvantage of hybrid polymer e-caps:

• limited service life due to evaporation

Advantages against MLCCs:

• no voltage dependent capacitance
• no microphonic
• higher capacitance values

Electrical characteristics

Series-equivalent circuit

Series-equivalent circuit model of an electrolytic capacitor

Capacitor electrical characteristics are harmonized by the international generic specification IEC 60384-1. In this standard, characteristics are described by an idealized series-equivalent circuit with electrical components that model all ohmic losses, capacitive and inductive parameters:

• C, capacitance
• R_ESR, the equivalent series resistance which summarizes all ohmic losses, usually abbreviated as "ESR"
• L_ESL, the equivalent series inductance which is the effective self-inductance, usually abbreviated as "ESL".
• R_leak, the resistance representing the leakage current

Rated capacitance, standard values and tolerances

Typical capacitance capacitor as a function of temperature for a polymer Al e-cap and two liquid Al e-caps

Capacitance depends on frequency and temperature. Electrolytic capacitors with liquid electrolytes show a broader variability over frequency and temperature ranges than polymer capacitors.

The standardized measuring condition for polymer Al-caps is an AC measuring method with 0.5 V at a frequency of 100/120 Hz and a temperature of 20 °C. For polymer Ta-caps a DC bias voltage of 1.1 to 1.5 V for types with a rated voltage ≤2.5 V, or 2.1 to 2.5 V for types with a rated voltage of >2.5 V, may be applied during the measurement to avoid reverse voltage.

The capacitance measured at the frequency of 1 kHz is about 10% less than the 100/120 Hz value. Therefore, the capacitance values are not directly comparable and differ from those of film capacitors or ceramic capacitors, whose capacitance is measured at 1 kHz or higher.

The basic unit of capacitance is the microfarad (μF). The value specified in manufacturer data sheets is called the rated capacitance C_R or nominal capacitance C_N. It is given according to IEC 60063 in values corresponding to the E series. These values are specified with a tolerance in accordance with IEC 60062, preventing overlaps.

E3-series	E6-series	E12-series
10-22-47	10-15-22-33-47-68	10-12-15-18-22-27 33-39-47-65-68-82
capacitance tolerance ±20%	capacitance tolerance ±20%	capacitance tolerance ±10%
letter code "M"	letter code "M"	letter code "K"

The actual measured capacitance value must be within the tolerance limits.

Rated and category voltage

Relation between rated voltage U_R and category voltage U_C and rated temperature T_R and category temperature T_C

Referring to IEC 60384-1, the allowed operating voltage for polymer e-caps is called the "rated voltage U_R". The rated voltage U_R is the maximum DC voltage or peak pulse voltage that may be applied continuously at any temperature within the rated temperature range T_R.

The voltage proof of e-caps decreases with increasing temperature. Some applications require a higher temperature range. Lowering the voltage applied at a higher temperature maintains safety margins. For some capacitor types, IEC specifies a "temperature derated voltage" for a higher temperature, the "category voltage U_C". The category voltage is the maximum DC voltage or peak pulse voltage that may be applied continuously to a capacitor at any temperature within the category temperature range T_C. The relation between voltage and temperature is given in the figure at right.

Applying a higher than specified voltage may destroy an e-cap. Applying a lower voltage may have a positive influence. A lower applied voltage can extend hybrid Al-caps' lifetimes.^[20] Lowering the voltage applied increases the reliability and reduces the expected failure rate of Ta-caps.^[50]

Rated and category temperature

The relation between rated temperature T_R and rated voltage U_R as well as higher category temperature T_C and derated category voltage U_C is given in figure at right.

Surge voltage

Polymer e-cap oxid layers are formed for safety reasons at higher than the rated voltage, called a surge voltage, for a limited number of cycles.

The surge voltage indicates the maximum peak voltage value that may be applied to capacitors for a limited number of cycles.^[20] The surge voltage is standardized in IEC 60384-1.

For polymer Al-caps the surge voltage is 1.15 times the rated voltage. For Ta-caps the surge voltage can be 1.3 times the rated voltage, rounded off to the nearest volt.

The surge voltage may influence the capacitor's failure rate.^[51][52][53]

Transient voltage

Transients are fast, high voltage spikes. Al-caps and Ta-caps cannot withstand transients or peak voltages higher than surge voltage. Transients may destroy the components.^[51][52]

Hybrid Al-caps are relatively insensitive to short-term, transient voltages higher than surge voltage, if the frequency and the energy content of the transients are low.^[17][41] This ability depends on rated voltage and component size. Low energy transient voltages lead to a voltage limitation similar to a zener diode.^[54] An unambiguous and general specification of tolerable transients or peak voltages is not possible. Transient voltage use cases must be individually assessed.

Reverse voltage

Polymer e-caps are polarized and generally require the anode voltage to be positive relative to the cathode voltage. Nevertheless, they can withstand a reverse voltage for limited cycles.^[55][56] A reverse voltage applied for too long leads to short-circuit and destruction.

Impedance and ESR

The impedance is the complex ratio of the voltage to the current in an AC circuit and expresses as AC resistance both magnitude and phase at a particular frequency. In data sheets only the impedance magnitude |Z| is specified. Regarding the IEC 60384-1 standard, the impedance values are measured and specified at 100 kHz.

In the special case of resonance, in which the both reactive resistances X_C and X_L have the same value (X_C=X_L), impedance will be determined by only ESR, which totals all resistive losses. At 100 kHz impedance and ESR have nearly the same value for polymer e-caps with capacitance values in the µF range. With frequencies above the resonance, impedance increases again due to ESL, turning the capacitor into an inductor.

Typical impedance characteristics over the frequency range for 100 µF e-caps compared with a 100 µF ceramic Class 2 - MLCC - capacitor.

Typical curve of the ESR as a function of temperature for polymer capacitors and "wet" Al e-caps

Impedance and ESR, as shown in the curves, depends on the electrolyte. The curves show the progressively lower impedance and ESR values of "wet" Al, MnO₂ tantalum, Al /TCNQ and tantalum polymer e-caps. The curve of a ceramic Class 2 MLCC capacitor, with still lower Z and ESR values is also shown, but whose capacitance is voltage-dependent.

An advantage of polymer over Al-caps with liquid electrolyte is low temperature dependence and almost linear ESR curve over the specified temperature range. This applies to all three polymer e-cap types. Impedance and ESR are also dependent on design and materials. Cylindrical e-caps have higher inductance resonant frequency than rectangular e-caps. This effect is amplified by multi-anode construction, in which individual inductances are reduced by their parallel connection^[45][46] and the "face-down" technique.^[47]

Ripple current

The superimposed (DC biased) AC ripple current flow across the smoothing capacitor C1 of a power supply causes internal heat generation corresponding to the capacitor's ESR.

A "ripple current" is the root mean square (RMS) value of a superimposed AC current of any frequency and any waveform of the current curve for continuous operation within the specified temperature range. It arises mainly in power supplies (including switched-mode power supplies) after rectifying an AC voltage and flows as charge and discharge current through the decoupling or smoothing capacitor.^[57]

Ripple currents generates heat inside the capacitor body. This dissipation power loss P_L is caused by ESR and is the squared value of the effective (RMS) ripple current I_R.

$P_{L} = I_R^2 \cdot ESR$

This internally generated heat, above the ambient temperature and other external heat sources, leads to a temperature differential of Δ T over the ambient. This heat has to be distributed as thermal losses P_th over the capacitor's surface A against the thermal resistance β to the ambient.

$P_{th} = \Delta T \cdot A \cdot \beta$

This heat is distributed by thermal radiation, convection and thermal conduction. The temperature must not exceed the maximum specified temperature.

The ripple current for polymer e-caps is specified as an effective value at 100 kHz at upper category temperature. Polymer capacitors' ESR stability over the frequency range allows the 100 kHz-value to apply across the frequency range. Typically, the specified value for maximum ripple current in datasheets is calculated for a core temperature differential of 20 °C. Use of polymer capacitors at higher temperature reduces the ripple current.

Non-sinusoidal ripple currents have to be analyzed and separated into their individual sinusoidal frequencies by means of Fourier analysis and summarized by squared addition.^[58]

$I_R=\sqrt{{i_1}^2 + {i_2}^2 + {i_3}^2 + {i_n}^2 }$

In polymer Ta-caps the heat generated by the ripple current influences reliability.^{[59][60][61][62]} Exceeding the limit can result in catastrophic failures with short circuits and burning components.

Ripple current heat affects the lifetimes of all three polymer e-cap types.^[57][63]

Current surge, peak or pulse current

Polymer Ta-caps are sensitive to peak or pulse currents.^[51][52] Solid Ta-caps that are exposed to surge, peak or pulse currents, for example, in highly inductive circuits, require voltage derating. If possible the voltage profile should be a ramp turn-on, as this reduces the peak current.

Polymer Al-caps have no restrictions on current surge, peak or pulse currents. However, the summarized currents must not exceed the specified ripple current.

Leakage current

The general leakage current behavior of electrolytic capacitors depend on the kind of electrolyte

The DC leakage current (DCL) is a unique characteristic for e-caps. It is the DC current that flows when a DC voltage of correct polarity is applied. This current is represented by the resistor R_leak in parallel with the capacitor in the series-equivalent circuit of e-caps. The main causes of DCL for solid polymer capacitors are points of electrical dielectric breakdown after soldering, unwanted conductive paths due to impurities or to poor anodization and for rectangular types, dielectric bypass due to excess MnO₂, due to moisture paths or cathode conductors (carbon, silver).^[64]

Datasheet leakage current specification is given by multiplication of the rated capacitance value C_R with the value of the rated voltage U_R together with an added figure, measured after 2 or 5 minutes:

$I_\mathrm{Leak} = 0{.}01\,\mathrm{{A}\over{ V \cdot F}} \cdot U_\mathrm R \cdot C_\mathrm R + 3\,\mathrm{\mu A}$

Leakage current in solid polymer e-caps generally drops fast but then remains steady. The value depends on the voltage applied, temperature, measuring time and moisture allowed by case sealing conditions.

Polymer e-caps have relatively high leakage current values. In solid polymer e-caps this cannot be reduced by "healing" in the sense of generating new oxide, because under normal conditions solid electrolytes cannot deliver oxygen for forming processes. Annealing of dielectric defects can only be carried out through local overheating and polymer evaporation. The leakage current values for polymer e-caps are between 0.2 C_RU_R to 0.04 C_RU_R. Thus the value of the leakage current for polymer capacitors is higher than for "wet" aluminum and MnO₂ Ta-caps.

This higher leakage current disadvantage is avoided by hybrid Al-caps. Their liquid electrolyte provides the oxygen that is necessary for the reforming of oxide defects, so that the hybrids achieve the same values as wet Al or Ta-caps.^[17][57]

Dielectric absorption (soakage)

Dielectric absorption occurs when a capacitor charged for a long time discharges only incompletely. Although an ideal capacitor would reach zero volts after discharge, real capacitors develop a small voltage from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage" or "battery action".

No figures for dielectric absorption are available for polymer capacitors.

Reliability and lifetime

Reliability (failure rate)

Bathtub curve with times of "early failures", "random failures" and "wear-out failures". The time of random failures is the time of constant failure rate

Reliability is a property that indicates how consistently a component performs its function over a time interval. It is subject to a stochastic process and can be described qualitatively and quantitatively, but is not directly measurable. Eurance tests reveal the failure rate. Reliability normally is shown as a bathtub curve an(figure on right) d is divided into three areas: early failures, constant random failures and wear out failures. Failure rates are the sum of short circuit, open circuit and degradation failures (exceeding electrical parameters). For Ta-caps the failure rate is influenced by the circuit series resistor, which is not required for Al-caps.

Billions of test unit-hours are needed to verify acceptable failure rates. This requires about a million units tested over a long period.^[65] Test failure rates are often complemented with feedback from large users (field failure rate), which mostly lowers failure rate estimates.

For historical reasons the failure rate units of Ta-caps and Al-caps are different. For Al-caps the reliability prediction is generally expressed in a failure rate λ, with the unit Failures In Time (FIT) at standard operating conditions 40 °C and 0.5 U_R during the period of constant random failures. This is the number of failures that can be expected in one billion (10⁹) component-hours of operation (e.g., 1000 components for 1 million hours, or 1 million components for 1000 hours which is 1 ppm/1000 hours) at standard operating conditions. This failure rate model implicitly assumes that failures are random. Individual components fail at random times but at a predictable rate. The reciprocal value of FIT is Mean Time Between Failure (MTBF).

For Ta-caps the failure rate "F_Ta" is specified with the unit "n % failures per 1000 hours" at 85 °C, U = U_R and a circuit resistance of 0.1 Ω/V. This is the failure percentage that can be expected in 1000 hours of operation at much more demanding operational conditions compared with the "FIT" model. The failure rates "λ" and "F_Ta" depend on operating conditions including temperature, voltage applied and environmental factors such as humidity, shocks or vibrations and capacitance.^[50] Failure rates are an increasing function of temperature and applied voltage.

Solid tantalum and "wet" Al-caps failure rates can be recalculated with acceleration factors standardized for industrial^[66] or military^[67] contexts. The latter is established in industry and often used for industrial applications. However, for polymer aluminum and Ta-caps no acceleration factors had been published as of 2015. An example of a recalculation from a Ta-cap failure rate F_Ta into a failure rate λ therefore only can be given by comparing standard capacitors. Example:

A failure rate F_Ta = 0.1%/1000 h at 85 °C and U= U_R shall be recalculated into a failure rate λ at 40 °C and U = 0,5 U_R.

The following acceleration factors from MIL-HDBK 217F are used:

F_U = voltage acceleration factor, for U = 0,5 U_R is F_U = 0.1

F_T = temperature acceleration factor, for T = 40 °C is F_T = 0.1

F_R = acceleration factor for the series resistance R_V, at the same value it is = 1

It follows

λ = F_Ta x F_U x F_T x F_R

λ = (0.001/1000 h) x 0.1 x 0.1 x 1 = 0.00001/1000 h = 1•10⁻⁹/h = 1 FIT

As of 2015 the published failure rate figures for polymer tantalum and polymer Al-caps are in the range of 0.5 to 20 FIT. These reliability levels are comparable with other electronic components and achieve safe operation for decades under normal conditions.

Lifetime, service life

The lifetime, service life, load life or useful life of e-caps is a special characteristic of liquid e-caps, especially liquid Al-caps whose liquid electrolyte can evaporate, leading to wear-out failures. MnO₂ Ta-caps have no wear-out mechanism so that the failure rate is constant up to the point all capacitors have failed. They don’t have a lifetime specification like liquid Al-caps.

Polymer Ta-caps and Al-caps do have a lifetime specification. The polymer electrolyte has a small conductivity deterioration by thermal polymer degradation. The electrical conductivity decreases as a function of time, in agreement with a granular metal type structure, in which aging is due to polymer grain shrinkage.^[63]

The useful life (load life, service life) is tested with a time accelerating endurance test according to IEC 60384-24/-25/-26^[68] with rated voltage at the upper category temperature. Passing the test requires no total failures (short circuit, open circuit) and degradation failures and capacitance loss by less than 20% and increased ESR and impedance by more than a factor of 2 compared to the initial value. These limits for degradation failures are much closer than for wet Al-caps. That means that lifetime behavior is much more stable than for wet Al-caps.

The lifetime for maximum voltage and temperature is specified in similar terms to the liquid electrolytic e-caps, but uses less stressful operational conditions that lead to much longer operational lifetimes.^[69][70][71] Polymer capacitor lifetimes for different operational conditions can be estimated by:

$L_x =L_\text{Spec}\cdot 10^\frac{T_0-T_A}{20}$

• L_x = lifetime to be estimated
• L_Spec = specified lifetime
• T₀ = upper category temperature
• T_A = temperature of the e-cap case or ambient temperature near the capacitor

This rule characterizes the change of thermic polymer reaction speeds within the specified degradation limits. According to this formula the theoretical expected service life of a 2000 h/105 °C polymer capacitor, operated at 65 °C, can be calculated (estimated) with 200,000 hours or more than 20 years.

For liquid hybrids, the 20-degree rule does not apply. The expected life of these hybrid e-caps can be calculated using the 10-degree rule.

Field crystallization

Polymer capacitors are reliable at the same level as other electronic components with low failure rates. However, all Ta-caps have a unique failure mode called "field crystallization".^[72] Field crystallization is the major reason for degradation and catastrophic failures of solid Ta-caps.^[73] More than 90% of (rare) Ta-cap failures are caused by short circuits or leakage current due to this failure mode.^[74]

The oxide film must be formed in an amorphous structure. Changing the amorphous structure into a crystallized structure increases conductivity reportedly 1000 times along with an enlarged oxide volume.^[23][75]

After application of a voltage at weakened spots in the oxide a localized higher leakage current is formed, which leads to a local heating of the polymer, whereby the polymer either oxidized and becomes highly resistive or evaporates.

Field crystallization followed by a dielectric breakdown is characterized by a sudden rise in leakage current, within a few milliseconds, from nano-ampere to ampere magnitude in low-impedance circuits. Increasing current flow can produce an "avalanche effect", rapidly spreading through the metal/oxide. This can result in damage ranging from small, burned areas on the oxide to zigzag burned streaks covering large areas of the pellet or complete oxidation of the metal.^[76][77] If the current source is unlimited, field crystallization may cause a short circuit. However, if the current source is limited, in Ta-caps with solid MnO₂ electrolyte a self-healing process takes place, reoxidizing MnO₂ into insulating Mn₂O_3.

In polymer Ta-caps combustion is not a risk. Field crystallization may occur, but the polymer layer is selectively heated and burned away by the leakage current, so that the faulty point is isolated. Without the polymer material, the leakage current can’t accelerate. The faulty area no longer contributes to the capacitance.

Self-healing

Polymer Al-caps exhibit the same self-healing mechanism as polymer Ta-caps. After application of a voltage at weakened spots in the oxide a localized higher leakage current is formed, which leads to localized polymer heating, whereby the polymer either oxidizes and becomes highly resistive or evaporates. Hybrids show this self-healing mechanism. Faulty spots not covered with a polymer film allow liquid electrolyte to deliver oxygen to build up new oxide.

Long-term electrical behavior, failure modes, self-healing mechanism and application rules

Type of e-caps	Long-term electrical behavior	Failure modes	Self-healing mechanism	Application rules
"Wet" Al-caps	Drying, capacitance ↓, ESR ↑	No unique determinable	New oxide formed under a voltage	Lifetime calculation 10 °C rule
Polymer Al-caps	Conductivity ↓, ESR ↑	No unique determinable	Dielectric fault Isolation by oxidation or electrolyte evaporation	Lifetime calculation 20 °C rule
MnO2 Ta-caps	Stable	Field crystallization [23][76]	Thermally induced dielectric fault isolation by electrolyte oxidization absent unlimited current	Voltage derating 50% Series resistance 3 Ω/V [77][78]
Polymer Ta-caps	Conductivity ↓, ESR ↑	Field crystallization [23][76]	Dielectric fault Isolation by oxidation or electrolyte evaporation	Voltage derating 20 % [77][78]
Hybrid polymer Al-caps	Drying, capacitance ↓, ESR ↑	No unique determinable	New oxide formed under a voltage	Lifetime calculation 10 °C rule

Standards

Electronic component and related technology standardization follow rules given by the International Electrotechnical Commission (IEC),^[79] a non-profit, non-governmental international standards organization.^[80][81]

The definition of the characteristics and the procedure of the test methods for capacitors for use in electronic equipment are set out in the generic specification:

• IEC/EN 60384-1—Fixed capacitors for use in electronic equipment

The tests and requirements to be met by aluminum and Ta-caps for use in electronic equipment for approval as standardized types are set out in the sectional specifications:

• IEC/EN 60384-24—Surface mount fixed Ta-caps with conductive polymer solid electrolyte
• IEC/EN 60384-25—Surface mount fixed aluminium e-caps with conductive polymer solid electrolyte
• IEC/EN 60384-26—Fixed aluminium e-caps with conductive polymer solid electrolytec

Commercial information

Capacitor symbol

Electrolytic capacitor symbols

Electrolytic capacitor	Electrolytic capacitor	Electrolytic capacitor

Polarity marking

Polarity marking

Polarity marking at the anode (plus)	Polarity marking at the cathode (minus)

Imprinted markings

Polymer e-caps, given sufficient space, have coded imprinted markings to indicate:

• manufacturer's name or trademark
• manufacturer's type designation
• polarity
• rated capacitance
• tolerance on rated capacitance
• rated voltage
• climatic category or rated temperature
• year and month (or week) of manufacture

For small capacitors no marking is possible.

The code of the markings vary by manufacturer.

Technological competition

ESR and ESL characteristics are converging to those of MLCC capacitors. Conversely, the specific capacitance of Class 2-MLCC capacitors is approaching that of tantalum chip capacitors.^[82][83] Other characteristics favor one or another type.^[84][85] e.g., Al-Polymer e-caps versus MLCC: Panasonic,^[86] MLCC versus Polymer and "wet" e-caps:Murata,^[87][88] Al-Polymer e-caps versus "wet" e-caps: NCC^[18] NIC^[16] andTa-Polymer e-caps against standard solid Ta-MnO₂ e-caps: |publisher=Kemet^[89]

Manufacturers and products

Worldwide operating manufacturers and their type spectrum

Manufacturer	Polymer Tantalum capacitors	Polymer Aluminum capacitors
	Rectangular SMD	Rectangular SMD	Cylindric leaded SMD, V-Chip	Cylindric Hybrid
AVX	X	-	-	-
AISHI	-	X	X	-
CapXon	-	-	X	-
CDE Cornell Dubilier	X	-	-	X
Elite	-	-	X	-
Elna	-	-	X	-
Illinois	-	X	X	-
Jianghai	-	-	X	-
KEMET	X	X	-	-
Lelon	-	-	X	-
Matsuo	X	X	-	-
Murata	-	X	-	-
Nippon Chemi-Con	-	-	X	X
NIC	X	-	X	X
Nichicon	-	X	X	-
Panasonic	X	X	X	X
PolyCap	-	-	X	-
ROHM	X	-	-	-
Rubycon	-	X	-	-
Samsung	X	-	-	-
Samwha	-	-	-	X
Sun Electronic (Suncon)	-	-	-	X
Teapo/Luxon	-	-	X	-
Vishay	X	-	-	-
Yageo	-	-	X

As of July 2015

See also

• Aluminum electrolytic capacitor
• Electrolytic capacitor
• Niobium capacitor
• SAL electrolytic capacitor
• Tantalum capacitor
• Capacitor types

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