Conductive Polymer Aluminum for Electrolytic Capacitor Technology

By Masashi Oshima, General Engineering Manager; Rubycon Carlit Japan

Figure 1: Chemical Structure of Polypyrrole

The last five years of growth and popularity of flat-panel monitors and television sets have dramatically driven the need for SMT conductive polymer aluminum electrolytic capacitors. Advancements in digitalization, miniaturization, and higher operating frequency have increased the popularity with high-heat receptivity and low-profile form factor with surface mounting. With this, key elements for capacitors used in digital circuitry are lower ESR and higher capacitance. In this article, I discuss the difficulties and solutions to the design and production of the Rubycon PC-CON Conductive Polymer Aluminum Solid Electrolytic Capacitors. These capacitors employ the chemical-polymerized polypyrrole as the cathode. Polypyrrole Pyrrole is a monomer material that is
easy to polymerize. It reacts with various chemical oxidants to form polypyrrole in a black powder form. (See
Figure 1.)

However, if pyrrole is mixed with oxidants in the room temperature state, the polymerization reaction starts immediately. Wi t h t h i s , polypyrrole begins to form even before the chemical solution diffuses into the bottom of narrow pits prepared on the surface of the anode aluminum foil. Thus, manufacturing aluminum solid electrolytic capacitors employing the chemical-polymerized polypyrrole has been difficult. The difficulty lies in controlling the polymerization process, such as cooling and diluting the chemical solution to lower its density, which in turn slows the reaction speed; however, this requires repeating the polymerization process multiple times. On the other hand, when a pair of electrodes is immersed into the pyrrole solution and supporting electrolyte and electric current is flown, polypyrrole forms on the surface of the anode electrodes by electrochemical polymerization. In this process, the anion of the supporting electrolyte is captured into the polypyrrole as dopant, resulting in the formation of a fine polypyrrole film with conductivity of approximately 100 siemens/ centimeters.

There are advantages in the electrochemical polymerization method. One is that the polymerization speed is controllable by current density of electrochemical polymerization. Another is that p r o c e s s i n g conditions are mild, such as temperature and the neutral non-acidic solution. Yet another advantage is when using mass-production techniques of electrolysis polymerization, polypyrrole forms on the pre-formed electro-conductive layer on the etched/magnified surface of the aluminum anode foil.

Manufacturing Process and Construction of PC-CON
The basic manufacturing process starts when the aluminum oxide layer, the dielectric, is formed on the surface of etched aluminum anode foil by electro-chemical forming treatment. An electro-conductive layer is formed on the surface of aluminum anode foil by chemical polymerization. Then the polypyrrole layer is formed on top by electrochemical polymerization. Carbon paste and silver paste are applied to form the cathode electrode. Cathode electrodes are connected by throughhole, and anode electrodes are connected by wire bonding, to the outer termination. Epoxy packaging and the aging and inspection processes complete the product manufacturing.

See the basic structure of PC-CON in Figure 2.

Figure 2: PC-CON Conductive Polymer Aluminum Chip

A uniquely developed etched aluminum foil, with the thickness of 200 to 300 μm, which is two to three times thicker than conventional foil, is used. This enlarges the capacitor’s value per unit surface area while conducting a deeper etching process. Thus, the number of stacks of the etched aluminum foils is approximately half of
normally etched aluminum foils.

Features of PC-CON:
Extremely low ESRs are achieved by the use of low resistance termination materials and by the superb conductivity of electrochemical polymerized polypyrrole, which is employed on the cathode electrode. ESR values at 100 kHz range from 9 to 50 milliohms in the SL/SLE series, from 9 to 15 milliohms in the SXB/SXE series, and from 7 to 10 milliohms on SW series. Figure 3 displays the typical frequency curves for the SW series of low ESR (<10 milliohms) and its stable capacitance at the resonant frequency. The polypyrrole used on cathodes has an electronic conduction property that promotes low resistance with minimal effect at low temperatures. Such features present stable temperature, capacitance, and ESR characteristics in extreme conditions reaching as low as –55°C. [caption id="attachment_572" align="alignright" width="275" caption="Figure 3: ESR/Impedance vs. Frequency"][/caption]
Excellent performance and durability in high humidity environments is achieved by using high-grade sealing materials. Also, electrochemical polymerized polypyrrole has excellent thermal stability with decomposition temperature over 300°C and can easily withstand the RoHS reflow soldering temperature profile (250°C peak for
5 seconds). The voltage dependency curve rated at 6.3 V. The solid line represents the leakage current when voltage with correct polarity is applied. Dielectric breakdown does not occur until the applied voltage reaches to double the rated voltage. The dotted line represents the leakage current when reverse voltage is applied. It indicates no breakdown until a significant voltage level. Thus, PC-CON has features that are more robust against reverse voltage compared to tantalum solid electrolytic capacitors. Furthermore, there is no required voltage derating, with no protection circuitry necessary. In addition, they are less susceptible to short circuit failures, which are a common mode for tantalum solid electrolytic capacitors.

Expansion Roadmap:
An expanded PC-CON lineup is planned through developing higher capacitance and voltage (up to 16 V), along
with lower ESR. High frequency noise reduction, especially in the GHz range, is becoming more important. In order to respond to the need for high frequency noise reduction, a low ESL feature is another theme for development.

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