Protective coating for electrolytic capacitors

4,864: 4,910. 4,934: 4,945. 4,984. 5,005: 5,017. 5,119: 5,135. 5,187: 5,198.

,472 ,645 ,033 ,452 ,134 ,107 ,272 ,274 ,618 ,650 ,968

9/1989 3/1990 6/1990 7/1990 1/1991 4/1991 5/1991 6/1992 8/1992 2/1993 3/1993

Yoshimura et al.
Jonas et al.
Harakawa et al.
Sturmer et al.
Locke

Kobashi et al.
Kamigawa
Kinuta et al.
Saiki et al.
Kudoh et al.
Galvagni

(List continued on next page.) FOREIGN PATENT DOCUMENTS

EP EP EP GB JP

0507315 0559109 0571329 1033020 6037114

10/1992 9/1993

11/1993 6/1966 2/1985

OTHER PUBLICATIONS

Abstract of Japanese Patent No. 01-253226.
Abstract of Japanese Patent No. 02-074016.

(List continued on next page.)

Primary Examiner—Anthony Dinkins

(74) Attorney, Agent, or Firm—Dority & Manning, PA.

(57)

ABSTRACT

A solid electrolytic capacitor having an anode that contains a valve-action metal (e.g., tantalum, niobium, and the like) and a dielectric film overlying the anode is provided. The capacitor also include a protective coating overlying the dielectric film, wherein the protective coating contains a relatively insulative, resinous material. For example, in one embodiment, the resinous material can be a drying oil, such as olive oil, linseed oil, tung oil, castor oil, soybean oil, shellac, and derivatives thereof. The capacitor also includes a conductive polymer coating overlying the protective coating. As a result of the present invention, it has been discovered that a capacitor can be formed that can have a relatively low leakage current, dissipation factor, and equivalents series resistance.

28 Claims, 1 Drawing Sheet

Page 2

U.S. PATENT DOCUMENTS

English Translation of Japanese Patent No. 04-023419.

OTHER PUBLICATIONS

Abstract of Japanese Patent No. 02-0740f 8. Abstract of Japanese Patent No. 02-07402f. Abstract of Japanese Patent No. 02-f535f 6. Abstract of Japanese Patent No. 02-2f 92f f. Abstract of Japanese Patent No. 02-235326. Abstract of Japanese Patent No. 02-2386f 3. Abstract of Japanese Patent No. 02-24922f. Abstract of Japanese Patent No. 02-2727f 7. Abstract of Japanese Patent No. 02-2980f 0. Abstract of Japanese Patent No. 03-034303. Abstract of Japanese Patent No. 03-0462f 5. Abstract of Japanese Patent No. 03-0640f 3. Abstract of Japanese Patent No. 03-0726f 5. Abstract of Japanese Patent No. 03-073509. Abstract of Japanese Patent No. 03-078222. Abstract of Japanese Patent No. 03-080522. Abstract of Japanese Patent No. 03-0932f 4. Abstract of Japanese Patent No. 03-0932f 6. Abstract of Japanese Patent No. 03-0932f 7. Abstract of Japanese Patent No. 03-096210. Abstract of Japanese Patent No. 03-1678f 6. Abstract of Japanese Patent No. 03-f 83f f f. Abstract of Japanese Patent No. 03-26f f24. Abstract of Japanese Patent No. 03-2805f 9. Abstract of Japanese Patent No. 03-280523. Abstract of Japanese Patent No. 03-28532f.

English Translation of Japanese Patent No. 05-2f7808.
Abstract of Japanese Patent No. 05-2f7809.
Abstract of Japanese Patent No. 05-234820.
Abstract of Japanese Patent No. 05-28329f.
Abstract of Japanese Patent No. 06-077093.
Abstract of Japanese Patent No. 06-084708.
Abstract of Japanese Patent No. 06-f f2094.
Abstract of Japanese Patent No. 06-f f2095.
Abstract of Japanese Patent No. 06-f f2096.
Abstract of Japanese Patent No. 06-f 96f f 6.
Abstract of Japanese Patent No. 52-079255.

"The Physical and Chemical Properties of Shellac," H.S. Cockeram and S.A. Levine, Journal of the Society of Cosmetic Chemists, May f2, f96f, pp. 3f6-323.

"Shape-Memory Alloys," Kirk Othmer Concise Encyclopedia of Chemical Technology, pp. f052-f053.

"Application of Shellac in Polymers," Maiti and Rahman, pp. 442-445.

"Heat Curing of Shellac," S. Ranganathan and R. W. Aldis, Bulletin No. f4, pp. f-fO.

"Tantalum Nitride: A New Substrate for Solid Electrolyte Cpacitors," Terrance B. Tripp, Richard M. Creasi, and Bonnie Cox, 2(fh Annual Meeting-Capacitor and Resistor Technology Symposium and Seminars.

"Comparisons of Multilayer Ceramic and Tantalum Capacitors," Jeffrey Cain, AVX Corporation.

"Basic Tantalum Capacitor Technology," John Gill, AVX Corporation.

"Glossary of Terms Used in the Tantalum Industry," John Gill, AVX Corporation.

"Analysis of Solid Tantalum Capacitor Leakage Current," R. W. Franklin, AVX Corporation.

* cited by examiner

1

PROTECTIVE COATING FOR
ELECTROLYTIC CAPACITORS

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application Ser. No. 60/297,410, filed on Jun. 11, 2001 now abandoned.

BACKGROUND OF THE INVENTION

Electrolytic capacitors (e.g., tantalum capacitors) are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. For example, one type of capacitor that has been developed is a solid electrolytic capacitor that includes an anode (e.g., tantalum), a dielectric oxide film (e.g., tantalum pentoxide, Ta205) formed on the anode, a solid electrolyte layer (e.g., manganese dioxide, Mn02), and a cathode. Various other layers can also be applied to the solid electrolyte layer, such as graphite and silver dispersion layers successively applied to the manganese oxide layer prior to welding the anode and cathode lead terminals onto the capacitor.

The solid electrolyte layer is generally designed to electrically connect the dielectric film and the cathode, and thus, must have a certain conductivity. In addition, the solid electrolyte layer is also designed to inhibit short-circuiting of the capacitor that results from the presence of defects in the dielectric film. For example, upon exposure to heat generated by a short-circuit current, a manganese oxide layer can be converted to an insulator and thereby inhibit further short-circuiting.

Nevertheless, despite the benefits of using manganese oxide as the solid electrolytic layer, other materials have also been utilized. For instance, some electrolytic capacitors have utilized a conductive polymer layer (e.g., polypyrrole, polythiophene, polyaniline, polyacetylene, poly-pphenylene, and the like) as the electrolytic layer. Examples of such capacitors are described in U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al, U.S. Pat. No. 5,729,428 to Sakata, et al, and U.S. Pat. No. 5,812,367 to Kudoh, et al.

For instance, Sakata, et al. '862 describes forming a conductive polymer layer by polymerizing an aniline monomer on a dielectric oxide film using an oxidant. Sakata, et al. '862 states, however, that because such conductive layers are thin, they become damaged by thermal stress generated upon mounting the capacitor, thereby increasing leakage current. Thus, Sakata, et al. '862 also describes forming a first conductive polymer layer formed on the oxide layer and a second conductive polymer layer formed on the first conductive polymer layer.

Moreover, Sakata, et al. '428 describes a capacitor having an electron donor organic compound layer covering the dielectric oxide film and a conductive polymer layer as the solid electrolytic layer. Sakata, et al. '428 states that the electron donor layer can reduce normalized leakage current at higher temperatures when using a conductive polymer as the electrolytic layer. Examples of such electronic donor organic compounds are said to be fatty acids, aromatic carboxylic acids, anionic surface agents (carboxylate or sulfonate), phenol and derivatives thereof, silane coupling agents, titanium coupling agents, and aluminum coupling agents.

Nevertheless, despite the benefits obtained by utilizing a conductive polymer layer, various problems still remain with the capacitors formed therefrom. For instance, capaci

2

tors utilizing a conductive polymer layer still tend to shortcircuit and have a relatively high equivalent series resistance ("ESR"), which refers to the extent that a capacitor acts like a resistor when charging and discharging in an electronic 5 circuit.

As such, a need currently exists for an improved electrolytic capacitor that inhibits short-circuiting and has decreased ESR.

10 SUMMARY OF THE INVENTION

In accordance With one embodiment of the present invention, a solid electrolytic capacitor is disclosed that comprises an anode that contains a valve-action metal (e.g., tantalum, niobium, and the like) and a dielectric film over

15 lying the anode. The capacitor also comprises a protective coating overlying the dielectric film, wherein the protective coating contains a relatively insulative, resinous material. In some embodiments, the resinous material is selected from the group consisting of polyurethane, polystyrene, esters of

20 unsaturated or saturated fatty acids, and combinations thereof. For example, in one embodiment, the resinous material can be a drying oil, such as olive oil, linseed oil, tung oil, castor oil, soybean oil, shellac, and derivatives thereof.

25

The capacitor also comprises a conductive polymer coating overlying the protective coating. For example, in some embodiments, the conductive polymer is selected from the group consisting of polypyrroles, polythiophenes,

3Q polyanilines, polyacetylenes, poly-p-phenylenes, and derivatives thereof.

As a result of the present invention, it has been discovered that a capacitor can be formed that has a relatively low leakage current, dissipation factor, and equivalents series

35 resistance. For example, in some embodiments, the capacitor has a normalized leakage current of less than about 0.1 fiAJfiF*W, in some embodiments less than about 0.01 fiAJfiF*W, and in some embodiments, less than about 0.001 fiAJfiF*W, where fiA is the measured leakage current of the

40 capacitor in microamps and fiF*W is the product of the capacitance and the rated voltage of the capacitor. In addition, the capacitor can also have a dissipation factor of less than about 10%, and in some embodiments, less than about 5%. Furthermore, the capacitor can have a equivalent

45 series resistance of less than about 1000 milliohms, in some embodiments less than about 500 milliohms, and in some embodiments, less than about 100 milliohms.

In accordance with another embodiment of the present invention, a method for forming a solid electrolytic capaci

50 tor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; forming a protective coating on the dielectric film, the protective coating containing a relatively insulative, resinous material; and forming a conductive

55 polymer coating. In some embodiments, for example, the protective coating is formed from a solution containing the relatively insulative, resinous material. If desired, the solution may further contain a non-aqueous solvent. During formation, one or more layers of the protective coating may

go be dried. For example, in some embodiments, one or more layers of the protective coating are dried at a temperature of from about 50° C. to about 150° C.

In accordance with yet another embodiment of the present invention, a method for forming a solid electrolytic capaci

65 tor is disclosed that comprises forming an anode that contains a valve-action metal; anodizing a surface of the anode to form a dielectric film; applying a solution to the anodized