User:Elcap/Tantalum capacitor



A tantalum electrolytic capacitor, a member of the family of electrolytic capacitors, is a polarized capacitor whose anode electrode (+) is made of tantalum on which an very thin insulating oxide layer originates by anodically oxidation (forming), which acts as the dielectric of the electrolytic capacitor. A solid or non-solid electrolyte which covers the surface of the oxide layer serves as the second electrode (cathode) (-) of the capacitor. Because of its very thin dielectric layer accompanied with it relatively high permittivity the tantalum capacitors distinguishes itself from other conventional and electrolytic capacitors in having high capacitance per volume (high volumetric efficiency) and weight.

Most of all tantalum capacitors are available as SMD chip capacitors with a solid manganese dioxide or a solid polymer electrolyte. The solid electrolyte ensures low ESR values, and a stable electrical behavior over a broad temperature range. The high specific capacitance of tantalum capacitors makes them particularly suitable for passing or bypassing low-frequency signals up to some mega-hertz and storing large amounts of energy to support the power lines of highly integrated circuits. Especially the SMD version (Ta-chip) with its small size and weight advantages make tantalum capacitors attractive for devices with flat design like mobile phones, laptops, tablets, and PDAs, and compact devices e.g. for automotive electronics. However, due to the expensive tantalum ore tantalum electrolytic capacitors are considerably more expensive than comparable used and a little bit larger aluminum electrolytic capacitors.

Special “wet” tantalum capacitors with non-solid electrolyte often have military approvals and are used in military or space applications.

Tantalum capacitors are polarized components by the manufacturing principle and may only be operated with DC voltage in correct polarity. Reverse voltage or ripple current higher than specified can destroy the dielectric and thus the capacitor. For safe operation of tantalum capacitors special circuit design rules are specified from the manufacturers.

Basic principle


Electrolytic capacitors are using a chemical feature of some special metals, earlier called “valve metals”, on which by anodically oxidation an insulating oxide layer serves as dielectric originates. Applying a positive voltage to the tantalum anode material in an electrolytic bath an oxide barrier layer with a thickness corresponding to the applied voltage will be formed (formation). This oxide layer serves as dielectric in an electrolytic capacitor. The properties of this oxide layer are given in the following table:

After forming a dielectric oxide on the rough anode structures a counter-electrode has to match the rough insulating oxide surface. This will be done by the electrolyte which acts as cathode electrode of an electrolytic capacitor. There are a lot of different electrolytes in use. Generally the electrolytes will be distinguished into two species, “non-solid” and “solid” electrolytes. Non-solid electrolytes as a liquid medium which have a ion conductivity by moving ions. Solid electrolytes have an electron conductivity and this makes solid electrolytic capacitors sensitive against voltages spikes or current surges. The anodic generated insulating oxide layer becomes destroyed if the polarity of the applied voltage changes.



Every electrolytic capacitor in principle forms a "plate capacitor" whose capacitance is greater, the larger the electrode area A and the permittivity ε are and the thinner the thickness (d) of the dielectric is.


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

The dielectric thickness of electrolytic capacitors is very thin in the range of nano-meter per volt. Otherwise the voltage strengths of these oxide layers are quite high. With this very thin dielectric oxide layer combined with a sufficient high dielectric strength the electrolytic capacitors can already achieve a high volumetric capacitance. This is one reason for the high capacitance values of electrolytic capacitors compared with other conventional capacitors.

All etched or sintered anodes have a much higher surface compared to a smooth surface of the same area or the same volume. That increases the later capacitance value, depending on the rated voltage, by the factor of up to 200 for solid tantalum electrolytic capacitors. The large surface compared to a smooth one is the second reason for the relatively high capacitance values of electrolytic capacitors.

One special advantage is given for all electrolytic capacitors. Because the forming voltage defines the oxide layer thickness the voltage proof of the later electrolytic capacitor can be produced very simple for the desired rated value. That makes electrolytic capacitors fit for uses down to 2 V applications in which other capacitor technologies must stay to much higher limits.

The volume of an electrolytic capacitor is defined by the product of capacitance and voltage, the so-called “CV-Volume”. However, comparing the permittivities of the different oxide materials it is seen that tantalum pentoxide has an approximately 3 times higher permittivity than aluminum oxide. Tantalum electrolytic capacitors of a given CV value therefore are smaller than aluminum electrolytic capacitors.

Basic construction of solid tantalum electrolytic capacitors
A typical tantalum capacitor is a chip capacitor and consists of tantalum powder pressed and sintered into a pellet as the anode of the capacitor, with the oxide layer of tantalum pentoxide as a dielectric, and a solid manganese dioxide electrolyte as the cathode.

Anode


Tantalum capacitors are manufactured from a powder of relatively pure elemental tantalum metal. A common figure of merit for comparing volumetric efficiency of powders is expressed in capacitance (C) in microfarads (µF) times voltage (V) in volts (V) per gram (CV/g). Since the mid-80’s, manufactured tantalum powders have exhibited around a ten-fold improvement in CV/g values (from approximately 20k to 200k). 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 takes on 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 sintering temperature, a specific capacitance/voltage rating can be designed. For example, a 220 μF 6 V 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 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 at the anode 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, one third grows out and two thirds 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).

The dielectric layer thickness generated by the forming voltage is direct proportional to the voltage proof of electrolytic capacitors. It is obvious, that electrolytic capacitors are manufactured with a safety margin in oxide layer thickness, which is the ratio between voltage used for electrolytical creation of dielectric and rated voltage of the capacitor, to ensure reliable functionality.

The safety margin for solid tantalum capacitors with manganese dioxide electrolyte is between factor 2 and 4. That means that for a 25 V tantalum capacitor with a safety margin of 4 the dielectric voltage proof can withstand 100 V to provide a more robust dielectric. This very high safety factor is substantiated by the failure mechanism of solid tantalum capacitors, “field crystallization”. For tantalum capacitors with solid polymer electrolyte the safety margin is much lower and lies approximately at factor 2.

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 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).

Production flow
The picture below shows the production flow of tantalum electrolytic chip capacitors with sintered anode and solid manganese dioxide electrolyte.



Styles of tantalum capacitors
Tantalum electrolytic capacitors are made of three different styles:


 * Tantalum chip capacitors: SMD style for surface mounting, 80 % of all tantalum capacitors are SMD’s
 * Tantalum „pearls“,resin-dipped, single ended style for PCB mounting
 * Axial-leaded tantalum capacitors, with solid and non-solid electrolyte, mostly used for military, medical and space applications.

Tantalum chip capacitors - case sizes
More than 90% of all tantalum electrolytic capacitors are manufactured in SMD style as tantalum chip capacitors. Enforced in this design compared to a dip lacquered version has the moulded version. It has the contact surfaces on the end faces of the case and is manufactured in different sizes mostly harmonized by EIA standard. The different dimensions could be identified by different case code letters. For some case sizes (A to E), which are manufactured for many decades, the dimensions and case coding over all manufactures are still largely the same. However, new developments in tantalum electrolytic capacitors such as the multi-anode technique to reduce the ESR or the "face down" technique to reduce the inductance have led to a much wider range of chip sizes and their case codes. Because the EIA meanwhile gave up their standardization the case codes between the individual manufacturers are no longer uniform.

An overview of the dimensions of conventional tantalum chip capacitors and their coding is shown in the following table.

Footnotes: 1) Case code AVX, 2) Case code Kemet

Wet tantalum capacitors


The main feature of modern non-solid (wet) tantalum electrolytic capacitors is its energy density compared with that of solid tantalum and wet aluminum related to the same temperature range. Due to their self-healing properties, because the non-solid electrolyte can deliver oxygen to form new oxide layer in weak positions of the dielectric, the dielectric thickness could be formed with much lower safety margins and consequently with much thinner dielectric than for solid types, resulting in a higher CV value per volume unit. Additional wet Ta-e-caps are able to operate at voltages in excess of 100V up to 630 V, have a relatively low ESR and the lowest leakage current of all electrolytic capacitors.

The original wet tantalum capacitors developed in the 1930s were axial capacitors, having a wound cell out of tantalum anode and cathode foil separated by a paper stripe soaked with an electrolyte, mounted in a silver case and non-hermetic elastomer sealed. Because of the inertness and stability of the tantalum dielectric oxide layer against strong acids the wet tantalum capacitors could use sulfuric acid as electrolyte providing them with a relatively low ESR.

Because the silver case in the past had some problems with silver migration and whiskers, which has led to an increasing leakage currents and short circuits, new styles of wet tantalum capacitors use a sintered tantalum pellet cell, a gelled sulfuric acid electrolyte mounted in an “all tantalum” case.

Due to their relatively high price wet tantalum electrolytic capacitors have no consumer applications. However, they are used in rough industrial applications f. e. in probes for oil exploration. Types with military approvals can provide the extended capacitance and voltage ratings, along with the high quality levels required for all avionics, military and space applications.

History
The phenomenon that can form an oxide layer on aluminum and other metals like tantalum, niobium, manganese, titanium, zinc, cadmium etc. in an electrochemical process, which block an electric current to flow in one direction but allow to flow in the other direction, however, it was discovered in 1875 by the French researcher and founder Eugène Ducretet. He coined for this kind of metals the term „valve metal“.

Charles Pollak (born Karol Pollak), a producer of accumulators, found out, that that the oxide layer on an aluminum anode remained stable in a neutral or alkaline electrolyte, even when the power was switched off. In 1896 he handed out this idea of an “Electric liquid capacitor with aluminum electrodes” as a patent of using the oxide layer in a polarized capacitor in combination with a neutral or slightly alkaline electrolyte.

The first industrial realized electrolytic capacitors in the 1930s and the following decades were aluminum electrolytic capacitors used mainly in radios.

The first tantalum electrolytic capacitors were developed in 1930 by Fansteel Metallurgical Corporation for military purposes. They adopt the basic construction of a wound cell and used a tantalum anode foil together with a tantalum cathode foil separated with a paper spacer impregnated with a liquid electrolyte. The relevant development of solid electrolyte tantalum capacitors began some years after William Shockley, John Bardeen and Walter Houser Brattain invented the transistor 1947. It was invented by Bell Laboratories in the early 1950s as a miniaturized, more reliable low-voltage support capacitor to complement their newly invented transistor. The solution R. L. Taylor and H. E. Haring from the Bell labs found for the new miniaturized capacitor found in early 1950 was based on experiences with ceramics. They grind down tantalum to a powder, pressed this powder into a cylindrical form and then sintered the powder particles at high temperature between 1500 and 2000 °C under vacuum conditions to a pellet (“slug”). These first sintered tantalum capacitors used a non-solid electrolyte, what don’t fit the concept of solid electronics. 1952 a targeted search in the Bell Labs for a solid electrolyte by D. A. McLean and F. S. Power led to the invention of manganese dioxide as a solid electrolyte for a sintered tantalum capacitor.

Although the fundamental inventions came from the Bell Labs the inventions for manufacturing commercially viable tantalum electrolytic capacitors was done by researchers of Sprague Electric Company. Preston Robinson, Spragues Director of Research is considered to be the actual inventor of tantalum capacitors 1954 His invention was supported by R. J. Millard, who introduced the “reform” step 1955 , a significant improvement in which the dielectric of the capacitor was repaired after each dip-and-convert cycle of MnO2 deposition which dramatically reduced the leakage current of the finished capacitors.

This first solid electrolyte manganese dioxide had a 10 times better conductivity than all other types of non-solid electrolyte. In the style of tantalum pearls they found very quick broad use in radio and new television devices.



With the beginning of the digitalization, 1971 launched Intel his first microcomputer MCS 4 and 1972 Hewlett Packard one of the first pocket calculator HP 35 the requirements for capacitors increases in terms of lower losses. The equivalent series resistance (ESR) for bypass and decoupling capacitors of standard electrolytic capacitors should be decreases.

Although solid tantalum capacitors offered capacitors with lower ESR values than the aluminum e-caps, a gambling at the stock exchange 1980 followed by a price shock the industry were somewhat cautiously towards tantalum. and switched back to use aluminum electrolytic capacitors.

The development of conducting polymers by Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa in 1975 was a break-through in point of lower ESR.. The conductivity of conductive polymers such as polypyrrole (PPy) or PEDOT are by a factor of 1000 better than that of manganese dioxide, and are close to the conductivity of metals. In 1993 NEC introduced his SMD polymer tantalum electrolytic capacitors called "NeoCap". 1997 followed Sanyo with the "POSCAP" polymer tantalum chips

A new conductive polymer for tantalum polymer capacitors was presented by Kemet at the "1999 Carts" conference. This capacitors used the new developed organic conductive polymer PEDT Poly(3,4-ethylenedioxythiophene), also known as PEDOT (trade name Baytron®)

This development to low ESR capacitors with high CV-volumes in chip style for the rapid growing SMD technology in the 1990s increases the demand on tantalum chips dramatically. However, an another price explosion for tantalum in 2000/2001 forces the new development of niobium electrolytic capacitors with manganese dioxide electrolyte which are available since 2002. The materials and processes used to produce niobium-dielectric capacitors are essentially the same as for existing tantalum-dielectric capacitors. The characteristics of this niobium electrolytic capacitors and tantalum electrolytic capacitors are roughly comparable.

Series-equivalent circuit


Tantalum electrolytic capacitors as discrete components are not ideal capacitors, they have losses and parasitic inductive parts. All properties can be defined and specified by a series equivalent circuit composed out of an idealized capacitance and additional electrical components which model all losses and inductive parameters of a capacitor. In this series-equivalent circuit the electrical characteristics are defined by:


 * C, the capacitance of the capacitor
 * Rleak, the resistance representing the leakage current of the capacitor
 * RESR, the equivalent series resistance which summarizes all ohmic losses of the capacitor, usually abbreviated as "ESR"
 * LESL, the equivalent series inductance which is the effective self-inductance of the capacitor, usually abbreviated as "ESL".

Using a series equivalent circuit instead of a parallel equivalent circuit is specified by IEC/EN 60384-1.

Capacitance standard values and tolerances
The electrical characteristics of tantalum electrolytic capacitors depend on structure of the anode and used electrolyte. This influences the capacitance value of tantalum capacitors which depends on measuring frequency and temperature. The basic unit of electrolytic capacitors capacitance is microfarad (μF).

The capacitance value specified in the data sheets of the manufacturers is called rated capacitance CR or nominal capacitance CN and is the value for which the capacitor has been designed. Standardized measuring condition for electrolytic capacitors is an AC measuring method with a frequency of 100/120 Hz. Herein, electrolytic capacitors differ from other capacitor types, whose capacitance is measured at 1 kHz or higher. AC voltage which shall not exceed 0,5 V  AC-RMS. For tantalum capacitors a DC bias voltage of 1.1 to 1.5 V for types with a rated voltage of ≤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 percentage of allowed deviation of the measured capacitance from the rated value is called capacitance tolerance. Electrolytic capacitors are available in different tolerance series, whose values are specified in the E series specified in IEC 60063. For abbreviated marking in tight spaces, a letter code for each tolerance is specified in IEC 60062.
 * rated capacitance, series E3, tolerance ±20%, letter code "M“
 * rated capacitance, series E6, tolerance ±20%, letter code "M“
 * rated capacitance, series E12, tolerance ±10%, letter code "K“

The required capacitance tolerance is determined by the particular application. Electrolytic capacitors, which are often used for filtering and bypassing capacitors don’t have the need for narrow tolerances because they are mostly not used for accurate frequency applications like oscillators.

Rated and category voltage


Referring to IEC/EN 60384-1 standard the allowed operating voltage for tantalum capacitors is called "rated voltage UR " or "nominal voltage UN". The rated voltage UR is the maximum DC voltage or peak pulse voltage that may be applied continuously at any temperature within the rated temperature range TR (IEC/EN 60384-1).

The voltage proof of electrolytic capacitors decreases with increasing temperature. For some applications it is important to use a higher temperature range. Lowering the voltage applied at a higher temperature maintains safety margins. For some capacitor types therefore the IEC standard specify a "temperature derated voltage" for a higher temperature, the "category voltage UC". 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 TC. The relation between both voltages and temperatures is given in the picture right.

Lower voltage applied may have positive influences for tantalum electrolytic capacitors. Lowering the voltage applied increases the reliability and reduce the expected failure rate.

Applying a higher voltage than specified may destroy tantalum electrolytic capacitors.

Surge Voltage
The surge voltage indicates the maximum peak voltage value that may be applied to electrolytic capacitors during their application for a limited number of cycles. The surge voltage is standardized in IEC/EN 60384-1. For tantalum electrolytic capacitors the surge voltage shall be 1.3 times of the rated voltage, rounded off to the nearest volt. The surge voltage applied to tantalum capacitors may influence the capacitors failure rate.

Transient Voltage
Transient voltage or a current spike applied to tantalum electrolytic capacitors with solid manganese dioxide electrolyte can cause some tantalum capacitors to fail and may directly lead to a short.

Reverse voltage
Tantalum electrolytic are polarized and generally require anode electrode voltage to be positive relative to the cathode voltage.

With a reverse voltage applied, a reverse leakage current flows in very small areas of microcracks or other defects across the dielectric layer to the anode of the electrolytic capacitor. Although the current may only be a few microamps, it represents a very high localized current density which can cause a tiny hot-spot. This can cause some conversion of amorphous tantalum pentoxide to the more conductive crystalline form. When a high current is available, this effect can avalanche and the capacitor may become a total short.

Nevertheless, tantalum electrolytic capacitors can withstand for short instants a reverse voltage for a limited number of cycles. The most common guidelines for tantalum reverse voltage are:
 * 10 % of rated voltage to a maximum of 1 V at 25 °C,
 * 3 % of rated voltage to a maximum of 0.5 V at 85 °C,
 * 1 % of rated voltage to a maximum of 0.1 V at 125 °C.

These guidelines apply for short excursion and should never be used to determine the maximum reverse voltage under which a capacitor can be used permanently.

But in no case for tantalum electrolytic capacitors, a reverse voltage may be used for a permanent AC application.

Impedance


Tantalum electrolytic capacitors, as well as other conventional capacitors, have two electrical functions. For timers or similar applications capacitors are seen as a storage component to store electric energy. But for smoothing, bypassing or decoupling applications like in power supplies the capacitors work additionally as AC resistor to filter undesired biased AC frequencies. For this (biased) AC function the frequency dependent AC resistance, the impedance "Z", is as important as the capacitance value.



The impedance is the complex ratio of the voltage to the current with both magnitude and phase at a particular frequency in an AC circuit. In this sense impedance is a measure of the ability of the capacitor to pass alternating currents and can be used like Ohms law
 * $$Z = \frac{\hat u}{\hat \imath} = \frac{U_\mathrm{eff}}{I_\mathrm{eff}}.$$

With other words, the impedance is a frequency dependent AC resistance and possesses both magnitude and phase at a particular frequency. In data sheets of electrolytic capacitors only the impedance magnitude |Z| is specified, and simply written as "Z". Regarding to the IEC/EN 60384-1 standard, the impedance values of tantalum electrolytic capacitors are measured and specified at 10 kHz or 100 kHz depending on the capacitance and voltage of the capacitor.

Besides measuring the impedance can be calculated using the idealized components out of a capacitor's series-equivalent circuit, including an ideal capacitor C, a resistor ESR, and an inductance ESL. In this case the impedance at the angular frequency ω therefore is given by the geometric (complex) addition of ESR, by a capacitive reactance XC


 * $$ X_C= -\frac{1}{\omega C}$$

and by an inductive reactance XL (Inductance)

$$ X_L=\omega L_{\mathrm{ESL}}$$.

Then Z is given by


 * $$Z=\sqrt{{ESR}^2 + (X_\mathrm{C} + (-X_\mathrm{L}))^2}$$.

In the special case of resonance, in which the both reactive resistances XC and XL have the same value (XC=XL), then the impedance will only be determined by ESR. With frequencies above the resonance the impedance increases again due to the ESL of the capacitor. The capacitor becomes to an inductance.

ESR and dissipation factor tan δ
The equivalent series resistance (ESR) summarizes all resistive losses of the capacitor. These are the terminal resistances, the contact resistance of the electrode contact, the line resistance of the electrodes, the electrolyte resistance, and the dielectric losses in the dielectric oxide layer.

ESR influences the remaining superimposed AC ripple behind smoothing and may influence the circuit functionality. Related to the capacitor ESR is accountable for internal heat generation if a flow over the capacitor. This internal heat may influences the reliability of tantalum electrolytic capacitors.

Generally the ESR decreases with increasing frequency and temperature.

For electrolytic capacitors, out of historical reasons sometimes the dissipation factor tan δ will be specified in the relevant data sheets, instead of ESR. The dissipation factor is determined by the tangent of the phase angle between the substraction of capacitive reactance XC and inductive reactance XL, and the ESR. If the capacitors inductance ESL is small, the dissipation factor can be approximated as:


 * $$\tan \delta = \mbox{ESR} \cdot \omega C$$

The dissipation factor tan δ is used for capacitors with very low losses in frequency determining circuits or resonant circuits where the reciprocal value of the dissipation factor is called the quality factor (Q) which represents a resonator's bandwidth.

Ripple current


A "ripple current" is the 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.

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


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

This internal generated heat, additional to the ambient temperature and possibly other external heat sources leads to a capacitor body temperature having a temperature difference of Δ T against the ambient. This heat has to be distributed as thermal losses Pth over the capacitors surface A and the thermal resistance β to the ambient.


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

The internal generated heat has to be distributed to the ambient by thermal radiation, convection, and thermal conduction. The temperature of the capacitor, which is established on the balance between heat produced and distributed, shall not exceed the capacitors maximum specified temperature.

The ripple current is specified as an effective (RMS) value at 100 or 120 Hz or at 10 kHz at upper category temperature. Non-sinusoidal ripple currents have to be analyzed and separated into their single sinusoidal frequencies by means of Fourier analysis and summarized by squared addition the single currents.


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

In solid tantalum electrolytic capacitors the heat generated by the ripple current influences the reliability of the capacitors. Exceeding the limit tends to result in catastrophic failures with shorts and burning components.

Current surge, peak or pulse current
Solid tantalum electrolytic capacitors are damageable against surge, peak or pulse currents. Tantalum capacitors, which are exposed to surge, peak or pulse currents should be used with a voltage derating up to 70% in highly inductive circuits. If possible the voltage profile should be a ramp turn-on, as this reduces the peak current seen by the capacitor.

Leakage current
The DC leakage current (DCL) is a special characteristic for electrolytic capacitors other conventional capacitors don’t have. This current is represented by the resistor Rleak in parallel with the capacitor in the series-equivalent circuit of electrolytic capacitors. The main causes of DCL for solid tantalum capacitors are f. e. electrical breakdown of the dielectric, conductive paths due to impurities or due to poor anodization, bypassing of dielectric due to excess manganese dioxide, due to moisture paths or due to cathode conductors (carbon, silver). This “normal” leakage current in solid electrolyte capacitors couldn’t be reduced by “healing” in the sense of generating new oxide because under normal conditions solid electrolytes don’t can deliver oxygen for forming processes. This statement should not be confused with the self-healing process during field crystallization, see.

Leakage current in solid MnO2 tantalum electrolytic capacitors generally drops very fast but than remain on the reached level. They normally have a very low leakage current, most much lower than specified.

The specification of the leakage current in datasheets often will be given by multiplication of the rated capacitance value CR with the value of the rated voltage UR together with an addendum figure, measured after a measuring time of 2 or 5 minutes, for example:
 * $$I_\mathrm{Leak} = 0{,}01\,\mathrm{{A}\over{ V \cdot F}} \cdot U_\mathrm R \cdot C_\mathrm R + 3\,\mathrm{\mu A}$$

The value of the leakage current depends on the voltage applied, on temperature of the capacitor, on measuring time, and on influence of moisture caused by case sealing conditions.

Dielectric absorption (soakage)
Dielectric absorption occurs when a capacitor that has remained charged for a long time discharges only incompletely when briefly discharged. 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".

Dielectric absorption may a problem in circuits, were very small currents are used for in the function of an electronic circuit such as long-time-constant integrators or sample-and-hold circuits. However, in most applications of tantalum electrolytic capacitors supporting power supply lines dielectric absorption is not a problem.

Reliability (failure rate]


The reliability of a component is a property that indicates how reliable this component performs its function in a time interval. It is subject to a stochastic process and can be described qualitatively and quantitatively; it is not directly measurable. The reliability of electrolytic capacitors are empirically determined by identifying the failure rate in production accompanying endurance tests, see Reliability engineering

The reliability normally is shown in a bathtub curve and is divided into three areas: Early failures or infant mortality failures, constant random failures and wear out failures. Failures totalized in a failure rate are short circuit, open circuit and degradation failures (exceeding electrical parameters).

The reliability prediction is generally expressed in a Failure rate λ, abbreviation FIT (Failures In Time]. 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 fixed working conditions during the period of constant random failures. These failure rate model implicitly assume the idea of "random failure". Individual components fail at random times but at a predictable rate. The standard operation conditions for the failure rate FIT are 40 °C and 0.5 UR.

The reciprocal value of FIT is MTBF (Mean Time Between Failures).

It is good to know that for tantalum capacitors often the failure rate is specified in essence at 85 °C and rated voltage UR as reference conditions and expressed as per cent failed components per thousand hours (n %/1000 h). That is “n” number of failed components per 105 hours or in FIT the ten-thousand-fold value per 109 hours.

For other conditions than the standard operation conditions 40 °C and 0.5 UR, for another  temperature and voltage applied, for current load, capacitance value, circuit resistance, mechanical influences and humidity the FIT figure can recalculated with acceleration factors standardized for industrial or military contexts. As higher f. e. temperature and applied voltage as higher is the failure rate.

The most often cited source for recalculation the failure rate is the MIL-HDBK-217F, the “bible” of failure rate calculations for electronic components. SQC Online, the online statistical calculators for acceptance sampling and quality control gives an online tool for short examination to calculate given failure rate values to application conditions.

Some manufacturers of tantalum capacitors may have their own FIT calculation tables, for tantalum capacitors. It should be noted that industrial produced tantalum capacitors nowadays are very reliable components. Continuous improvement in tantalum powder and capacitor technologies have resulted in a significant reduction in the amount of impurities present which formerly have caused most of the field crystallization failures. Commercial available industrial produced tantalum capacitors now have reached as standard products the high MIL standard “C” level which is 0.01 %/1000h at 85 °C and UR or 1 failure per 107 hours at 85 °C and UR. Recalculated in FIT with the acceleration factors coming from MIL HDKB 217F at 40 °C and 0.5 UR is this failure rate for a 100 µF/25 V tantalum chip capacitor used with a series resistance of 0.1 Ω the failure rate is 0.02 FIT.

Life time
The life timelife time, service life, load life or useful life of electrolytic capacitors is a special characteristic of non-solid electrolytic capacitors, especially non-solid aluminum electrolytic capacitors which liquid electrolyte can evaporate over the time leading to wear-out failures. Solid tantalum capacitors with manganese dioxide electrolyte have no wear-out mechanism so that the constant failure rate least up to the point all capacitors have failed. They don’t have a life time specification like non-solid aluminum electrolytic capacitors.

Tantalum capacitors with non-solid electrolyte, the “wet tantalums” also don’t have a life time specification because they are hermetically sealed and evaporation of electrolyte is minimized.

However, solid polymer tantalum electrolytic capacitors do have a life time specification. The polymer electrolyte have a small deterioration of conductivity by a thermal degradation mechanism of the conductive polymer. The electrical conductivity decreased, as a function of time, in agreement with a granular metal type structure, in which aging is due to the shrinking of the conductive polymer grains. The life time of polymer electrolytic capacitors is specified in similar terms like non-solid e-caps but it’s life time calculation follows other rules leading to much longer operational life times.

Failure modes, self-healing mechanism and application rules
Tantalum capacitors show a different behavior in point of electrical long-term behavior related to the used electrolyte. Application rules for types with an inherent failure mode are specified to ensure capacitors high reliability and long life.

Tantalum capacitors are reliable on the same very high level as other electronic components with very low failure rates. However, they have a single unique failure mode called “field crystallization" . Field crystallization is the major reason for degradation and catastrophic failures of solid tantalum capacitors . More than 90% of the todays rare failures in tantalum solid-state electrolytic capacitors are caused by shorts or increased leakage current due to this failure mode.

The extremely thin oxide film of a tantalum electrolytic capacitor, the dielectric layer, must be formed in an amorphous structure. Changing the amorphous structure into a crystallized structure the conductivity is reported to be 1000 times higher combined with an enlargement of the oxide volume. The field crystallization followed by an dielectric breakdown is characterized by a sudden rise in leakage current, within a few milliseconds, from nano-ampere magnitude to ampere magnitude in low-impedance circuits. Increasing current flow can be accelerate as an “avalanche effect”, and rapidly spread through the metal/oxide. This can result in various degrees of destruction from rather small, burned areas on the oxide to zigzag burned streaks covering large areas of the pellet or complete oxidation of the metal. If the current source is unlimited a field crystallization may cause a capacitor short circuit.

Impurities, tiny mechanical damages or imperfections in the dielectric can affect the structure changing it from amorphous into crystalline structure lower the dielectric strength. The level of purity of the tantalum powder is one of the most important parameters defining its risk for field crystallization. Since the mid-80’s, manufactured tantalum powders have exhibited around a ten-fold improvement in CV/g values (from approximately 20k to 200k). Together with this dimension improvement in point of impurities a quality improvement has taken place.

Surge currents after soldering-induced stresses are often attributed to start a field crystallization followed by an insulation breakdown. The only way to avoid catastrophic failures is to limit the current which can flow from the source in order to reduce the breakdown point to a limited area. With a limitation this current only flow over the crystallization point causes a localized Joule heating in the manganese dioxide (MnO2) cathode opposite the fault. At increased temperatures a chemical reaction then reduce the surrounding conductive manganese dioxide to the insulating manganese(III) oxide (Mn2O3) and insulate the crystallized oxide in the tantalum oxide layer. The current flow stops by an insulating self-healing process.

The prevalent failure moment for solid tantalum capacitors to fail with field crystallization followed by a short circuit and a burning component is the power-on moment. It is believed that the voltage across the dielectric layer is the trigger mechanism for the breakdown and that the switch-on current pushes the collapse to a catastrophic failure.

To prevent tantalum capacitors for sudden failures like this some general rules for using solid manganese dioxide tantalum capacitors in electronic circuits are recommended
 * 50% application voltage derating against rated voltage
 * using a series resistance of 3 Ω/V or
 * using of circuits with slow power-up modes (soft-start circuits).

The effect of voltage derating in steadystate conditions is well known, but derating has been empirically shown to reduce the number of failures in dynamic switch-on applications also. The function of series resistance and the soft-start modus is to slowly ramp up the voltage applied to limit transient capacitor-damaging fault current.

Capacitor symbols
Electrolytic capacitor symbols

Parallel connection
Smaller or low voltage electrolytic capacitors may be connected in parallel without any safety correction action. Large sizes capacitors, especially large sizes and high voltage types should be individual guarded against sudden energy charge of the whole capacitor bank due to a failed specimen.

Series connection
Some applications like AC/AC converters with DC-link for frequency controls in three-phase grids needs higher voltages aluminum electrolytic capacitors usually offer. For such applications electrolytic capacitors can be connected in series for increased voltage withstanding capability. During charging, the voltage across each of the capacitors connected in series is proportional to the inverse of the individual capacitor’s leakage current. Since every capacitor differs a little bit in individual leakage current the capacitors with a higher leakage current will get less voltage. The voltage balance over the series connected capacitors is not symmetrically. Passive or active voltage balance has to be provided in order to stabilize the voltage over each individual capacitor.

Polarity marking


All tantalum capacitors are in general polarized components, with distinctly marked positive or negative terminals. When subjected to reversed polarity (even briefly), the capacitor depolarizes and the dielectric oxide layer breaks down, which can cause it to fail even when later operated with correct polarity. If the failure is a short circuit (the most common occurrence), and current is not limited to a safe value, catastrophic thermal runaway may occur.

Tantalum electrolytic capacitors with solid electrolyte are marked at their positive terminal with a bar or a "+". Tantalum electrolytic capacitors with non-solid electrolyte (axial leaded style) are marked on the negative terminal with a bar or a “-“ (minus). The polarity better can be identified on the shaped side of the case, which has the positive terminal. The different marking can cause dangerous confusion.

Imprinted markings
Tantalum capacitors, like most other electronic components and if enough space is available, have imprinted markings to indicate manufacturer, type, electrical and thermal characteristics, and date of manufacture. But most tantalum capacitors are chip types so the reduced space limits the imprinted signs to capacitance, tolerance, voltage and polarity.

Smaller capacitors use a shorthand notation. The most commonly used format is: XYZ J/K/M “V”, where XYZ represents the capacitance (calculated as XY × 10Z pF), the letters K or M indicate the tolerance (±10 % and ±20 % respectively) and “V” represents the working voltage.

Examples:


 * 105K 330V implies a capacitance of 10 × 105 pF = 1 µF (K = ±10%) with a working voltage of 330 V.
 * 476M 100V implies a capacitance of 47 × 103 pF = 47 µF (M = ±20%) with a working voltage of 100 V.

Capacitance, tolerance and date of manufacture can be indicated with a short code specified in IEC/EN 60062. Examples of short-marking of the rated capacitance (microfarads): µ47 = 0,47 µF, 4µ7 = 4,7 µF, 47µ = 47 µF

The date of manufacture is often printed in accordance with international standards.


 * Version 1: coding with year/week numeral code, "1208" is "2012, week number 8".
 * Version 2: coding with year code/month code. The year codes are: "R" = 2003, "S"= 2004, "T" = 2005, "U" = 2006, "V" = 2007, "W" = 2008, "X" = 2009, "A" = 2010, "B" = 2011, "C" = 2012, "D" = 2013, “E” = 2014 etc. Month codes are: "1" to "9" = Jan. to Sept., "O" = October, "N" = November, "D" = December. "X5" is then "2009, May"

For very small capacitors no marking is possible. Here only the traceability of the manufacturers can ensure the identification of a type.

Standardization
The standardization for all electrical, electronic components and related technologies follows the rules given by the International Electrotechnical Commission (IEC), a non-profit, non-governmental international standards organization.

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 tantalum electrolytic capacitors for use in electronic equipment for approval as standardized types are set out in the following sectional specifications:


 * IEC/EN 60384-3—Surface mount fixed tantalum electrolytic capacitors with manganese dioxide solid electrolyte
 * IEC/EN 60384-15—fixed tantalum capacitors with non-solid and solid electrolyte
 * IEC/EN 60384-24—Surface mount fixed tantalum electrolytic capacitors with conductive polymer solid electrolyte

Tantalum ore
Tantalum capacitors are the main use of the element tantalum. Tantalum ore is one of the conflict minerals. Some non-governmental organizations are working together to raise awareness of the relationship between consumer electronic devices and conflict minerals.

Market
The market of tantalum electrolytic capacitors in 2008 was approximately US$2.2 billion, that was roughly 12% of the total capacitor market.

Uses
The low leakage and high capacity of tantalum capacitors favor their use in sample and hold 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 makes use of tantalum capacitors.

An especially common application for low-voltage tantalum capacitors is power supply filtering on computer motherboards and in peripherals, due to their small size and long-term reliability.