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

2002-11-21

2003-12-30

Dinkins, Anthony (Department: 2836)

Electricity: electrical systems and devices

Electrolytic systems or devices

Solid electrolytic capacitor

C361S524000

Reexamination Certificate

active

06671167

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 has comprised an anode, a dielectric layer, an electrolyte layer, and a cathode. Generally, a solid electrolytic capacitor has had 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, an electrolyte layer as a semiconductor layer formed on the dielectric layer, and a cathode (made of graphite or the like) formed on the electrolyte 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.
With respect to aluminum (Al), a foil is generally used as the anode, and with respect to Ta, a porous body prepared by sintering a Ta-based powder is used as the anode.
Of those solid electrolytic capacitors, a solid electrolytic capacitor having a porous sintered body type is particularly adaptable to miniaturization and capable of being adapted to have a high capacity, and hence there is strong demand therefor as a part which meets needs of miniaturization of a cellular phone, information terminal equipment or the like.
In the following, a structure and a preparation method of a conventional Ta solid electrolytic capacitor will be described with reference to the drawings.
FIG. 4
is a sectional view showing a structure of a conventional Ta solid electrolytic capacitor.
As shown in
FIG. 4
, the conventional solid electrolytic capacitor
1
using Ta comprises an anode body
11
which is provided with an element lead wire
11
a
implanted therein and which Is formed by sintering a Ta-based mixed powder, a dielectric layer
12
formed over the surface of the anode body
11
, an electrically conductive polymer layer as an electrolyte layer
13
which is formed on the surface of the dielectric layer
12
, a graphite paste layer
14
as a cathode body which is formed on the electrolyte layer
13
as a semiconductor layer, and a silver (Ag)-containing 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)-containing 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
52
out.
In the next place, a method for preparing a conventional 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 Ta Porous Body (S
1
)
(i) Preparation of Ta-based Powder
To improve press-moldability, a binder is added to a Ta powder, and the addition is followed by mixing.
(ii) Press Molding and Sintering
An element lead wire for an anode is partially inserted in the 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 1,300degreeC. to 2,000degreeC. under high vacuum (10
−4
Pa or lower pressure) to form a Ta porous body, i.e., an anode body.
(2) Formation of Dielectric Layer (S
2
)
Chemical Conversion Treatment (S
2
a
)
The 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 Ta layer as a dielectric layer over the surface of the Ta porous body. (anodic oxidation method)
The thickness of the dielectric layer (oxidized Ta layer) is dependent upon the chemical conversion voltage (Vf: formation voltage) when an electric current is fixed, and characteristics as a capacitor are in turn dependent upon the thickness of the oxidized 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 Electrolyte Layer (S
3
)
On the oxidized layer formed over the Ta porous body in the preceding step, a solid electrolyte layer is formed. (S
3
a
)
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.2001-160318 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.2000-216061 by one of the present inventors.
(4) Re-Treatment for Chemical Conversion (S
4
)
In the step of forming the solid electrolyte layer, the dielectric layer is likely to be damaged. To mend the damaged portions of the dielectric layer, the anode body with the sequentially formed dielectric and solid electrolyte layers is subjected again to the chemical conversion treatment.
(5) Formation of Cathode Body (S
5
)
Formation of Graphite Paste Layer (S
5
a
), and Formation of Silver (Ag)-Containing Paste Layer (S
6
)
A graphite layer as a cathode layer is formed on the solid electrolyte layer, and a silver (Ag)-containing paste layer is formed thereon. With respect to the formation of the graphite layer, a method disclosed in Japanese Unexamined Patent Publication NC. 1999-297574 by one of the present inventors 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 (Ag)-containing 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 Ta solid electrolytic capacitor having a structure as shown in FIG.
4
.
However, the Ta solid electrolytic capacitor prepared through the above-described steps has problems which affect basic characteristics thereof, i.e., formation of so-called defective portions and lowering of electrical conductivity of the electrolyte layer.
In the first place, description will be given with respect to the defective portions of the dielectric layer.
The defective portions of the dielectric layer mean portions where the dielectric layer has insufficient thicknesses, and specifically, mean locally concave areas where the dielectric layer has smaller thicknesses such as cracks or concave areas having thicknesses smaller than the intended thickness of the dielectric layer such as portions which have undergone exfoliation of the dielectric layer.
Causes of the formation of defective areas include (A) inclusion of impurities in Ta, (B) irregularity of current in the chemical conversion step and (C) external mechanical stress, and it is believed that any of these causes cannot completely be prevented from occurring in the existing methods for preparing a solid electrolytic capacitor.
Since thicknesses of the dielectric layer in the defective portions are smaller than that in the other area, local intensifications of field strength are likely to occur in such defective portions. Locally elevated te

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