Electricity: electrical systems and devices – Electrolytic systems or devices – Solid electrolytic capacitor
Reexamination Certificate
2001-09-26
2004-10-19
Gupta, Yogendra N. (Department: 1751)
Electricity: electrical systems and devices
Electrolytic systems or devices
Solid electrolytic capacitor
C252S510000, C252S518100, C252S500000, C361S271000, C361S434000, C361S529000, C361S527000
Reexamination Certificate
active
06807049
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a solid electrolytic capacitor utilizing a specified electrically conducting polymer as a solid electrolyte. More specifically, the present invention relates to a solid electrolytic capacitor capable of realizing down-sizing, high capacity and low impedance of the electrolytic capacitor and having good moisture-resistant load characteristic and excellent heat resistance as well as excellent thermal stress relaxation characteristic and to a method for the production of the same.
BACKGROUND ART
A solid electrolytic capacitor is a device which comprises a metal foil subjected to etching treatment and having a large specific surface area (anode substrate) on which is formed an oxide dielectric layer and a solid semiconductor layer (hereinafter simply referred to as a solid electrolyte) as an opposing electrode outside the oxide dielectric layer and preferably further an electric conductor layer such as an electrically conducting paste. The device has an cathode lead terminal connected to the metal foil and an anode terminal connected to the electric conductor layer and as a whole is completely sealed by an epoxy resin or the like and is put into use as a capacitor part in electric products over a wide range.
As the method for forming a solid electrolyte layer, there have conventionally been known a method of fusing a solid electrolyte onto a dielectric layer which has been formed on a metal surface and has a porous or fine void structure to form a solid electrolyte layer on the dielectric layer and a method by producing a solid electrolyte on a dielectric layer.
According to recent digitalization of electronic equipment, an increase in the operation speed of personal computers, a compact capacitor having a high capacity and a low impedance in high-frequency regions is being demanded. Hitherto, as a capacitor having a high capacity, there have been known electrolytic capacitors such as an aluminum electrolytic capacitor and a tantalum electrolytic capacitor. However, an aluminum electrolytic capacitor has problems that it has a high impedance in high frequency regions because it uses an ion conducting liquid electrolyte and also has a poor temperature characteristic. A tantalum capacitor, which uses manganese oxide as the electrolyte, has a problem that it has a high impedance in high frequency regions since the manganese oxide has a relatively high specific resistance.
To cope with the problems, it has been heretofore proposed to use an electrically conducting polymer having an electron conductivity as the solid electrolyte. For example, there has been known use of polymers such as an intrinsic electrically conducting polymer having an electric conductivity of 10
−3
to 10
3
S/cm [JP-A-1-169914 (the term “JP-A” as used herein means laid-open publication of unexamined Japanese patent application) (U.S. Pat. No. 4,803,596)], polyaniline (JP-A-61-239617), polypyrrole (JP-A-61-240625), polythiophene derivative (JP-A-2-15611 (U.S. Pat. No. 4,910,645)), and polyisothianaphthene (JP-A-62-118511). Many of these electrically conducting polymers comprising a &pgr;-conjugated system are used as a composition containing a dopant. Further, recently, not only dopants are used singly but also they are used in combination with manganese dioxide (JP-B-6-101418 (the term “JP-B” as used herein means publication of examined Japanese patent application) (U.S. Pat. No. 4,959,753)) or a filler (JP-A-9-320901).
Thus, the reason why electrically conducting polymers are drawing an attention as the solid electrolyte is that they may be improved to have a sufficiently high electric conductivity. However, there is a problem in that if the electric conductivity is higher than a proper range, the leakage current greatly increases to cause short circuit, whereas if it is lower than the proper range, the frequency properties are deteriorated to cause a large reduction in the capacity. Accordingly, it is a subject of development how to control the electric conductivity of the solid electrolyte in a proper range and attain the heat resistance and thermal stability thereof.
On the other hand, as the method for forming a solid electrolyte layer, there have hitherto been known a method of forming by fusion a solid electrolyte layer on a dielectric layer of a valve acting metal surface having a microfine porous or void structure and a method of producing the above-mentioned electrically conducting polymer on a dielectric layer. More specifically, for example, in the case of using a polymer of a 5-membered heterocyclic compound such as pyrrole or thiophene, there have been known a method where an anode foil is dipped in a solution of a 5-membered heterocyclic compound in a lower alcohol/water, and then dipped in an aqueous solution having dissolved therein an oxidizing agent and an electrolyte to give rise to chemical polymerization, thereby forming an electrically conducting compound (JP-A-5-175082), and a method where a 3,4-dioxyethylenethiophene monomer and an oxidizing agent each preferably in the form of a solution are applied separately differing in time or simultaneously on an oxide cover layer of a metal foil to thereby form a solid electrolyte layer (JP-A-2-15611 (U.S. Pat. No. 4,910,645) and JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)). In particular, JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)) discloses polymers of a monomer selected from pyrrole, thiophene, furan, aniline and derivatives thereof and doped with an aromatic polysulfonic acid having a plurality of sulfonic acid groups in the molecular structure and also discloses as a production method, a polymerization method in which a monomer is introduced after a mixed solution of the above-mentioned monomer and an oxidizing agent is coated and dried or after an oxidizing agent is introduced. Also, JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)) discloses a production method in which the dopant of the aromatic polysulfonic acid is utilized as a constituent component of the oxidizing agent (ferric salt) and describes that the solid electrolytic capacitor provided therewith has an advantage that it is excellent in high temperature resistance and prevention of deterioration of static capacity. As the oxidizing agent used in the prior art in the case of chemical polymerization of 5-membered aromatic cyclic compounds, for example, thiophene, there have been known iron (III) chloride, Fe(ClO
4
)
3
, organic acid iron (III) salt, inorganic acid iron (III) salt, alkyl persulfate, ammonium persulfate (hereafter, abbreviated as APS), hydrogen peroxide, K
2
Cr
2
O
7
, etc. (JP-A-2-15611 (U.S. Pat. No. 4,910,645)), cupric compounds, silver compounds, etc. (JP-A-10-32145 (European Patent Application Laid-open No. 820076(A2)).
More specifically, the capacitor comprising a solid electrolyte of the manganese dioxide is disadvantageous in that the oxide layer is ruptured at the thermal decomposition of manganese nitrate and the impedance property is unsatisfactory. Use of lead dioxide must be accompanied with a consideration on the environment.
The solid electrolytic capacitor using a tetracyanoquinodimethane (TCNQ) complex salt has good heat fusion workability and excellent electric conductivity but the TCNQ complex salt itself is said to have a problem in the heat resistance and in turn, a poor reliability in the soldering heat resistance.
In order to overcome these problems, the above-mentioned electrically conducting polymer such as polypyrrole is applied to the solid electrolyte on a dielectric surface by electrochemical polymerization or chemical polymerization but the conventional capacitors with electrically conducting polymer such as polypyrrole has a problem that their capacitor characteristics vary greatly depending on the humidity resistance load.
Further, as associated with humidity resistance load, heat resistance is highly demanded. For example, soldering heat resistance (reflow characteristic) when a capacitor element is mo
Furuta Yuji
Ikenoue Yoshiaki
Konuma Hiroshi
Monden Ryuji
Ohata Hideki
Gupta Yogendra N.
Hamlin D. G.
Showa Denko K.K.
Sughrue & Mion, PLLC
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