Tantalum Polymer Capacitors Achieve Higher Voltage Ratings

Aug 1, 2008 12:00 PM
By Jayson Young, Technical Product Manager, KO-CAP/AO-CAP; Jake Qiu, Senior Scientist, Conductive Polymer Development; and Randy Hahn, Development Director, Tantalum Materials, KEMET Electronics, Simpsonville, S.C.

Tantalum surface-mount capacitors have gained widespread favor for bulk decoupling use in both conventional and switch-mode power supplies (SMPSs) since their introduction more than 20 years ago. Today, tantalum surface-mount capacitors are primarily used in SMPSs across multiple industry segments, most often in applications that have space restrictions, long stable life expectancy and high-reliability requirements. The characteristics of high-volumetric efficiency, stable performance and the absence of a wear-out mechanism continue to drive their popularity in SMPSs, despite aggressive competition from other alternative dielectric materials such as aluminum and ceramic.

In addition to their benefits, tantalum capacitors have traditionally had two weaknesses — a susceptibility to ignition when they fail and higher equivalent series resistance (ESR) than capacitors based on other dielectrics. These drawbacks were overcome with the introduction of a new cathode material — an intrinsically conductive polymer — as a replacement for the conventional manganese dioxide (MnO₂) cathode. But while surface-mount tantalum capacitors built using the MnO₂ cathode could be used at voltages up to 28 V, the tantalums built using polymer cathode material were previously only usable up to 19 V.

However, recent advancements in polymer technology have permitted development of tantalum polymer capacitors for continuous duty at 20 V to 28 V, which enables them to address a wider range of power-supply input requirements. To assess the usability of the new tantalum polymer capacitors in power-supply applications, their electrical performance and reliability are compared against existing MnO₂ tantalum capacitors.

Tantalum Pros and Cons

All capacitor technologies have their advantages and disadvantages. Issues such as voltage coefficient and the potential cracking of high-capacitance ceramics or the dry-out concerns and incompatibility of aluminum electrolytics to reflow temperatures are weighted and compared against their advantages to arrive at a technology solution that best meets the needs of the power-supply design. Primary among the disadvantages of tantalum capacitors in SMPS use are the potential for an ignition failure mode and higher ESR when compared to some alternative dielectrics.

Both these disadvantages are linked to a single construction material within the tantalum capacitor, namely the use of MnO₂ as the cathode. High amounts of oxygen present in the MnO₂ cathode can provide a localized fuel source that, under the right failure conditions, can result in an ignition failure.

Also, since MnO₂ is a semi-conductive material, it is the largest contributor to the component's total ESR value. It is these two characteristics that most often lead power-supply designers to consider other dielectric solutions.

In the late 1990s, however, the undesirable features of tantalum surface-mount capacitors were overcome by the introduction of intrinsically conductive polymer as a replacement for the MnO₂ cathode. The use of conductive polymer offered a new material set 1000 times more conductive than MnO₂ with an absence of readily available oxygen that could potentially lead to an ignition failure.

With the risk of ignition failure addressed and ESR values significantly lower than any offerings coming from a traditional MnO₂-style tantalum capacitor, tantalum polymer capacitors rapidly gained popularity throughout the design community as engineers quickly took advantage of this new technology to replace MnO₂ tantalum capacitors and multilayer ceramic chip (MLCC) capacitors on the power supply's output rails.

Yet another advantage gained with the removal of MnO₂ was the improvement in the voltage derating requirements of tantalum surface-mount capacitors. To fully optimize the reliability of the tantalum dielectric (Ta_2O5), a voltage derating is recommended. Over many years of reliability analysis, the tantalum capacitor industry, in conjunction with reliability experts from the military sector, concluded that a 50% voltage derating of MnO₂ tantalum capacitors yielded acceptable reliability even in the most demanding applications such as aerospace.

Extensive studies concluded that the primary contributor to failures was damage induced on the dielectric during the board mounting process.^[1-2] This damage was the result of coefficient of thermal expansion mismatches in the material sets, which placed mechanical stresses on and produced fault sites in the dielectric. The physical properties of the MnO₂ play a role in this since the material is rigid and in direct contact with the dielectric, offering no protection from the expansion and contraction of the other materials around the anode.

When the MnO₂ was replaced with the softer and more elastic conductive polymer, researchers found that the dielectric condition after board mounting was much improved and the applied voltages could be greatly increased with no negative impact on predicted reliability.^[2] Over time, the recommended voltage derating for tantalum polymers was established at 20% voltage derating or less (depending on manufacturer and voltage rating). The predicted reliability of the tantalum polymer capacitor with a 20% derating was equal to that of a MnO₂ tantalum capacitor with a 50% derating.

While multiple improvements were realized with the replacement of MnO₂ with conductive polymer, tantalum polymer capacitors did have at least one weak point with regard to SMPS applications, which was the inability of manufacturers to produce a reliable design for working application voltages much above 19 V. This limited the use of tantalum polymer capacitors to the lower-voltage output applications of SMPS.

At the time tantalum polymer capacitors were introduced, MnO₂ tantalum capacitors were being safely used in voltage input applications up to 28 V. With an industry demand for higher-voltage tantalum polymer capacitors, manufacturers quickly began development activities to provide solutions for the 20-V to 28-V input voltages to replace MnO₂ and high-capacitance MLCCs with these higher-performance solutions. Despite industry pressures for a higher-voltage rating in tantalum polymer capacitors, technical challenges prevented capacitor manufacturers from delivering the higher-voltage ratings.

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