Solid electrolytic capacitor and method for preparing the same

Details Solid electrolytic capacitor and method for preparing the same Solid electrolytic capacitor and method for preparing the same

2002-10-17

2003-07-15

Dinkins, Anthony (Department: 2811)

Electricity: electrical systems and devices

Electrolytic systems or devices

Solid electrolytic capacitor

C361S525000, C361S528000

Reexamination Certificate

active

06594141

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for preparing the same.
2. Description of the Prior Art
Heretofore, a solid electrolytic capacitor comprises an anode, a dielectric, a semiconductor layer (solid electrolyte layer), and a cathode.
Generally, a solid electrolytic capacitor has a structure comprising an anode made of a metal exhibiting valve action (valve metal), an oxidized layer as a dielectric layer formed over the surface of the anode, a semiconductor layer (solid electrolyte layer) formed on the dielectric layer, and a cathode formed on the semiconductor layer.
In this connection, the valve metal means a metal capable of forming an oxidized layer whose thickness can be controlled by anodic oxidation. Specifically, valve metal includes niobium (Nb), aluminum (Al), tantalum (Ta), titanium (Ti), hafnium (Hf) and zirconium (Zr). Actually, however, aluminum and tantalum are mainly used.
In the following, a structure and a preparation method of a conventional tantalum (Ta) solid electrolytic capacitor will be described with reference to the drawings.
FIG. 4
is a sectional view showing a structure of a conventional tantalum (Ta) solid electrolytic capacitor.
As shown in
FIG. 4
, the solid electrolytic capacitor
1
using tantalum (Ta) comprises an anode body
11
which is provided with an element lead wire
11
a
partially inserted therein and which is formed by sintering a tantalum (Ta)-based mixed powder, a dielectric layer
12
formed over the surface of the anode body
11
, an electrically conductive polymer layer
131
as a semiconductor layer
13
which is formed on the surface of the dielectric layer
12
, a graphite paste layer
141
as a cathode body which is formed on the semiconductor layer
13
, and a silver (Ag) paste layer
15
formed on the cathode body
14
.
To the element lead wire
11
a
of the anode body
11
and the silver (Ag) paste layer
15
, lead frames
52
are connected, respectively. The resultant is sheathed with a resin by molding with end portions of the lead frames out.
In the next place, a method for preparing a conventional tantalum (Ta) solid electrolytic capacitor will be described with reference to FIG.
5
.
FIG. 5
is a flow chart showing a method for preparing a conventional solid electrolytic capacitor.
(1) Formation of Tantalum (Ta) Porous Body (S
1
)
i) Preparation of Tantalum (Ta)-based Powder
To improve press-moldability, a binder is added to a tantalum (Ta) powder, and the addition is followed by mixing.
ii) Press Molding and Sintering
An element lead wire of an anode is partially inserted in the tantalum (Ta)-based powder, and the resultant was press-molded into a cylindrical or parallelepipedonal shape.
Then, the press-molded product is sintered by heating at a temperature of 1,300° C. to 2,000° C. under high vacuum (10.sup.−4Pa or higher vacuum) to form a tantalum (Ta) porous body, i.e., an anode body.
(2) Formation of Dielectric Layer (S
2
)
Chemical Conversion Treatment (S
2
a
)
The tantalum (Ta) porous body as an anode was soaked in an electrolytic aqueous solution such as a phosphoric acid aqueous solution together with a counter electrode, and a chemical conversion voltage (formation voltage) is applied to thereby form an oxidized tantalum (Ta) layer as a dielectric layer over the surface of the tantalum (Ta) porous body. (anodic oxidation method)
The thickness of the dielectric layer (oxidized tantalum (Ta) layer) is dependent upon the condition of the chemical conversion voltage (Vf: formation voltage), and characteristics as a capacitor are in turn dependent upon the thickness of the oxidized tantalum (Ta) layer. As the electrolytic solution, there may be used an aqueous solution of phosphoric acid of which concentration is adjusted to 0.6%, or the like.
(3) Formation of Semiconductor Layer (Electrolyte Layer) (S
3
)
On the oxidized layer formed over the tantalum (Ta) porous body in the preceding step, a solid electrolyte layer is formed as a semiconductor layer.
As the solid electrolyte, there may be used manganese dioxide, or an electrically conductive polymer obtained by polymerizing a monomeric material such as pyrrole, thiophene or a derivative thereof.
For example, when a pyrrole polymer is used as the solid electrolyte, a solid electrolyte layer is formed on the dielectric layer formed over the surface of the anode body by effecting chemical polymerization or electrolytic polymerization using a pyrrole monomer solution and a solution of an oxidizing agent such as iron trichloride, as disclosed in Japanese Unexamined Patent Publication No.2001-160318A by Fukunaga et al.
For forming the electrically conductive polymer, a process may be employed which comprises preliminarily applying an oxidizing agent to the surface of the dielectric layer, and then soaking the resultant in a monomer solution to effect polymerization reaction, as disclosed in Japanese Unexamined Patent Publication No.2000-216061A by the present inventor.
When manganese is used as the solid electrolyte, the anode body with the dielectric layer formed over the surface thereof is soaked in manganese nitrate and heat-treated. The soaking, the heat-treatment and the like are sequentially conducted to thereby form a solid electrolyte layer.
(4) Re-Treatment for Chemical Conversion (S
4
)
In the step of forming the semiconductor layer (solid electrolyte layer), the dielectric layer is likely to be damaged by the heat-treatment conducted in the step. It is particular when manganese is selected as a material of the semiconductor layer (solid electrolyte layer). To mend the damaged portions of the dielectric layer, the anode body with the sequentially formed dielectric and semiconductor (solid electrolyte) layers is soaked in the liquid for chemical conversion.
(5) Formation of Cathode Body (S
5
)
Formation of Graphite Paste Layer (S
5
a
), and Formation of Silver (Ag) paste layer (S
6
)
A graphite layer as a cathode layer is formed on the semiconductor layer (solid electrolyte layer), and a silver (Ag) paste layer is formed thereon. With respect to the formation of the graphite layer, a method disclosed in Japanese Unexamined Patent Publication No.1999-297574 by the present inventor may be employed.
(6) Connection of Lead Frames (S
7
), and Sheathing by Molding (S
8
)
Then, a lead frame for the anode is connected to the element lead wire of the anode body by spot welding, and a lead frame for the cathode is connected to the silver paste layer with an electrically conductive adhesive.
Finally, the resulting capacitor element is sheathed with a resin by molding with end portions of the lead frames out to complete a tantalum (Ta) solid electrolytic capacitor having a structure as shown in FIG.
4
.
However, the tantalum (Ta) solid electrolytic capacitor prepared through the above-described steps has the following problems.
In the step of soaking the anode body, which has been soaked in the liquid for chemical conversion and is thereby provided with the dielectric layer formed over the surface thereof, in the oxidizing agent-containing solution, and air-drying the resultant, the oxidizing agent-containing solution tends to gather around the edges of the anode body with the dielectric layer formed over the surface thereof because of its surface tension. As a result, the semiconductor layer, i.e., solid electrolyte layer which is formed on the dielectric layer is likely to have a non-uniform thickness.
If the semiconductor layer (solid electrolyte layer) has non-uniformity in thickness, the semiconductor layer is liable to be damaged by heat-treatment conducted in the step of sheathing with a resin to cause separation between the layers and/or cracking of the layer.
Further, there is an undesired possibility that if the semiconductor (solid electrolyte layer) is damaged, the dielectric layer is damaged due to the damage of the semiconductor layer. This causes a drawback that leakage current (hereinafter referred to as LC) is increased by influence of heat

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