JP4648202B2 - Method for manufacturing anode element for solid electrolytic capacitor - Google Patents

Description

  The present invention relates to a method of manufacturing an element that serves as an anode of a solid electrolytic capacitor in which an anode is formed of a sintered body of valve action metal powder such as tantalum or niobium.

In recent years, in order to reduce the size of a solid electrolytic capacitor and increase its capacitance as much as possible, the particle size of the valve metal powder constituting the anode is made as small as possible to increase the specific surface area of the powder, that is, the powder. It is widely practiced to select a high CV value.
However, it is also known that when the specific surface area of the valve action metal powder increases, the oxygen concentration contained in the valve action metal powder generally increases and adversely affects the leakage current value of the solid electrolytic capacitor.

  In addition, when an electrode lead wire is embedded in the anode element, when filling a predetermined molding die with the valve action metal powder, the wire is inserted together and then pressure molding is performed.

In the above background, as a method of reducing the leakage current by suppressing the oxygen concentration in the element, a valve element metal powder having an electrode lead wire embedded therein is pressure-molded and sintered in vacuum to obtain a sintered body element. After that, a method of reducing the internal oxygen by heat-treating the sintered body element together with a reducing agent such as magnesium has been proposed, and the anode element subjected to the reduction process is fired again at a temperature lower than the sintering temperature. As a result, a method for stabilizing the element has also been proposed. (For example, see Patent Document 1)
With this technique, the oxygen concentration of the sintered body element can be reduced and the leakage current can be reduced.

On the other hand, in a solid electrolytic capacitor, the anode element is anodized in a chemical conversion solution, an oxide film layer serving as a dielectric is formed on the element surface, and then impregnated with a solid electrolyte layer such as manganese dioxide or a conductive polymer. Alternatively, the cathode is formed by coating, but it is also conceivable to use a valve metal powder having a high CV value in order to reduce the overall size and increase the capacity.
However, if the CV value is simply increased, the specific surface area is increased, so that the surface of the sintered anode element and the internal voids are reduced. For this reason, it is known that the formation of the solid electrolyte layer on the surface of the oxide film layer becomes non-uniform, and as a result, the electrical characteristics of the solid electrolytic capacitor, particularly tan δ and ESR, are adversely affected.

Also, in order to reduce tan δ and ESR, in general, the density of the anode element is reduced, that is, the amount per unit volume of the powder body during pressure molding is reduced to increase the void inside the sintered element. There is a method of homogenizing the formation state of the solid electrolyte on the surface of the metal oxide after the chemical conversion treatment.
JP 07-142290 A

  In recent years, the expansion of the specific surface area of valve metal powder (high CV of valve metal powder) has progressed, and after sintering at a high sintering temperature as before, the sintered body element is formed. There was a problem that the capacitance was lowered.

  One way to prevent this decrease in capacitance is to lower the sintering temperature and the reduction temperature. However, the CV of the valve metal powder has increased, and the oxygen concentration of the valve metal powder has decreased. It has also been found that in the situation where the oxygen adsorption amount of the sintered body element increases and the reduction temperature is lowered, there is a problem that if the reduction temperature is lowered, the reduction ability is lowered and the oxygen concentration reduction effect cannot be sufficiently achieved.

  On the other hand, if the reduction temperature is too high, the whole becomes brittle, and the electrical connection between the reduced valve metal powder and the electrode lead wire is reduced, and the reduced electrical connection is recovered by subsequent sintering. In addition, there is a problem that the contact resistance between the anode and the lead wire increases.

  Furthermore, as described in [0006], as a method for reducing tan δ and ESR, the molding density when pressure-molding the valve metal powder is lowered (the amount of powder filling during molding is reduced), It is common to increase the void inside the sintered body element, but if the molding density is lowered, the strength of the entire element will be weakened, so the joint strength after sintering of the valve metal and electrode lead wire will decrease. However, there is also a problem that the leakage current characteristic is deteriorated due to thermal and physical stress during the production of the solid electrolytic capacitor.

  An object of the present invention is to provide a method for manufacturing an anode element for a solid electrolytic capacitor having a high electrostatic capacity, which solves the above-described problems and has a good leakage current characteristic and a good tan δ · ESR characteristic.

  In addition to the improvement of the electrical characteristics, there is provided an effective element manufacturing method for providing a very dense and strong anode body for an electrolytic capacitor with high bonding strength between the valve action metal and the electrode lead wire. .

  In view of the above, the present invention includes a step of pressure-molding a valve-acting metal powder in which an electrode lead wire is embedded, and a step of performing a first sintering of the formed body in a vacuum to form a pre-sintered element. The step of heat-treating the temporary sintered body element in the presence of a reducing substance and the second sintering of the temporary sintered body element subjected to the reduction treatment in vacuum at a temperature higher than the first sintering temperature. And a step of sequentially performing the steps.

As a preferred embodiment of the present invention, when the first sintering temperature is T1, the heat treatment temperature during reduction is T2, and the second sintering temperature is T3, these temperature conditions are T2 <T1 <T3.
A method of manufacturing a capacitor element selected for the relationship is provided.

  Furthermore, the manufacturing method of the capacitor | condenser element which selects the heat processing temperature T2 at the time of a reduction | restoration in the range of 800-900 degreeC on the said temperature conditions is provided.

  Further, the present invention is a manufacturing method that is particularly effective for an anode element in which the particle size of the valve action metal powder is selected to be 70 kCV or more.

  As described above, in the present invention, after pressure-molding the valve action metal powder into a predetermined shape, the first sintering step of the molded body is a preliminary sintering step at a temperature lower than that during the second sintering. Since the second sintering after the reduction process is re-sintered at a temperature higher than the first sintering temperature as the main sintering process, the anode body is not affected even when the specific surface area of the valve action metal powder is increased. An increase in oxygen concentration is suppressed, and deterioration of leakage current characteristics can be prevented.

  Furthermore, as shown in the second and third inventions, the heat treatment temperature for reduction with a reducing substance is selected in the range of 800 to 900 ° C., and the temporary sintering temperature is slightly higher than this and lower than the main sintering temperature. By selecting, reduction of the oxygen concentration by a reduction process is achieved effectively, and the increase in oxygen concentration by this sintering is also suppressed.

  Furthermore, since the capacitance does not decrease even if the main sintering is performed at a high temperature in a state where the metal powder having a high CV value is firmly and densely pressed, without reducing the molding density of the valve action metal powder, A dense and strong sintered body can be obtained, the bonding strength with the electrode lead wire and the electrical continuity can be improved, resulting in a decrease in leakage current and an improvement in tan δ and ESR.

  In addition, as the valve metal that can be used in the present invention, all of the anode metals conventionally used for this type of solid electrolytic capacitor such as niobium and tantalum can be used, and the particle size of the powder is as small as 80 kCV. Since it can be used effectively, a small and large-capacity solid electrolytic capacitor having better electrical characteristics can be obtained with the conventional materials.

  As the reducing substance used in the reduction step, various reducing agents such as magnesium, carbon, aluminum, and hydrogen can be used. The reducing atmosphere is preferably performed in an argon stream, but may be in a vacuum.

  If the heat treatment temperature during this reduction is less than 800 ° C., if the specific surface area of the anode element block is large, the reduction action becomes insufficient and an effective reduction in oxygen concentration cannot be obtained. On the other hand, when the temperature exceeds 900 ° C., the electrical connection between the valve action metal powder and the electrode lead wire is deteriorated, and as a result of the experiment, it has been found that this cannot be improved by the subsequent main sintering as shown in FIG. The reduction temperature is most preferably 800 to 900 ° C.

  As described above, by processing the anode element by the manufacturing method of the present invention, a solid electrolytic capacitor having a small size and a high capacity, a small leakage current, and an excellent tan δ · ESR characteristic can be obtained.

Hereinafter, a preferred embodiment in the case where tantalum is used as the valve action metal will be outlined.
First, tantalum powder with a fine particle size of about 70 kCV is put into a molding machine and pressure-molded by a normal method. By pressing and compressing the aggregate, a molded body in which the wire is embedded is obtained. (Pressure forming process)

  Next, this wire-attached tantalum powder compact is pre-sintered by heating in a vacuum furnace at a temperature of 1250 to 1275 ° C. (a temperature lower by 25 to 50 ° C. than the main sintering temperature, which is a subsequent step) for 5 to 10 minutes. To do. (First sintering step)

  This temporary sintered body element is heat-treated with a reducing substance such as magnesium and aluminum in a vacuum or an inert gas such as argon to reduce and remove oxygen in the temporary sintered body element. (Reduction process)

  Then, the reduction-treated temporary sintered body element is again subjected to main sintering at a main sintering temperature, that is, 1300 to 1350 ° C. for 10 to 15 minutes in a vacuum. (Second sintering step)

Hereinafter, supplementary matters in the basic embodiment will be described using data drawings confirmed by experiments.
According to the experiments by the present inventors, a capacitor anode element having a higher capacity can be obtained when the first sintering temperature is lower. When the sintering temperature is lowered, the valve action metal powder and the electrode lead electrode are extracted after reduction. It was found that the electrical connection with the wire was lowered and the contact resistance was increased.
Therefore, as a condition for maintaining a high capacity and reducing the contact resistance with the wire, a temperature lower than the main sintering temperature of the valve action metal, that is, a temperature lower by about 25 to 50 ° C. than the main sintering temperature 1300 to 1350 ° C. in the case of tantalum. It was found that 1250 to 1275 ° C. is effective, and the sintering time is preferably about 5 to 10 minutes because the sintering progresses and the capacitance decreases with increasing time. Therefore, this process was positioned as a pre-sintering process.

In addition, the higher the heat treatment temperature in the reduction step is, as shown in FIG. However, it was also demonstrated that during the second main sintering, the reduced pre-sintered element again adsorbs oxygen and the oxygen concentration increases by about 800 ppm.
Therefore, it is necessary to select a reduction temperature range in which the oxygen concentration of the sintered body element after the main sintering does not exceed the oxygen concentration of the pre-reduction sintered body element. That is, as can be seen from FIG. 1, the oxygen concentration of the device can be kept low after the main sintering if it is reduced at a heat treatment temperature of 750 ° C. or higher.
On the other hand, if the reduction temperature is too high, as shown in FIG. 2, the electrical bonding between the pre-sintered valve action metal powder and the electrode lead wire is lowered, and the contact failure rate is rapidly increased. Moreover, it has been experimentally confirmed that this contact failure is not recovered even by the second sintering, that is, main sintering.

  Therefore, this reduction temperature is extremely important, and the present inventors consider the conditions of both the effect of reducing the oxygen concentration and ensuring the tight junction between the electrode lead wire and the anode element block, and the temperature of 800 to 900 ° C. It was found that reducing heat treatment in the range (in the case of tantalum powder) is most effective.

  Thereafter, the second main sintering temperature is heated and sintered at a normal sintering temperature of the valve action metal powder, that is, about 1300 to 1350 ° C. in the case of tantalum.

As described above, in the best embodiment of the present invention, when the first sintering temperature is T1, the reduction heat treatment temperature is T2, and the second sintering temperature is T3, the temperature condition of each step is T2 = 800 to 900 ° C. <T1 <T3
It is desirable to satisfy this relationship. As a result, a very dense and strong capacitor element with good electrical connection with the wire can be obtained, and the electrical characteristics of the solid electrolytic capacitor manufactured with this element can be remarkably improved.

Next, Table 1 compares the sintered element characteristics (oxygen concentration and sintered body CV value) of the anode element for a solid electrolytic capacitor manufactured by the above-described method of the present invention and the sintered element characteristics of a conventional equivalent product. Show.
The anode element shown in Table 1 is a molded product for a 2012 size product (rectangular size with a length of 2.0 mm and a width of 1.2 mm) obtained by pressure-molding tantalum powder with a nominal 70 kCV at a molding density of 5.70 g / cc. The tantalum sintering temperature is 1300 ° C., but the element of the present invention is an element obtained by performing the above-described preliminary sintering step and reduction step before this sintering. The element indicated as the anode element is an element that has not undergone the preliminary sintering step and the reduction step.
The sintered body CV value was expressed as the product of the capacity per unit weight of the element and the voltage.

  As can be seen from Table 1, it can be seen that the anode element of the present invention has substantially the same CV value and reduced oxygen concentration as compared with the sintered element obtained by the conventional method.

(Example)
Hereinafter, the leakage current, tan δ, and ESR of an example of a solid electrolytic capacitor using a capacitor element in which an oxide film layer and a cathode layer are formed on a surface of an anode element manufactured by the manufacturing method of the present invention by a normal method are shown. The measured results are shown in Table 2.

  Example 1 embeds an electrode lead wire, press-molds a nominal 70 kCV tantalum powder, and sinters in a vacuum at a first sintering temperature of 1250 ° C. for 5 minutes to obtain a temporary sintered body element. Then, this temporary sintered body element was heat-treated with magnesium at 850 ° C. for 60 minutes in a vacuum to reduce and remove oxygen in the sintered body element. After the reduction, the temporarily sintered body element was acid-washed with sulfuric acid, and then sintered in vacuum at a second sintering temperature of 1300 ° C., which is the sintering temperature of the valve action metal powder, for 10 minutes. Anodized to form an oxide film layer, impregnation into manganese nitrate aqueous solution, thermal decomposition is repeated a plurality of times to form a solid electrolyte layer composed of manganese dioxide, and then a carbon lead layer and a cathode lead layer composed of a silver layer are sequentially formed. Formed. Subsequently, the anode lead and the anode lead frame are connected by resistance welding, and after the cathode lead layer and the cathode lead frame are connected by a conductive adhesive, the resin is sheathed by transfer molding, and the frame is bent along the sheathing resin. In the example of the completed solid electrolytic capacitor, the size of the finished product is set to 2012 size (length 2.0 mm × width 1.2 mm).

  In Example 2, the size was 3216 size (3.2 mm × 1.6 mm) under exactly the same conditions as in Example 1, and in Example 3, a 3528 size product having a larger size under the same conditions as in Example 1 ( 3.5 mm × 2.8 mm).

  Example 4 is an example in which fine particles of valve action metal powder of 80 kCV are used and manufactured under the same conditions as in Example 1, and the size of the finished product is a large size of 3528 as in Example 3. It is.

(Conventional example)
Table 3 shows the measurement data of the conventional example. For comparison with the embodiment of the present invention, the tantalum powder having the same CV value as that of the embodiment of the same size was manufactured and the electrical characteristics were measured. However, the anode element is an example manufactured by the conventional method shown in the following [0042] [0043].

  Conventional Examples 1 and 2 are 2012 size products manufactured from nominal 70 kCV tantalum powder, Conventional Examples 3 and 4 are 3216 size products manufactured from nominal 70 kCV tantalum powder, and Conventional Examples 5 and 6 are from nominal 70 kCV tantalum powder. The manufactured 3528 size product, Conventional Examples 7 and 8 are 3528 size products manufactured from tantalum powder having a nominal 80 kCV.

Conventional examples 1, 3, 5, and 7 are pressure-molded with the same density (5.70 g / cc) as in Examples 1 to 4 to each tantalum powder in which an electrode lead wire is embedded, and calcined. A solid electrolytic capacitor was produced in the same manner as in Examples 1 to 4 except that the sintering step and the reduction step were not performed and the valve action metal powder was sintered at 1300 ° C. for 10 minutes in a vacuum.
In the conventional examples 2, 4, 6, and 8, each tantalum powder in which an electrode lead wire is embedded is pressed into each of the above sizes at a density (5.40 g / cc) lower than those in Examples 1 to 4, and calcined. A solid electrolytic capacitor was produced in the same manner as in Examples 1 to 4 except that the sintering step and the reduction step were not performed and the valve action metal powder was sintered at 1300 ° C. for 10 minutes in a vacuum.
Table 3 shows the leakage current, tan δ, and ESR characteristics of the eight types of products thus obtained.

  When comparing the electrical characteristics of Examples 1 to 4 in Table 2 and Conventional Examples 1, 3, 5, and 7 in Table 3 having the same molding density, the solid electrolytic capacitor produced from the element of the present invention is as follows. It can be seen that the leakage current is low and the tan δ · ESR is also low. Further, even when the electrical characteristics of Examples 1 to 4 in Table 2 and Conventional Examples 2, 4, 6, and 8 in Table 3 with a reduced molding density are compared, it can be seen that tan δ · ESR is low.

  As is clear from this result, it was proved that the product manufactured by the element of the embodiment of the present invention has a low leakage current, and tan δ · ESR is reduced without reducing the molding density.

A graph showing the relationship between the heat treatment temperature during reduction and the oxygen concentration in the temporary sintered body after reduction after reducing the sintered body obtained by pressure-molding and sintering tantalum powder. is there. It is a graph which shows the heat processing temperature at the time of the reduction | restoration process of a sintered compact element, and the relationship of the electrical joining defect rate of the valve action metal powder and the electrode extraction wire.

Claims (3)

A step of forming a molded body by press-molding a tantalum powder in which an electrode lead wire is embedded, and a first sintering step of performing a first sintering of the molded body in a vacuum to form a sintered body element And a reduction heat treatment step in which the sintered body element is heat- treated at a heat treatment temperature in the range of 800 to 900 ° C. in the presence of a reducing substance, and the sintered body element subjected to the reduction heat treatment is subjected to second sintering in a vacuum. In the method of manufacturing a sintered body element of a capacitor by sequentially performing the second sintering step to be performed,
A method of manufacturing a capacitor element, wherein the first sintering step is a pre-sintering step of sintering at a temperature lower than the second sintering temperature. Said first sintering temperature T1, and the heat treatment temperature for the reducing T2, when the second sintering temperature is T3, these temperature conditions T2 <T1 <T3
The capacitor element manufacturing method according to claim 1, wherein the relationship is selected. 2. The first sintering step, the reduction heat treatment step, and the second sintering step are sequentially performed in a state where a fine powder size of 70 kCV or more is selected in advance. 2. A method for producing a capacitor element according to 2 .

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