CA1217248A

CA1217248A - Electrolytic capacitor

Info

Publication number: CA1217248A
Application number: CA000482349A
Authority: CA
Inventors: William P. Montgomery
Original assignee: Sprague Electric Co
Current assignee: Sprague Electric Co
Priority date: 1984-07-30
Filing date: 1985-05-24
Publication date: 1987-01-27
Also published as: FR2568403B1; JPS6142909A; US4538212A; FR2568403A1

Abstract

ELECTROLYTIC CAPACITOR

Abstract of the Disclosure An electrolytic capacitor has a porous valve-metal anode pellet, a tri-metal housing, and a hermetic glass-metal seal. A thin layer of a ductile and solder-able metal on the inner surface of the housing extends inwardly from the mouth of the housing to at most the bottom of the glass-metal seal which is sealed to the layer.

Description

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ELECTROLYTIC CAPACITOR
This invention relates to an electrolytic capac-itor, and more particularly to such a capacitor having a thin layer of a ductile solderable metal to which a glass-metal seal is sealed.
In glass-metal compression seals used in prior art tantalum electrolytic capacitors, the outer compres-sion ring is made of a metal different than tantalum and has a coefficient of e~pansion which is about twice that of tantalum. In the past, the compression ring has been plated or clad with a metal that is resistant to corro-sion by the electrolyte used, which is frequently sulfuric acid.
Silver cases have enjoyed popularity as cathode cans for tantalum capacitors, but migration of silver and plating out of silver onto the anode from the cathode have been serious problems i~ prolonged reverse ~oltage is applied. For this reason a tantalum case or a case with a tantalum interior is pre~erred, but there have been problems in joining seal rings to tantalum cases.
In a drawing which illustrates embodiments of this invention, The sole Figure is a cross-section of the capac-itor of the present invention.

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-4~3 In accordance with this invention an electro-lytic capacitor having a porous valve-metal pellet anode, a hermetic glass-metal seal 3 and a metal can as cathode, has a thin layer of a ductile and soldera~le metal which extends from the mouth of ~he can to at most the bottom of the glass-metal seal. I`he seal is firmly attached to this layer.
The metal cathode can of this invention is preferably the tri-metal can that is described by Rogers in copending Canadian application 4~0,680 filed August 9, lg84. This can is drawn from a tantalum-copper-nickel composite, with tantalum orming the inner surface of the can and nickel the outer.
When the glass-metal seal is a compression seal, i.e. the glass is compressed against the central tube by a ring, the ring generally has a coe~ficient of expansion (or contraction) about twice that of the glass and the central tube. For a tantalum capacitor and seal, this compression ring is frequently nickel, M~nel, stainless steel, etc.
When the cathode can is tantalum or has a tanta-lum interior, joining the compression ring to the tantalum poses problems. By providing the thin layer or wash of a solderable metal of this invention at the mouth of the can, the metal compression ring can be joined to the tan-talum can much more easily. However, this layer must not extend into the can beyond the seal, as it might plate onto the anode under reverse voltage conditions.
The inner tantalum layer of the tri-metal can is relatively thin, so that a weld to it requires even more care and is costly. While the metal wash can be applied to just the interior of the can mouth, it is preferable to use a metal which is ductile as well as solderable and to apply the wash over the tantalum surface befo-re that can is drawn. If this is done, this metal acts as a lubricant during the roll bonding and the drawing of the can. 'rhe wash is then removed from all but the mouth area.
Other methods of providing a ring of ductile solderable ~L2~7~4k~
metal could be used, but the preferred method is to apply a wash, draw th~ can, and remove all but a ring of the wash.
The electrolytic capacitor of this invention has a porous anodized valve-metal pellet anode 10, pre~erably tantalum, with an associated riser 11. Anode 10 is seated in an insulating plastic gasket 12 located in the bottom of cathode can 20. Riser 11 passes through an upper insulating gas~et 13 which is compressed against anode 10 and in turn compresses anode 10 against bottom gasket 12.
A resilient 0-ring 14 surrounds a portion of gasket 13 and is compressed between the gasket and can 20. A
second resilient 0-ring 15 surrounds riser 11 where the riser emerges ~rom the top of gasket 13. 0-ring 15 is lS compressed against riser 11 by hermetic seal 30.
Can 20 is preferably a tri-metal can consisting of an outer layer of nickel 21, an intermediate layer of copper 22, and an inner layer of tantalum 23, whereby tantalum forms the interior cathode surface of can 20.
A layer of finely divided carbon 25 overlies tantalum la~er 23. Electrolyte 16 fills can 20 to the bottom of gasket 13O
At the upper end of can 20, tantalum layer 23 bears a thin layer 24 of ductile solderable metal which extends on the interior of can 20 from the mouth of the can to at most the bottom of hermetic seal 30.
Hermetic seal 30 has a central hollow tube 34 through which anode riser 11 passes and is sealed off at weld 35. A mass of glass 32 surrounds tube 34 and is compressed against the tube by metal ring 31. Ring 31 is sealed to can 20 by solder 33 between the upper edge of ring 31 and metal layer 24. Anode lead 36 and cathode lead 40 complete the capacitor.

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-- 4. --Metal layer 24 may be one of a variety of ductile solderable metals, e.g., copper, silver, bronze, lead, tin, etc., but is preferably copper. Metal layer 24 extends at most to the lower edge of seal 30 and S preferably not that far, so that the me~al layer will not come into contact with electrolyte 16 which might work its way around gasket 13 or along riser 11. It is easy to solder to copper, the preferred metal for layer 24, and its presence facilitates the sealing of capacitors having a tantalum or part tantalum case and a glass-metal compression seal.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an electrolytic capacitor having a porous anodized valve-metal pellet anode, a hermetic glass-metal seal having a metal ring forming its periphery, and a housing in the form of a can, the improvement comprising a thin layer of a ductile and solderable metal on the inner surface of the mouth of said can, said layer extends into said can at most as far as the bottom of said seal, and said ring is sealed to said thin layer.

2. A capacitor according to claim 1 wherein said valve-metal is tantalum and said can is at least in part made of tantalum.

3. A capacitor according to claim 2 wherein said thin layer is bonded to a tantalum surface of said can.

4. A capacitor according to claim 2 wherein said can is a tri-metal clad can having a tantalum inner surface.

5. A capacitor according to claim 1 wherein said thin layer is copper.

6. A capacitor according to claim 1 wherein said ring is soldered to said thin layer.

7. A capacitor according to claim 1 wherein said thin metal extends into said can only partially the depth of said seal.