Tantalum and Niobium capacitors are belonging to electrolytic capacitor types and they are known for its high capacitance in small dimensions (high energy and power density), reliability and stability of its parameters.
Traditionally, tantalums’ advantages over aluminum electrolytics capacitors have been found in terms of capacitance per volume, parameter stability over temperature, and longevity; tantalums in general do not suffer from dry-out problems or issues of dielectric degradation when stored discharged for long periods of time. However, tantalums are generally more costly, have a more limited range of available capacitance and voltage values.
Cross section schematics of typical SMD tantalum chip construction is shown in Figure 4. and 5.
Tantalum metal has a dielectric constant of approximately 27. The price for solid tantalums is relatively high due to mainly the high cost of the tantalum powder. The solid electrolyte theoretically has no limitations in operation and storage time. The capacitance range extends up to some thousands µF. The capacitors are characterized by a high CV product per volume unit, especially when high CV powders above 200k CV/g are introduced for low voltages and above 80k CV/g for higher voltages.
The conventional solid electrolyte is MnO2, however conductive polymer types are more and more favorite due to its lower ESR and reduced ignition features. On the other hand the MnO2 types are more robust against thermo-mechanical stress, stable electrical parameters under high temp & high humid environment, thus high reliability and longer operation life time applications still uses mostly established reliability MnO2 types as the main tantalum solid capacitor technology.
Physically, the capacitor also has a high density. Thus, lead mounted components always should be fixed mechanically against their substrate. This type requires large margins to the forming voltage that usually amounts to 2 to 4 times VR. For automotive and other high reliability applications ratios higher than 4 is required while for consumer electronics the ratio around 2 may be quite sufficient. The higher ratio favors not only higher reliability but also a better capacitance stability.
On the other hand higher VF/VR ratios means a correspondingly lower volumetric efficiency (capacitance is inversely proportional to the dielectric thickness). The maximum VR stops at 125 V DC for capacitors with polymer electrolyte and 100V DC for MnO2 types. However derating rules are different. It is recommended to derate MnO2 types by 50% for current surge circuits (directly on battery or DC/DC input) and 20% for non-high surge current applications (DC/DC output, timing, coupling/decoupling …). Tantalum polymer derating is recommended 10% for up to 16V capacitors and 20% for >16V capacitors for all kind of applications.
The capacitance stability is good (∆C/C ≤ ± 5%) and the tolerances range from (±5) ±10% to ±20%. The reverse voltage should, at a maximum 85 °C be limited to the least value of the alternatives: 10 % of VR or 1V. The continuous reverse voltage operation is not recommended, however small uneven spikes, such as diode overshoot voltage is acceptable. Chip designs are becoming very common and are well documented, with a functioning international standardization some basic case sizes have been established. However the market need and manufacturers feedback was faster then standardization and there are number of other case variants with different letter codes per manufacturing that may be very confusing in some cases – as the same letter from one manufacturer has different dimensions to the other.
As an example AVX X case is 1.5mm low profile variant of standard D case 7343 chip (AVX X case is equivalent to KEMET’s W case), however KEMET X case is a taller 4.3mm variant of standard D case 7343 part. (KEMET X case is equivalent to AVX E case).
The case size variants from manufacturers on market is in fact so wide that they are now using nearly complete alphabet options and began to use also numbers or combination of letters and numbers for different case size dimensions. The following table shows the common few sizes of a series that has acquired international acceptance under the same letter code:
SMD Chip Construction Variants
The most common construction design shown on Figure 5. is called J-lead type as per the termination shape. There are also some other construction designs on the market – see Figure 6. below.
J-lead – most common, cost effective tantalum capacitor design with J-lead robust termination that works like a springs absorbing mechanical shocks and vibrations
Undertab (facedown) – termination placed at the bottom of the case. This allows high density board mounting (no solder on capacitor sides) and also maximize its energy density / maximum capacitance and CV by inner space for larger anode. It does not allow solder quality visual inspection so this solution is not preferred by high rel applications.
Diced design – such as TACmicrochip on figure below – proprietary AVX construction combining tantalum wafer and dice through technology allowing miniaturization down to 0201 case size. Diced designs are available also by other vendors such as Kemet/NEC or Vishay – most in combination with undertab configuration to maximize its CV.
Conformal – original Spraque design – Vishay/Nichicon (AVX now) of dipped coated tantalum anodes. Advantage is in large, high CV anode, unmolded package is however not so flat with potential high speed pick and place issues.
Failure modes, Self-healing, Burn-in, Acceleration Factor (AF) and Derating
At first it has to be said that tantalum capacitors have been developed are representing one of the most reliable capacitor technology, historically linked with high rel applications. Nevertheless, solid tantalums sometimes are randomly stricken by short circuits. If the circuit has a high impedance it may happen that the failure disappears. A kind of self-healing has occurred. This is called sometimes a “scintillation”. Under certain conditions this kind of self-healing may occur also in circuits where the impedance is low. The reasons for this type of failure have been the subject of comprehensive researches and tests. After the forming the oxide film contains a number of weak sites caused by impurities in the tantalum metal or at sites of crystalline defects. If a voltage is applied gradually these sites will be the place for local higher currents that generated heat and heal the defect site. This is true for both MnO2 and conductive polymer while the mechanisms and effectiveness are different.
The local current generates a heat that creates temperatures in the micro spot of 400 to 500 °C. At that temperature the manganese dioxide in the case of MnO2 types is reduced to a lower degree of oxide (Mn2O3) with a several orders of magnitude higher resistivity. An insulating “patch” is formed over the leakage site, the local leakage current is choked and the capacitor self heals. This type of self-healing is provoked during the production phase by a voltage load, a so called voltage aging, that for every eliminated leakage site successively forces the leakage current down.
MnO2 self-healing considerations:
microscopic localized mechanism with “no” influence to Capacitance, ESR
“no” impact to DCL
100% hard surge current screening mandatory for ESA/MIL
self healing and capacitor recovery may be limited in combination of following factors:
high circuit resistance circuits >100kOhm
aggressive reflow (=higher thermal damage to dielectric)
very high derating >70%
MnO2 Tantalum Capacitors Design Considerations
While tantalum self-healing process is bringing high reliability for the MnO2 based tantalum capacitors, the ignition if-overloaded, pose an important point for a careful evaluation of its circuit applications. The following are general guidelines regarding the application of Ta-MnO2 capacitors:
Use series resistance – if applicable: limiting the external current available to a fault greatly reduces the chance of a fault site reaching the critical ignition temperature. Historically a series resistance of 1 to 3 ohms per applied volt has been recommended, this is not any more required. Modern designs may not tolerate this much ESR, and larger devices may contain sufficient electrical energy when charged to self-ignite should a fault suddenly appear. In these cases, de-rating and device screening are particularly important.
Derate voltage: Derating is the most effective way to increase steady-state reliability, derate to 50% of rated voltage, and as much as 70% when series resistance is extremely low, on the order of 0.01 ohm per applied volt or less. If currents are externally limited – in low/no surge applications, 20% derating may be sufficient.
Burn-in carefully: Many tantalum failures occur on first power up of an assembled device, as a result of assembly-induced dielectric faults. Facilitating successful self-healing through gradual application of voltage via a current-limited source may avert some of these failures. Subsequent exposure to maximum expected electrical and environmental stresses will serve as a proof test, since Ta-MnO2 capacitors that survive a given set of stresses once are likely to survive them almost indefinitely.
Limit transient current: Current flows in excess of the manufacturer’s stated surge current limit are to be avoided, including those arising from non-routine events, such as hot-plugging of batteries or power supplies, short-circuit faults of system outputs, etc. In absence of a surge current specification, a value Imax<Vrated/(1+ESR) has been suggested.
Observe ripple current/temperature limits: Ripple current ratings are typically based on the amount of ripple required to produce a given rise in device temperature above ambient. Excepting cases where the resulting waveforms would violate voltage or surge current limits, ripple current limits are a thermal management issue. Evaluate the test conditions under which the datasheet ripple limit figures are specified, and adapt those limits according to actual application conditions.
Polymer Capacitors Self-healing and Derating
There is some self-healing effect also on tantalum polymer types as due to the high spot temperature the conductive polymer may evaporate / peel off (delaminate) or oxidize to high insulating state, thus the failure spot is eliminated. In effect, the capacitor reduce its high leakage through the defective site. In comparison, the self-healing on MnO2 is still considered as more effective, especially at lower current. On the other hand the self-healing in MnO2 parts is releasing some free oxygen that may extend the ignition failure mode. This is also one of the reason for higher derating (50%) of MnO2 types for high current circuit applications.
Polymer capacitors are much more robust against ignition and surge failures compare to MnO2 types. It is thus also reflected in its application derating recommendations usually as low as 10% up to 16V capacitors and 20% for >16V. See below applied voltage vs component rated voltage recommendation table:
polymer self-healing by evaporation
polymer self-healing by oxidization
ESR can be reduced by change of MnO2 solid electrolyte to more conductive polymer material, however there are also other ways to reduce ESR by change of the construction and anode shape.
One way to reduce the ESR is to shorten the current path to the capacitor elements in the center of a pellet. Several methods to achieve this are shown in the following schematic of different designs. In Figure 9. design example No. 2 the rough surface of the coated anode is covered with the outer silver paint. Compared to the conventional anode in design No. 1 the average distance between the equipotential silver paint plain and the dielectric is reduced which means a reduced ESR.
A further ESR reduction is achieved by dividing the tantalum pellet in several elements that are connected in parallel – so called multi anode technology – where the current paths are reduced considerably and the conductive paint surface is even larger than in design No. 2. The method is introduced in recent years and will certainly be developed further. Roughly the ESR is inversely proportional to the number of elements in parallel.
Note: When improvement of “ESR” is mentioned we mean ESR value measured at standard measurement conditions = 100kHz “high frequency” important for SMPS output capacitor performance. The multiple anode or rough anode design is in fact using a “skin” effect that improves ESR at higher frequences while ESR at lower freq. such as 120Hz is not impacted or slightly higher as it is driven mostly by dielectric losses.
The ESR (skin effect) depends also on the layers coating the manganese dioxide electrolyte or conductive polymer. First the buffer between the electrolyte and the silver paint, i.e., the carbon layer, then the silver paint that is bonded to the leadframe by a silver adhesive. Layer thickness and distribution, material and treatment during production, all contribute to a low ESR that has to remain low even after environmental exposure.
Solid tantalum electrolytics are considered as being able to stand more reverse voltage than wet electrolytics. The diode function is in other words not equally evident. The diode in the equivalent circuit ought to be supplemented with a Zener-diode. What, then, the Zener voltage is varies according to certain short time investigations from 15 to 40% of VR, depending on lot and manufacturer. Long term investigations point to more conservative values. This is underlined by the fact that advanced powder technology has increased the sensitivity to reverse voltages. Some leading manufacturers may have different recommendations for reverse voltage. If nothing is said about reverse voltage limitations, we should for high reliability applications choose the least of applicable values from following alternatives:
Temp. range & Derating
-55/+25°C: 15% VR,
+25→+55°C: 15%→ 10% VR,
+55→+85°C: 10%→ 5% VR,
+85→+125°C: 5%→ 1% VR,
of short duration maximum 1 V,
continuous operations maximum 0.5 V
Solid Tantalum Capacitors Properties
Capacitance versus temperature
Capacitance versus frequency
The higher the porosity and the finer the powder and worse conductivity (MnO2) the sooner the capacitor starts loosing capacitance due to the increasing time constant of the farthest in localized capacitor elements in the pellet. On the contrary, when the ESR is reduced the time constants decrease. That means that the so called capacitance roll-off is moved toward higher frequencies and the capacitance decrease will be correspondingly smaller. The effect of multianode is demonstrated in the following figure on MnO2 part that shows examples from 470 µF / 6.3 V tantalums of different designs. The impact of multianode construction for polymer tantalum is not so big change in capacitance vs frequency characteristics (as it is already more-less flat), but to the absolute ESR value.
There is practically no ESR temperature dependence on polymer electrolyte and small dependence on manganese dioxide electrolyte – see Figure 14.
ESR versus temperature
ESR versus frequency
Tan δ DF versus frequency
Note the difference in frequency dependence of the two loss characteristics ESR and Tan δ in Figures 15. and 16. The influence of the linearly decreasing ESR in Figure 15. is visible at lower frequencies in Figure 16. but becomes more and more negligible when the frequency increases, just as the formula predicts: Tan δ = ESR x ωC.
DCL Leakage current versus ambient temperature
The MnO2 DCL specification limit for standard parts is usually given as:
DCL = 0.01 x CV
,where C=capacitance, V=rated voltage … thus 100μF 10V MnO2 capacitor has a DCL standard limit as 10μA at rated voltage. In practice, by rule of thumb, typical leakage current is about ten times lower, so in real measurement we can expect DCL to be around 1μA for the 100μF 10V part.
The basic DCL leakage current level on MnO2 capacitors is typically 10x lower compare to the polymer capacitors. Thus for polymer tantalum capacitors, DCL specification limits follow:
DCL = 0.1 x CV
the 100μF 10V polymer capacitor has a DCL standard limit as 100μA and typically we can measure around 10μA at room temperature.
Important note is that due to the continuous improvement leakage current limits may be specific to series and manufacturer. Low leakage series or consumer highest CV series (with higher DCL) are available on the market that differ with DCL specification limits to the above general equations. It is always a good idea to check the manufacturer specification datasheet.
Normalized DCL leakage current plot as shown in Fig 17. is valid for both MnO2 and polymer types, however the absolute DCL values differ !
Leakage current versus voltage
The Figure 18. above shows drop of the DCL with applied voltage (valid for both MnO2 and polymer types). By rule of thumb, DCL drop in one range with 50% derating. So taking an example of the 100μF 10V MnO2 capacitor with a DCL limit as 10μA at 10V rated voltage, we can assume the maximum leakage current values as 1μA at 5V applied and typical values would be around 0.1μA.
Leading manufacturers are providing now high fidelity simulation tools, where electrical characteristics of CAP, ESR, IMP, DF, DCL and S-parameters can be followed for individual capacitor part number.
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2.Polymer and Manganese Dioxide MnO2Solid Tantalum Capacitors