Tantalum Capacitor History Part 3: The Conductive Polymer Cathode

2025-01-20

Philip Lessner, James Q. Chen, and Hideaki Sato

This post is based on a presentation given at the Tantalum and Niobium International Study Center (T.I.C) 65^th General Assembly in Tokyo, Japan, September 2024. In the last post we traced the technological progress of Solid Tantalum capacitors based on the MnO₂ cathode which by the mid 1990s were showing their limitations as digital circuit speeds continued to increase. In this post, we'll look at a fundamental technology breakthrough from the 1970s that resulted in significant improvement in the performance of solid Tantalum capacitors and continues to make them a relevant technology in the 21^st century.

Intrinsically Conducting Polymer Cathodes

In the late 1970s a startling discovery was made: polymers which had heretofore been considered as insulating materials could be electrically conductive, if they had conjugated structures and were doped with certain molecules. The discovery of polyacetylene by Shirakawa, et al. ¹ opened a whole new field of conductive materials. Shirakawa, MacDiarmid, and Heeger were awarded the Nobel Prize in 2000 for this discovery (Figure 1).

Figure 1: The 2000 Nobel Prize in Chemistry

Conductive polymers, while not as conductive as metals, were orders of magnitude more conductive than inorganic semiconductors such as manganese dioxide (Figure 2).

Figure 2: Conductivity of Cathode Materials

Conductive polymers shared a key property with manganese dioxide in that they became non-conductive (or ‘burned away’) when they were heated to high temperatures. Thus, they could provide a self-healing mechanism at dielectric fault sites. Unfortunately, polyacetylene was too unstable in air to be of practical use as a cathode for a tantalum capacitor. However, there was significant research effort in this new field and more stable conductive polymers were soon developed (Figure 3). Two that were eventually applied as cathode materials in capacitors were polypyrrole (PPy) and polyethylenedioxythiophene (PEDOT).

Figure 3: Some Conductive Polymer Materials

The first commercially successful capacitor with a conducting polymer cathode was an aluminum electrolytic capacitor introduced by Matsushita (Panasonic) in 1991 ² and the cathode was PPy deposited by an electrodeposition process. The capacitor was dubbed “SP-Cap” for specialty polymer capacitor (Figure 4).

Figure 4: The Panasonic SP-Cap

The Energy Devices Division of NEC (which had been in the tantalum capacitor business since 1955 and the solid tantalum capacitor business since 1970) followed soon after with a tantalum capacitor, NEOCAPACITOR, based on chemically deposited PPy (Figure 5) ³.

Figure 5: The NEC NEOCAP

A second-generation device based on the PEDOT conductive polymer developed by Dr. Fredrich Jonas and colleagues at Bayer AG ⁴ was introduced a few years later ⁵.

The substitution of conductive polymer for manganese dioxide as the cathode material had a dramatic impact on the ESR of the tantalum capacitor. Devices with ESRs below 30mΩ could be manufactured with single anodes instead of the more costly MAT construction and with a MAT single digit ESR could be achieved. Due to continuing material advances over the last 30 years, single digit ESRs are possible even with single anodes in 7.3x4.3mm case sizes and small case size ESRs below 50mΩ are possible.

Capacitor manufacturers recognized the potential of this “Tantalum-Polymer” capacitor. Sanyo, who was manufacturing aluminum capacitors with the conductive salt TCNQ, entered the market with its POSCAP device. KEMET Electronics entered the market in 1998 signing a licensing/co-development agreement with NEC (Figure 6). The new capacitor was dubbed “KO-CAP” for KEMET Organic Capacitor.

Figure 6: On June 10, 1998 Charles Culbertson of KEMET and Nakata-san of NEC Signed an Agreement for KEMET to License/Co-Develop with NEC Tantalum-Polymer Capacitors

Lower ESR was the biggest benefit of the Tantalum-Polymer capacitor, but other benefits over the traditional manganese dioxide solid tantalum capacitor were soon discovered. Because the internal ESR of the part was lower, capacitance decreased less with increasing frequency and this ‘cap retention’ allowed Tantalum-Polymer capacitors to be used in higher frequency circuits. Tantalum-MnO₂ capacitor had the reputation for some ppm level of ‘ignitions’ in failed capacitors where the oxygen ‘donated’ by the MnO₂ would react with the tantalum metal and a flame would shoot out from the failed capacitor. Because of the lack of this ‘free oxygen’ in conductive polymers, they did not have this failure mechanism. Because of concerns about power-on surge failures, Tantalum-MnO₂ capacitors were typically only used at 50% of their rated voltage (so, for example, a 10V capacitor would be used on a 5V rail). Tantalum-Polymer capacitors were much more robust to surge, and manufacturers recommend that they be used at 80-90% of their rated voltage.

When the Tantalum-Polymer capacitor was introduced, it found immediate application in notebook computers where space was at a premium and high capacitance and low ESR was needed to support the power delivery. Initial voltage ratings were 10V and below which limited the applications to power supply outputs. It was difficult to manufacture reliable product with ratings of 16V and above using the in-situ chemical polymerization of monomer to polymer (which KEMET and NEC practiced) or electrochemical polymerization (Sanyo). Tantalum-MnO₂ capacitors, by contrast, were available in voltage ratings up to 63V in surface mount packages and to 125V in through-hole hermetic seal packages.

Bayer AG introduced a pre-polymerized aqueous dispersion of PEDOT (Baytron P) as an antistatic coating for photographic films ⁶ (they owned Afga at time). Qiu, Hahn, and Brenneman at KEMET Electronics discovered that substituting this pre-polymerized dispersion for some of the in-situ chemical polymerization cycles significantly increased the breakdown voltage ⁷. KEMET introduced a 35V rated Tantalum-Polymer in 2009 and today ratings are available to 75V.

Since the introduction of Tantalum-Polymer capacitors in the mid-1990s the application space has continued to expand. ESR decreases have continued. Voltage ratings have increased. Series qualified to AEC-Q200 for automotive were introduced ⁸. The defense and aerospace industry are conservative and slow to adopt new technologies, but now high reliability series are available ⁹ and a MIL-SPEC series was recently introduced ¹⁰.

The market for Tantalum-Polymer capacitors continues to grow. Figure 7 shows that while the overall tantalum capacitor market has remained between ~$1.5-$2B the share of Tantalum-Polymer capacitors surpassed that of Tantalum-MnO₂ capacitors around 2019. Tantalum-MnO₂ capacitors are still used where the application is cost sensitive and the lowest ESR is not required, where low DC leakage currents are needed, and where high temperature operation is needed.

Figure 7: (a) MnO₂ and Polymer Cathode Market Share (b) Overall Tantalum Capacitor Market Value

The conductive polymer cathode was a "game changer" for the solid Tantalum capacitor and has made this type of capacitor the choice for many applications in the 21^st century. In the next (and last) post in this series, we'll take a look at how the Tantalum capacitor industry has reshaped itself to meet electronics industry challenges, some performance challenges with Tantalum-Polymer capacitors, and what the cutting-edge applications for these capacitors are.

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