High-Reliability Solid Tantalum Capacitors

Basic Construction

Solid tantalum capacitors are electrolytic capacitors, and all electrolytic capacitors are polar, meaning that current will only pass from the positive end (the anode) to the negative end (the cathode). The three major elements of an electrolytic capacitor are the anode, dielectric, and cathode. The tantalum anode consists of particles of very pure tantalum powder that are pressed and sintered into a sponge-like structure. Traditional versions have a tantalum wire embedded within the structure to create a positive connection to the circuit. The surface of the anode is covered with a layer of tantalum pentoxide (Ta2O5), which functions as the capacitor dielectric. In traditional solid tantalum capacitors, the cathode is manganese dioxide (MnO2). This material is deposited over the dielectric, followed by other materials — typically carbon and silver — to establish a connection with the other capacitor components.

Solid tantalum capacitors are frequently used in power supply filtering applications, which exhibit improved efficiency with lower capacitor resistance. To address the needs of these types of applications, AVX introduced the first molded, low-ESR, surface-mount tantalum capacitors — the TPS Series — in 1992. This series proved to be extremely popular and helped expand the market for tantalum capacitors. For instance, AVX and others soon incorporated surge testing into their production sequences to address concerns with power-on failures. Over the years, other product enhancements have also been introduced, including multianode tantalum capacitors (like the COTS-Plus TBM Series) for even lower ESR.

More recently, tantalum capacitors with conductive polymer counter-electrodes in place of the traditional MnO2 cathode system have become popular in consumer electronics and automotive applications, since polymer materials have a lower resistance than MnO2. However, there can be tradeoffs between providing low ESR and high reliability in solid tantalum capacitors. The stated reliability of conductive polymer capacitors is poorer than that of MnO2 based systems because the polymer materials lack the self-healing properties of MnO2 systems. This also causes the leakage of the polymer capacitors to be significantly higher than MnO2-cathode capacitors.

In demanding applications where low ESR and very long product life is critical, such as satellite electronics, low-ESR MnO2-cathode capacitors are still the preferred choice. AVX developed the space-level SRC9000 TBM Series multianode capacitors, which utilize proven MnO2 cathode technology to achieve the lowest possible ESR and incorporate the latest processing and testing techniques, Weibull burn-in and statistical screening for DC leakage, to remove any outliers from the population. This methodology ensures that the low ESR required for aerospace circuits is maintained over the long lifetime required for space platforms and has been successful in meeting the demanding needs of the aerospace industry. Capacitors produced in this way have been incorporated in most US space systems, including the Mars Curiosity Rover.

For more technical information about ESR, please reference “Equivalent Series Resistance of Tantalum Capacitors” by R.W. Franklin.

Direct Current Leakage

Traditionally, tantalum capacitor DCL has been specified as 0.01 x capacitance x voltage in microamps (µA). For example, the DCL of a 10µf, 10V capacitor would be:

0.01 X 10µF X 10V = 1.0µA

The most common commercial applications, such as cell phones, are easily recharged. So, capacitor leakage is not a major issue in these applications, and only becomes a problem if the capacitor fails catastrophically. However, in critical, implantable medical device applications, which are not as readily recharged as consumer applications, capacitor leakage is a real detriment to battery life. If a capacitor has high leakage resulting from dielectric defects, high-temperature reflow processes can aggravate the defects and significantly increase the capacitor’s leakage.

There are several factors that contribute to capacitor leakage, but the most important is the thickness of the dielectric. Tantalum pentoxide (Ta2O5) has a dielectric constant that is lower than other capacitor types, but it can be grown as a film over the surface of tantalum metal. This significantly contributes to the small size of tantalum capacitors, as it allows a very thin film to create a functioning dielectric layer. The Ta2O5 dielectric is created by passing current through the sintered tantalum anode in the presence of a liquid electrolyte. This process builds a layer over the surface of the tantalum particles, taking tantalum from the anode and oxygen from the electrolyte to form Ta2O5. The voltage applied during formation will determine the thickness of the dielectric layer. Thicker dielectrics have higher voltage handling capabilities and will have lower DCL at a given applied voltage than thinner dielectrics. In the capacitor industry, the dielectric thickness is usually expressed as the formation ratio compared to voltage. For example, a formation ratio of 3 would mean that the dielectric is formed at 30V for a 10V capacitor.

For high-reliability capacitors, AVX typically specifies a formation ratio of at least 3 to provide optimal reliability and low leakage. It is not unusual for commercial capacitors to have much lower formation ratios.

Reliability

Some electrolytic capacitors depend on a wet electrolyte, or on a level of moisture content in the cathode system, but these can dry out over time and lead to capacitor failure. The long-term reliability of these capacitor types can be illustrated with the familiar bathtub curve, which shows an eventual increase in failure rate as the cathode system degrades. Tantalum capacitors with conductive polymer cathodes and aluminum capacitors with wet electrolytic systems exhibit this wear-out mechanism.

Figure 6: The “Bathtub Curve” illustrates the reliability of a component with a wear out mechanism.

Unlike those technologies, solid tantalum capacitors with MnO2 cathodes have no wear-out mechanism. The cathode is a solid system that is not subject to degradation over time, and the self-healing mechanism exhibited by this technology ensures that small dielectric defects do not lead to catastrophic failure in normal operation. The result of these factors is a failure rate that improves over time, and is a major reason that MnO2-based tantalum capacitors are popular in aerospace and medical life-support applications.

Figure 7: Solid tantalum capacitors with MnO2 cathodes have no wear out mechanism and improving reliability over time.

Although tantalum capacitor manufacturing processes have significantly improved over time, a small number of weaker parts can exist in any production batch. As mentioned, preconditioning (i.e., burn-in) methods are used to identify and remove these infant mortals from the population. The type of burn-in varies depending on intended applications, but normally includes high temperatures and/or voltages above the capacitors’ ratings. For commercial capacitors, the burn-in time is typically short to minimize cycle time and cost. The traditional method of burn-in for high-reliability tantalum capacitors is known as Weibull burn-in due to the statistical method used to calculate reliability. These test conditions are highly accelerated (e.g., 85°C and a multiplier of rated voltage) and failures are counted over time. Using the Weibull distribution as the assumed model of batch behavior, the calculated reliability must reach a predetermined level and the failure rate must be declining. This system has been used for many decades to characterize high-reliability capacitors, and is used to state reliability as a maximum failure percentage per thousand hours of operation at rated voltage with a 90% confidence level. The reliability level of commercial capacitors is normally projected at 1% per thousand hours, while high-reliability capacitors typically offer reliability ranging from 0.1% to 0.001% per thousand hours with 90% confidence. However, it is important to note that the actual reliability exhibited within intended applications is much better than projected by burn-in methods, and is typically expressed in very low failures-in-time (FIT) rates.

Reaching such high reliability levels requires that infant mortals are identified and removed from the population in the early stages of burn-in, as, after that, the failure rate declines sharply. Tantalum capacitors that are reliability graded through Weibull burn-in are successfully employed in the most demanding applications in the electronics industry, including satellites and implantable medical devices.

After many years of supplying high-reliability tantalum capacitors to the medical device industry using Weibull burn-in for reliability grading, AVX explored other potential options. The primary reason for this shift was that, as our production processes improved, it became more difficult to create the infant mortals required for Weibull grading. Voltage acceleration factors had to be increased to generate failures, but that caused issues with capacitors’ electrical parameters. Since most customers in the medical device industry use 125°C system-level burn-in, we explored using capacitor-level burn-in at that condition with lower voltage acceleration and discovered that 125°C burn-in at near-rated-voltage, in conjunction with statistical screening for other parameters, is as effective as Weibull at removing infant mortals, but produces a lower DCL distribution than Weibull. This led to the development of our patented Q-Process™. For detailed information about the Q-Process, please refer to “Reaching the Highest Reliability for Tantalum Capacitors” by James Bates, Marc Beaulieu, Michael Miller, and Joseph Paulus.

Additionally, the traditional derating rules for solid tantalum capacitors — that capacitors be used at 50% of their rated voltage to achieve optimal reliability — do not have to be applied to Q-Process capacitors. Capacitors produced with the Q-Process can be operated at full rated voltage, which allows for the selection of smaller capacitors with lower rated voltages, in most applications.

Figure 8: AVX Q-Process reduces or eliminates the need for voltage derating.