Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Utilizing nonaqueous bath
Reexamination Certificate
2000-11-13
2002-06-25
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Utilizing nonaqueous bath
C205S324000, C205S332000
Reexamination Certificate
active
06409905
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method of and electrolyte for anodizing aluminum substrates for solid capacitors.
BACKGROUND OF THE INVENTION
Electrolytic capacitors have long been recognized as the most volumetrically efficient (i.e., as having the highest capacitance×voltage product for a given volume) type of capacitor available. This high capacitance×voltage product (commonly called“CV”) is made possible by the extremely thin anodic oxide dielectric layer present in electrolytic capacitors.
Capacitors containing very high surface area electrodes and employing polarization/depolarization of the so-called Helmholtz double layer formed by the contact of these high surface area electrodes and a conductive liquid or gel electrolyte may have extremely high capacitance values per unit volume but these double-layer capacitors are limited to very low voltages (1-3 volts) per cell by the electrolytic decomposition voltage of the electrolyte. The fine pore structure of the electrodes combined with the electrical conductivity limitations of liquid electrolytes results in relatively high equivalent series resistance (ESR) for double-layer capacitors. Modem electronic circuits generally require low-ESR devices, thus electrolytic capacitors remain the devices of choice for applications requiring capacitors having a high capacitance×voltage (CV) product per unit volume.
The introduction of so-called “solid” tantalum capacitors (consisting of an anodized porous tantalum compact impregnated with manganese dioxide cathode material) in the early 1950's made possible the use of electrolytic capacitors in higher frequency circuits requiring low equivalent series resistance as well as high CV density (i.e., high CV product per unit volume) devices. The lower ESR characteristic of these capacitors is due, in large part, to the 1-2 order of magnitude higher conductivity of the manganese dioxide cathode material compared with the liquid electrolyte cathode material used in older “wet” electrolytic capacitor designs.
The density of component placement on circuit boards increased greatly with the widespread adoption of “surface mount” circuit board construction in the 1980's. Surface mount devices attach directly to the circuit board conductive traces, via solder or conductive adhesive attachment, thereby reducing resistive losses and inductance associated with components fabricated with wire leads. Surface mount solid capacitors were introduced by major manufacturers in the 1970's and grew to be the dominant form of solid capacitors by the end of the 1980's.
Surface mount solid capacitors have traditionally been fabricated from porous powder metallurgy tantalum compacts which have been anodized, impregnated with manganese dioxide, and coated with carbon and conductive paint (usually containing silver powder) before final encapsulation.
Surface mount tantalum capacitors are fabricated in two general configurations, molded body and conformally-coated devices. Molded body devices have the general construction described in U.S. Pat. No. 4,288,842 which teaches a silver paint-coated tantalum anode body encapsulated in a molded, insulating material case having a pair of wrapped electrical leads extending from the case walls and connected to the encapsulated anode body via welding or conductive adhesive, etc. Conformally-coated surface mount tantalum capacitors fall into two sub-catagories depending on the type of electrode terminations employed. One type of termination follows the general construction described in U.S. Pat. No. 4,093,972 in which metallic end caps are attached to the insulating polymer conformally coated anode body to provide external electrode connections. The other type of termination follows the general construction described by U.S. Pat. No. 4,203,194 in which the insulating polymer conformally coated body is provided with external electrode connections via plating processes.
The surface mount solid capacitors, described above, employ anode bodies fabricated from powder metallurgy tantalum anode compacts. It has long been recognized that “solid” capacitors containing etched and anodized aluminum foil anode coupons in place of the powder metallurgy porous tantalum anodes bodies would not only have the advantage of the much lower cost of aluminum as an anode material but would also exhibit a low equivalent series resistance (ESR) due to the very short electrical path length (generally on the order of 0.001 to 0.002 inch) from the inner to the outer portion of the etch structure of the aluminum foil compared to the generally much longer electrical path length present in powder metallurgy tantalum anodes. A “solid” aluminum capacitor is described in U.S. Pat. No. 1,906,691 in which the liquid electrolyte traditionally present in aluminum electrolytic capacitors is replaced with a semi-conducting solid such as cuprous oxide or sulfide. Such fabrication methods were expensive and difficult to control. The product produced by these methods is variable, depending upon the exact stoichiometry of the semi-conductor coating, etc.
When “solid” tantalum capacitors having manganese dioxide cathodes produced via pyrolysis of manganese nitrate-solutions contained within the porous anode bodies were introduced in the 1950's, attempts were made to coat anodized aluminum capacitor foil coupons with manganese dioxide via the same pyrolysis method used to fabricate solid tantalum capacitors. The high moisture, temperature, and acidity associated with the pyrolysis process proved to be excessively aggressive and the electrical performance of the resulting devices was found to be inadequate to meet the demands of the electronics marketplace.
With the development of reasonably stable organic semi-conductors and intrinsically conductive polymers in the 1970's and 1980's, practical “solid” aluminum capacitors became possible and a line of solid aluminum capacitors having a cathode material consisting of an amine/TCNQ charge-transfer salt, organic semi-conductor was introduced in the early 1980's by the Sanyo Corporation (the “OS-Con” capacitor line). More recently, intrinsically conductive polymers such as polyaniline, polypyrrole, polythiophene, and their derivatives, doped with strong organic acids, have been utilized as cathode materials in both tantalum and aluminum electrolytic capacitors.
Organic charge-transfer salts and intrinsically conductive polymers, while more expensive than the manganese dioxide cathode material present in conventional solid tantalum capacitors, offer several advantages as solid cathode materials. They do not support combustion as does manganese dioxide, they have a range of conductivity such that they may be made significantly more conductive than manganese dioxide, and as stated above, they may be applied under conditions which are not so destructive to aluminum anode materials.
The high conductivity organic polymer cathode material and the short conductive path length inherent with etched, and anodized aluminum anode foil have been combined to yield solid capacitors having very low equivalent series resistance. Devices of this type have been constructed having ESR values below 0.005 ohm and having a volume of only a small fraction of a cubic centimeter.
The preferred surface mount configuration of solid aluminum capacitors having conductive polymer cathodes usually consist of a stack of etched and anodized aluminum foil coupons partially coated with conductive polymer, graphite, and conductive paint layers and with the cathode coatings bonded together and attached to a lead frame with conductive adhesive to form a negative terminal. The uncoated ends of the etched and anodized coupons are welded to each other and to a portion of a lead frame to form a positive terminal after encapsulation and singulation of the device. The devices are usually encapsulated by molding using a non-conductive polymer, usually an epoxy compound. The individual devices are singulated by post-molding removal of the non-e
Harrington Albert K.
Kinard John T.
Lessner Philip M.
Melody Brian J.
Wheeler David A.
Banner & Witcoff , Ltd.
Kemet Electronics Corporation
Leader William T.
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