Military/High Reliability/Fail Safe
Some practical improvements on the basic Ta-MnO2 capacitor technology have been made, and mechanisms for mitigating or at least quantifying the risk of failure are available. Products designated as military and procured under a MIL-spec part number are produced and tested according to the prescriptions of the referenced MIL-spec, which typically include lot testing and screening procedures to establish a statistical assurance of reliability. MIL-specs often also call for a (non-RoHS compliant) lead bearing terminal finish, which benefits system reliability overall due to reduced risk of tin whisker formation and lower peak temperatures during assembly. High reliability parts are often built as MIL-spec materials with different labeling and terminal finish, but may also incorporate technological improvements not yet adopted by MIL-spec governing bodies. In any event, Hi-Rel product worthy of the name will have been screened, tested, and/or burnt-in to offer a statistical assurance of reliability. Fail-safe devices incorporate some type of fusing mechanism, in order to convert short-circuit failures into open-circuit failures before they can progress into open-flame failures. These mechanisms are not perfect, but they do reduce the risk of incendiary failures by a few decimal places.
Tantalum polymer capacitors dispense with the manganese dioxide altogether and use a conductive polymer as the cathode material instead, which pretty much eliminates the risk of pyrotechnic failures. Due to the lower resistance of the polymer materials used relative to MnO2, tantalum polymer caps generally have better ESR and ripple current specifications, as well as better performance at high frequency relative to their MnO2-based counterparts. The disadvantages of polymer cathode systems include a more limited temperature range, greater sensitivity to moisture, and a reduced efficacy of self-healing that contributes to higher leakage currents.
Wet tantalum capacitors, as the name suggests, use a liquid electrolyte in their cathode systems. Since it’s hard to solder to a liquid, a cathode counter-electrode is needed to complete the circuit through the sintered tantalum anode slug, and the design of this counter-electrode is one of the differentiators among different lines of wet tantalum devices. Modern devices use hermetically sealed/welded tantalum cases, which are less prone to electrolyte leakage and more tolerant of incidental voltage reversals than earlier devices incorporating a silver case material and elastomer seal. The chief advantages of wet tantalum devices are their reliability and relatively high specific capacitance; the liquid electrolyte provides a continuous self-healing action for the dielectric, leading to low leakage currents and a higher range of applicable operating voltages. Due to the resistance of the liquid electrolyte however, the ESR of most wet tantalums is not particularly good, resulting in loss of capacitance at relatively low frequencies. Wet tantalums are also quite costly, roughly 100x that of an aluminum electrolytic device of comparable ratings. Taken together, these factors render wet tantalums something of a niche technology, found mostly in the sort of applications where failure is not an option and money is not an object; space/satellite applications, life-critical avionics systems, etc.
Failure mechanisms and design considerations
For Tantalums in General
The dominant cause of dielectric faults in tantalum capacitors is impurities in the tantalum powders from which the anode slugs are formed. Like the gaps that occur when highway crews don’t bother to move roadkill out of the way when painting lines on roads, impurities in the tantalum result in flaws in the dielectric layer. Since the dielectric in a tantalum capacitor is only a few nanometers thick to start with, even very small impurities can cause problems.
Other dielectric faults in tantalum capacitors are mechanically induced. Being a somewhat brittle, glass-like substance, the tantalum pentoxide dielectric is prone to fracturing when mechanical stress is applied. Particularly significant are thermal expansion stresses during soldering operations when parts are assembled onto a board. Because these stresses can cause faults that did not exist (and are therefore undetectable) at the time of production, failure of tantalum capacitors on first application of power following assembly is a known phenomenon. Because of the softer, more pliable nature of polymer cathode materials (and obviously, liquid cathodes) relative to manganese dioxide, these types have an advantage over MnO2-based capacitors in terms of infant mortality for this reason.
For MnO2-Based Devices
The self-healing mechanism at work in Ta-MnO2 capacitors is based on thermal decomposition of the MnO2 material into the much less conductive Mn2O3. When the leakage current near a fault site causes the local temperature to rise high enough, the region of MnO2 cathode material supplying current to the fault breaks down, thus insulating the fault from further current flow. Unfortunately, this process generates loose oxygen: 2(MnO2) + (energy) --> Mn2O3 +O. The difference between a successful self-healing event and a pyrotechnic failure is whether or not this oxygen finds tantalum metal at a high-enough temperature to auto-ignite. Ambient temperature and the amount of electrical fault current available to cause ohmic heating at a fault site are both factors that influence the outcome.
Mn02 Design Considerations
While a careful study of manufacturers’ application literature is recommended, the following guidelines regarding the application of Ta-MnO2 capacitors are offered for the impatient:
- Use series resistance: 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. 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.
- De-rate voltage: To (significantly) increase steady-state reliability, de-rate devices by half from 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, as little as 20% de-rating may suffice. A further (compounding) de-rating factor for temperature is recommended, increasing linearly from 0 @ 85°C to 33% @ 125°C, though high-temp product series may differ.
- 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.
For Polymer and Wet Tantalums
When they do fail, tantalum polymer capacitors tend to become a warm resistor, rather a rapidly expanding cloud of hot gasses and shrapnel. Because of this and the reduced risk of assembly-induced defects, their application rules of thumb are a bit simpler: de-rate voltage by 20%, observe recommended ripple current limits, and follow the manufacturer’s recommended de-rating schedule at elevated temperatures.
For wet tantalums, the sort of applications that can justify the cost of the parts are also likely to require a detailed reliability analysis of the system on a part-by-part basis, rendering rules of thumb less valuable than they might be in other applications. With that, a 20% standard de-rating factor is suggested, and users are advised to be mindful of the relatively low frequency response characteristics common among these devices.