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PC-CON Technical Notes

Introduction

Rubycon Carlit Co., Ltd. is a joint venture that was established in July 2005 by Rubycon Corporation and Japan Carlit Co., Ltd. in order to further develop the two companies’ business in the area of PC-CONs (conductive polymer aluminum solid electrolytic capacitors). Rubycon Corporation brings to the new venture its manufacturing technology for electrolytic capacitors and its extensive sales network, while Japan Carlit Co., Ltd. brings its expertise in conductive polymer materials and its business experience with PC-CONs.
As electronic devices become increasingly digitalized, compact, and high-frequency, there is a growing demand for miniaturized capacitors that can offer low ESR and low impedance in high frequency zones. The trend toward lead-free soldering for mounting to PCBs means that superior heat resistance is also required. In response to these requirements, Rubycon Carlit Co., Ltd. is engaged in the design, manufacture, and sale of PC-CONs that employ the highly conductive and heat resistant conductive polymer polypyrrole (PPy) for the cathode.

Conductive Polymers

The PC-CON employs electropolymerized PPy as the solid electrolyte (Fig. 1).

The conductivity of PPy is characterized by resistance that is several hundred times lower than the electrolyte used in conventional electrolytic capacitors (Fig. 2).

Thanks to the extremely low resistance of its electrolyte, the PC-CON offers low ESR and low impedance despite its compact size. PPy is also suited for lead-free reflow soldering (reflow at 260℃) thanks to its superior thermal stability; its decomposition temperature is over 300℃, higher than TCNQ salt, another solid electrolyte. In addition, compared to the polythiophene (PEDOT) used in products such as OS-CONs, characteristics are more stable, with little fluctuation in leakage current, when observed in an environment to test resistance to moisture.

PC-CON Structure and Features

The figure at left shows the basic structure of a PC-CON. On top of the aluminum oxide dielectric layer, the electrolyte PPy layers are formed. Then the cathodes are formed by applying first carbon paste and then silver paste. The layered structure of the PC-CON capacitor element means that PC-CONs can achieve a wide range of capacities by varying the number of layers.

One distinctive feature of the PC-CON is that the anodes are connected by gold wire. Another is that the etched surface of the capacitor element is not used. This is necessary to ensure the safety and reliability of the PC-CON, because the self-healing properties of the film used in conductive polymer aluminum solid electrolytic capacitors are inferior to those of the film used in aluminum electrolytic capacitors.

Product Line-up

The D size (7.3 mm x 4.3 mm) is used for PC-CONs. There are three series available: the SLB/SLE series (height: 1.4 mm), the SXB/SXE series (1.9 mm), and the SW series (2.7/2.9 mm).
The rated voltages and capacitance ranges are as follows: 2 – 6.3 V and 10 – 100 µＦ for the SLB/SLE series, 2 – 8 V and 15 – 220 µＦ for the SXB/SXE series, and 2 – 8 V and 33 – 390 µＦ for the SW series.

PC-CON Capacitor Characteristics

The PC-CON achieves low ESR by combining the superior conductivity of electropolymerized PPy with low-resistance structural materials. Figure 4 shows the frequency characteristic of the 2 V – 100 μF PC-CON (SLE). The impedance is lowest at the resonance frequency. In addition, the ESR characteristic is stable and low, at less than 9mΩ in the high frequency zone (100 kHz or more). The capacity characteristic is also stable, with hardly any variation across the low to high frequency zones.

Because the PC-CON uses aluminum oxide as the dielectric and the electron conductive PPy as the electrolyte, there is hardly any variation in either the capacitance or the ESR characteristic across the broad temperature range of from -55℃ to 105℃. There is also little variation in capacity with respect to DC bias voltage (Fig. 5).

Figure 6 shows the current/voltage curve for the PC-CON. The solid line shows the leakage current when positive voltage is applied, while the broken line shows the current behavior when reverse voltage is applied.

When positive voltage is applied, electrical breakdown does not occur until double the rated voltage is applied. When reverse voltage is applied, leakage current is greater than when positive voltage is applied, but electrical breakdown does not occur until the vicinity of the product’s rated voltage.

Product reliability is designed into the PC-CON, in the form of an ample withstand voltage margin for the rated voltage. Provided that the PC-CON is used within the rated voltage, the voltage derating that is required for tantalum solid electrolytes is not necessary, although this depends to some extent on the application. The PC-CON also offers high reliability in rapid charging and discharging, eliminating the need for the protection circuitry used in conventional tantalum solid electrolytic capacitors. Further, in the event of a short mode failure due to overvoltage, the PC-CON is much less likely to burn than the tantalum solid electrolytic capacitor.

Future Development

Rubycon Carlit intends to continue its development of PC-CONs so as to provide even higher capacities in ever smaller packages, to further increase withstand voltage levels and to enhance the product line-up. Another priority is high-frequency noise reduction, especially in the GHz range. This is becoming ever more important as digital circuits become increasingly high-frequency and the trend to ubiquitous computing gathers pace. As a means of reducing noise at high frequencies, Rubycon Carlit plans to develop a low ESL PC-CON with reduced capacitor inductance.