Solid tantalum capacitors offer infinitely useful lifetimes and unparalleled volumetric efficiency, making them a smart choice for high-reliability applications including military, aerospace, and medical devices.
Solid tantalum capacitors are among the most popular types of small, surface-mount capacitors for electronic applications across the consumer, automotive, aerospace, and medical device markets. This paper will provide some context on the development of tantalum capacitor technology and address issues frequently faced by users, including the need for low equivalent series resistance (ESR) in filtering applications and the need for the highest possible reliability and long-lifetime performance in aerospace and medical applications.
A HISTORY OF TANTALUM CAPACITORS
The history of solid tantalum capacitors started in the 1950s when the concept was invented by Bell Labs to provide small capacitors to accompany their transistors. The structure of a tantalum capacitor is sponge-like, with a very high surface area available for dielectric formation. This provides high capacitance in a much smaller package than other capacitor technologies.
Over the course of the following decades, tantalum capacitor technology evolved to include several form factors. Axial- and radial-leaded configurations that are compatible with automated insertion processes for through-hole technology dominated until the 1980s. At that time, assembly technology evolved to embrace surface-mount technology (SMT) and SMT tantalum capacitors of various sizes were developed and widely adopted.
The widespread employment of surface-mount technology exposed tantalum capacitors to conditions outside of those experienced by their axial- and radial-leaded counterparts. High-temperature, board-level reflow processes can initiate defects in the thin-film dielectric layers of tantalum components, and can occasionally lead to catastrophic failure and even ignition.
As a result of this experience, tantalum capacitor manufacturers adopted in-line reflow conditioning and other measures to help the capacitors survive mounting and reflow conditions. These efforts have been successful at mitigating the risk of catastrophic failures, enabling solid tantalum capacitors to provide reliability that is suitable for any application.
Demand for tantalum capacitors grew dramatically with the development of reliable SMT versions in the late 1980s and continued into the 1990s. Surface-mount solid tantalums were the smallest capacitors in the 1–100μF range, and became the industry standard for many applications. The introduction of cellular phones and personal computers, along with the incorporation of extensive electronic equipment in automobiles, were the primary market drivers of this growth.
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 the 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.
The MnO2 cathode has a property that significantly contributes to tantalum capacitor reliability. Defects in the Ta2O5 dielectric cause local heating at the defect site during capacitor operation, which changes the nearby MnO2 to Mn2O5 — a non-conductive phase of manganese oxide. This non-conductive site serves to remove that portion of the capacitor from the circuit, effectively correcting the defect. This characteristic is called self-healing, and it allows tantalum capacitors with MnO2 cathodes to have a declining failure rate over time. Throughout their history, tantalum capacitor manufacturers have performed preconditioning (i.e., burn-in) through exposure to elevated voltage and temperature intended to facilitate this self-healing characteristic and remove weaker parts from the population. For more information about the construction of solid tantalum capacitors, please refer to “Basic Tantalum Capacitor Technology” by John Gill.
Most surface-mount tantalum capacitors are constructed as shown above, but there are also versions that use a conformal epoxy coat as the outside surface. These can be somewhat smaller than molded capacitors, but at the cost of reduced mechanical strength. To optimize use of available board space, AVX developed and patented the TACmicrochip® construction in 1995. This construction has a tantalum wafer substrate with tantalum powder pressed and sintered onto the surface. Individual anodes are defined on the surface using a sawing operation, and the wafers are processed through dielectric formation and cathode deposition. Next, a lid is placed over the structure and epoxy is flowed into the channels between anodes. This is followed by a dicing operation that separates the capacitors, which then proceed through burn-in, test, and packaging processes. The TACmicrochip and its high-reliability counterpart, the COTS-Plus TBC Series microchip, have become very popular in applications where space is at a premium, including handheld electronics and implantable medical devices.
EQUIVALENT SERIES RESISTANCE
The inherent equivalent series resistance (ESR) of tantalum capacitors is higher than some competing technologies. AVX and other manufacturers in the industry have done much to address this issue.
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.