GB2032691A - Anode assembly for electrolytic capacitors - Google Patents

Abstract

The anode assembly 22 comprises a sintered anode 12, an anode riser 14 connected to and extending from the anode and an electrically insulating sleeve 20 bonded both to the anode and to the anode riser around the connection. The sleeve is of a fluorinated polymer (other than PTFE). A method of making the anode assembly is also disclosed. <IMAGE>

Description

SPECIFICATION Anode assembly for capacitors The invention relates to an anode assembly for capacitors, a method of making such an anode assembly and capacitors containing such an anode assembly.

Many capacitors, including solid tantalum capacitors, use a porous metal pellet as an anode. These pellets are manufactured by pressing and then sintering a metal powder into a rigid mass. The powder is usually the powder of a film-forming metal, such as tantalum, aluminium, titanium, zirconium or niobium. Such powders can be formed into a porous pellet for use as an anode by a sintering process which is similar to the vacuum or inert gas sintering process employed with other metals.

Either before or after sintering, a wire riser is inserted into or attached to the pellet to form an anode assembly. The wire riser conducts electricity to and from the pellet during use.

After the attachment of the riser and the sintering of the pellet, the anode assembly is ready for further manufacturing steps, such as anodizing the anode, manganese dioxide (MnO2) deposition, riser-wire cut off, and welding of the riser wire to a terminal. The anode assembly is further processed by encapsulating it into an electronic device.

Some of the above manufacturing steps are severe and can cause the anode assembly to fail. More specifically, many failures occur because of physical damage to the junction of the wire riser and the anode, which can cause high direct current leakage and device shorting. Electrical shorts can often be traced to the presence of solid electrolyte on the riser which causes the device to become shorted after a contact lead is attached to the riser.

Various washers, sleeves and similar means of protection have been used in the past to keep the riser wire clean and thus prevent shorting at the riser. But since these means of protection do not adhere to the riser or to the anode, they cannot completely protect the riser from the electrolyte, or the junction of the riser and anode from physical abuse. It has been found that a small quantity of epoxy resin applied to and cured around the egress or junction of the riser wire and anode after processing, but before assembly and encapsulation, reduces the chances of physical damage and produces better direct current leakage distribution among the electrical devices.

However, the application of an epoxy or similar cured coating requires additional manufacturing steps which increase the manufacturing costs and complexity of the electrical device.

The ideal time to apply a protective coating is before anode anodization, because a coat ing applied before anodization accomplishes the dual purpose of preventing physical damage to the riser egress and preventing solid electrolyte build up on the riser. In order to put the coating in place prior to anodization, the coating must be inert to anodization and to any subsequent manufacturing steps, especially manganese dioxide (MnO2) deposition from manganese nitrate. An additional requirement of the material used in the coating is that it should not cause the anode or other components of the electrical device to degrade, such as by reducing the wettability of the anode.

We have now developed an improved sleeve for such an anode assembly.

According to the invention, there is provided an anode assembly for a capacitor, which comprises a sintered anode, an anode riser connected to and extending from the anode, and an electrically insulating sleeve bonded both to the anode and to the anode riser around the connection therebetween, the sleeve comprising a fluorine-containing polymer having repeating units of the formula

in which X1, X2, X3 and X4 are each substituents, a predominant proportion (as defined herein) of which are fluorine, at least one of the substituents being other than fluorine when the polymer is not a tetrafluoroethylene copolymer.

By ''predominant portion" it is meant that at least fifty percent of the substituents are fluorine when only one other type of substituent is present, and that, in other cases, there is an excess of fluorine over any other substituent present.

The polymer can be applied to the anode and anode riser in the molten state to form the sleeve, but in order to avoid additional manufacturing steps, it is preferably applied as a solid, such as a preform of definite shape.

The anode assembly according to the invention is therefore preferably made by a method comprising connecting the anode riser to the anode, disposing a preform of the polymer around the connection, melting the polymer and allowing the molten polymer to flow around the connection to form the insulating sleeve bonded both to the anode and to the anode riser around the connection.

The preform is preferably in the form of a washer or sleeve placed around the riser wire.

Because the polymer flows on heating, the washer or sleeve may have an aperture two to three times larger than the anode riser. This permits the use of automated equipment to place the washer around the riser and on the anode. The compatibility of the solid polymer with automated equipment helps to reduce the manufacturing costs of the anode assembly.

The polymers used are capable of withstanding subsequent manufacturing steps, such as anodization, and are adherent to both the anode and the riser; they do not degrade or adversely affect the anode by reaction of the anode with degradation by-products as may be the case with other polymers. Further, these polymers do not shrink or separate from the riser during anodization or manufacture.

As mentioned above, a completely coated riser is desirable since the coating prevents shorting within the electrical device during use.

In the polymers used in the present invention those substituents in the repeating units which are not fluorine are preferably chlorine, bromine, hydrogen, -RY, or-ORYm, or mixtures thereof, wherein each Y represents halogen or hydrogen, R is a chain having 1-6 carbon atoms, and m represents the requisite number of hydrogen and/or halogen atoms.

Suitable polymers for use in the present invention include fluorinated ethylene-propylene copolymers, (such as the copolymers sold under the trademark Teflon FEP by E.l. Du Pont de Nemours and Co., Wilmington, Delaware); copolymers of ethylene and tetrafluoroethylene (such as those sold under the trademark of TEFZEL by E.l. DuPont de Nemours and Co.), chlorotrifluoroethylene resins, such as "KEL-F" available from 3M Co. or "Plaskon" sold by Allied Chemical Corp.; a chlorotrifluoroethylene copolymer such as a copolymer of ethylene and chlorotrifluoroethylene (for example, the polymers sold under the trademark of HALAR by Allied Chemical Corporation, Morristown New Jersey), polymers having a fluorocarbon backbone and a perfluoroalkoxy side chain (perfluoroalkoxy polymers, such as the polymers sold under the trademark of TEFLON PFA by E.l.DuPont de Nemours and Co.) and homopolymers of vinylidene fluoride, (such as the polymers marketed as "KYNAR" by Pennwalt Corp., Philadelphia, Pennsylvania).

Polytetrafluoroethylene cannot be used in the present invention since this does not flow well when heated, has a very high melt viscosity and will not adhere to the anode or anode riser on solidification and therefore cannot form the required adherent sleeve at the junction of the riser and anode.

Nonhalogenated polymers, such as polyes ter or nylon, are also unsuitable, since they decompose when subjected to the reaction products of the thermal conversion reaction of manganese nitrate to manganese dioxide. The reaction by-products produced by the decomposition of such polymers will significantly reduce the wettability of the anode, which in turn significantly degrades the resulting electrical device.

In general, the fluorine-containing polymer used should be capable of forming a melt and of flowing when heated to between about 150"C and 450"C (of course, each of the preferred polymers has a different temperature at which it will flow, and a different preferred softening period). For example, for fluorinated ethylene-propylene copolymers, the preferred temperature is from 300"C to 350"C and the heating will preferably continue for 10 to 1 5 minutes.Chlorotrifluoroethylene polymers are generally heated to 275"C to 325"C for 3 to 10 minutes, while the preferred heating procedure for copolymers of ethylene and tetrafluoroethylene is from 275 to 325"C for a period of from 3 to 10 minutes. The preferred heating of perfluoroalkoxy polymers is from 350"C to 400"C for a period of from 5 minutes to 1 5 minutes.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of the anode assembly prior to melting of 3 preform of the the fluorine-containing polymer; Figure 2 is a cross-sectional view of the anode assembly of Fig. 1 after the formation of the sleeve; and Figure 3 is a cross-sectional view of the anode assembly incorporated into an electrical device, here a capacitor.

Referring now to Fig. 1, there is shown an anode 1 2 which has been formed by pressing a powder, preferably of a film forming metal such as tantalum, into a pellet and then sintering it. An anode riser 14, which can also be made from a film forming metal is shown pressed into the anode 1 2. Alternatively the anode riser 1 4 can be welded (not shown) to the anode 12.

A preform 16, here shown in the shape of a washer, is in place around the anode riser 14.

The preform 1 6 is comprised of a fluorinated polymer as described above, such as a fluorinated ethylene-propylene copolymer ("Teflon FEP"). The preform 1 6 is shown having an aperture 1 8 of approximately twice the width of the anode riser 1 4. Such a loose fit permits the preform 1 6 to be put in place by automated equipment (not shown).

Referring to Fig. 2, there is shown an anode assembly 22 according to the invention after the formation of the sleeve 20. The droplet of the polymer, which had formed on the melting of the preform 1 6, closed in around the anode riser 14 through surface tension and on cooling adhered to the riser 14. The droplet also filled in the area surrounding the egress or junction 24 betwee..

the riser 14 and the anode 1 2. On cooling, the droplet formed a sleeve 20 which adhered to both the riser 14 and anode 1 2 and protected the junction 24 from physical damage.

Referring to Fig. 3 the anode assembly 22 is shown encapsulated within a capacitor 26.

An anode terminal 28 is welded 30 to the riser 14. A layer of a solid electrolyte 32, here manganese dioxide, is applied to the anode 1 2. The electrolyte layer 32 is surrounded by a graphite coat 34, which is in turn surrounded by a metallic coat 36, here of silver.

The anode assembly 22 and the combination of layers described above are held in place within the case 38 by solder 40, which also electrically connects the metallic coat 36, which is the cathode, to the case 38. A cathode terminal 42 is connected to the case 38 to provide the second terminal of the capacitor 26.

The case 38 is sealed from the environment by a cover 44 comprised of a steel ring 46, an eyelet 48 through which the terminal 28 passes and to which the terminal 28 is soldered, and a ring of compressed glass 50 between the steel ring 46 and the eyelet 48.

The glass 50 electrically insulates the terminal 28 from the case 38 thereby preventing electrical shorts.

In order that the present invention may be more fully understood, the following Example is given by way of illustration only. In this Example, reference is made to Figs. 1 and 2 of the accompanying drawings.

EXAMPLE An anode assembly was made by first pressing powdered tantalum into a pellet. An anode riser 14 (Fig. 1) also made from tantalum, was pressed into the pellet at the same time and the pellet was sintered to form an anode 1 2. Alternatively the anode riser 14 could be welded to the anode 1 2 after sintering.

Once the sintering was complete, a washershaped preform 1 6 made from a fluorinated ethylene-propylene copolymer (Teflon FEP), was placed around the anode riser 14. The preform 16 had an outside diameter of 0.22 centimetre, a thickness of 0.075 centimetre and a central aperture having a diameter of 0.15 centimetre.

Once the preform 1 6 was in place, the anode assembly was heated in air at 300"C for five minutes to melt the preform 16.

When the preform 1 6 melted, the liquid polymer formed a droplet which rests on the anode 1 2 and surrounds much of the anode riser 14. The polymer also flowed into the area around the egress 24 of the anode riser 14 from the anode 1 2. The polymer adhered to the anode riser 14, anode 1 2 and the area of the egress 24.

On the cooling of the anode assembly, the droplet formed a sleeve 20, which adhered to the riser 14, anode and egress 24, (see Fig.

2). The anode assembly was then ready for anodization and further processing.

Claims (14)

1. An anode assembly for a capacitor, which comprises a sintered anode, an anode riser connected to and extending from the anode, and an electrically insulating sleeve bonded both to the anode and to the anode riser around the connection therebetween, the sleeve comprising a fluorine-containing polymer having repeating units of the formula

in which X,, X2, X3 and X4 are each substituents, a predominant proportion (as defined herein) of which are fluorine, at least one of the substituents being other than fluorine when the polymer is not a tetrafluoroethylene copolymer.

2. An anode assembly according to claim 1, in which at least one of X1, X2, X3 and X4 is a chlorine, bromine or hydrogen atom or an alkyl, haloalkyl, alkoxy or haloalkoxy group having 1 to 6 carbon atoms.

3. An anode assembly according to claim 1, in which the polymer is a fluorinated ethylene-propylene copolymer, an ethylenetetrafluoroethylene copolymer, a chlorotrifluoroethylene copolymer or a cpolymer having a fluorocarbon backbone and a perfluoroalkoxy side chain.

4. An anode assembly according to any of claims 1 to 3, in which the anode comprises a film-forming metal.

5. An anode assembly according to claim 4, in which the metal is tantalum.

6. An anode assembly according to any of claims 1 to 5, in which the anode is anodised.

7. An anode assembly for a capacitor, substantially as herein described with reference to Fig. 2 of the accompanying drawings.

8. A method of making an anode assembly according to any of claims 1 to 5, which comprises connecting the anode riser to the anode, disposing a preform of the polymer around the connection, melting the polymer and allowing the molten polymer to flow around the connection to form the insulating sleeve bonded both to the anode and to the anode riser around the connection.

9. A method according to claim 8, in which the preform is in the shape of a washer, and preform being disposed adjacent the anode with the anode riser passing therethrough.

10. A method according to claim 8 or 9, in which the polymer is melted at 1 50 to 450"C.

11. A method according to any of claims 8 to 10, in which the anode is subsequently anodised.

1 2. A method of making an anode assem bly, substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

1 3. A method of making an anode assembly, substantially as described herein with reference to the foregoing Example.

14. An anode assembly when made by a method according to any of claims 8 to 11.

1 5. A capacitor which comprises an anode assembly according to any of claims 1 to 7 and 14 a cathode and an electrolyte layer between the anode and the cathode.

1 6. A capacitor according to claim 15, in which the electrolyte layer is of manganese dioxide.

1 7. An electronic device which comprises a capacitor according to claim 1 5 or 16, the capacitor being encapsulated and provided with cathode and anode terminals connected to the cathode and anode, respectively.

1 8. An electronic device according to claim 1 7, substantially as herein described with reference to Fig. 3 of the accompanying drawings.