GB2121604A

GB2121604A - Method of making a solid electrolytic capacitor

Info

Publication number: GB2121604A
Application number: GB08214420A
Authority: GB
Inventors: Ralph Barton Dean; Christopher Frank Powling
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-05-18
Filing date: 1982-05-18
Publication date: 1983-12-21

Abstract

Rows 2 of anode areas (4, Fig. 3, not shown) are formed on a valve metal foil 1 (eg by anodising sintered tantalum power screen printed onto the foil) and conductive metal (eg nickel) strips 5 are then welded to the foil 1 with each strip 5 being positioned adjacent an associated one of the rows 2 (see Fig. 2). The rows 2 with their associated strips 5 are then cropped off one by one and placed sequentially into a process frame (Fig. 4, not shown). Manganising, electrolytic reforming and dipping in colloidal graphite solution and silver-based paint may then be effected with the rows 2 still in the process frame and after dipping in conductive epoxy to provide a base for a cathode termination the resulting block is release from the frame. After a further conductive epoxy dip the block is cut into individual capacitors. The foil may be of tantalum or niobium. <IMAGE>

Description

SPECIFICATION Solid electrolytic capacitors This invention relates to solid electrolytic capacitors and particularly to improvements in production techniques to facilitate the manufacture of such capacitors.

Our co-pending application 81 36165 (Serial No. ) describes a batch process for making solid electrolytic capacitors by etching a tantalum foil to form a number of rows of teeth, silkscreen printing tantalum powder onto the teeth, processing the teeth through sintering, anodising and manganesing stages, severing the rows and sequentially encapsulat-.

ing opposite edges of the rows in conductive epoxy and separating individual capacitors from the rows.

The present invention aims to improve the technique for handling and processing the capacitors.

According to the present invention there is provided a method of making solid electrolytic capacitors comprising providing parallel rows of discrete anode areas in a valve metal foil, providing a strip frame of conductive metal parallel strips, welding the strips of the frame to one side of the respective rows of the foil, separating each row with its welded strip from the foil and from the frame, and subsequently processing the anode areas to form complete capacitors.

According to another aspect of the invention there is provided a method of making solid electrolytic capacitors comprising forming a plurality of laminar rows of partially processed capacitor anodes, clamping the rows in a processing frame which has a plurality of insulating spacers, each row being clamped between a respective pair of said spacers so that all the components project from a common surface of the frame defined by an edge of the spacers, and further processing the components by dipping them in a dipping bath while held in the frame.

According to yet another aspect of the invention there is provided a process frame comprising a plurality of insulating spacers, a plurality of parallel guides on which the spacers are supported, and spring biassing means for biassing the spacers together whereby components to be processed may be clamped between the spacers and dipped in a dipping bath.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Figure 1 shows a. nickel frame and etched tantalum foil having rows of conductive areas and screen printed tantalum anodes; Figure 2 shows the frame and etched foil assembled together for welding; Figure 3 shows on an enlarged scale a detail of the foil 1 of Fig. 1; Figure 4 shows a process frame for carrying rows of capacitors for processing; Figure 5 shows a section through the frame of Fig. 4 on a larger scale and carrying a row of capacitors, and Figure 6 shows the anode rows held in the nickel frame and being sequentially cropped into the processing frame.

Referring to Fig. 1 of the drawings there is shown an etched foil 1 of valve metal, in this example of tantalum, which has rows of teeth such as 2. Other valve metal foils could be used such as niobium. The foil would be no thicker than five thou and in this Furthermore instead of etching the pattern in the foil, the foil could be stamped.

The foil has been screen-printed with a pattern of tantalum powder, and this is shown in greater detail in Fig. 3. In Fig. 3 tantalum powder, made up in a vehicle of organic solvent and binder into a screen-printing ink, is deposited onto one side of the foil in specific areas such as 4 on respective teeth such as 2a of a row 2 of teeth. As an example the patterned area of foil 1 measures about 100 mm X 50 mm and contains 26 rows, each row forming 24 capacitors. The size could vary however and anything from 100 to 2000 capacitor bodies could be formed on one foil.

The foil and the screened powder has been sintered to remove the residue of the ink vehicle and to bond the powder to itself and the foil. For tantalum a sintering temperature similar to that used in the manufacture of conventional compressed tantalum slugs, would be used, such as 1600"C in an inert atmosphere.

Also the sintered powder has been anodised (forming stage) to form a dielectric (tantalum pentoxide) of the desired thickness throughout the sintered powder surface.

Also shown in Fig. 1 and aligned with the foil 1, is an etched nickel sheet providing a pattern of nickel strips such as 5 and supporting nickel frame 6. The spacing between the nickel strips 5 corresponds exactly to the spacing between the rows of teeth 2 in the tantalum foil 1 and registration holes 7 and 8 in the nickel frame 6 correspond to registration holes 9 and 10 in the tantalum foil 1.

The nickel sheet could also be made by a selective plating process.

The next stage is to lie the tantalum foil on a vacuum chuck and remove the right hand side portion of the foil so that the right hand ends of the rows are no longer connected to the side portion. This can either be done by a guillotine or by laser cutting.

Then in the manufacturing process the nickel frame with strips 6 is taken and laid over the tantalum foil 1 with the registration holes 7, 9 and 8, 10 in register on the chuck acting as a jig and a bottom electrode and then we spot-weld each nickel strip 5 to the corresponding row 2 of the tantalum foil which supports the adjacent set of teeth, as shown in Fig. 2. The spot-welding can be done by a multiple wheel machine or alternatively the nickel strips can be individually and sequentially welded. Thus preferably the strips are welded all at once; otherwise they are welded serially. Laser welding can alternatively be used. It may be advantageous to weld a second similar nickel frame to the opposite side of the tantalum foil so each row has a nickel strip welded to each side.

Then the combination of nickel strips 5 welded to the tantalum foil rows 2 is taken to a cutting machine and the individual strips 5 are cropped off one by one and this machine puts each individual strip sequentially into a process frame. This process frame is shown in Fig. 4.

As the machine crops off the nickel strips one by one and assembles them sequentially into adjacent openings in the process frame, the nickel strips being magnetic are held magnetically in position in the process frame and for this purpose the datum bars 21 are magnetised, referred to later.

Referring now to Fig. 4 the process frame comprises a pair of tie bars 24 and 25 carrying compression springs 26 and 27 respectively compressed to a desired compression by thumb screws 28 and 29 respectively.

The tie bars are carried in a pair of end blocks 22 and 23 which, by means of the springs 26 and 27, compress between them a number of insulating spacers 20 only some of which are shown for clarity.

Three datum bars 21 extend through the end blocks 22 and 23 and also through the insulating spacers.

Adjacent each end block is a respective insulating buffer having the same shape as a spacer 20 but several times as thick. Buffers are referenced 30 and 31.

The individual rows of teeth and their associate nickel strips 5 are sequentially cropped and magnetically held against the datum bars 21 in respective spaces between adjacent spacers 20. In order to facilitate entry of each strip between two adjacent spacers 20, each spacer can have an inclined edge surface so that two adjacent spacers 20 can be wedged apart by insertion of the strip or by preinsertion of a wedge followed by insertion of a respective strip and attached row of teeth.

Fig. 6 shows the rows 2 of anodes 2a in the nickel frame and held by side portions 6a and 6b. As the bottom row 2 approaches the space between two PTFE spacers 20 two guillotine blades represented diagrammatically to arrows A and B cut the row 2 from the sides 6a and 6b and allow the row to fall into the space, aided by magnetic attraction between the magnetised bars 21 and the nickel strip or strips 5 on the row 2. Then the knurled nuts are tightened to squeeze the spacers via the end blocks and springs.

When the process frame has been loaded the strips occupy a position as shown in Fig.

5 with tails 5a projecting from the side of the process frame.

Tails 5a are connected to the main portion of the strip 5 by a weakened portion or neck 5b so that when no longer required i.e. after the manganising and reform is complete, the tails can easily be manually broken away from the strip 5 and the attached row 2 of capaci tors, while the capacitors are still held in the process frame.

The process frame holds the rows of capaci tor bodies so that only the teeth project be yond the plane of the frame determined by the lower edges 20a of the insulating PTFE spacers 20 which form a seal against the teeth and close against each other across the gaps between adjacent teeth. Thus the frame is supported on the sides of a dipping bath holding the manganising solution (manganous nitrate) and the level of the solution is pre vented from reaching above the edges 20a up the teeth. For the manganising stage this will be a level which just covers the sintered and anodised tantalum pads 4, and there could be seven or eight separate "dips" in the manga nous nitrate solution followed by baking out at about 250"C to form manganese dioxide.

At least one electrolytic reforming stage is employed in which the teeth are dipped into an acetic acid solution or aqueous nitric acid solution while the "tails" are connected to a forming voltage. The current is of the order of 10 mA per gram of tantalum powder and the voltage is continuously increased to between 1.5 times and 3 times the ultimate working voltage of the capacitor; the depth of immer sion of the teeth is the same for the reforming operation as for the manganising stages. The reformed anodes are subsequently washed with distilled water and dried.

Then the anodes, still in the process frame, are dipped into a colloidal graphite solution to cover the powder on each tooth, so that the graphite contacts the manganese dioxide com pletely. This is then dried. Following this the anodes are dipped into a silver-based paint which is subsequently dried and then a con ductive epoxy dip is made which substantially seals the anodes and provides a base for a cathode termination. At this point the capaci tor bodies are all united at the tips by the epoxy and the process frame screws are loosened and the "block" of capacitors re moved from the frame.

The block is then inverted and a second conductive epoxy dip is made to provide con nection to the nickel strip and anode tantalum foil ends of the teeth in contact with the nickel.

Then an insulating epoxy "flood" is pro vided which joins the two conductive epoxy blocks and unifies each capacitor and seals it.

The "solid" block is cut into individual capacitors.

A portion of the nickel strip 5 remains in each capacitor and serves to magnetically identify the anode and also serves to improve electrical connection between the anode foil of tantalum and the second conductive epoxy.

The conductive epoxy ends are then plated such as by electroless plating and finally tinned.

Claims

1. A method of making solid electrolytic capacitors comprising providing parallel rows of discrete anode areas in a valve metal foil, providing a strip frame of conductive metal parallel strips, welding the strips of the frame to one side of the respective rows of the foil, separating each row with its welded strip from the foil and from the frame, and subsequently processing the anode areas to form complete capacitors.

2. A method as claimed in claim 1, wherein the strips of the frame are integrally connected at their ends with side portions of the frame.

3. A method as claimed in claim 1 or claim 2, wherein the frame is etched or plated nickel.

4. A method as claimed in claim 1, claim 2, or claim 3, wherein a second frame similar to the first-mentioned is provided and welded to the other side of the foil so that each row has a strip welded to each side.

5. A method as claimed in any preceding claim, wherein each capacitor retains part of the or each magnetic strip when finished.

6. A method as claimed in any preceding claim, wherein a strip has an extension beyond the end of a row and an electric potential is applied via extension during a reform stage.

7. A method as claimed in claim 6, wherein the extension of the strip has a weakened portion, whereby it can be easily broken away from the main portion connected to the row.

8. A method as claimed in any preceding claim, wherein a strip is connected to a row by seam welding or laser welding.

9. A method as claimed in any preceding claim, wherein the rows and associated welded strips are sequentially cropped from the strip frame and foil as they are loaded one by one into a processing frame in which they are held in side by side spaced apart relationship with the anode areas projecting from one side of the processing frame for subsequent processing.

10. A method of making capacitors substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of making solid electrolytic capacitors comprising forming a plurality of laminar rows of partially processed capacitor anodes, clamping the rows in a processing frame which has a plurality of insulating spacers, each row being clamped between a respective pair of said spacers so that all the components project from a common surface of the frame defined by an edge of the spacers, and further processing the components by dipping them in a dipping bath while held in the frame.

1 2. A method as claimed in claim 11, wherein longitudinal datum bars in the processing frame determine the extent to which the anodes project beyond the surface of the frame.

13. A method as claimed in claim 12, wherein at least one of the bars is magnetised and attracts the laminar rows on to the bars.

14. A method as claimed in claim 11, 12, or 13, wherein the spacers limit or prevent unwanted coverage of the components by the content of the bath.

1 5. A method as claimed in any of claims 11 to 14, wherein the components undergo a heated drying or baking stage while held in the process frame.

16. A method as claimed in any of claims 11 to 15, wherein the spacers are made of PTFE.

1 7. A method as claimed in any of claims 11 to 16, wherein the components are solid electrolytic capacitors and while held in the frame, are dipped in a manganising bath and are reformed by applying a reform voltage to the components.

18. A process frame comprising a plurality of insulating spacers, a plurality of parallel guides on which the spacers are supported, and spring biassing means for biassing the spacers together whereby components to be processed may be clamped between the spacers and dipped in a dipping bath.

1 9. A frame as claimed in claim 18, wherein the spring bias force is adjustable.

20. A frame substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.

21. A capacitor made by a method or apparatus according to any preceding claim.