Solid electrolytic capacitors comprising a TCNQ complex salt, polypyrrole or like high polymer serving as a solid electrolyte and an anode of aluminum foil, thin sheet or the like have found wide use in various electronic devices. There is in recent years a rapidly growing demand for solid electrolytic capacitors having higher voltage resistance. With such solid electrolytic capacitors, a dielectric oxide film or layer is formed over the surface of the anode as is well known. As a method of giving increased voltage resistance to solid electrolytic capacitors, i.e., to the dielectric oxide film thereof, it is practice to increase the voltage (formation voltage) to be applied to the aluminum material for use as an anode or to be made into an anode in the anodic oxidation step of forming the dielectric oxide film. In the common conventional process for fabricating solid electrolytic capacitors, the formation voltage for anodic oxidation is set at a level approximately three times the rated voltage of the capacitor.
However, if a voltage higher than this value is used for anodic oxidation, the solid electrolytic capacitor obtained has the problem of becoming susceptible to marked leakage current and to short-circuit failure. To overcome this problem, it is practice to form on the anode a dielectric oxide film from amorphous alumina instead of crystalline alumina (see, for example, JP 5-343267A). The dielectric oxide film of crystalline alumina undergoes volumetric shrinkage during formation to develop defects, whereas dielectric oxide film of amorphous alumina remains almost free of volumetric shrinkage during formation and is greatly diminished in defects. The leakage current or short-circuit failure of solid electrolytic capacitors is attributable to the defects in the dielectric oxide film, so that the formation of the dielectric oxide film from amorphous alumina provides solid electrolytic capacitors having high voltage resistance, diminished in leakage current and less prone to short-circuit failure.
The aluminum material to be used as or made into an anode, and the anode are subjected to a bending stress, tensile stress and like mechanical stresses (physical stresses) in the process for fabricating solid electrolytic capacitors. For example, in the case of solid electrolytic capacitors of the rolled-up type, a dielectric oxide film is formed on aluminum foil having a large width and to be made into anodes, followed by a cutting step, in which the aluminum foil of large width is cut into separate pieces of aluminum foil of reduced width, namely, into separate anodes. After the cutting step, a lead tab is joined to the anode by crimping, and the anode is connected to a lead wire by the lead tab terminal. The anode is then rolled up along with a cathode and separator paper to make a capacitor element.
If the anode is subjected to a mechanical stress in the cutting step, joining step or rolling-up step described above, the dielectric oxide film on the anode will be thereby injured to develop defects anew. If the defects thus subsequently occurring result in increased leakage current and more serious short-circuit failure in the solid electrolytic capacitor, the advantage of the dielectric oxide film of amorphous alumina becomes impaired. The present invention, which has overcome such problems, provides a solid electrolytic capacitor wherein the anode is provided with a dielectric oxide film of a structure less susceptible to damage or faults due to a mechanical stress and which is smaller in leakage current and less prone to short-circuit failure than conventional solid electrolytic capacitors, and a process for fabricating the capacitor.