Solid electrolytic capacitor and method for manufacturing same

A solid electrolytic capacitor (A1) includes a porous sintered body (1) of valve metal, and a metal case (2) accommodating the porous sintered body. The metal case (2) and the porous sintered body (1) are electrically connected to each other to serve as an anode. The porous sintered body (1) is formed with a dielectric layer and a solid electrolyte layer. The solid electrolyte layer serves as a cathode.

TECHNICAL FIELD

The present invention relates to a solid electrolytic capacitor and a method of manufacturing the same.

BACKGROUND ART

An example of solid electrolytic capacitor is disclosed in Patent Document 1 described below. The prior art solid electrolytic capacitor includes a porous sintered body made of so-called “valve metal”. The sintered body is sealed in a resin package after a dielectric layer and a solid electrolyte layer are formed thereon.

For instance, the solid electrolytic capacitor having the above-described structure is used as a bypass capacitor connected between an electronic device (such as a CPU) and a power supply circuit. In accordance with the recent speed increase and digitalization of electronic devices, a power supply system which operates stably and responds at high speed is demanded. Accordingly, also with respect to a solid electrolytic capacitor used for noise cancellation and the stabilization of a power supply system, excellent noise cancellation performance and high responsiveness in supplying power are demanded. Further, a large capacitance and high reliability for preventing firing are also demanded.

The capacitance of a solid electrolytic capacitor can be increased by increasing the surface area or volume of the porous sintered body. However, to merely increase the capacitance causes the degradation of frequency characteristics. Specifically, the frequency characteristics of a capacitor are generally determined by two factors of 1/ωCR and ωL. Herein, ω=2 Πf (f represents frequency), C represents capacitance, R represents resistance and L represents inductance. Of the two factors, the frequency characteristics of most solid electrolytic capacitors are substantially determined by 1/ωCR. Therefore, in doubling the capacitance, R need be cut in half to avoid the degradation of the frequency characteristics. Further, when the size of the porous sintered body is merely increased, the ESR (internal resistance, equivalent series resistance) increases. Therefore, in increasing the capacitance, the increase of ESR and the degradation of the frequency characteristics need be prevented. Particularly, when the thickness of the porous sintered body is increased to increase the size of the porous sintered body, the resistance in the electrical path from the obverse surface to the interior increases, which degrades the frequency characteristics. Further, the treatment liquid for forming a dielectric layer or a solid electrolyte layer in the porous sintered body becomes unlikely to permeate into the entire interior of the porous sintered body, so that the productivity of the solid electrolytic capacitor is degraded. Moreover, since a porous sintered body is made by sintering powder of niobium or tantalum, the reliability for preventing firing may be deteriorated by increasing the size of the porous sintered body.

Conventionally, therefore, to solve the above-described problems, the capacitance is increased by connecting a large number of small capacitors in parallel. However, such use of a large number of capacitors requires a large space for mounting the capacitors and also increases the manufacturing cost.

As means for increasing the capacitance without causing such disadvantages, to reduce the thickness of the porous sintered body maybe considered. When the thickness of a porous sintered body is reduced, the distance between electrodes is reduced. As a result, the impedance in the capacitor is reduced, which achieves low ESR. However, when the thickness of a porous sintered body is reduced, the length and the width are increased. Therefore, the possibility that the porous body warps in the sintering process or cracks increases. Moreover, even when the thickness of the porous sintered body is reduced, the heat generation in use is increased, because the entire volume is increased. Therefore, the performance of the capacitor itself may be reduced or the reliability for preventing firing may be degraded.

DISCLOSURE OF THE INVENTION

The present invention is conceived under the above-described circumstances. It is, therefore, an object of the present invention to provide a solid electrolytic capacitor whose capacitance is increased without degrading the frequency characteristics and which is unlikely to warp or crack. Another object of the present invention is to provide a method for manufacturing such a solid electrolytic capacitor.

To solve the above-described problems, the present invention takes the following technical measures.

According to a first aspect of the present invention, there is provided a solid electrolytic capacitor comprising a porous sintered body of valve metal, and a metal case accommodating the porous sintered body.

With such a structure, the porous sintered body is protected by the metal case and does not easily warp or crack even when the porous sintered body has a relatively small thickness. Since the metal case also serves to dissipate heat generated in the porous sintered body to the outside, the temperature rise at the porous sintered body in use is suppressed. Therefore, the solid electrolytic capacitor according to the present invention has a large capacitance and excellent frequency characteristics as a result of increasing the size of the porous sintered body while reducing the thickness thereof and is reliably prevented from firing.

Preferably, the solid electrolytic capacitor according to the present invention further comprises a dielectric layer and a solid electrolyte layer which are formed at the porous sintered body. The solid electrolyte layer acts as a cathode. The metal case is made of valve metal, and the metal case and the porous sintered body are electrically connected to each other to act as an anode.

With such an arrangement, the metal case also acts as an anode similar to the porous sintered body, which is advantageous for increasing the entire capacitance.

Preferably, the metal case includes a main plate portion, and a side plate portion standing from the periphery of the main plate portion. The main plate portion and the side plate portion define a hollow for accommodating the porous sintered body. With such a structure, the main plate portion and the side plate portion of the metal case surround the porous sintered body, whereby the porous sintered body is reliably protected.

Preferably, the porous sintered body is flat and has a thickness which is smaller than the depth of the hollow of the metal case. With such a structure, the porous sintered body does not project out from the metal case in the thickness direction, so that the porous sintered body is reliably protected. Further, a space is left in the metal case which can be utilized for loading sealing resin or storing the treatment liquid poured into the metal case to form a dielectric layer or a solid electrolyte layer so that the treatment liquid gradually permeates into the porous sintered body, which will be described later.

Preferably, the porous sintered body includes a first surface, and a second surface opposite to the first surface, and the first surface is bonded to the main plate portion of the metal case directly. Alternatively, the first surface of the porous sintered body is bonded to the main plate portion of the metal case via a bonding material containing valve metal powder. With such an arrangement, the porous sintered body is reliably fixed and held in the metal case, and the electrical connection between the metal case and the porous sintered body to make them act as an anode is reliably achieved.

Preferably, the metal case is provided with at least one anode terminal extending outward from the metal case. With such a structure, the soldering of the anode terminal to an intended mount region can be properly performed. Since the anode terminal is provided by utilizing the metal case, the entire structure is simplified.

Preferably, the metal case is provided with a plurality of anode terminals extending outward from the metal case so that a current can flow through the metal case via the anode terminals. With such a structure, the circuit current flows through the metal case and the porous sintered body, and the equivalent series inductance thereof blocks high-frequency noises. Therefore, the noise cancellation performance for a high frequency band is enhanced. When the capacitor is used for power supply, the equivalent series inductance becomes lower than that in a conventional structure, so that high response speed in power supply can be achieved.

Preferably, the anode terminal is integrally formed on the side plate portion of the metal case. With such a structure, the anode terminal is provided without increasing the number of parts, which is preferable for suppressing the manufacturing cost.

Preferably, the solid electrolytic capacitor further comprises a metal member made of the same material as the metal case and bonded to the metal case. Part of the metal member serves as the anode terminal. With such a structure, the metal case is reinforced by the metal member, and the metal member can also act as an anode.

Preferably, part of the solid electrolyte layer is provided on the second surface of the porous sintered body. The solid electrolytic capacitor further comprises a metallic connecting member made of metal and bonded to the part of the solid electrolyte layer. Part of the metallic connecting member serves as a cathode terminal. With such a structure, the provision of the cathode terminal can be achieved by a simple structure.

Preferably, the metal case is formed with a cutout, and part of the metallic connecting member extends from the inside to the outside of the metal case by passing through the cutout. With such an arrangement, the cathode terminal can be properly arranged outside the metal case while preventing undesirable electrical connection between the metal member and the metal case.

Preferably, the second surface of the porous sintered body includes a periphery formed with an insulating layer, and the part of the solid electrolyte layer on the second surface is formed at a region surrounded by the insulating layer. With such an arrangement, undesirable electrical connection between the solid electrolyte layer and the metal case can be properly prevented by a simple structure.

Preferably, the insulating layer is made of resin, and part of the resin is impregnated into a peripheral portion of the porous sintered body. With such an arrangement, the part of the solid electrolyte layer which is formed in the porous sintered body is easily and properly prevented from being connected to the metal case. At the periphery of the porous sintered body, particularly at the corners, the degree of sintering may be lower than at other portions. The resin insulates such portions with lower degree of sintering and reinforces such portions so as not to be easily damaged.

Preferably, the metal case includes an irregular inner surface, and the inner surface is bonded to the porous sintered body. In such a case, to the inner surface of the metal case, a metal member made of valve metal may be welded to form a projection. Alternatively or additionally to the above, a plurality of recesses and a plurality of burrs corresponding to the recesses may be formed at the inner surface of the metal case. The inner surface of the metal case may be provided with a plurality of projections formed by partially bulging the metal case. With such arrangements, the bonding strength between the porous sintered body and the metal case is increased.

Preferably, the metal case includes an opening which is closed with resin. With such an arrangement, the interior of the metal case is properly protected by the resin.

Preferably, the metal case includes an outer surface which is at least partially covered with resin. With such an arrangement, the protection and electrical insulation of the metal case is achieved properly.

Preferably, the solid electrolytic capacitor according to the present invention further comprises a dielectric layer and a solid electrolyte layer formed at the porous sintered body, an anode wire partially extending into the porous sintered body, a metal member electrically connected to the anode wire and including a portion serving as an anode terminal, and a cathode terminal electrically connected to the solid electrolyte layer. In this case, the metal case is electrically connected to the solid electrolyte layer, and the cathode terminal is provided at the metal case.

According to a second aspect of the present invention, there is provided a method for manufacturing a solid electrolytic capacitor. The solid electrolytic capacitor includes a metal case and a porous sintered body accommodated in the metal case. The manufacturing method comprises a first step of preparing the metal case, and a second step of preparing the porous sintered body.

Preferably, the second step includes compacting valve metal powder put in the metal case to provide a porous body, and heating the porous body together with the metal case to provide a porous sintered body.

Preferably, the second step includes bonding a porous body of valve metal powder into the metal case by using a bonding material containing valve metal powder, and heating the porous body with the metal case to provide a porous sintered body.

Preferably, the second step includes bonding a porous sintered body of valve metal powder into the metal case by using a bonding material containing valve metal powder.

Preferably, the first step includes subjecting a metal frame to drawing.

Preferably, the manufacturing method of the present invention further comprises the step of forming a dielectric layer and a solid electrolyte layer at the porous sintered body. The porous sintered body includes a bonding surface bonded to the metal case and a non-bonding surface which is not bonded to the metal case. The step of forming the dielectric layer and the solid electrolyte layer comprises forming the dielectric layer and the solid electrolyte layer at an interior and the non-bonding surface of the porous sintered body.

Preferably, the metal case includes an opening defined by a plurality of side plate portions, and the step of forming the dielectric layer and the solid electrolyte layer is performed by setting the metal case to be open upward and pouring treatment liquid for forming the dielectric layer or the solid electrolyte layer into the metal case through the opening.

Preferably, the manufacturing method of the present invention further comprises the step of forming an insulating layer at the periphery of the non-bonding surface of the porous sintered body before forming the solid electrolyte layer. The insulating layer prevents the solid electrolyte layer from being formed at the periphery of the non-bonding surface.

Preferably, the manufacturing method further comprises the step of providing, after the formation of the dielectric layer and the solid electrolyte layer, a metal member at the non-bonding surface of the porous sintered body so that the metal member is electrically connected to the solid electrolyte layer. Part of the metal member is extended out of the metal case to act as a cathode terminal.

Preferably, the manufacturing method of the present invention further comprises the step of loading resin into the metal case to seal part of the metal member with the resin after the metal member is provided at the non-bonding surface.

Preferably, the manufacturing method of the present invention further comprises the step of covering an outer surface of the metal case with resin.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2show a solid electrolytic capacitor (indicated by reference sign A1) according to a first embodiment of the present invention. The solid electrolytic capacitor A1includes a porous sintered body1, a metal case2and an auxiliary metal plate3.

The porous sintered body1is formed by compacting powder of so-called “valve metal” such as niobium or tantalum and the subsequent compacting, and is in the form of a flat rectangular plate. A dielectric layer and a solid electrolyte layer, which will be described later, are formed in the porous sintered body1and at a lower surface10bof the porous sintered body1.

The metal case2is formed by pressing a metal plate of niobium, for example. The case2comprises a main plate portion20in the form of a rectangular flat plate, and four side plate portions21extending downward from the periphery of the main plate portion20. The main plate portion20and the four side plate portions21define a hollow22which opens downward. The porous sintered body1is accommodated in the hollow22. The porous sintered body1has a thickness t which is smaller than the depth d of the hollow22. Thus, a space is provided under the porous sintered body1in the metal case2for arranging the auxiliary plate3, and a conductive layer50and resin42, which will be described later. The porous sintered body1has an upper surface10awhich is in direct contact with a lower surface of the main plate portion20of the metal case2. The porous sintered body1and the metal case2serve as an anode. A plurality of wires23made of niobium is welded to the lower surface of the main plate portion20. The wires23are embedded in the porous sintered body1, whereby the bonding strength between the porous sintered body1and the metal case2is enhanced.

An insulating resin layer40is formed on the outer surface of the metal case2. The resin layer40is made of thermosetting resin such as epoxy resins, for example. As shown inFIG. 3B, two of the side plate portions21of the metal case2are integrally formed with anode terminals24, respectively. Each of the anode terminals24extends from the lower edge of the side plate portion21to project out from the metal case2to be suitable for the surface mounting of the solid electrolytic capacitor A1.

As shown inFIG. 4, the porous sintered body1includes sintered portions11provided by sintering niobium powder, and a slight gap is defined between the sintered portions11. On the surfaces of the sintered portions11, a dielectric layer12made of niobium oxide, for example, is formed. On the surfaces of the dielectric layer12, a solid electrolyte layer13as a cathode is formed. The solid electrolyte layer13is made of manganese dioxide or conductive polymer, for example, and preferably, so formed as to completely fill the gap. (InFIG. 4, part of the solid electrolyte layer13is omitted.) However, the peripheral portion of the porous sintered body1is impregnated with insulating resin41a, and the solid electrolyte layer13is not formed at the resin-impregnated portion. Part of the resin41abulges downward from the lower surface10bof the porous sintered body1to provide an insulating layer41. The insulating layer41is in the form of a frame extending along the periphery of the surface10b. Of the solid electrolyte layer13, the portion13aon the surface10bof the porous sintered body1is formed limitedly so as not to cover the entirety of the insulating layer41. The insulating layer41prevents the solid electrolyte layer13from coming into contact with the side plate portions21of the metal case2and provides insulation between the solid electrolyte layer13and the metal case2. In the present invention, an electrolytic polymerization film may be formed on the portion13aof the solid electrolyte layer13.

The auxiliary metal plate3is electrically connected to the solid electrolyte layer13and in the form of a rectangular flat plate. The auxiliary metal plate3may be made of valve metal or may be made of copper alloy (or nickel alloy). The auxiliary plate3is bonded to the portion13aof the solid electrolyte layer13via the conductive layer50. The conductive layer50comprises a graphite layer51and a solidified silver paste layer52, for example. As shown inFIGS. 2 and 3, the auxiliary metal plate3is formed with a cathode terminal34extending from the inside to the outside of the metal case2by passing through a cutout25formed at the side plate portion21of the metal case2. The sealing resin42is provided in the metal case2to cover portions of the auxiliary metal plate3other than the cathode terminal34. The sealing resin42closes the downward opening of the metal case2.

An example of method for manufacturing the solid electrolytic capacitor A1will be described below.

First, a metal frame F′ having a shape as shown inFIG. 5Ais prepared. The metal frame F′ may be prepared by punching a flat plate made of niobium, for example, and includes a plurality of sections2′ each as the original form of a metal case2. The metal frame F′ is subjected to deep drawing to provide a metal frame F as shown inFIG. 5B. The metal frame F includes a plurality of metal cases2connected to each other via connecting portions24ain the form of a strip. After the metal frame F is prepared in this way, wires23made of niobium are welded to the main plate portions20of the metal cases2. (In the illustrated example, two wires23are welded to each of the cases2.) Preferably, before the welding of the wires23, the metal frame F is washed with hydrofluoro-nitric acid, for example. The washing may be performed after the welding of the wires23.

Subsequently, as shown inFIG. 6A, niobium powder11ais put into each of the metal cases2. Then, the powder11ais compacted using an appropriate pressing member65as shown inFIG. 6B. By the compacting, a porous body1A of niobium is provided. In the present invention, the porous body1A may be formed by performing the compacting operation a plurality of times instead of forming the porous body by single compacting operation. Specifically, as shown inFIG. 7, a porous sintered body1A′ having a thickness which is smaller than an intended thickness is formed by the first compacting operation. Subsequently, after niobium powder is added to the porous sintered body1A′, the second compacting operation is performed. Generally, in forming a porous body1A by compacting powder such as niobium powder, the degree of compaction is higher at portions close to the center of the porous body and lower at portions close to the periphery of the porous body. Therefore, to form a porous body having large length and width, it is preferable to make the degree of compaction generally uniform throughout the porous body by performing the adding and compacting of niobium powder a plurality of times.

After the porous body1A is formed, the porous body1A is heated in the state housed in the metal case2, as shown inFIG. 8. By sintering the niobium powder in this way, a porous sintered body1is provided. Preferably, the heating is performed in e.g. an argon gas atmosphere to prevent oxidation and nitriding.

After the porous sintered body1is formed, conversion treatment is performed to form a dielectric layer12in the porous sintered body1and at the inner surface of the metal case2. For instance, the conversion treatment is performed by pouring an aqueous solution of phosphoric acid12′ into the metal case2, as shown inFIG. 9. With the metal case2open upward, the aqueous solution of phosphoric acid12′ stored in the metal case2gradually permeates into the porous sintered body1from the upper portion toward the inner portion. As a result, the inside of the porous sintered body1and the inner surface of the metal case2are oxidized, whereby the dielectric layer12is formed. After the dielectric layer12is formed, the aqueous solution of phosphoric acid12′ can be easily discharged from the metal case2by turning over the metal case2, for example. Alternatively, in the present invention, the dielectric layer can be formed by a conventional technique, i.e., by immersing the porous sintered body in an aqueous solution of phosphoric acid stored in a vessel.

After the conversion treatment, an insulating layer41is formed on the periphery of the porous sintered body1, as shown inFIG. 10. Specifically, resin41ahaving flowability, for example, is applied onto the periphery of the porous sintered body1. Part of the resin41ais caused to sufficiently permeate into the periphery of the porous sintered body1. Thereafter, the resin41ais hardened, whereby the insulating layer41is provided.

Subsequently, a solid electrolyte layer13is formed. As shown inFIG. 11, this operation is performed by pouring treatment liquid13′ such as manganese nitrate solution or conductive polymer liquid into the metal case2. With this technique, similarly to the formation of the dielectric layer12described with reference toFIG. 9, the treatment liquid13′ stored in the metal case2gradually permeates into the porous sintered body1from the upper portion toward the inner portion. As a result, a solid electrolyte layer13made of manganese dioxide or conductive polymer is formed in the porous sintered body1and on the upper surface of the porous sintered body. In pouring the treatment liquid13′ into the metal case2, the level of the treatment liquid is kept lower than the resin layer41. This is because, when the level of the treatment liquid13′ becomes higher than the resin layer41, the resulting solid electrolyte layer13comes into contact with the side plate portions21of the metal case2, and the insulation therebetween cannot be achieved. In this way, the resin layer41serves to properly provide insulation between the solid electrolyte layer13and the metal case2.

As shown inFIG. 12, after the solid electrolyte layer13is formed, a conductive layer50is formed, and then an auxiliary metal plate3is bonded to the conductive layer. Subsequently, as shown inFIG. 13, sealing resin42for covering the auxiliary metal plate30is provided in the metal case2, and then a resin layer40is formed on the outer surface of the metal frame F. The sealing resin and the resin layer can be easily formed by loading or applying resin and then hardening the resin. By this operation, there is provided an aggregate of a plurality of solid electrolytic capacitors A1connected to each other via connecting portions24aof the metal frame F. Then, as shown inFIG. 14, each of the connecting portions24ais cut. By this cutting operation, each of the connecting portions24abecomes two anode terminals24, and a plurality of individual solid electrolytic capacitors A1are provided.

In the above-described manufacturing method, to provide the porous sintered body1accommodated in the metal case2, niobium powder is directly put into the metal case2and then compacted and sintered. Therefore, the productivity of the solid electrolytic capacitor A1is enhanced. Further, each of the dielectric layer12and the solid electrolyte layer13is formed by pouring the treatment liquid necessary for forming the layer into the metal case2to cause the treatment liquid to permeate into the porous sintered body1. This operation is easy and reliable, and the waste of the treatment liquid is small. Moreover, by the use of a metal frame F including a plurality of metal cases2, a plurality of solid electrolytic capacitors A1are obtained from the single frame F, which also enhances the productivity. Therefore, the manufacturing cost of the solid electrolytic capacitor A1can be advantageously reduced.

Advantages of the solid electrolytic capacitor A1will be described below.

The porous sintered body1is accommodated in the metal case2and hence protected by the metal case2. The porous sintered body1is reliably prevented from warping or cracking. Therefore, the porous sintered body1can be made large in length and width and flat to provide a solid electrolytic capacitor A1having a large capacitance and excellent frequency characteristics. The metal case2is made of niobium, similarly to the porous sintered body1, and acts as the anode. Therefore, the entire capacitance can be increased by the provision of the metal case2.

The metal case2has excellent heat dissipation ability and also serves to dissipate heat generated in using the solid electrolytic capacitor A1to the outside. In the illustrated example, the resin layer40is formed on the outer surface of the metal case2, preventing the metal case2from coming into direct contact with the outside air. However, since the case2is made of metal and has a high strength, the thickness of the resin layer40as the protective layer need not be large. Therefore, the resin layer40does not unduly hinder the heat dissipation by the metal case2. With such a structure, the temperature rise of the porous sintered body1is suppressed, so that the firing or fuming of the porous sintered body1is reliably prevented. Since the porous sintered body1is accommodated in the metal case2and further covered by the sealing resin42, its contact with air is prevented, which prevents the firing further reliably.

In the solid electrolytic capacitor A1, the metal case2serving as the anode includes the paired anode terminals24, and a circuit current can flow through the metal case2. Therefore, the noise cancellation performance is enhanced, as will be described below.

As shown inFIG. 15, the solid electrolytic capacitor A1is connected, in use, between a power supply71and a circuit72. The circuit72may comprise a CPU or an IC, for example. The paired anode terminals24of the solid electrolytic capacitor A1is connected in series to the wiring70aof the positive pole side from the power supply71to the circuit72. The cathode terminal34is connected to the wiring70bof the negative pole side. With such an arrangement, all the current flowing through the wiring70aof the positive pole side flows into the metal case2. The equivalent series inductance L1of the metal case2of the solid electrolytic capacitor A1is connected in series to the wiring70a. The equivalent series inductance L1acts as a resistor relative to an alternating current, and the resistance (impedance) is proportional to the frequency. Therefore, the higher the frequency of the noise the current flowing through the solid electrolytic capacitor A1includes, the larger resistance to the noise the equivalent series inductance L1provides. Thus, the solid electrolytic capacitor A1produces a large insertion loss in a high frequency band and hence can properly cancel noises in a high frequency band.

The solid electrolytic capacitor A1is electrically equivalent to a plurality of capacitors C1aof a small capacitance connected to each other as shown inFIG. 16. When a current including noises of a high frequency band flows through the solid electrolytic capacitor A1, the assembly of a small number of capacitors C1aact equivalently to a capacitor having a small capacitance and inductance. Therefore, the noises flow toward the cathode side through these capacitors C1aand are removed. On the other hand, when a current including noises of a low frequency band flows, a large number of capacitors C1aact as a capacitor having a large capacitance. In a low frequency band, the power loss of a capacitor is determined depending on the impedance which depends on the capacitance. The impedance is inversely proportional to the capacitance, and a larger capacitance provides a lower impedance with respect to a low frequency band. Therefore, in the solid electrolytic capacitor A1, noises of a low frequency band can also be properly removed.

Moreover, since the metal case2and the porous sintered body1have a small thickness, the current path in the thickness direction is short, so that the equivalent internal series resistance R1a, R2ais low. Therefore, the noises of the alternating current component readily flow to the cathode side, which also enhances the noise cancellation performance.

FIGS. 17-26show variations of the first embodiment described above. In these figures, the elements which are identical or similar to those of the first embodiment are designated by the same reference signs as those used for the first embodiment.

In the structure shown inFIG. 17, the metal case2is formed with four anode terminals24. The auxiliary metal plate3is formed with four cathode terminals34. The metal case2is formed with cutouts25for allowing the four cathode terminals34to pass therethrough. (In this figure, the illustration of parts other than the metal case2and the auxiliary metal plate3is omitted.)

In the structure shown inFIG. 17, four cathode terminals34are provided. Therefore, in flowing a current from the metal case2to the cathode side, the current can be distributed to the four cathode terminals34. Therefore, the internal resistance is reduced, which leads to the suppression of heat generation and the enhancement of the frequency characteristics. These advantages are not limited to the structure including four cathode terminals34and can be obtained by the provision of at least two cathode terminals34. One of the four anode terminals24may be connected to the input side of the wiring of the positive pole, whereas the other three of the four anode terminals24may be connected to output side of the wiring of the positive pole. This arrangement is equivalent to the arrangement in which respective inductances of the three anode terminals24are connected in parallel, and the inductance of the entirety of the output side becomes small. As a result, the speed of current output and the responsiveness in using the solid electrolytic capacitor for supplying power are improved. The number of anode terminals24is not limited to four, and the advantages can be obtained by providing at least three anode terminals. (One is connected to the input side, whereas the other two are connected to the output side.)

In the structure shown inFIG. 18, a metal member29in the form of a strip is welded to an outer surface of the metal case2. The longitudinally opposite ends of the metal member29provide anode terminals24which are bent to extend outward from the metal case2. Similarly to the metal case2, the metal member29may be made of niobium.

In the structure shown inFIG. 18, since the metal member29is welded to the metal case2, the metal case2is reinforced by the metal member29. Particularly when the metal member29extends from one end to the other end of the metal case2as shown in the figure, the metal case2is effectively reinforced. Therefore, it is possible to make the metal case2using a thin metal plate, which leads to the material cost reduction.

In the structure shown inFIG. 19, the opening of the metal case2is closed by a plate44made of resin. The plate44is formed with holes44ainto which respective base portions of the anode terminals24are inserted so that the plate44does not come off from the metal case2. This structure can be obtained by forming anode terminals24having a straight configuration as indicated by the phantom lines in the figure, inserting the anode terminals24into the holes22aof the plate44, and then bending the anode terminals24as indicated by the solid lines in the figure. With this structure, the plate44protects the interior of the metal case2. Further, the plate44reinforces the metal case2.

In the structure shown inFIG. 20, the entirety of the metal case2except for the anode terminals24and the cathode terminal34are sealed in sealing resin45. The opening of the metal case2is also closed by the resin45. With such a structure, both of the insulation of the outer surface of the metal case2and the closing of the opening are achieved by the sealing resin45alone. Therefore, as compared with the structure in which two separate resin members are used for such purposes, the manufacturing steps and hence the manufacturing cost can be reduced. Moreover, the metal case2and other intended portions can be sealed without leaving any gap.

In the structure shown inFIG. 21, a plurality of recesses26are formed at the lower surface of the main plate portion20of the metal case2. Each of the recesses26may be formed by cutting the main plate portion20, and the edge of the recess is formed with a burr27. The burr27cuts into the porous sintered body1. Part of the porous sintered body1enters the recess26.

With such a structure, similarly to the wires23described before, the burrs27have an anchoring effect, so that the bonding strength between the main plate portion20and the porous sintered body1is enhanced. Further, since part of the porous sintered body enters the recesses26, the bonding strength is further enhanced. The burrs27are inevitably formed in cutting the main plate portion20to form the recesses26by. Therefore, the operation to form the burrs27need not be performed separately from the operation to form the recesses26. The formation of the recesses26(and hence, the formation of the burrs27) is easier than the welding of the wires23to the metal case2. Moreover, since the formation of the recesses26and the burrs27does not require any member other than the metal case2, the manufacturing cost can be advantageously reduced.

In the present invention, the compaction of niobium powder11amay be performed by the method shown inFIG. 22A. Specifically, the niobium powder11aput into the metal case2is compacted by using an upper mold member75A and a lower mold member75B. In the compacting process, part of the main plate portion20of the metal case2is pressed upward by a plurality of pressurizing rods76bprovided at the lower mold member75B, whereby projections28are formed at the main plate portion20. Further, by using a plurality of pressuring rods76aprovided at the upper mold member75A, the niobium powder11aat portions above the projections28and the nearby portions is pressed more strongly than other portions. As a result, as shown inFIG. 22B, the porous body1A is obtained which is formed with recesses19at positions corresponding to the projections28of the metal case2. The porous body1A is then baked to become the porous sintered body1.

With the above method, the niobium powder11ais densely compacted at portions around the projections28of the metal case2to embed the projections28. Therefore, the bonding strength between the porous sintered body1and the metal case2is enhanced. Since the bonding strength is enhanced without welding an additional member to the metal case2or particularly working the metal case2by a process other than the pressing, the manufacturing cost can be reduced.

In the structure shown inFIG. 23, the porous body1A is prepared separately from the metal case2. Thereafter, the porous body1A is bonded to the metal case2via conductive paste77containing valve metal powder. The porous body1A is then heated while being accommodated in the metal case2, whereby the porous sintered body1is obtained. Also with this method, a porous sintered body which the present invention intends to provide can be manufactured.

Unlike the above method, the porous sintered body1after having undergone the sintering process may be bonded to the metal case2via the conductive paste77.

FIGS. 24-26illustrate a solid electrolytic capacitor A2according to a second embodiment of the present invention. The capacitor A2includes a porous sintered body1and an anode wire69penetrating through the porous sintered body1. The structure of the porous sintered body1itself is similar to that of a conventional porous sintered body made of niobium, and includes a dielectric layer and a solid electrolyte layer (not shown) formed on the surfaces of the sintered body of niobium powder. The metal case2is made of copper alloy or nickel alloy, for example, and bonded to the solid electrolyte layer on the upper surface of the porous sintered body1via a bonding material78ahaving an electrical insulating property. Similarly to the first embodiment, the porous sintered body1acts as an anode. However, the metal case2is insulated from the porous sintered body1and does not act as an anode.

The porous sintered body1is accommodated in the metal case2and is sealed in the sealing resin49. The anode wire69has opposite ends each of which a metal plate68is bonded to. Part of each metal plate68extends out of the metal case2, whereby a pair of anode terminals68ais provided. An auxiliary metal plate3is bonded to the solid electrolyte layer on the lower surface of the porous sintered body1via a conductive bonding material78b. Part of the auxiliary metal plate extends out of the metal case2to serve as a cathode terminal34.

In the solid electrolyte capacitor A2again, the porous sintered body1is accommodated in the metal case2. Therefore, similarly to the solid electrolytic capacitor A1, a large capacitance and excellent frequency characteristics can be obtained by making the porous sintered body large in length and width and flat while preventing the warping or cracking of the porous sintered body1. Moreover, since the anode wire69penetrates through the porous sintered body1, all the circuit current can be caused to flow into the anode wire69. Therefore, in the solid electrolytic capacitor A2again, the same effects as those described with reference toFIG. 15can be obtained, whereby the noise cancellation performance at a high frequency band is enhanced.

FIGS. 27 and 28show a solid electrolytic capacitor A3according to a third embodiment of the present invention. Similarly to the solid electrolytic capacitor A2, the solid electrolytic capacitor A3includes a porous sintered body1and an anode wire69penetrating through the sintered body. In the solid electrolytic capacitor A3, the metal case2is bonded to the electrolyte layer (not shown) on the upper surface of the porous sintered body1via a conductive bonding material78c. (That is, the metal case2is electrically connected to the solid electrolyte layer as the cathode.) The metal case2is integrally formed with a pair of cathode terminals34.

With such a structure, the same advantages as those of the solid electrolytic capacitor A2are obtained. Further, since the anode terminals34are integrally formed on the metal case2, the auxiliary metal plate3, which is provided in the solid electrolytic capacitor A2, is not necessary, whereby the manufacturing cost can be reduced.

The present invention is not limited to the foregoing embodiments. For instance, although the anode wire69of the solid electrolytic capacitors A2and A3penetrates through the porous sintered body1, the anode wire can be inserted in the porous sintered body1only partially so as not to penetrate through the porous sintered body1. Further, instead of providing the single anode wire69, a plurality of anode wires69may be provided.