JP2008182056A - Solid electrolytic capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid electrolytic capacitor which improves a quality of a dielectric film, has a small leakage current and has a high withstand voltage. <P>SOLUTION: In the solid electrolytic capacitor, a porous molded body is formed under pressure from a valve action metal powder which has been heated and a sintered body formed by sintering this porous molded body is used as an anode body. The valve action metal powder before forming under pressure is assorted into a secondary particle thermally aggregated or a primary particle not aggregated yet. Thereafter, the primary particle not aggregated yet is selectively removed, and the primary particle contained in the valve action metal powder is set at 20 wt.% or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid electrolytic capacitor used in various electronic devices, and more particularly to a solid electrolytic capacitor with a small leakage current and a high withstand voltage.

  In conventional solid electrolytic capacitors, single particles (primary particles) that are valve metal powders such as tantalum or niobium are thermally agglomerated by high-temperature and high-vacuum heat treatment to form secondary particles (granulation). A porous molded body is formed by pressure forming. Then, after sintering the porous molded body to form a porous sintered body, a dielectric coating is formed on the porous sintered body, and a solid electrolyte layer made of a conductive polymer or manganese dioxide as a cathode And a cathode lead layer composed of a graphite layer and a silver layer.

  An ideal solid electrolytic capacitor has an infinite impedance when a DC voltage is applied, so no current flows. However, in practice, even when a DC voltage is applied, the impedance does not become infinite, and a current (leakage current) flows. Leakage current is generated at a portion where the dielectric film is thin or at a minute defect portion where a part of the dielectric film which should be amorphous is crystallized and becomes conductive.

  In addition, the non-uniform dielectric film as described above also causes a decrease in withstand voltage of the solid electrolytic capacitor.

  As a means for improving the nonuniformity of the dielectric film of this solid electrolytic capacitor, it is known that reducing the oxygen concentration in the porous sintered body is extremely effective, and various attempts have been made. ing.

  For example, in patent document 1, the porous molded object which pressure-molded the valve action metal powder is sintered in a sintering chamber, and is forcedly cooled under a high vacuum. Then, the pressure of the sintering chamber was changed to atmospheric leak → re-vacuum → atmospheric leak → re-vacuum →.. The content of oxygen is reduced by removing the porous sintered body from the substrate.

According to this method, it was possible to reduce the concentration of oxygen contained in the porous sintered body by preventing natural oxidation of the sintered porous sintered body, but the oxygen contained in the valve action metal powder itself. Could not be reduced. Therefore, when the oxygen contained in the valve action metal powder itself is large, the oxygen concentration in the porous sintered body is still high, and it is difficult to improve the quality of the dielectric film.
JP 2001-135552 A

  Accordingly, an object of the present invention is to improve the quality of the dielectric film by paying attention to the oxygen concentration of the valve action metal powder, and to provide a solid electrolytic capacitor with low leakage current and high withstand voltage.

  In order to solve the above-mentioned problems, the present inventor suppresses the binding of primary particles to each other by the oxidized layer formed on the surface in the secondary particles generated by thermally aggregating the primary particles that are the valve action metal powder. Focusing on the existence of primary particles that have not been converted into secondary particles because they have been removed, pressure removal and sintering are performed after removing them, and a porous sintered body having a low oxygen concentration can be obtained. The headline and invention were completed.

The present invention provides a sintered body obtained by pressure-molding a heat-treated valve action metal powder and sintering the obtained pressure-formed body as an anode body, a dielectric film, a solid electrolyte layer, and a cathode lead A solid electrolytic capacitor in which layers are sequentially formed,
The non-aggregated single particles contained in the valve action metal powder are 20 wt% or less.

  In the present invention, the valve metal powder is preferably tantalum or niobium.

  According to the present invention, since a porous sintered body having a low oxygen concentration is obtained, the quality of the dielectric film can be improved, and a solid electrolytic capacitor having a low leakage current and a high withstand voltage can be provided.

  In each example and each comparative example, first, tantalum powder subjected to heat treatment at high temperature and high vacuum was subjected to thermal aggregation of secondary particles (average particle size: about 2 to 300 μm) and unaggregated primary particles (average) (Particle size: about 0.3 to 2.0 μm), and using a sieve.

[Example 1]
In Example 1, only the tantalum powder made into secondary particles was used.
This powder is pressure-molded together with a tantalum lead wire to produce a porous molded body, which is vacuum sintered at a high temperature to produce a porous sintered body. Then, an anodic oxidation voltage of 20 V is applied to the porous sintered body in an acidic solution for 3 hours to form a dielectric film inside the pores. Next, as a cathode, a solid electrolyte layer made of manganese dioxide, a graphite layer as a cathode lead layer, and a silver layer were sequentially formed to constitute a capacitor element.

  Subsequently, the lead wire and the anode terminal are connected to the capacitor element by welding, and further, the silver layer and the cathode terminal of the capacitor element are connected by a conductive adhesive, and then transfer molding is performed, and the rating is 6.3 V-10 μF. A chip-type solid electrolytic tantalum capacitor was manufactured.

[Example 2]
In Example 2, a tantalum powder containing 20 wt% of primary particles was used, and a chip-type solid electrolytic tantalum capacitor having a rating of 6.3 V-10 μF was produced in the same manner as in Example 1 except for this.

[Comparative Example 1]
In Comparative Example 1, a tantalum powder containing 30 wt% primary particles was used, assuming a conventional tantalum powder in which primary particles and secondary particles were not separated, and the other conditions were the same as in Example 1 except that the rating was 6 A chip type solid electrolytic tantalum capacitor of 3 V-10 μF was produced.

[Comparative Example 2]
In Comparative Example 2, a tantalum powder containing 50 wt% of primary particles was used as a more extreme conventional example than Comparative Example 1, and the other conditions were the same as in Example 1 except that the chip shape was rated 6.3 V-10 μF. A solid electrolytic tantalum capacitor was produced.

  Table 1 shows the results of comparison of the oxygen concentration of the porous sintered body in each example and each comparative example.

  As described above, the non-aggregated primary particles remaining after the heat treatment are those that are not converted into secondary particles because the bonding between the primary particles is suppressed by the oxide layer formed on the surface thereof. That is, as is apparent from Table 1, the concentration of oxygen contained in the porous sintered body can be reduced by selectively removing unaggregated primary particles from the tantalum powder after heat treatment.

  Next, a leakage current test (FIG. 1), a withstand voltage test (FIG. 2), and a high temperature load test (FIG. 3) were performed on the chip-type solid electrolytic tantalum capacitors (100 each) of each example and each comparative example. The results will be described.

FIG. 1 is a graph showing the results of a leakage current test. In this test, the leakage current immediately after a DC voltage of 6.3 V was applied for 1 minute between the cathode and anode terminals was measured.
From this result, it was confirmed that when the concentration of oxygen contained in the porous sintered body exceeds around 2500 ppm (between Example 2 and Comparative Example 1), the leakage current rapidly increases.

FIG. 2 is a graph showing the results of the withstand voltage test. In this test, the DC voltage applied between the cathode and anode terminals was gradually increased, and the applied voltage (withstand voltage) when the dielectric film was damaged was measured.
From this result, it was confirmed that the withstand voltage sharply decreased when the oxygen concentration in the porous sintered body exceeded 2500 ppm (between Example 2 and Comparative Example 1).

FIG. 3 is a graph showing the results of the high temperature load test. In this test, first, the leakage current (initial value) before performing the heat treatment in the reflow furnace was measured, and then the leakage current (after reflow) after passing through the 260 ° C. reflow furnace was measured. Subsequently, the leakage current after applying a rated DC voltage of 6.3 V at 125 ° C. for a predetermined time (500, 1500, 2000, 2500 hours) was measured. The leakage current measurement conditions were the same as the leakage current test of FIG.
From this result, it was confirmed that in Comparative Examples 1 and 2 in which the concentration of oxygen contained in the porous sintered body exceeded 2500 ppm, the leakage current deteriorated with time. On the other hand, in Examples 1 and 2 in which the content of primary particles is small, that is, the oxygen concentration in the porous sintered body is low, after applying a rated DC voltage of 6.3 V at 125 ° C. for 2500 hours, However, it can be seen that the leakage current hardly changes from the initial value.

  As described above, the tantalum powder after heat treatment is separated into secondary particles and unaggregated primary particles, and the primary particles contained in the tantalum powder to be pressure-molded are reduced to 20 wt% or less, thereby reducing leakage current. Further, it was confirmed that a solid electrolytic capacitor having a high withstand voltage can be obtained.

  In each of the above examples, tantalum was used as an example of the valve action metal, but other known valve action metals such as niobium could achieve the same effect.

It is a graph which shows the leakage current test result of a solid electrolytic capacitor. It is a graph which shows the withstand voltage test result of a solid electrolytic capacitor. It is a graph which shows the high temperature load test result of a solid electrolytic capacitor.

Claims (2)

Press-molded heat-treated valve action metal powder, and the sintered body obtained by sintering the obtained pressure-molded body is used as an anode body, and a dielectric film, a solid electrolyte layer, and a cathode lead layer are sequentially formed. A solid electrolytic capacitor comprising:
A solid electrolytic capacitor characterized in that unaggregated single particles contained in the valve action metal powder are 20 wt% or less.   The solid electrolytic capacitor according to claim 1, wherein the valve action metal powder is tantalum or niobium.

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