Tantalum electrolytic capacitor

Axially-bucket, radial-bead-shaped and surface-mountable tantalum electrolytic capacitors compared in size with a match

A tantalum electrolytic capacitor , also known as tantalum capacitor or tantalum electrolytic capacitor , is an electrolytic capacitor whose anode (+) is made of tantalum , on which anodic oxidation, also known as " formation ", creates an even, extremely thin, electrical, adapted to the nominal voltage insulating oxide layer is generated, which forms the dielectric of the capacitor. A mostly solid electrolyte , which geometrically adapts to the surface structure of the anode, forms the cathode (-) of the electrolytic capacitor.

Tantalum electrolytic capacitors have an anode surface that is greatly enlarged by sintering tantalum powder to form an anode block in order to increase the capacitance . Together with the relatively high permittivity of the dielectric tantalum pentoxide and the possibility of adapting the very thin oxide layer to the desired dielectric strength, they achieve a higher specific capacitance compared with ceramic , plastic film capacitors and also with aluminum electrolytic capacitors , which, however, is much lower than that is of supercapacitors .

Most tantalum electrolytic capacitors are manufactured in the SMD design with a solid electrolyte, which consists of either manganese dioxide or a conductive polymer . These low-resistance electrolytes have very low ESR values , a very low temperature dependence of their electrical parameters and a long service life. Due to the large specific capacity, the low ESR and the available flat SMD designs, Ta-Elkos are particularly suitable for devices with a flat design such as laptops , mobile phones , digital cameras and flat screens. Here they are used to decouple unwanted frequencies from the double-digit Hertz range up to a few megahertz, to smooth rectified voltages in switched-mode power supplies and to buffer the power supply of digital circuits in the event of a sudden power requirement.

Axial tantalum electrolytic capacitors with liquid or solid electrolytes in hermetically sealed housings are required for industrial applications with high requirements, for military and for space applications.

Tantalum electrolytic capacitors are polarized components that may only be operated with direct voltage . Any superimposed alternating voltage must not cause polarity reversal. Incorrect polarity, too high a voltage or ripple current overload can destroy the dielectric and thus also the capacitor. The destruction can have catastrophic consequences (short circuit, fire ). Tantalum electrolytic capacitors are also sensitive to fast switching edges. For the safe operation of tantalum electrolytic capacitors, the manufacturers therefore stipulate special rules for circuit design.

Basics

Plate capacitor

Tantalum electrolytic capacitors are basically plate capacitors, the capacity of which is greater, the larger the electrode area and the relative permittivity and the smaller the distance between the electrodes. ${\ displaystyle A}$ ${\ displaystyle \ varepsilon}$ ${\ displaystyle d}$

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

To increase the capacity of the later capacitor, the anode is roughened, whereby the surface is significantly larger than that of a smooth surface, but nothing changes in the principle of the plate capacitor.

The dielectric constant ε is made up of the electric field constant and the material- specific permittivity of the dielectric : ${\ displaystyle \ varepsilon _ {0}}$ ${\ displaystyle \ varepsilon _ {\ mathrm {r}}}$

{\ displaystyle \ varepsilon = \ varepsilon _ {0} \ varepsilon _ {\ mathrm {r}}}

.

This value then determines the specific capacitance of the tantalum electrolytic capacitor.

Anodic oxidation (formation)

Schematic representation of anodic oxidation

Tantalum electrolytic capacitors are based on the electrochemical effect of anodic oxidation ( formation ). In this case , an electrically insulating oxide layer is formed on the surface of so-called valve metals ( aluminum , tantalum , niobium, etc.) by applying the positive pole of a direct current source in a bath connected to the negative pole and filled with a liquid electrolyte , which acts as the dielectric of a capacitor can be used.

These oxide layers on the anode (+) are very thin and have a very high dielectric strength , which is in the nm / V range. The capacity of this capacitor is determined as to a capacitor plate from the geometry of the anode surface and the thickness of the oxide layer. This is determined with the forming voltage and can thus be adapted to the requirements of the respective application, whereby an optimization of the specific capacity is possible.

Material data of tantalum pentoxide and niobium pentoxide in comparison
Anode material	dielectric	Oxide structure	Relative permittivity	Dielectric strength (V / micron)	Oxide layer thickness ( nm / V)
Tantalum	Tantalum pentoxide Ta ₂ O ₅	amorphous	27	625	1.6
Niobium or niobium oxide	Niobium pentoxide Nb ₂ O ₅	amorphous	41	400	2.5

The dielectric strength of tantalum pentoxide is very high at around 625 V / µm. Since any desired dielectric strength can be achieved through the formation, the thickness of the oxide layer varies with the nominal voltage of the later capacitor. A 10 V tantalum electrolytic capacitor would therefore have a dielectric with a layer thickness of only about 16 nm , unless a safety margin is taken into account . In real tantalum capacitors, however, the oxide layer thicknesses are formed considerably more than the later nominal voltage of the capacitor would require.

When comparing the Ta and Nb electrolytic capacitors that are in competition with each other, it can be seen that niobium pentoxide (Nb ₂ O ₅ ) has a relative permittivity of around 40% higher than that of tantalum pentoxide, but with 400 V / µm a relative permittivity of around 30% lower dielectric strength. This means that a CV value of roughly the same size could be produced from the same amount of material. However, the currently available niobium oxide powder does not have the same small powder grain size as tantalum powder, so that the same energy densities cannot yet be achieved in reality. Real niobium electrolytic capacitors are therefore somewhat larger than tantalum electrolytic capacitors with the same CV product.

Materials and Manufacturing

The following descriptions of the materials and the manufacture focus on tantalum electrolytic capacitors with MnO ₂ electrolytes.

Anode (tantalum powder)

Tantalum powder with different specific capacitance values

The anode material of tantalum electrolytic capacitors consists of the metal tantalum , the purity of which is decisive for the quality of the capacitors made from it.

The metal is turned into a powder in a chemical process. Since the capacitance of a tantalum capacitor is proportional to the surface of the anode and the anode surface depends on the size of the powder particles used for the same structural volume, the particle size of the tantalum powder defines the capacitance of the capacitor: the smaller the particle size of the powder, the larger the surface of the powder manufactured body.

The typical grain size of such powders is between 0.1 and 10 µm. The grain size is specified in a unit that contains the specific capacity per weight, usually in "µF · V / g". Tantalum powders are offered with values between around 20,000 and 300,000 µFV / g. With a tantalum powder with the specific capacity of 200,000 µFV / g, anodes with a surface area of about 4 m ² / g can be produced. For comparison, the surface of activated carbon for supercapacitors is between 1000 and 3000 m ² / g, which is roughly a factor of 1000 larger.

Sintering

Sintered tantalum anode

The tantalum powder is mixed with a binding agent for further processing and pressed into a block ( pellet ) together with a tantalum wire, the later anode connection of the capacitor . This block is then sintered in a vacuum at high temperatures, typically 1200 to 1800 ° C. The contact surfaces of the grains are metallically baked together. A large number of pores remain in the sintered tantalum block and run through the entire sintered block. This gives the block a sponge-like structure with a large inner surface, the tantalum grains being connected to one another in an electrically conductive manner with the tantalum wire. The sintering also results in a very high mechanical strength of the block, which makes Ta electrolytic capacitors very robust.

As a result of this process, the surface of the anode block has become many times larger than the surface of a smooth block. For comparison. a 220 μF / 6.3 V capacitor in an SMD-C housing (6 × 3.2 × 2.6 mm) has an anode surface of around 350 cm², which is slightly larger than a DIN A5 sheet, the surface of which is smooth Tantalum block is only about 0.8 cm². The degree of surface enlargement through the use of fine tantalum powder is dependent on the required nominal voltage of the capacitor. High nominal voltages require coarser tantalum powder for the thicker oxide layers.

Formation of the dielectric

Formed anode

After sintering, the tantalum anode is anodically oxidized or formed . The anode block is immersed in an electrolyte bath and connected to a direct voltage with the correct polarity. Initially, the formation is controlled with a current limitation until the desired dielectric strength is reached, then the voltage remains applied until the current has dropped to almost zero, thus creating an even layer of tantalum pentoxide ( tantalum (V) oxide , Ta ₂ O) ₅ ) is guaranteed over the entire surface of the anode. This oxide layer is electrically insulating and forms the dielectric of the capacitor. The thickness results from the applied forming voltage. The oxide layer must have an amorphous structure.

The chemical processes of anodic oxidation are described with the following formulas:

Anode:

2 Ta → 2 Ta ⁵⁺ + 10 e−

2 Ta ⁵⁺ + 10 OH- → Ta ₂ O ₅ + 5 H ₂ O

Cathode:

10 H ₂ O + 10 e → 5 H2 ↑ + 10 OH

Since the thickness of the oxide layer is in the order of magnitude of the wavelength of the light, a different dielectric strength can be estimated on the basis of the color of the oxidized (formed) sintered block. The colors shift according to the Bragg equation with the refractive index.

The thickness of the anodically formed oxide layer is proportional to the voltage applied during the forming process. It is specifically adapted to the desired dielectric strength of the capacitor. However, for reasons of reliability, tantalum electrolytic capacitors are manufactured with a large safety margin with regard to the thickness of the oxide layer. This safety margin is typically a factor of 4 for Ta electrolytic capacitors with a manganese dioxide cathode. This means that a 25 V capacitor is formed with a forming voltage of 100 V in order to enable reliable operation.

This very high safety factor is due to the typical failure mechanism of tantalum electrolytic capacitors, the so-called "field crystallization".

For Ta electrolytic capacitors with polymer cathodes, the safety margin is slightly lower, around a factor of 2.

Two “bottlenecks” limit the miniaturization of tantalum capacitors, the metallically conductive connection between the tantalum grains must not be interrupted by the formation and the pore openings in the sintered block must be large enough to be able to guarantee impregnation with the electrolyte.

About one third of the growth of the oxide from the metal to the amorphous Ta ₂ O ₅ occurs during the forming process into the metal and about two thirds out of the metal, since the oxide is less dense than the metal. This means that the metallically conductive area, the sinter necks , in which the tantalum grains are conductively connected to one another through sintering, is reduced in size. This can u. Under certain circumstances, with small grains and high dielectric strength, the metallic connection between the particles disappears completely, in which case these tantalum powder grains no longer contribute to the capacity.

The remaining pore channels in the sintered body after sintering and forming can result in a further limitation. The pores must be large enough to allow the electrolyte to penetrate into the sintered body. The size of the channels between the pores determines the resulting capillary forces and the discharge of the displaced air or gas.

Thus, there is a relationship among the structure of the sintered body, the particle size of the tantalum powder and the forming stress. This means that with tantalum electrolytic capacitors there is an optimal particle size for the anode for every nominal capacitor voltage.

It is therefore a major technical challenge for the mass production of Ta electrolytic capacitors to find the difficult balance between the smallest possible pore channels and conductive sintered connections in order to obtain the largest possible specific capacity. For this purpose, the tantalum powders must be able to be produced with a high degree of homogeneity of the grain particle sizes.

Cathode (electrolyte)

Manganization, introduction of the solid electrolyte

The electrolyte in an electrolytic capacitor is always the cathode of the respective capacitor. It has to adapt mechanically as perfectly as possible to the inner porous sintered structure of the oxidized tantalum anode, so that the capacity of the anode can also be used as fully as possible. In the case of liquid electrolyte systems, the introduction of an electrolyte is quite problem-free; in the case of the solid electrolyte manganese dioxide , a liquid precursor of the electrolyte is only converted into the solid substance in situ.

The liquid precursor of the solid electrolyte manganese dioxide is manganese nitrate (Mn (NO ₃ ) ₂ ). The sintered and formed tantalum block is immersed in this liquid and then converted into the solid manganese dioxide MnO ₂ in a pyrolytic process at around 250 ° C. The chemical formula for this is:

{\ displaystyle \ mathrm {Mn (NO_ {3}) _ {2} \ cdot 6 \ H_ {2} O \ {\ xrightarrow {\ Delta T}} \ MnO_ {2} +2 \ NO_ {2} +6 \ H_ {2} O}}

This process must be repeated several times, until the surface of the entire anode, inside and outside, is covered as completely as possible with MnO ₂ .

Manganese dioxide is a hard, black crystalline substance and has quite good electrical conductivity. It has excellent long-term stability, has only a slight temperature dependence of the electrical parameters even at low temperatures and is inexpensive.

In addition to manganese oxide as a solid electrolyte, tantalum capacitors can be manufactured with a solid conductive polymer as the electrolyte, see polymer electrolytic capacitor, or with a liquid electrolyte.

Cathode contact

Cross-section through the layer sequence of a tantalum electrolytic capacitor with solid manganese dioxide electrolyte with the connection contact via a graphite and a silver layer.

The manganized capacitor cell, called a pill ( pellet ), still has to be provided with a connection that electrically connects the electrolyte with the environment. Manganese dioxide as a ceramic substance cannot simply be soldered to a connection. Therefore, the manganised pill is first dipped into a dispersion of graphite, then into a solution of electrically conductive silver lacquer, which hardens to form a silver layer. The graphite layer prevents direct contact between manganese dioxide and silver. Such a direct contact would result in a chemical reaction that transforms the conductive manganese dioxide MnO ₂ into the high-resistance manganese (III) oxide , which would increase the ESR of the capacitor. The capacitor is then electrically connected to the environment by soldering a cathode connection to the silver layer.

Manufacturing process

The most common type of tantalum electrolytic capacitors with sintered anodes and solid manganese dioxide electrolytes is the SMD capacitor (SMD chip) for surface mounting. It consists of an anode made of high-purity pressed and sintered tantalum powder. After the formation of this anode, the creation of the dielectric, it is provided with the solid electrolyte manganese dioxide or a conductive polymer. This capacitive cell is then contacted with a graphite and a silver layer in order to establish an electrically conductive connection to the cathode connection. The casing usually consists of a plastic over-molding, but can consist of a simple paint or a hermetically sealed metal cup, depending on the requirements. The manufacturing process for the manufacture of tantalum electrolytic capacitors ends with the 100% final inspection. For tantalum electrolytic capacitors with guaranteed reliability, various screening or burn-in processes are also carried out to reduce the failure rate .

Schematic representation of the production of tantalum electrolytic capacitors with sintered anode and solid manganese dioxide electrolytes.

Types and forms

Tantalum electrolytic capacitors are available in three different types , also called families:

Tantalum electrolytic capacitors with sintered anode and solid manganese dioxide electrolyte
Tantalum electrolytic capacitors with sintered anode and polymer electrolytes, see polymer electrolytic capacitor
Tantalum electrolytic capacitors sulfuric acid electrolytes, see # Tantalum electrolytic capacitors with liquid electrolytes

Types of tantalum electrolytic capacitors

The three types of tantalum electrolytic capacitors are manufactured and supplied in different designs :

SMD design with MnO₂ or polymer electrolytes (tantalum chips) for surface mounting on circuit boards or substrates
Radial design , tantalum electrolytic capacitors with solid MnO ₂ electrolytes with radial wire connections (led out on one side) for upright installation on printed circuit boards, dip-coated in bead shape or over-molded with plastic.
Axial design , tantalum electrolytic capacitors with solid MnO ₂ electrolyte or with liquid electrolyte for horizontal installation on circuit boards.

There are also versions in larger cuboid cups or in button-like cells.

Tantalum chip capacitors with MnO ₂ electrolytes

Basic structure of an SMD chip for manganese dioxide-tantalum electrolytic capacitor
The tantalum anode of a tantalum electrolytic capacitor consists of sintered tantalum powder.
Cross-section through the structure of the sintered tantalum anode with the oxide, the manganese dioxide electrolyte and the contacting of the electrolyte via a graphite and a silver layer.
Structure of a typical SMD tantalum chip electrolytic capacitor with solid electrolyte.

More than 90% of all tantalum electrolytic capacitors worldwide are manufactured as "tantalum chips" in the SMD design for surface mounting. These tantalum chip electrolytes with manganese dioxide electrolytes are cheaper than with polymer electrolytes. Their electrical properties are also stable over long periods of time and show no drift. The structure is described under the paragraph #Manufacturing process .

A typical tantalum SMD chip capacitor

The development of this design up to the form that is common today took place through different sheaths and different shapes of the two connections, through a lacquer covering with soldered-on caps (CWR 06) or lacquer covering with tin-plated connection surfaces (CWR 10) up to plastic encapsulation with pressed-in connection surfaces ( CWR 09). The version molded with plastic, which offers sufficient protection for most industrial requirements, is most frequently used today. However, for requirements with high humidity or harsh climatic conditions, SMD Ta chips are also manufactured with a hermetically sealed casing.

A very special version of tantalum SMD chip capacitors is the design with a built-in fuse. This version was developed in order to disconnect the capacitor from the voltage quickly enough in the event of a short circuit in the capacitor, which reduces the risk of fire and the consequential damage.

Tantalum chip capacitors with polymer electrolytes

New Ta-Chip Constructions - Reduction of ESR and ESL

Multi-anode technology

In the event of a sudden power requirement from a downstream circuit, the supply voltage is reduced by voltage drops across the ESL, ESR and by a loss of capacity.

With the multi-anode construction, several tantalum sintered anodes are connected in parallel, which reduces both ESR and ESL.

The development of digital electronic devices in flat design such as laptops , flat screens and mobile phones required an ever more precise power supply with increasing supply currents in the double-digit ampere range but decreasing supply voltages, now often below 2 V. These requirements are a major challenge for the capacitors in the power supplies, because the series equivalent resistance ESR of the capacitor results in a voltage drop of ΔU = ESR · I in the event of a sudden power requirement , which can impair the functionality of the downstream circuit. In addition, the series inductance ESL of the capacitor delays the rapid supply of the circuit with the required current via the derivative di / dt. The aim for all capacitor developments for these applications is therefore to reduce the ESR and, if possible, also the ESL of the capacitors.

Tantalum chip capacitors have been used in these applications right from the start. However, the increasing requirements made new developments necessary. Tantalum electrolytic capacitors with polymer electrolytes were one of these new developments. However, design measures can also have a major influence on the electrical parameters of capacitors. Lower ESR values can be achieved, for example, by connecting several conventional capacitor cells in parallel in one housing. Three capacitors connected in parallel with an ESR of 60 mΩ each then result in a total ESR of 20 mΩ. This construction is called multi-anode technology and is used in both cheaper Ta capacitors with manganese oxide and somewhat more expensive Ta capacitors with polymer electrolytes. In such Ta-Chip capacitors, up to six individual anodes are connected together in one housing. Such tantalum multi-anode chips have ESR values in the single-digit milliohm range.

Face-down technique

With the "face-down" technique, the current path is structurally reduced, which reduces the parasitic impedance (ESL), which means that the resonance shifts to higher frequencies.

The parasitic inductance of the capacitor can also be reduced by design changes. Since the length of the supply lines makes up a large proportion of the total inductance ESL of the capacitor, the internal supply lines can be reduced by asymmetrical arrangement of the anode connection in the tantalum anode, thereby reducing the ESL. With this "face-down" construction, the resonance of the capacitor shifts to higher frequencies, whereby the consequences of faster load changes are taken into account with the ever higher switching frequencies of digital circuits. Tantalum chip electrolytic capacitors have achieved properties that approach those of MLCC capacitors more and more through these design improvements that reduced both the ESR and the ESL.

Chip package sizes

Tantalum chips are available in many different housing sizes. The sizes are marked by the manufacturers in accordance with the American standard EIA -535-BAAC, which has since been discontinued , with a capital letter and a code resulting from the inch. For sizes A to E, which have been manufactured for many decades, the dimensions (without tolerances) are still largely identical for the respective manufacturers.

Frame size A: 3.2 mm × 1.6 mm × 1.6 mm, customs code: 1206
Frame size B: 3.5 mm × 2.8 mm × 1.9 mm, customs code: 1210
Frame size C: 6.0 mm × 3.2 mm × 2.2 mm, customs code: 2312
Frame size D: 7.3 mm × 4.3 mm × 2.9 mm, customs code: 2917

New developments in tantalum electrolytic capacitors with smaller dimensions or with very high capacitance values as well as such. For example, the multiple anode technology to reduce the ESR or the "face down technology" to reduce the inductance have meanwhile led to a large number of other chip sizes. Many sizes also have different heights with the same footprint. This older EIA code has therefore been standardized by the EIA through a new, metric coding without a letter code, for example: EIA 3216-12 has the nominal dimensions 3.2 mm × 1.6 mm × 1.2 mm. However, the chip package sizes are usually still marked by the manufacturers in the respective data sheets with capital letters. However, confusion can arise because the manufacturers have not standardized the dimensions of their SMD chips in a uniform manner.

Definitions of the dimensions of a tantalum chip capacitor

The following table provides an overview of the dimensions of some tantalum chip capacitors and their coding:

Housing sizes and coding of tantalum SMD chip capacitors
Housing size according to EIA metric	L ± 0.2 mm	B ± 0.2 mm	H max mm	Case size, inch code	Housing code AVX	Housing code Kemet	Housing code Vishay
EIA 1608-08	1.6	0.8	0.8	0603	J	-	-
EIA 1608-10	1.6	0.85	1.05	0603	L.	-	M, M0
EIA 2012-12	2.05	1.35	1.2	0805	R.	R.	W.
EIA 2012-15	2.05	1.35	1.5	0805	P	-	R.
EIA 3216-10	3.2	1.6	1.0	1206	K	I.	Q, A0
EIA 3216-12	3.2	1.6	1.2	1206	S.	S.	-
EIA 3216-18	3.2	1.6	1.8	1206	A.	A.	A.
EIA 3528-12	3.5	2.8	1.2	1210	T	T	N
EIA 3528-15	3.5	2.8	1.5	1210	H	-	T
EIA 3528-21	3.5	2.8	2.1	1210	B.	B.	B.
EIA 6032-15	6.0	3.2	1.5	2312	W.	U	-
EIA 6032-20	6.0	3.2	2.0	2312	F.	-	-
EIA 6032-28	6.0	3.2	2.8	2312	C.	C.	C.
EIA 7343-15	7.3	4.3	1.5	2917	X	W.	-
EIA 7343-20	7.3	4.3	2.0	2917	Y	V	V
EIA 7343-30	7.3	4.3	3.0	2917	N	-	-
EIA 7343-31	7.3	4.3	3.1	2917	D.	D.	D.
EIA 7343-40	7.3	4.3	4.0	2917	-	Y	-
EIA 7343-43	7.3	4.3	4.3	2917	E.	X	E.
EIA 7360-38	7.3	6.0	3.8	2923	-	E.	W.
EIA 7361-38	7.3	6.1	3.8	2924	V	-	-
EIA 7361-438	7.3	6.1	4.3	2924	U	-	-

Radial leaded tantalum electrolytic capacitors

Radial leaded tantalum electrolytic capacitors
Tantalum capacitor in teardrop shape (tantalum pearl)
Radial tantalum capacitor with plastic extrusion

Radial-wired tantalum electrolytic capacitors for circuit board assembly are nowadays largely replaced by the SMD chip design. Nevertheless, they can still be found in the delivery programs of major manufacturers. The best known are the pearl-shaped tantalum electrolytic capacitors. Nowadays they are mostly only used by amateur electronics enthusiasts. The capacitors have a sintered anode cell, impregnated with the solid electrolyte manganese dioxide. The cathode contact is made via a layer sequence made of graphite and silver. The capacitors are dip-coated and marked with a stamp. A color coding of the capacity and the nominal voltage has not been used since 1970.

Another type of radially wired Ta electrolytic capacitor is the version molded with a plastic. The plastic sheathing results in better mechanical strength, more precise positioning of the component on the circuit board and better protection against environmental influences.

Axial tantalum electrolytic capacitors with solid MnO ₂ electrolytes

Axial tantalum electrolytic capacitors

Axial tantalum electrolytic capacitors with solid manganese dioxide electrolytes have a sintered and oxidized anode. The electrolyte is introduced into the anode structure in the pyrolytic process described above and surrounds the anode block. The cathode is contacted via a graphite and a silver layer. The silver-plated block is built into a metal cup and electrically connected to the cup by means of a solder. The metal cup is then usually provided with a hermetic seal for leading out the anode connection. In the case of versions with a plastic cover, the metal cup is not required and the cathode connection is soldered directly to the silver layer.

Bipolar tantalum electrolytic capacitors contain a second oxidized and contacted anode block which is electrically connected to the first block via a solder. This connects two anodes in a series circuit.

The axial tantalum electrolytic capacitors with solid MnO ₂ electrolytes are mainly manufactured in accordance with the military standard MIL-PRF-39003 in accordance with one of the many "CSR" series and delivered with an established reliability . The versions of axial Ta electrolytic capacitors with a plastic cover are manufactured and supplied in accordance with the MIL-PRF-49137 standard.

These tantalum electrolytic capacitors are used in industrial areas with very high requirements in terms of reliability, robustness and temperature resistance, such as B. geoprobes for petroleum exploration, in medical devices, in all military areas and in space travel .

Axial tantalum electrolytic capacitors with liquid electrolyte

Sectional view through a hermetically sealed axial tantalum electrolytic capacitor with sintered anode and liquid electrolyte installed in a tantalum cup.

Axial tantalum capacitors with a winding of tantalum foil soaked in liquid electrolyte and built into a metal cup were the first industrially manufactured tantalum electrolytic capacitors. In the military standards MIL-C-39006/1 to 4, these capacitors are still present as polarized or non-polarized capacitors with etched or smooth tantalum foils. However, the production of these capacitors with wound foils has since been discontinued.

Axial tantalum electrolytic capacitors with liquid electrolytes are nowadays produced with a sintered body as the anode. The electrolyte is usually sulfuric acid . Tantalum and tantalum pentoxide are insensitive to this strong acid. These "wet slug" axial tantalum electrolytic capacitors are provided with a hermetically sealed silver or tantalum cup. A platinum black or a special tantalum sintered cathode reduces the contact resistance of the electrolyte and helps improve the switching resistance.

The advantage of the liquid electrolyte is that it can supply the oxygen for the self-healing processes of the dielectric. This means that failures due to field crystallization do not appear with “wet” Ta-Elkos. As a result of self-healing, the dielectric can be designed with lower safety margins and thus much thinner than the dielectric for tantalum electrolytic capacitors with solid electrolytes. This results in the main feature of tantalum electrolytic capacitors with liquid electrolytes, their high specific capacity compared to the Ta electrolytic capacitors with solid electrolytes but also to aluminum electrolytic capacitors with liquid electrolytes. Due to the self-healing process, “wet” Ta capacitors have the lowest residual current values of all electrolytic capacitors. The liquid electrolyte with its ionic conductivity also makes these capacitors relatively insensitive to fast inrush or surge currents. Disadvantages are the slightly higher ESR compared to manganese dioxide or polymer electrolytes and the greater temperature dependence of the electrical parameters, especially at low temperatures.

Although the axial design of Ta capacitors with liquid electrolyte is the predominant design, "wet" Ta capacitors are also offered in SMD housings or in cuboid cups.

"Wet" tantalum electrolytic capacitors can be manufactured with high nominal voltages of up to 125 V and for very high temperatures of up to 200 ° C. They are used for applications with the highest quality requirements in very special areas of industry, in the military sector and in space travel.

Comparison of the characteristic values of different Ta-Elko types

The following table shows the capacitance and nominal voltage ranges as well as the max. Temperature listed for the different Ta-Elko types:

Characteristic values of the different tantalum electrolytic capacitors
Anode material	electrolyte	Design	Capacity range ( µF)	Nominal voltage range (V)	Max. Category temperature (° C)
Tantalum sintered body	solid, manganese dioxide	chip	0.1 ... 1,500	2.5 ... 63	105/125/150/175/200/230
Tantalum sintered body	solid, manganese dioxide	Radial	1.0 ... 680	3… 50	85/105/125
Tantalum sintered body	solid, manganese dioxide	Axially	0.1 ... 1000	6… 125	105/125 / 150
Tantalum sintered body	liquid, sulfuric acid	Axially	0.1 ... 4,700	2.5… 125	105/125/150/200
Tantalum sintered body	solid, conductive polymer	chip	0.47 ... 3,300	2.5… 125	105/125

history

Tantalum is a relatively "young" metal. It was first obtained in pure form in 1903 by Werner von Bolton . In the years that followed, its properties were thoroughly investigated and it was established that tantalum belongs to the group of so-called " valve metals ". The high melting point of 2996 ° C prevented industrial use for a long time.

Early development

The first tantalum electrolytic capacitors with wound tantalum foils and liquid electrolyte were manufactured in 1930 by Tansitor Electronic Inc. USA for military purposes.

Development after the Second World War

The decisive development of tantalum electrolytic capacitors in the form best known today took place after the Second World War, a few years after Shockley, Barden and Brattain invented the transistor in 1947 . It was promoted from 1950 by Bell Laboratories in search of smaller and more reliable capacitors for low voltages to support the new circuits with transistors. Since tantalum was difficult to etch in order to enlarge the anode surface, the researchers RL Taylor and HE Haring came up with the idea in 1950 to grind tantalum into a powder, then press the powder into a powder block and then sinter this block at high temperatures . The result resulted in a compact, mechanically strong tantalum block with a sponge-like structure with many pores, whereby the individual tantalum particles are connected to one another in a metallic and electrically conductive manner in a space lattice, creating a very large anode surface.

The search for a solid electrolyte took place at the beginning of 1950 in parallel to the research group of Taylor and Haring, also in Bell Laboratories, by DA McLean and FS Power. In 1952 this search was crowned with a success. The researchers soaked the tantalum sintered body with the liquid manganese nitrate (Mn (NO ₃ ) ₂ ) and then converted this liquid into the solid semiconducting manganese dioxide (MnO ₂ ) in a pyrolytic process . Manganese dioxide, and manganese dioxide called, is a hard, black crystalline substance has a better by a factor of 10 electrical conductivity than the former liquid electrolytes and has a stable temperature characteristics, particularly at low temperatures as well as excellent long term stability.

In 1954 Preston Robinson succeeded in producing the first functional tantalum capacitor with MnO ₂ electrolytes at the Sprague Electric Company (today: Vishay ) . The new technology was quickly developed and perfected to such an extent that very soon numerous manufacturers, including in Japan and Europe, started large-scale production. The development was particularly favored by the design of the tantalum drop capacitor (Ta beads), which was quickly used in large series, especially in radio and television sets. The higher price of the tantalum capacitors compared to aluminum electrolytic capacitors was often annoying, but because of the smaller design of the tantalum bead, the smaller ESR, the better low-temperature behavior and the much better residual current behavior, decisions were often made in favor of the tantalum capacitors.

This changed when, in 1980, speculation on the stock exchange caused the price of tantalum to explode. The boom in tantalum electrolytic capacitors in the entertainment sector and also in industrial electronics then subsided almost suddenly. Only with the trend towards ever greater miniaturization of electronic devices were tantalum SMD capacitors used again in large numbers in industry from the mid-1980s. In 2000, further speculation drove tantalum prices up again. However, the manufacturers of the tantalum powder then managed to calm the situation down through long-term supply contracts.

Miniaturization

By increasing the specific capacity of tantalum powders with ever smaller grain sizes, the construction volume of tantalum chip capacitors has been reduced considerably in the last few decades.

The size of tantalum capacitors depends crucially on the size of the tantalum powder grains, which had not changed significantly between 1960 and 1990. It was not until the mid-1990s that a new chemical process was developed at HC Starck , Germany, which made it possible to produce tantalum powder with extremely small grain sizes. As a result of this new production process, a tenfold increase in the specific powder capacity could be achieved by 2015, whereby the capacity of a tantalum capacitor also increased by a factor of 10 for a given construction volume.

In addition to the downsizing of the housing, the design also developed further. Multiple anode blocks in one housing, the so-called “multi-anode technology”, resulted in a significant reduction in internal losses, and the ESR value became smaller. With a further design in the so-called "face-down" technology, the inductance of the capacitors was also reduced.

Polymer electrolyte

Tantalum electrolytic capacitors with polymer electrolytes were brought onto the market in 1993 by NEC with its "NeoCap" SMD tantalum electrolytic capacitors with polypyrrole electrolyte. In 1997 Sanyo followed with the "POSCAP" tantalum chips, see polymer electrolytic capacitor . With this development, Ta-Elkos are now achieving values that in some applications compete directly with ceramic multilayer film capacitors (MLCC) .

Conflict mineral coltan

Coltan is a tantalum ore and therefore the raw material for tantalum electrolytic capacitors. Coltan is classified as a conflict mineral. The high profits for the corporations and the lack of state surveillance during the civil war in the Democratic Republic of the Congo led to completely haphazard overexploitation, which resulted in serious environmental damage and inhumane working conditions. Because of the negative consequences, numerous companies that process coltan have decided not to process coltan from the DRC . Above all, the companies that are suppliers, manufacturers or processors of Ta-Elkos are involved in this waiver, for example HC Starck , Traxys, Samsung , Apple and Intel .

In the meantime (2018) the “EU regulation on conflict minerals” came into force, so that in the future even companies that have not adhered to the voluntary agreement will now be legally forced to renounce conflict-affected minerals.

Electrical characteristics

Equivalent circuit diagram

The electrical properties such as capacity, losses and inductance of real capacitors are determined according to the basic specification IEC 60384-1, which in Germany is called DIN EN 60384-1; VDE 0565-1 has been published, described with the help of an idealized series equivalent circuit diagram.

Series equivalent circuit diagram of an electrolytic capacitor

Here are:

${\ displaystyle C}$ , the capacitance of the capacitor,
${\ displaystyle R_ {ESR}}$ , the equivalent series resistance or equivalent series resistance, it summarizes all ohmic losses of the component. This effective resistance is generally only called "ESR" ( Equivalent Series Resistance )
${\ displaystyle L_ {ESL}}$ , the equivalent series inductance or substitute series inductance, in it all inductive parts of the component are summarized, it is generally only called "ESL" ( Equivalent Series Inductivity L).
${\ displaystyle R _ {\ mathrm {Leak}}}$ , the parallel resistance to the ideal capacitor, which represents the residual current (leakage current) of the electrolytic capacitor.

Capacity and capacity tolerance

The usual unit of capacitance for tantalum electrolytic capacitors is " µF " (microfarad).

The capacity of an electrolytic capacitor is frequency and temperature dependent. It is measured with an alternating voltage of 0.5 V and a frequency of 100/120 Hz at room temperature of 20 ° C. The capacity value measured in this way is about 10 to 15% lower than the value corresponding to the stored charge. In terms of the measuring frequency, electrolytic capacitors differ from ceramic and plastic film capacitors , whose capacitance is measured at 1 kHz.

Tantalum electrolytic capacitors with solid electrolytes occasionally have areas at the boundary layer between oxide and electrolyte that behave like n-semiconductors, similar to a Schottky barrier. This semiconducting behavior of the anodically generated barrier layer means that a positive DC voltage has to be applied to measure correct capacitance values of electrolytic capacitors with solid electrolyte in order to avoid polarity reversal, since otherwise a meaningful measurement is not possible and values that are far too high could be simulated . For this reason, tantalum electrolytic capacitors with solid electrolytes must have a DC voltage of 1.1 to 1.5 V for capacitors with a nominal voltage of ≤ 2.5 V or 2.1 to 2.5 V for capacitors with a nominal voltage of> 2.5 V be applied.

The specified in the data sheets of the manufacturer capacitance value for electrolytic capacitors is the "nominal capacity C _R " ( Rated capacitance C _R ), also known as "design capacity". According to DIN EN / IEC 60063, it is specified in values corresponding to the E series . This nominal value is specified in accordance with DIN EN / IEC 60062 with a permissible deviation, the capacity tolerance, in such a way that no overlaps occur.

E3 series	E6 series	E12 series
10-22-47	10-15-22-33-47-68	10-12-15-18-22-27 33-39-47-56-68-82
Capacity tolerance ± 20%	Capacity tolerance ± 20%	Capacity tolerance ± 10%
Code letter "M"	Code letter "M"	Code letter "K"

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

Nominal voltage and category voltage

Relationship between nominal voltage U _R and category voltage U _C with the nominal temperature range of T _R and the category temperature range T _C

The dielectric strength of tantalum electrolytic capacitors can be produced specifically for the desired nominal voltage of the capacitor via the anodic oxidation (formation) of the dielectric. Therefore, even very small nominal voltages such as B. 2.5 V, which is not possible with foil or ceramic capacitors. Such small voltages are increasingly required in modern integrated circuits.

The dielectric strength of the respective oxide layer decreases with increasing temperature. Therefore be specified in tantalum electrolytic capacitors with solid electrolytes often two voltages, the "nominal voltage U _R " ( Rated voltage U _R ), which is the maximum DC voltage constant at any temperature within the nominal temperature range of T _R "( temperature Rated T _R may abut) and the "Category voltage U _C " ( Category voltage U _C ) which is the maximum DC voltage constant at any temperature within the category temperature range T _C "( Category temperature T _C may abut). The picture on the right shows this relationship.

The sum of a constant DC voltage applied to the capacitor and the peak value of a superimposed AC voltage must not exceed the voltage specified for the capacitor. Exceeding the specified voltage can destroy the capacitor.

The operation of Ta-Elkos with a voltage lower than the specified nominal voltage has a positive influence on the expected failure rate, it is lower.

Nominal temperature and category temperature

The relationship between the nominal temperature range T _R and the nominal voltage U _R as well as the extended category temperature range T _C and the reduced category voltage U _C is explained in the figure above.

Peak voltage

For safety reasons, electrolytic capacitors are formed with a higher voltage than just the nominal voltage. Therefore, they can during the operation for a short time for a limited number of cycles of a so-called "peak voltage U _S " ( surge voltage U _S ) are exposed. The peak voltage is the maximum voltage value that is applied during the entire operation of the capacitors via a protective resistor of 1 kΩ or RC = 0.1 s with a frequency of 1000 cycles with a dwell time of 30 seconds and a pause of five minutes and 30 seconds without visible damage or a capacity change of more than 15%.

The permissible peak voltage is specified in DIN / EN IEC 60384-1. For Ta electrolytes with solid electrolytes, the peak voltage is specified as 1.3 times the nominal voltage. However, the peak voltage can lead to an increased failure rate.

Transients

Transients are fast, mostly low-energy surge peaks.

Electrolytic capacitors with solid electrolytes are generally sensitive to overvoltages and transients, since the solid electrolyte, as an electron conductor, transmits electrical changes without delay. These rapid overvoltage peaks can therefore cause changes in the oxide of the dielectric in tantalum electrolytic capacitors with solid electrolytes. The changes in the oxide can lead to a short circuit under certain circumstances.

Polarity reversal (reverse polarity)

Tantalum electrolytic capacitors are generally polarized capacitors, the anode of which must be operated with a positive voltage compared to the cathode.

If a polarity reversal voltage is applied to a tantalum electrolytic capacitor with solid electrolyte, a current begins to flow from a type-dependent threshold value. This current initially flows in local areas where there is contamination, broken oxide or defects. Although the currents are very small, this creates a local thermal load that can destroy the oxide layer. A longer time on the Ta-Elko polarity reversal or reverse polarity voltage above the type-dependent threshold value inevitably leads to a short circuit and thus to the destruction of the capacitor.

To minimize the risk of incorrect polarity when equipping, all electrolytic capacitors are marked with a polarity mark, see # polarity markings

Impedance Z and equivalent series resistance ESR

The mathematical description of these terms, taking into account the particularities applicable to electrolytic capacitors in the specification in the respective data sheets, see this section .

The impedance is specified in the data sheets of tantalum electrolytic capacitors as an impedance without a phase angle. The prescribed measuring frequency of the impedance is 100 kHz. The impedance measured at this frequency usually corresponds to the 100 kHz ESR value. ${\ displaystyle Z}$

Typical impedance curves of 100 µF electrolytic capacitors with different electrolytes in comparison with a 100 µF ceramic class 2 MLCC capacitor.

The impedance or ESR of electrolytic capacitors depends on the materials and the structure of the capacitor. A high specific capacitance of a Ta electrolytic capacitor, which can be achieved with very fine-grained Ta powder, has a higher ESR than capacitors with a lower specific capacitance due to the thinner current paths in the anode. The ESR is also influenced by the conductivity of the electrolyte. Polymer electrolytes have a better conductivity than the MnO ₂ electrolyte. Special designs such as #multianode technology or #face-down technology also influence the impedance / ESR behavior of Ta electrolytic capacitors.

The impedance and the ESR are frequency and temperature dependent. The ESR decreases with increasing frequency and with increasing temperature up to the resonance point of the capacitor. Ta capacitors with solid electrolytes show a doubling of the ESR and the impedance at −40 ° C compared to the value at room temperature.

Current carrying capacity

Ripple current

A rectified alternating voltage causes charging and discharging processes in the downstream smoothing capacitor, which cause the capacitor to heat up as a "ripple current" .

{\ displaystyle I_ {R} ^ {2} \ cdot {\ text {ESR}}}

An alternating voltage superimposed on the direct voltage and applied to a capacitor causes charging and discharging processes in it. This results in an alternating current, the ripple current ( ripple current ) is called. It flows as RMS on the ESR of the capacitor and is frequency-dependent electrical losses result ${\ displaystyle I_ {R}}$ ${\ displaystyle P_ {V _ {\ text {el}}}}$

{\ displaystyle P_ {V {\ text {el}}} = I_ {R} ^ {2} \ cdot {\ text {ESR}}}

which heat it up from the inside out and lead to an increase in temperature. This internally generated temperature is added with any other heat sources to the operating temperature of the condenser, which then differs by the value from the ambient temperature. ${\ displaystyle \ Delta T}$

This temperature difference is dissipated as thermal power loss through thermal conduction , radiation and convection via the surface and the heat transfer resistance of the capacitor to the environment ${\ displaystyle \ Delta T}$ ${\ displaystyle P_ {V _ {\ text {th}}}}$ ${\ displaystyle A}$ ${\ displaystyle \ beta}$

{\ displaystyle P_ {V _ {\ text {th}}} = \ Delta T \ cdot A \ cdot \ beta}

If the electrical losses and the thermal power loss are in thermal equilibrium, the temperature difference between the capacitor and the environment is calculated as follows: ${\ displaystyle P_ {V _ {\ text {el}}}}$ ${\ displaystyle P_ {V _ {\ text {th}}}}$

{\ displaystyle \ Delta T = I_ {R} ^ {2} \ cdot {\ text {ESR}} / A \ cdot \ beta}

The data sheet value of the ripple current for tantalum electrolytic capacitors is given as a sinusoidal effective value at 100–120 Hz or 100 kHz for a type-dependent temperature increase of the capacitor compared to the environment at the upper nominal temperature. Non-sinusoidal operating currents with other frequencies must therefore be measured or calculated as an effective value. Series-specific conversion tables are provided by many manufacturers. ${\ displaystyle \ Delta T}$

The ripple current for tantalum electrolytic capacitors is given as a 100 kHz effective value mostly for a temperature increase of the capacitor compared to the environment of 2 to 6 ° C at the upper nominal temperature. For the operation of Ta electrolytic capacitors at lower temperatures, a higher effective value is often specified; for applications in the extended range of the category temperature, the specified ripple current is reduced. Since the ESR of Ta electrolytic capacitors is frequency-dependent and increases at lower frequencies, the 100 kHz ripple current value at lower frequencies must be converted to the permissible value using appropriate conversion factors. Series-specific conversion tables are provided by many manufacturers.

Since a ripple current flowing through the capacitor leads to the heating of the component and the temperature of the capacitor influences the failure rate, the ripple current has an influence on the reliability of the capacitors. If the ripple current exceeds the specified limits, it can lead to a total failure with a short circuit and fire.

Charge, discharge, inrush current

Tantalum and niobium electrolytic capacitors with solid electrolytes react principle sensitive to high current peaks ( Current surge ) at the loading or unloading or at high inrush currents ( inrush current ) can adversely affect the reliability of Ta capacitors. Since the solid electrolyte, as an electron conductor, transmits electrical changes with steep current flanks di / dt without delay, there are rapid changes in the field strength in the dielectric. Defects, the tiniest mechanical damage or impurities in the dielectric heat up more quickly than the rest of the dielectric with rapid changes in the electrical field. As a result, the oxide structure can change selectively from an amorphous to a crystalline structure. This process is known as "field crystallization", which under certain circumstances can lead directly to a short circuit.

Tantalum electrolytic capacitors must therefore be used in accordance with specified application rules, e.g. B. operated with a voltage derating or with a current limitation.

In addition, the specified maximum ripple current must not be exceeded due to a load with charging and discharging currents or frequent peak currents.

Residual current

The switch-on behavior of the residual current of electrolytic capacitors depends heavily on the type of electrolyte

A special feature of all electrolytic capacitors is the so-called leakage current ( leakage current ) I _leak , formerly leakage current called. The residual current of an electrolytic capacitor is the direct current that flows through it when a direct voltage of the correct polarity is applied. It is shown in the equivalent circuit as a parallel resistance to the capacitance. The residual current is caused by local defects or weaknesses in the dielectric due to impurities that form local conductive bridges, due to moisture or cracks in the dielectric that occur during the soldering process.

The residual current is usually specified by multiplying the nominal capacitance value C _R in µF by the nominal voltage U _R in V, to which a small fixed value is often added. For example, here is a typical residual current formula:

{\ displaystyle I _ {\ text {Leak}} = 0 {,} 01 \, \ mathrm {\ frac {A} {V \ cdot F}} \ cdot C _ {\ mathrm {R}} \ cdot U _ {\ mathrm {R}} +3 \, \ mathrm {\ mu A}}

This value can be reached or fallen below within a prescribed measuring time of, for example, 2 or 5 minutes.

Tantalum electrolytic capacitors with solid electrolytes reach their typical residual current value after a relatively short switch-on time. However, during operation they do not provide any oxygen for healing defects in the oxide. Once a value has been reached, it remains at this value for approximately the entire operating time. In Ta electrolytes with liquid electrolytes, however, defects are healed. With these electrolytic capacitors, the residual current becomes smaller and smaller, the longer the capacitor is on.

The residual current of an electrolytic capacitor depends on the voltage and temperature. At 85 ° C it can reach ten times the value compared to the 20 ° C value. On the other hand, the residual current is about a factor of 10 smaller when the operating voltage is about 50% below the nominal voltage.

Recharge effect (dielectric absorption)

The dielectric absorption ( latin absorbere "aspirate, absorb") describes the dielectric properties of a non-conductor as a function of frequency . In the case of tantalum electrolytic capacitors, the effect is responsible on the one hand for the dielectric losses in AC voltage operation and on the other hand for the increase in the residual current when the electrolytic capacitor is switched on and for the occurrence of a voltage on the capacitor after the electrolytic capacitor has been switched off and discharged . This effect is also called the reload effect.

The voltage that can arise after switching off and discharging due to the dielectric relaxation at the connections of tantalum electrolytic capacitors can reach quite high values, see table.

Capacitor type	Dielectric absorption
Tantalum electrolytic capacitors with solid electrolytes	1 to 5%, 10%
Aluminum electrolytic capacitors with liquid electrolyte	about 10%

Notes on operation

reliability

The reliability of a component is a property that indicates how reliably ( failure rate ) this component will fulfill its respective function in a time interval ( service life ). It is subject to a stochastic process and can be described qualitatively and quantitatively; it is not directly measurable.

Failure distribution (bathtub curve)

With the so-called bathtub curve , the behavior over time of failures of a batch of similar components is divided into three areas. Only the range of the constant failure rate in which only random failures occur is used to specify a failure rate λ .

The temporal behavior of failures in a batch of similar components is shown as a so-called bathtub curve, which has three areas: 1) area of early failures, 2) area of constant failure rate (random failures) and 3) area of wear failures (change failures). In the case of tantalum electrolytic capacitors, early failures are mostly removed by the manufacturer during formation and subsequent screening processes. In the area of the constant failure rate, only "random failures" occur. This range applies to the specification of the failure rate λ . The range generally ends with the occurrence of wear failures (change failures). However, since there are no changes in the electrical parameters of tantalum electrolytic capacitors with MnO ₂ electrolytes, there are no wear failures. Therefore area 3) has no meaning for these capacitors.

Failure rate

The failure rate is a statistical value about the probable functionality of components in a time interval. It cannot be measured directly and is determined for tantalum electrolytic capacitors via the failures in the production-accompanying continuous voltage tests ( endurance test ), in which the components are tested with the applied nominal voltage at the upper nominal temperature. Both total failures ( short circuit , interruption) and change failures (exceeding characteristic value limits) are rated as failures .

The failure rate λ is obtained by dividing the failures C that have occurred by the number of test objects n multiplied by the test time t :

{\ displaystyle {\ text {Failure rate}} \ lambda = {\ frac {C} {n \ cdot t}}}

It indicates how many capacitors will fail on average in a unit of time and is given in 1 / time, i.e. failure per unit of time. As a statistical value, the failure rate is still at a confidence level ( confidence interval , confidence level subject), usually 95%. If the failure rate is constant, then the reciprocal value of the failure rate is the mean operating time until failure MTTF ( Mean Time To Failure ) and is used to calculate a survival probability for a desired device service life in combination with other components involved.

The failure rate λ depends on the temperature, the applied voltage, various environmental influences such as humidity, shocks or vibrations, the capacitance of the capacitor and, if applicable, the series resistance in the circuit. For this reason, the failure rate determined in the continuous voltage tests is converted to specific reference conditions. There are two definitions for this. For electrolytic capacitors with solid electrolytes, the internationally known and widespread definition of a reference failure rate λ _{ref (MIL)} according to MIL-HDBK-217F is mostly used. This set of rules also defines the reference failure rate

Failure rate λ _{ref (MIL)} in " n% failures per 1000 h at 85 ° C and U = U _R " and with a series resistance of 0.1 Ω / V

This standard comes from the military sector, but is also used in other industrial sectors.

The second definition of a reference failure rate is standardized according to IEC [DIN EN] 61709 and is mainly used in the industrial sector. The reference failure rate λ _{ref (FIT)} with the unit FIT ( Failure In Time ) is used here.

Failure rate λ _{ref (FIT)} in " n failures per 10 ⁹ h at 40 ° C and U = 0.5 or 0.8 U _R ".

To compare the numerical values, the respective reference failure rates must be converted to the desired value with the help of so-called acceleration factors. There are various models such as MIL-HDBK-217 F or Bellcore / Telcordia. The electrolytic capacitor manufacturers also provide their own calculation models, e.g. B. Vishay, Kemet and NEC / TOKIN.

Example of a conversion for tantalum capacitors with a basic failure rate of λ _{ref (MIL)} = 0.1% / 1000 h (85 ° C, U = U _R ) into a failure rate λ _{ref (FIT)} at 40 ° C and U = 0.5 U _R .

The conversion from λ _{ref (MIL)} to λ _{ref (FIT)} is carried out using correction factors taken from MIL-HDBK-217F:

λ _{ref (FIT)} = λ _{ref (MIL)} × λ _V × λ _T × λ _R × λ _B with

λ _U = voltage correction factor, for U = 0.5 U _R , λ _U = 0.1

λ _T = temperature correction factor, for T = 40 ° C, λ _T = 0.1

λ _R = correction factor for the series resistor R _V , with the same value = 1

λ _B = specified failure rate at U = U _R , T = T _max , R _V = 0.1 Ω / V

The specified failure rate of λ _{ref (MIL)} = 0.1% / 1000 h (85 ° C, U = U _R ) becomes

λ _{ref (FIT)} = 0.001 / 1000 h × 0.1 × 0.1 × 1 = 0.00001 / 1000 h = 1 · 10 ⁻⁹ / h = 1 FIT (40 ° C, 0.5 UR)

After manufacture, tantalum electrolytic capacitors are subjected to various additional tests in addition to 100% measurement of capacitance, impedance and residual current. After these tests, for example the surge current test, failed capacitors are sorted out. The different classes of failure rates can be achieved with screening methods of different severity.

Typical failure rates for tantalum electrolytic capacitors with approvals according to MIL-PRF-55365 is the specification of a failure rate in classes that are marked with a letter:

B = 0.1% / 1000 h, C = 0.01% / 1000 h, D = 0.001% / 1000 h

Commercially-shelf and available tantalum capacitors ( commercial off-the-shelf (COTS) ) have now very high military "C" achieved as standard products level are 0.01% / 1000 h at 85 ° C, and U _R . With the model according to MIL HDKB 217F, this is 0.02 FIT at 40 ° C and 0.5 U _R for a 100 µF / 25 V tantalum chip capacitor with a series resistance of 0.1 Ω.

In order to determine these already very low failure rates in the continuous voltage tests accompanying production, billions of component test hours are required. This requires a large amount of personnel and considerable financing. Even smaller numerical values can no longer be achieved with the help of tests. That is why failure rates are often mentioned that come from failure feedback from customers. These "field failure rates" are usually significantly lower than the failure rates determined in the tests.

lifespan

Tantalum electrolytic capacitors with solid manganese dioxide electrolytes have no specification of a " useful life " ( useful life, load life, service life ) in the data sheets because they do not have an operating time that is limited by drying processes or changes.

Tantalum electrolytic capacitors with liquid electrolytes also have no specification of a "service life" in the data sheets, provided the housing is hermetically sealed.

Causes of failure, self-healing and application rules

Cause of failure "field crystallization"

The tantalum electrolytic capacitors manufactured today and used in devices meet the high quality requirements of industry in almost all areas. They are reliable components whose failure rate is on the same low level as other electronic components. However, solid electrolyte tantalum electrolytic capacitors have an inherent failure mechanism called "field crystallization". More than 90% of the failures of tantalum electrolytic capacitors, which have become very rare today, are caused by field crystallization, which causes an increased residual current and can lead to a short circuit.

Burned out tantalum electrolytic capacitor

The cause of the field crystallization lies in the structure of the dielectric oxide layer of tantalum pentoxide. This extremely thin oxide layer must be in an amorphous structure. If the amorphous structure of the oxide changes to a crystalline structure, for example at a point contamination, a break in the oxide or an insufficiently formed point, the conductivity of the oxide increases by a factor of 1000 and the volume of the oxide increases. Such a punctual structural change in the tantalum pentoxide can lead to a sudden increase in the residual current from the order of magnitude of nanoampere to the ampere range within a few milliseconds. A punctual breakdown occurs, which has different effects depending on the degree of current limitation.

In the event of a punctual electrical breakdown in the dielectric, the defect heats up due to the short-circuit current. From around 450 ° C, the conductive MnO ₂ releases oxygen. Without current limitation, the defect can continue to heat up. From around 500 ° C, the insulating amorphous Ta ₂ O _{5 changes} into its crystalline form and becomes conductive. The current extends to neighboring areas. The reactive tantalum of the anode combines with the released oxygen to form Ta ₂ O ₅ , with further heat being released. This additional heat leads to an avalanche effect and can ignite the tantalum and cause the capacitor to burn. With current limitation, the heating is limited selectively and an avalanche effect is avoided.

self-healing

In the case of MnO ₂ -Ta electrolytic capacitors with current limitation, the conductive electrolyte MnO _{2 is} thermally converted into the insulating Mn ₂ O ₃ in the event of a punctual breakdown and the defect is switched off.

All electrolytic capacitors actually tend to self-heal their oxide layer in the case of spot contamination, oxide fractures or weakened oxide areas, provided the electrolyte can supply the oxygen to build up the oxide. However, in contrast to liquid electrolytes, solid electrolytes cannot supply oxygen to build up a new oxide layer. In addition, the field crystallization of tantalum electrolytic capacitors with MnO ₂ electrolytes is an inherent cause of failure that lies in the structure of the anode oxide and cannot be cured by building up a new oxide layer. Here only a current limitation can bring about self-healing. This has the effect that in the case of tantalum electrolytic capacitors with manganese dioxide electrolytes, in the event of a punctual breakdown in the dielectric, the fault does not heat up to more than 450 ° C due to the short-circuit current. At this temperature the conductive manganese dioxide MnO ₂ releases oxygen and converts to insulating Mn ₂ O ₃ . The flaw is isolated and the current stops flowing. This area then no longer contributes to the total capacity.

There is no risk of fire with polymer tantalum electrolytic capacitors. Field crystallization can also occur with the polymer-tantalum electrolytic capacitors, but in this case the polymer layer is heated selectively, whereby the polymer either becomes oxidized and high-resistance or evaporates, depending on the type. The defect is isolated. The area around the defect is exposed and no longer contributes to the capacitance of the capacitor.

Application rules

The effects of defects in the dielectrics in tantalum electrolytic capacitors with MnO ₂ and with polymer electrolytes lead to different application rules for these capacitors. The following table shows the relationships between these different types of capacitors.

Failure mechanisms, self-healing mechanisms and application rules for tantalum electrolytic capacitors.
Electrolytic capacitor type	Failure mechanism	Self-healing mechanism	Application rule
MnO ₂ tantalum electrolytic capacitor	Field crystallization	Isolation of imperfections by oxidation of MnO ₂ into insulating Mn ₂ O ₃ with current limitation	Voltage reduction 50%, series resistance 3 Ω / V or "soft-start circuit"
Polymer tantalum electrolytic capacitor	Field crystallization	Isolation of defects by oxidation or evaporation of the polymer electrolyte	Voltage reduction 20%

Tantalum electrolytic capacitors with liquid electrolytes do not require any special application rules.

More information

Parallel and series connection

Parallel connection of electrolytic capacitors

If, in a parallel connection of electrolytic capacitors, one specimen gets a short circuit, the entire circuit is discharged through this defect. In the case of larger capacitors with a high energy content, this can lead to quite large discharge phenomena. Therefore, in such a case, measures should be taken to limit the short-circuit discharge current. This can e.g. B. be done by individually protecting each capacitor via an overcurrent protection device.

Series connection of electrolytic capacitors

When electrolytic capacitors are connected in series or in series, there is a distribution of the total voltage over the individual capacitors connected in series, which results from the individual residual currents of the capacitors. In the case of different residual currents, after applying a voltage, an uneven voltage distribution results, which is inversely proportional to the individual residual current and can be quite large under certain circumstances. This can u. U. the maximum permissible voltage for individual copies in the capacitor bank may be exceeded. Therefore, in particular, larger electrolytic capacitors with a high energy content or for higher voltages must be balanced with balancing resistors or with active voltage balancing with push-pull transistors.

standardization

The conditions for the tests and measurements of the electrical parameters of the tantalum electrolytic capacitors for industrial applications are specified in the basic specification:

DIN EN / IEC 60384-1 (VDE 0565-1) , fixed capacitors for use in electronic devices

as well as in the frame specifications:

IEC / DIN EN 60384-3 - Surface- mountable tantalum capacitors with solid manganese dioxide electrolyte

IEC / DIN EN 60384-15 - Tantalum electrolytic capacitors with solid or liquid electrolytes

IEC / DIN EN 60384-24 - Surface- mountable tantalum electrolytic capacitors with conductive polymer solid electrolytes

In the USA, a number of standards have been created specifically for tantalum electrolytic capacitors, which mostly apply to capacitors with specified reliability and are therefore mainly intended for military and space applications:

MIL-PRF-39003 - Capacitors, Fixed, Electrolytic (Solid Electrolyte), Tantalum, Established Reliability, General Specification (Axial Style, Hermetically Sealed, CSR)
MIL-PRF-39006 - Capacitor, Fixed, Electrolytic (Nonsolid Electrolyte), Tantalum, Established Reliability, General Specification (Axial Style, Hermetically Sealed, CLR)
MIL-PRF-49137 - Capacitors, Fixed, Electrolytic (Solid Electrolyte), Tantalum, Molded, Conformal Coated (Pearl dipped) and metal Cased with Plastic End-Fill, Nonhermetically Sealed, General Specification (Axial and Radial Style CX)
MIL-PRF-55365 - Capacitor, Fixed, Electrolytic (Tantalum), Chip, Nonestablished Reliability, Established Reliability, General Specification (Chip Style CWR)

Circuit symbols

The electrical circuit symbols of electrolytic capacitors are standardized according to IEC / DIN / EN 60617-4.

Circuit symbols for electrolytic capacitors

polarized capacitor
polarized capacitor
polarized capacitor
Bipolar electrolytic capacitor

Type identification

If there is enough space, the capacitors should be labeled with the following:

Polarity, nominal capacity, tolerance, nominal voltage, nominal temperature range, date of manufacture, manufacturer, series designation

Uncoded labeling is possible for larger components. In the case of chip capacitors, however, this is not possible because of their small size. Capacity, tolerance and date of manufacture can therefore be marked with abbreviations in accordance with IEC / DIN EN 60062.

Example of a short designation of the nominal capacity with a unit symbol (microfarad):

µ47 = 0.47 µF, 4 µ7 = 4.7 µF, 47 µ = 47 µF

Example of a short designation of the nominal capacity with a number for the unit:

476

The first two digits indicate the value in picofarads, the third the number of subsequent zeros. 476 means 47 × 10 ⁶ pF = 47,000,000 pF = 47 µF.

The tolerance is marked with a letter:

K = ± 10%, M = ± 20%

The nominal voltage can be coded with a letter. There are no uniform rules here.

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

Version 1: Coding with year / week, "0708" is then 2007, 8th calendar week
Version 2: Coding with year code / month code

Year code: "R" = 2003, "S" = 2004, "T" = 2005, "U" = 2006, "V" = 2007, "W" = 2008, "X" = 2009, "A" = 2010, "B" = 2011, "C" = 2012, "D" = 2013, "E" = 2014, "F" = 2015, "G" = 2016 etc.

Month code: "1" to "9" = Jan. to Sept., "O" = October, "N" = November, "D" = December

"A5" is then 2010, May

The marking of the capacity and the voltage of tantalum electrolytic capacitors has not been color-coded since 1970.

Polarity marking

Marking of polarity for tantalum electrolytic capacitors

The positive pole of tantalum electrolytic capacitors is marked regardless of whether a solid or a liquid electrolyte is used.

SMD housing: The positive connection is identified by a bar (colored line) on the housing. This bar must not be confused with a minus sign.
In the vertical design (radial design, tantalum bead) the positive pole is marked with one or more "+" signs.
With the axial / horizontal design , the negative pole is connected to the housing, the positive pole is insulated. The positive pole can be identified by the fact that it is centered and led out of the mostly hermetic seal. On the positive side there is often a notch all around and sometimes one or more “+” signs. Occasionally, however, similar to "wet" axial aluminum electrolytic capacitors, a circumferential black line is used to mark the negative pole.

Applications

Typical applications for tantalum electrolytic capacitors with MnO ₂ electrolytes, especially the SMD designs, are the smoothing and backup capacitors in power supplies and DC / DC converters, especially in miniaturized devices, e.g. B. in mobile phones, laptops and in medical devices such as pacemakers and hearing aids, in vehicle electronics and in navigation devices and sensors. They can also be found in aerospace, military, and industrial equipment.

Tantalum electrolytic capacitors with liquid electrolytes are used in the aerospace industry, in military and medical devices as well as in oil exploration in geo-probes.

Technological competition

The ESR and ESL properties of tantalum capacitors, especially those with polymer electrolytes, are increasingly approaching those of MLCC capacitors. On the other hand, the specific capacitance of class 2 MLCC capacitors is approaching that of tantalum chip capacitors more and more. However, there are differences that speak for or against certain types of capacitors.

With this increasing comparability it becomes necessary to compile arguments for or against certain capacitor technologies. Here is a small selection of special comparisons for or against certain capacitor technologies:

Capacitors for switching power supplies: Kemet
Analog Circuit Capacitors: Analog Devices Inc.
MLCC Compared with Ta-Ekos, polymer electrolytic capacitors and “wet” Al electrolytic capacitors: Murata Kemet, AVX, Kemet / Texas Instruments
Ta-polymer electrolytic capacitors compared to Ta-MnO ₂ electrolytic capacitors: Kemet

market

The total market for capacitors in 2010 was around US $ 18 billion with around 1.4 trillion units. The market for tantalum electrolytic capacitors accounted for around US $ 2.2 billion (12%) and around 24 billion units (2%).

Tantalum capacitors in the SMD design could be found in almost all electronic devices in the flat design in 2012. They accounted for more than 80% of tantalum capacitor production, which is about 40% of the world's tantalum demand.

Within this design, size A takes up the majority with 45% of the number. The market leaders in tantalum capacitors are AVX Corporation and KEMET Corporation , both of which have roughly equal market shares. NEC is in 3rd place, the Vishay group takes 4th place.