Stacked solid electrolytic capacitor

A stacked solid electrolytic capacitor has a substrate, a capacitor element, and a metal cap. The substrate has electrical conductivity. The capacitor element is provided on the substrate. The metal cap is coupled to the substrate, covers the capacitor element and is electrically conducted to the substrate. A cathode of the capacitor element is electrically conducted to the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a stacked solid electrolytic capacitor.

2. Description of the Related Art

A solid electrolytic capacitor attracts attentions because the capacitor is superior in frequency property compared to other electrolytic capacitors. A roll-formed solid electrolytic capacitor, a stacked solid electrolytic capacitor and so on are used as the solid electrolytic capacitor. An exterior of the stacked solid electrolytic capacitor is, generally, molded with epoxy resin. There is, however, some defects with respect to the epoxy molding.

Transfer molding is, generally, used as an epoxy molding method. The epoxy resin is heated to more than 150 degrees C. and the epoxy resin is put into with a pressure of more than few atmospheres, in the method. Polymerized element is subjected to much stress. And it is possible that a leakage current is increased and electrical short circuit is generated. The epoxy resin of high temperature breaks into between electrode foils of the polymerized element. And the property is possibly degraded because of separation of a polymer.

And so, Japanese Patent Application Publication No. 2005-116713 (hereinafter referred to Document 1) discloses a molding method as a method other than the transfer molding. In the method, a thermocompression tape where a single liquid epoxy resin is impregnated is pasted to the polymerized element. And the epoxy resin dissolved with heat is molded.

It is, however, difficult to obtain humidity resistance because a molding material is epoxy resin, with respect to a solid electrolytic capacitor manufactured following the art of Document 1.

SUMMARY OF THE INVENTION

The present invention provides a thinned stacked solid electrolytic capacitor having high humidity resistance.

According to an aspect of the present invention, preferably, there is provided a stacked electrolytic capacitor including a substrate, a capacitor element, and a metal cap. The substrate has electrical conductivity. The capacitor element is provided on the substrate. The metal cap is coupled to the substrate, covers the capacitor element and is electrically conducted to the substrate. A cathode of the capacitor element is electrically conducted to the substrate.

With the above-mentioned configuration, the capacitor element is covered with the metal cap. The metal cap has high sealing performance, and shields against external environment. In this case, it is possible to obtain high humidity resistance. It is possible to restrain a degradation of property of the solid electrolytic capacitor. It is possible to reduce ESL of the solid electrolytic capacitor because the metal cap and the substrate act as a cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanying drawings, of embodiments of the present invention.

First Embodiment

FIG. 1AthroughFIG. 1Cillustrate a stacked solid electrolytic capacitor100in accordance with a first embodiment of the present invention.FIG. 1Aillustrates a cross sectional view of the solid electrolytic capacitor100.FIG. 1Billustrates a top view of the solid electrolytic capacitor100.FIG. 1Cillustrates a bottom view of the solid electrolytic capacitor100. As shown inFIG. 1A, the solid electrolytic capacitor100has a structure in which a capacitor element200is packaged in a case10.

As shown inFIG. 1AandFIG. 1B, the case10has a structure in which a metal cap11is provided on a base substrate13. The metal cap11may be seam welded to the base substrate13. The metal cap11is composed of a metal such as copper, aluminum, SPC steel, cobalt steel or stainless steel.

The base substrate13has electrical conductivity, can be soldered easily, and is composed of a material having low moisture permeability. The base substrate13is, for example, composed of a metal such as copper, aluminum, SPC steel, cobalt steel or stainless steel, or a ceramics having a metal layer plated on the surface thereof. An insulating layer12is coated on an inner face of the metal cap11. And it is possible to restrain an electrical short circuit between the capacitor element200and the metal cap11. The insulating layer12is, for example, composed of a resin, nylon, polyethylene terephthalate (PET) that have insulating property.

As shown inFIG. 1AandFIG. 1C, a through hole is formed near both ends of the base substrate13. An anode terminal31is provided in one of the through holes, and another anode terminal31is provided in the other. An insulating material32is formed between the anode terminal31and the through hole. And it is possible to restrain an electrical short circuit between the anode terminal31and the base substrate13.

The anode terminal31is composed of a conductive material that can be soldered easily. The anode terminal31is, for example, composed of SPC steel or cobalt steel. The anode terminal31is coupled to an extractor portion of an anode foil21mentioned later. The base substrate13has a convex portion33on a bottom face thereof between the through holes. The convex portion33acts as a cathode terminal, as mentioned later. An insulating sheet34is formed between the convex portion33and the anode terminal31. It is therefore possible to restrain an electrical short circuit between the convex portion33and the anode terminal31.

The insulating material32is, for example, composed of a glass such as a hard glass or a soft glass, or a rubber. The insulating material32is, preferably, composed of a soft glass, in a case where the base substrate13is composed of a material such as SPC steel having relatively high linear coefficient of thermal expansion. On the other hand, the insulating material32is, preferably, composed of a hard glass, in a case where the base substrate13is composed of a material such as cobalt steel having relatively low linear coefficient of thermal expansion. In these cases, it is possible to improve sealing performance of the case10.

Next, a description will be given of the capacitor element200, with reference toFIG. 1AandFIG. 2.FIG. 2illustrates a cross sectional view of the capacitor element200taken along a line A-A ofFIG. 1A. As shown inFIG. 1AandFIG. 2, the capacitor element200has a structure in which a plurality of unit elements20are stacked. In the embodiment, the capacitor element200has a structure in which two unit elements20are stacked on the base substrate13by an adhesive agent25having electrical conductivity. It is possible to control the capacitance of the capacitor element200by controlling the stacking number of the unit element20.

The adhesive agent25is composed of a conductive material such as silver. The unit element20has a structure in which a solid electrolyte layer22, a carbon paste layer23and an extractor cathode layer24are stacked on the anode foil21in order. The anode foil21is composed of a valve metal having a dielectric oxide layer formed on a surface thereof. The valve metal used for the anode foil21is a metal such as aluminum. It is possible to form the dielectric oxide layer by subjecting the surface of the valve metal to an etching treatment and a chemical conversion treatment.

It is possible to form the anode foil21by cutting a valve metal having a dielectric oxide layer formed on a surface thereof into a given shape. In the cutting process, the valve metal at the end face of the anode foil21is exposed, and a defect is formed in the dielectric oxide layer. It is therefore necessary to form a dielectric oxide layer on the exposed valve metal. It is possible to form the dielectric oxide layer on the exposed valve metal by carrying out a chemical conversion treatment and a thermal treatment few times after the cutting. The chemical conversion treatment is carried out at a voltage near a formation voltage of the dielectric oxide layer, using chemical liquid mainly containing 0.5 wt % to 2 wt % ammonium adipate.

The solid electrolyte layer22has a separator. In the solid electrolyte layer22, a solid electrolyte is formed in the separator and between the separator and the anode foil21. The separator is mainly composed of synthetic fiber having more than one polymer fiber such as PET fiber or acrylic fiber. The solid electrolyte is composed of 3,4-polyethylene dioxythiophene (PEDT) or the like. It is possible to form the solid electrolyte by impregnating polymerizable monomers and an oxidizer into the separator. A description will be given of a forming method of the solid electrolyte.

A compound liquid including a monomer to be the solid electrolyte and an oxidizer is provided on the surface of the anode foil21and on the separator. The monomer is a compound solvent including a volatile solvent. Concentration of the monomer in the compound solvent is within a range 1 wt % to 50 wt %. The concentration is, preferably, within a range 10 wt % to 35 wt %. The oxidizer is contained in an alcohol solvent by 40 wt % to 60 wt %. In the embodiment, a solvent containing 60 wt % oxidizer is used. Next, the compound liquid on the anode foil and in the separator is subjected to a heat polymerization, and the solid electrolyte layer22is formed.

In addition, an insulating layer26is formed on an exposed area of the solid electrolyte layer22. And it is prevented that the solid electrolyte exudes from the solid electrolyte layer22. The insulating layer26is, for example, composed of an insulating synthetic resin such as silicon resin, epoxy resin, polyamide resin, or polyimide resin.

The extractor cathode layer24is, for example, composed of silver paste. In the embodiment, the extractor cathode layer24of the unit element20at lower side is electrically coupled to the base substrate13through the adhesive agent25. And the base substrate13and the metal cap11act as a cathode.

The solid electrolytic capacitor100in accordance with the embodiment has a high humidity resistance, because the capacitor element200is sealed with the metal cap11and the base substrate13that have high sealing performance and shield against external environment. It is therefore possible to restrain property degradation of the solid electrolytic capacitor100. Further, it is possible to reduce ESL of the solid electrolytic capacitor100, because whole of the case10acts as a cathode.

Second Embodiment

FIG. 3AthroughFIG. 3Cillustrate a solid electrolytic capacitor100ain accordance with a second embodiment of the present invention.FIG. 3Aillustrates a cross sectional view of the solid electrolytic capacitor100a.FIG. 3Billustrates a top view of the solid electrolytic capacitor100a.FIG. 3Cillustrates a bottom view of the solid electrolytic capacitor100a. As shown inFIG. 3AthroughFIG. 3C, the solid electrolytic capacitor100ahas a capacitor element200ainstead of the capacitor element200, being different from the solid electrolytic capacitor100.

The capacitor element200ahas a structure in which a plurality of unit elements20aare stacked. The unit element20ahas a structure in which the solid electrolyte layer22and a cathode foil27is stacked on the upper face and on the lower face of an anode foil21a. Two unit elements20aare stacked in the capacitor element200ain the embodiment. The capacitor element200ais adhered to the base substrate13with the adhesive agent25. And the metal cap11and the base substrate13act as a cathode.

The anode foil21ais different from the anode foil21inFIG. 1Ain shape. Details are mentioned later. The cathode foil27is composed of a metal such as aluminum, tantalum or niobium. In the embodiment, the cathode foil27is aluminum foil. The surface of the cathode foil27is subjected to an evaporation process or a physical adsorption process of carbide. A carbide grain thus adsorbs to the surface of the cathode foil27.

The metal composing the cathode foil27is in touch with the solid electrolyte layer22, not directly by through the carbide of organic. And adhesiveness between the cathode foil27and the solid electrolyte layer22is improved. In addition, the solid electrolyte in the solid electrolyte layer22is formed effectively, because airspace between the carbide grains is larger than an etching pit of a normal oxide layer. The interference resistance between the cathode foil27and the solid electrolyte layer22is reduced, and tan δ and ESR are reduced. And the frequency property is improved.

The cathode foil27, the carbide grain and the solid electrolyte layer22are electrically conducted to each other, when the solid electrolytic capacitor100ais energized. Therefore, the electrical capacitance at anode side is that of the solid electrolytic capacitor100a, because the carbide grain and the solid electrolyte layer22does not affect the capacitance of the cathode of the solid electrolytic capacitor100a. It is possible to reduce the stacking number of the unit element20a, because the capacitance of the solid electrolytic capacitor100ais increased. And it is possible to reduce the thickness of the solid electrolytic capacitor100a. It is possible to control the capacitance of the solid electrolytic capacitor100aby controlling the stacking number of the unit element20a.

The carbide grain is not limited and may be a material including carbon. The carbide grain is, for example, composed of carbon, graphite, carbon nitride, carbide or carbon compound. The carbide grain may be held by a whisker formed on the surface of the cathode foil27.

An extractor portion is formed on the cathode foil27. The extractor portion of each cathode foil27is coupled to each other through a welding portion28. And each cathode foil27is coupled electrically. The welding portion28is formed with a laser welding, a resistance welding, or an ultrasonic welding. The anode foil21is coupled to the anode terminal31through the extractor portion.

FIG. 4AthroughFIG. 4Cillustrate a shape of the anode foil21and the cathode foil27.FIG. 4Aillustrates a perspective view of the solid electrolytic capacitor100aviewing from upper side.FIG. 4Billustrates a top view of the anode foil21a.FIG. 4Cillustrates a top view of the cathode foil27. As shown inFIG. 4BandFIG. 4C, the anode foil21aand the cathode foil27are a foil having a plate shape. An extractor portion is formed integrally on each anode foil21aand on each cathode foil27. The extractor portion of the anode foil21aand the extractor portion of the cathode foil27are arranged so as not to overlap when the extractor portions are viewed from a direction vertical to each foil.FIG. 3Amentioned above is a cross sectional view taken along a line C-C ofFIG. 4A.FIG. 5illustrates a cross sectional view taken along a line B-B ofFIG. 4A.

The solid electrolytic capacitor100ain accordance with the embodiment has a high humidity resistance, because the capacitor element200ais sealed with the metal cap11and the base substrate13that have high sealing performance and shield against external environment. It is therefore possible to restrain property degradation of the solid electrolytic capacitor100a. Further, it is possible to reduce ESL of the solid electrolytic capacitor100a, because whole of the case10acts as a cathode.

EXAMPLES

In an example 1, the solid electrolytic capacitor100shown inFIG. 1AthroughFIG. 1Cwas fabricated. Aluminum foil, which was subjected to an etching treatment and a chemical conversion treatment, was used as the anode foil21. The aluminum foil was cut out into the anode foil21. The anode foil21was subjected to a chemical conversion treatment at a voltage near a formation voltage of the dielectric oxide layer of the anode foil21using chemical liquid mainly containing 0.5% to 2% ammonium adipate by weight, and was subjected to a thermal treatment in a temperature range 200 degrees C. to 280 degrees C. The thickness of the anode foil21was 100 μm to 110 μm.

Next, the solid electrolyte layer22was formed on both faces of the anode foil21. A solvent containing 25 wt % monomer and a solvent containing 60 wt % oxidizer were provided on the both faces of the anode foil21and in the separator. The solvents were heated from 30 degrees C. to 150 degrees C. The thickness of the solid electrolyte layer22was 30 μm to 50 μm.

After that, the carbon paste layer23and the extractor cathode layer24were formed on the solid electrolyte layer22. The insulating layer26was formed on both ends of the solid electrolyte layer22. And the unit element20was fabricated. The thickness of the carbon paste layer23was less than 10 μm. Silver paste was used as the extractor cathode layer24. Two unit elements20were stacked on the base substrate13by the adhesive agent25, and the capacitor element200was fabricated. Silver was used as the adhesive agent25.

After that, the anode foil21was coupled to the anode terminal31. The capacitor element200was covered with the metal cap11. The base substrate13was coupled to the metal cap11with projection welding. The height of the metal cap11was 1.7 mm. The base substrate13and the anode terminal31were composed of cobalt steel. The insulating material32was composed of hard glass. The thickness of the base substrate13was 0.7 mm. The capacitance of the solid electrolytic capacitor in accordance with the example 1 was 2.5 V 1000 μF.

In an example 2, the solid electrolytic capacitor100ashown inFIG. 3AthroughFIG. 5was fabricated. Aluminum foil, which was subjected to an etching treatment and a chemical conversion treatment, was used as the anode foil21a. The thickness of the anode foil21awas 100 μm to 110 μm. The aluminum foil was cut out into the anode foil21a. Aluminum foil having a carbide grain held at the surface thereof and having a thickness of 50 μm was used as the cathode foil27. The aluminum foil was cut out into the cathode foil27. Next, the anode foil21awas subjected to a chemical conversion treatment at a voltage near a formation voltage of the dielectric oxide layer of the anode foil21ausing chemical liquid mainly containing 0.5% to 2% ammonium adipate by weight, and was subjected to a thermal treatment in a temperature range 200 degrees C. to 280 degrees C.

Next, the solid electrolyte layer22was formed on one surface of the anode foil21a. A solvent containing 25 wt % monomer and a solvent containing 60 wt % oxidizer were provided on the both faces of the anode foil21aand in the separator. The solvents were heated from 30 degrees C. to 150 degrees C. The thickness of the solid electrolyte layer22was 30 μm to 50 μm.

After that, the cathode foil27was pasted to the solid electrolyte layer22, and the unit element20awas fabricated. A plurality of the unit elements20awere stacked with an adhesive agent, and the capacitor element200awas fabricated. After that, the anode foil21awas coupled to the anode terminal31, and each extractor portion of the cathode foil27was coupled with ultrasonic welding. The capacitor element200awas covered with the metal cap11. The base substrate13was coupled to the metal cap11with projection welding. The height of the metal cap11was 1.7 mm. The base substrate13and the anode terminal were composed of cobalt steel. The insulating material32was composed of hard glass. The thickness of the base substrate13was 0.7 mm. The capacitance of the solid electrolytic capacitor in accordance with the example 2 was 2.5 V 1000 μF.

Comparative Example

In a comparative example, the capacitor element200in accordance with the example 1 was attached to a conventional lead frame. The capacitor element was covered with epoxy resin by transfer molding, and a solid electrolytic capacitor was fabricated. The capacitance of the solid electrolytic capacitor in accordance with the comparative example was 2.5 V 1000 μF.

Table 1 shows an electrical capacitance, the tan δ, the leakage current, and the ESR of the solid electrolytic capacitors in accordance with the examples 1 and 2 and the comparative example. Thirty capacitors in accordance with the examples 1 and 2 and the comparative example were fabricated, and each value in Table 1 shows average value thereof.

As shown in table 1, with respect to the solid electrolytic capacitors in accordance with the examples 1 and 2, the electrical capacitance was increased, and the tan δ, the leakage current and the ESR were reduced considerably, compared to the solid electrolytic capacitors in accordance with the comparative example. In particular, the leakage current was reduced considerably. This is because the capacitor element was sealed with the metal case and the base substrate.

Next, the property change of the solid electrolytic capacitor in accordance with the examples 1 and 2 and the comparative example was shown in Table 2, in a case where the capacitors were left at 90 degrees C., in an atmosphere of 95 Rh %, for 1000 hours.

As shown in Table 2, all of the solid electrolytic capacitors in accordance with the comparative example shorted out. It was not possible to measure the capacitance, the tans and the ESR. On the other hand, the capacitance, the tans, the ESR and the leakage current were not changed remarkably, with respect to the electrolytic capacitors in accordance with the examples 1 and 2. This is because the capacitor element was sealed with the metal case and the base substrate, and the humidity resistance in a condition of high temperature and high humidity was improved.

While the preferred embodiments of the prevent invention have been illustrated in detail, the invention is not limited to the specific embodiments above. In addition, it will be appreciated that the invention is susceptible of modification, variation and change without departing from the proper and fair meaning of the accompanying claims.

The present invention is based on Japanese Patent Application No. 2006-014467 filed on Jan. 23, 2006, the entire disclosure of which is hereby incorporated by reference.