WO1998050929A1

WO1998050929A1 - Process for manufacturing a solid electrolytic capacitor

Info

Publication number: WO1998050929A1
Application number: PCT/GB1998/001263
Authority: WO
Inventors: Peter Harris; William Millman
Original assignee: Avx Limited
Priority date: 1997-05-01
Filing date: 1998-04-30
Publication date: 1998-11-12
Also published as: AU7222898A; GB9708944D0

Abstract

This application concerns methods of manufacturing capacitors, and in particular solid state electrolytic capacitors, such as those made from porous tantalum. According to the present invention, there is provided a method of manufacturing one or more solid state electrolytic capacitors comprising: (i) forming an anode comprising a porous sintered anode body with a riser wire protruding therefrom, (ii) forming a dielectric layer on the anode body, (iii) applying a resist layer to a portion of the riser wire to be kept clear of a cathode layer material, (iv) forming a cathode layer on the anode body by a method including immersion of the anode body into a cathode-layer-forming solution, (v) causing the resist layer to become detached thereby to remove unwanted cathode material contamination from the riser wire. In this way any splashed cathode precursor solution is removed along with the resist layer, the resist layer acting to protect the riser wire in the region where contamination is sought to be prevented.

Description

PROCESS FOR MANUFACTURING A SOLID ELECTROLYTIC CAPACITOR

The present invention concerns methods of manufacturing capacitors., and in particular solid state electrolytic capacitors.

Solid state electrolytic capacitors typically comprise an anode of a sintered valve action metal, such as tantalum. The anode comprises an anode body formed around a riser wire. The riser wire permits connection of the anode body to a capacitor terminal. Once the anode has been manufactured further processing steps are carried out to form the capacitor.

The processing involves applying a dielectric layer to the anode, typically by anodising the surface of the porous anode material. Next a cathode layer of, for example, manganese dioxide, is applied by dipping the anode body into a suitable cathode layer-forming solution. In the case of the formation of a manganese dioxide layer, manganese nitrate solution is typically used. The solution applied to the anode body is converted from the nitrate to the oxide by heating. The dipping and heating must typically be repeated many times to build up a cathode layer of suitable thickness and integrity. The dipping process is usually carried out simultaneously on a plurality of anodes, each anode mounted in series on a carrier strip.

During the- dipping process it is necessary to keep the riser wire, which is also anodised, substantially clear of the cathode layer-forming material otherwise the device will become a short circuit when, during further assembly, the riser wire and anode body are released from the carrier strip and the riser wire is connected to the anode terminal of the capacitor. Hence for ease of processing it is desired to make the riser wire long so that at least the free end of the riser wire will be clear of cathode layer material and therefore not prone to short the capacitor. However, in the interests of volumetric efficiency of material use, the wire length between the body and the anode terminal has to be kept to a minimum, so the length of the riser wire tends to be shortened as far as possible without risking failure of the capacitor. Because of this great care and precision is required during the dipping process to prevent unwanted contamination of the riser wire by the cathode layer precursor solution. As the dipping process involves repeated dipping events, in this aspect of capacitor manufacture the riser wire is particularly vulnerable to contamination. In an attempt to overcome the problem of contamination of the riser wire, it is known to apply a hydrophobic fluoro-polymer barrier to the tantalum anode riser wire either before or after connection of the riser wire to the carrier strip. The barrier material is configured and positioned so that when the anode body is dipped into manganese nitrate or other processing solutions, the liquid should not wet the wire above the fluor-polymer barrier. However, it has been found that the cathode material may still sometimes splash onto the riser wire above the fluor-polymer barrier, and this has to be removed by very careful immersion of the wires and carrier strips into a dissolving/cleaning liquid to remove the unwanted spots and splashes of cathode material. Another disadvantage of the use of the barrier is that the hydrophobic nature of the fluoro-polymer can also cause the cathode material to be repelled from a critical area of the anode; where the wire enters the anode body. If this area is not properly coated with cathode material a potential failure site in the final capacitor is created.

Once the cathode layer has been applied, the riser wire is detached from the carrier strip for assembly of the capacitor by connection to the respective anode and cathode terminals . Each terminal is detached from a lead frame. The lead frame comprises an elongate metal strip or ribbon which is formed with a repeating pattern, each repeat pattern corresponding to the shape of one capacitor anode and/or cathode terminal. The lead frame is typically formed by stamping and bending to produce the required repeating configuration.

The riser wire is connected to the anode terminal, usually by welding. A conducting layer is then applied to the anode body, which body is then attached to the cathode terminal, for example, by use of a conducting adhesive. The resultant capacitor is then encapsulated in plastics resin material to provide protection, with distal ends of each terminal protruding as the anode and cathode terminals of the capacitor.

The present invention seeks, inter alia, to provide an improved method of manufacture of a solid state capacitor without the aforementioned problems of the prior art.

According to one aspect of the present invention there is provided a method of manufacturing a solid state electrolytic capacitor comprising: - (i) forming an anode comprising a porous sintered anode body with a riser a riser wire protruding therefrom, (ii) forming a dielectric layer on the anode body, (iii) applying a resist layer to a portion of the riser wire to be kept clear of a cathode layer material, (iv) forming a cathode layer on the anode body by a method including immersion of the anode body into a cathode-layer-forming solution,

(v) causing the resist layer to become detached thereby to remove unwanted cathode material contamination from the riser wire.

In this way any splashed cathode precursor solution is removed along with the resist layer, the resist layer acting to protect the riser wire in the region where contamination is sought to be prevented. Because of the protection provided by the resist layer the immersion steps of the process do not have to be carried out to such close tolerances as compared with the prior art methods. This is especially convenient in view of the repeated dipping steps normally involved in step (iv) above.

The method may further comprise attaching the riser wire to an anode terminal member and the body portion to a cathode terminal member, and optionally encapsulating the resultant component in a plastics resin with distal portions of the terminal members presented for electrical contact. In this way a finished capacitor is created which is ready to be attached to and incorporated in an electronic circuit.

According -to another aspect of the present invention there is provided a method of manufacturing multiple solid state electrolytic capacitors comprising: (i) forming a plurality of anodes each comprising a porous sintered anode body having a riser wire protruding therefrom, (ii) forming a dielectric layer on each anode body,

(iii) providing a carrier strip and attaching the riser wire of each anode to the carrier strip whereby the anodes are disposed in series along the length of the carrier strip, (iv) applying a resist layer to a portion of the riser wire to be kept clear of cathode layer and optionally to the carrier strip,

(v) forming a cathode layer on the anode body by a method including immersion of the anode body into a cathode layer forming solution,

(vi) causing the resist layer to become detached thereby to remove cathode material contamination on the resist layer from the riser wire.

In a preferred embodiment of the invention only those portions of the carrier strip to be given a cathode layer are left uncoated with resist layer. In this way the immersion of the anode body may comprise immersion of the whole carrier strip.

The anodes may be arranged in series on the strip to facilitate manipulation of the anodes during processing.

In another aspect of the invention the carrier strip comprises a lead frame formed with a plurality of anode terminal features and wherein each riser wire is attached to a corresponding anode terminal feature. In this embodiment preferably at least a portion of the lead frame and at least a portion of each riser wire are coated in resist material.

The combination of the use of a resist layer and the substitution of the lead frame for the conventional carrier strip provides a significant simplification of the process. The step of detachment of the riser wire from the carrier strip and attachment to the anode terminal is removed since the riser wire of each capacitor can be attached to the anode terminal before further processing begins.

A portion of, or the entirety of, the lead frame may be coated with resist material. The coated portions may be immersed into the cathode layer precursor without fear of contamination of the lead frame components.

The resist layer is preferably also applied to the lead frame. Conventionally, the lead frame must be kept separate from the anode during processing because of the corrosive and potentially contaminating conditions of the manufacturing process. However, by using a resist layer to protect the lead frame, the lead frame may be incorporated into the carrier strip. Because the lead frame is already attached to the anode riser wire, the riser wire may be made significantly shorter than would otherwise be required. Prior art methods involve attachment of the riser wire to an anode terminal after the anode body has had a cathode layer applied, and this requires a long riser wire in order to permit detachment from the carrier strip (by e.g. cutting) and attachment to a separate lead frame, such as by overlapping and welding. The volumetric savings in riser wire material create a significant economic advantage and environmental benefit by reducing waste material discarded after cutting.

In yet another aspect of the invention the lead frame may be formed with a plurality of cathode terminal features, each cathode terminal feature associated with an anode terminal feature, the arrangement being such that each anode body, when attached to the lead frame, is located adjacent a cathode terminal feature. In this embodiment the method may further comprise attaching each cathode terminal feature to the body portion of the anode after the cathode layer has been applied.

Once the cathode terminal features have been attached to the cathode layer of the processed anode body, the anode terminal feature and the cathode terminal features may be detached from the lead frame to form components each comprising: an anode terminal feature, an anode body and a cathode terminal feature (where present on the lead frame) . The components may be detached by cutting, shearing or stamping.

The method may further comprise encapsulating each component in a plastics resin, either before detachment from the lead frame or after detachment. The encapsulation leaves distal portions of the terminal features presented for electrical contact.

The carrier strip or lead frame may be provided with a series of sprocket holes which are configured to engage with sprockets of a drive mechanism for moving the carrier strip.

The cathode layer-forming solution may comprise a cathode layer material precursor solution and the wetted anode body may be treated after immersion to convert the precursor material to the cathode material. The precursor solution may comprise manganese nitrate, which solution is treated by heating to convert the nitrate to manganese dioxide by decomposition of the nitrate. The dipping process may be repeated to build up gradually the desired thickness of cathode layer.

In another aspect of the invention a portion of the riser wire adjacent the anode body is left uncoated with resist layer, thereby permitting a cathode layer to form on the said portion of wire adjacent the anode body. In this way it may be ensured that an anode body top surface in the region where the riser wire enters the anode body is fully and correctly coated with cathode material. In this way the integrity and quality of the anode body top surface in the region of riser wire entry into the anode body is assured.

The resist layer may be applied by spraying, painting, dipping, or transferring from a printing wheel. The resist layer may dried and/or cured after application. Transferring from a printing wheel has the advantage of being amenable to massed production methods in which a carrier strip is used, and provides good accuracy of application of the resist layer.

The resist layer is applied before applying the cathode layer. The resist layer may be applied before the dielectric layer is applied to the anode, by for example anodization.

The resist layer should be selected to be capable of withstanding the heating steps of the cathode layer formation process.

The resist layer may comprise any known resist material which may be formed into a layer resistant to degradation during the cathode layer formation process. The resist layer should also be easily removable once the cathode layer has been applied. Suitable resist materials will be known to the man skilled in the art. Examples include liquid-resist or photo-resist materials such as those commonly used for PWB or semiconductor fabrication.

In a preferred embodiment the resist layer comprises a resist applied in liquid form and which then sets to form a protective layer. In one example the resist layer is removable by dissolution in a caustic solution.

The anode material in a preferred aspect of the invention comprises -tantalum. However, any valve action material may be used in anodes sub ect to the process of the present invention. Naturally the resist layer material will have to be compatible with the anode material and The anode material must not be deleteπously affected by the resist removal process. Examples of suitable valve action metals are aluminium, niobium and titanium.

Following is a description by way of example only and with reference to the figures of the drawings of methods of putting the present invention into effect.

In the drawings : -

Figure 1 is a side elevation of an anode attached to a carrier strip in a method according to a first embodiment of the present invention.

Figure 2 shows the anode of figure 1 coated with a cathode layer.

Figure 3 shows the anode of figure 2 after removal of the resist layer. Figure 4 shows a side elevation of a lead frame involved in a process according to a second embodiment of the present invention.

Figures 5 to 7 show the lead frame of figure 4 in sequential stages of the second embodiment of the invention.

Figure 8 shows an anode processed according to the second embodiment and formed into a capacitor.

Figure 9 shows the capacitor of figure 8 after encapsulation in plastics resin.

Figure 10 shows the capacitor of figure 9 in which the terminals have been configured by bending.

First embodiment

In figure 1 a carrier strip 1 is an elongate metal ribbon to which is attached a plurality of anodes 7 (one shown for clarity) . Each anode comprises a rectilinear generally cube-shaped body 5, although other suitable shapes will be known to the man skilled in the art. The body is formed from a pressed particulate article comprising tantalum powder and a binder/lubricant. The lubricant is removed and the body is then sintered. During pressing the body is formed around a riser wire 2. The riser wire protrudes from a top end 9 of the anode body. An end of the riser wire distal from the anode body is welded to the carrier so that the anode depends therefrom.

A resist layer 3 is applied to the riser wire to form a generally annular sleeve around a middle portion of the riser wire. In one example the resist material consists of Eccocoat 7508 (trade name) produced by Emmerson & Cummings . A Lower portion 4 of the riser wire adjacent the top surface 9 of the anode body is kept clear of resist material. The anode body is anodised to form a dielectric layer thereon (not shown) . The anodization may be effected by controlled oxidation of the tantalum in an oxygen-controlled environment, thereby forming a layer of tantalum pentoxide. The dielectric layer thickness may be controlled to produce the required voltage capacity in the final capacitor.

The anode body is then dipped into a bath of manganese nitrate solution. The manganese nitrate wets the anode to form a layer, and this layer is then heated to form a layer of manganese dioxide, as shown schematically in figure 2. The wetting and heating steps may be repeated as necessary to form the required quality of layer. It will be evident from figure 2 that the manganese dioxide layer may be allowed to extend over a lower portion of the resist layer.

Once the manganese dioxide layer has been formed, the resist is removed. The resist may be removed by dissolution in a suitable resist stripper and/or by exposure to ultra violet energy to cause degradation thereof. In the case of Eccocoat 7508 removal is by dipping in a caustic solution eg 10% potassium hydroxide solution. In the process of removal, the manganese dioxide layer juxtaposed the resist layer falls away to leave the anode body fully coated in manganese dioxide, including the top portion 9 of the anode body, and the portion 4 of the resist wire adjacent thereto. The capacitor assembly is then washed clean and dried. The anode and cathode terminals may then be attached, by welding to the riser wire in the case of the anode terminal. In the case of the cathode terminal, a conducting layer of, for example carbon and silver paint, is applied to a portion of the anode body and the terminal is attached thereto by the use of, for example, a conducting adhesive. The capacitor is then encapsulated in a plastics resin protective covering, leaving the terminals exposed for use. Second embodiment

In Figure 4, two of a series of anodes (one of which is indicated -generally at 10) are shown attached to a lead frame 11. The anode 10 comprises a cube of sintered tantalum 12 which has been formed around an upstanding tantalum riser wire 13. An end 14 of the riser wire, distal from the anode body 12, is welded to the anode terminal area 15 of a lead frame 11. The anode terminal area is a shaped planar member, configured in the appropriate shape for the anodic terminal of the capacitor. The lead frame 11 is provided with a plurality of sprocket holes 17 which are sized for engagement with the sprockets of a drive mechanism (not shown) for moving the anodes through steps of the manufacture process. By the use of a driven lead frame, multiple anode / lead frame assemblies may be processed in series through different stages of the manufacturing process .

The manufacturing process is illustrated in Figures 5-10. First of all the anodes are attached to the lead frame, preferably by welding. A resist layer is then applied to the riser wire and lead frame, at least in those areas prone to contamination during subsequent processing. Examples of suitable resist materials are those commonly used for P B or semiconductor fabrication, such as photoresists or liquid resists. Other examples and particular compositions will be known to the man skilled in the art. The resist material is applied to form a layer on the lead frame carrier strip 11 and an upper portion 20 of the riser wire distal from the anode body 12, as shown in figure 5. A portion 21 of the riser wire adjacent the point of entry of the riser wire into the anode body 12 is left un-coated with resist material to ensure that a good cathode material layer can be over the whole of the anode body.

The resist layer is then dried and cured by, for example, heating or exposure to ultra-violet light. A dielectric layer is applied to the anode body and the exposed portion of the riser wire, by anodization. A cathode layer 22 in figure 6 is then applied to the anode body and riser wire by dipping in manganese nitrate solution and heating to convert the wetted layer of manganese nitrate to manganese dioxide. The dipping may be repeated several times to apply the required cathode layer. Once the cathode layer has been applied, the resist layer is then removed by dipping or spraying with a resist stripper which is substantially inert to the manganese dioxide layer and to the material of the lead frame. Once the resist has been removed, as shown in figure 7, the lead frame and depending portion 20 of riser wire become un-coated as shown at 20,23. The anode body and the portion of the riser wire 21 are both coated to the required extent in manganese dioxide. The anode is now ready for final manufacture into a capacitor.

The anode body 12 is partially dipped into carbon and silver paint to cover a lower portion of the anode body 24 distal from the point of entry of the riser wire (see figure 8) . A cathode terminal 25, which comprises a shaped planar rectilinear member taken from a cathode lead frame (not shown) , is attached to the painted portion 24 of the anode body. The terminal 25 is attached by the use of a conducting adhesive or another conventional method known to those skilled in the art. Next the riser wire and anode body and connection points between the anode and cathode terminals are cut from their respective lead frames and are encapsulated in plastics resin as shown in figure 9. The cathode terminal 26 and the anode terminal 27 which each project from opposite ends of the resin body 28 are then folded as shown in figure 10 to form the final capacitor.

By the use of a resist layer the risk of unwanted contamination of various parts of the capacitor by manganese dioxide during manufacture is considerably reduced. In addition, a uniform and effective cathode coating may be applied to the upper surface of the anode body, without danger of contamination of the riser wire. By incorporation of the anode terminal into the carrier strip, the production process is considerably simplified, since the steps of removal of the riser wire from the carrier strip and re-attachment to a separate lead frame are by-passed. There will also be a reduction in waste caused by off-cuts riser wire not used after removal of the anode from the carrier strip. In addition, because there is no danger of contamination by cathode layer precursor solution, the riser wire itself may be made shorter, with a corresponding increase in economic advantage. A further advantage is that by reducing the length of the riser wire a larger capacitive body may by used in a given size of encapsulated capacitor.

In another embodiment (not illustrated) the lead frame is formed with both anode and cathode terminal members. The anodes are laser welded to respective anode terminal portions of the lead frame before application of the cathode layer. The body portions of the respective anodes are adjacent the cathode members, but not in contact. The resist layer is applied to the whole lead frame except the portion of the anode to be provided with a cathode layer. Once the cathode layer application process is complete the resist layer is removed by dipping in 10% potassium hydroxide solution. The anode body portions may be attached to the cathode terminals by the use of conductive- adhesive and by pressing each anode body and each corresponding adjacent cathode terminal together. The foregoing method facilitates automation of the capacitor manufacture process by minimizing the handling and movement of the capacitor components during manufacture .

Claims

1. A method of manufacturing a solid state electrolytic capacitor comprising: - (i) forming an anode comprising a porous sintered anode body with a riser a riser wire protruding therefrom,

(ii) forming a dielectric layer on the anode body,

(iii) applying a resist layer to a portion of the riser wire- to be kept clear of a cathode layer material ,

(iv) forming a cathode layer on the anode body by a method including immersion of the anode body into a cathode-layer-forming solution, (v) causing the resist layer to become detached thereby to remove unwanted cathode material contamination from the riser wire.

2. A method as claimed in claim 1 and further comprising attaching the riser wire to an anode terminal member and the body portion to a cathode terminal member, and optionally encapsulating the resultant component in a plastics resin with distal portions of the terminal members presented for electrical contact.

A method of manufacturing multiple solid state electrolytic capacitors comprising:

(i) forming a plurality of anodes each comprising a porous sintered anode body having a riser wire protruding therefrom, (ii) forming a dielectric layer on each anode body, (iii) providing a carrier strip and attaching the riser wire of each anode to the carrier strip whereby the anodes are disposed in series along the length of the carrier strip, (iv) applying a resist layer to a portion of the riser wire to be kept clear of cathode layer and optionally to the carrier strip,

4. A method as claimed in claim 3 wherein the carrier strip comprises a lead frame formed with a plurality of anode terminal features and wherein each riser wire is attached to a corresponding anode terminal feature.

5. A method as claimed in claim 4 wherein at least a portion of the lead frame and at least a portion of each riser wire is coated in resist material.

6. A method as claimed in claims 4 or claim 5 wherein the lead frame-, is formed with a plurality of cathode terminal features, each cathode terminal feature associated with an anode terminal feature, the arrangement being such that each anode body, when attached to the lead frame, is located adjacent a cathode terminal feature.

7. A method as claimed in claim 6 and further comprising attaching each cathode terminal feature to the body portion of the anode after the cathode layer has been applied.

8. A method as claimed in any of claims 3 to 7 and further comprising detaching from each lead frame a component comprising: an anode terminal feature, an anode body and preferably a cathode terminal feature.

9. A method as claimed in claim 7 or claim 8 and further comprising encapsulating the component in a plastics resin with distal portions of the terminal features presented for electrical contact.

10. A method as claimed in any of claims 4 to 9 wherein the carrier strip is provided with a series of sprocket holes which are configured to engage with sprockets of a drive mechanism for moving the carrier strip.

11. A method as claimed in any preceding claim wherein the cathode layer- orming solution comprises a cathode layer material precursor solution and the wetted anode body is treated after immersion to convert the precursor material to the cathode material.

12. A method as claimed in claim 11 wherein the precursor solution comprises manganese nitrate which applied solution is treated by heating to convert the nitrate to manganese dioxide .

13. A method as claimed in claim 8 wherein a portion of the riser wire adjacent the anode body is left uncoated with resist layer, thereby permitting a cathode layer to form on the said portion of wire adjacent the anode body.

14. A method as claimed in any preceding claim wherein the resist layer is applied by spraying, painting, dipping, or transferring from a printing wheel.

15. A method as claimed in any preceding claim wherein the resist layer is dried and/or cured after application.

16. A method as hereinbefore described and with reference to figures 1 to 3 or figures 4 to 10 of the drawings.