Solid electrolyte capacitor with improved cathode lead

#This invention relates to a solid electrolytic capacitor and more particularly to an improved cathode especially for a small solid electrolytic capacitor body. Numerous means are known for forming a cathode electrode and attaching a cathode lead wire thereto. The typical solid electrolytic capacitor has a porous valve-metal anode pellet, usually cylindrical, with one anode lead attached; a dielectric film is formed over the surfaces of the pellet and this is covered by a solid electrolyte; next a layer of graphite is applied; then the graphite layer is metallized and a cathode lead wire is attached to the metallized layer.

#One known method for attaching the cathode lead wire is to butt solder it, another consists in placing an end of the cylindrical wire flush with the surface of the metallized surface and applying solder, and yet another method employs a lead wire that is spirally wrapped around the graphite coated body and is attached thereto by an overcoat of solder.

#Of the first two methods for attaching the cathode lead wire to a small pellet (e.g. a pellet having a diameter less than about five times that of the cathode lead wire to be attached), the solder connection tends to crack and the solder joint between lead and pellet consequently exhibits a high resistance during subsequent handling such as in treating, testing, and/or encapsulating. This typically shows up as a high dissipation factor or open circuit and usually is cause for rejecting the part. The last mentioned means for connecting a cathode lead wire to a pellet is less prone to crack, however all three methods of cathode lead attachment are difficult to implement especially when employing a small pellet since accurate registration of the wire and the pellet prior to soldering cannot be accomplished reliably and surely on a ganged or multiple capacitor basis as in a high speed large scale manufacturing operation. The first two mentioned cathode lead structures are therefore produced with less than desirable final yields and the yield diminishes as the size of the pellet becomes smaller. The latter mentioned method of spiralling the wire about the body can only be accomplished one at a time and tends to produce a capacitor of large diameter.

#FIG. 1 shows a solid electrolyte capacitor body with an anode lead wire.

#FIG. 2 shows a cathode lead of this invention.

#FIG. 3 shows a cross-sectional side view of the assembled body and cathode lead of FIGS. 1 and 2, respectively.

#A conventional solid electrolyte capacitor body 10 having an anode lead wire 11 is shown in FIG. 1. In FIG. 2 there is shown a novel metal cathode lead having a round wire portion 20a and a "c" shaped strip portion 20b. This lead is not necessarily round but may be any other convenient shape such as oval or rectangular. The strip portion 20b is conveniently made by flattening and shaping an end portion of the wire 20a by means of a standard cold forming process. The wire 20a is preferably made of nickel having an outer surface of tin or tin-lead solder. This lead however may be made of metals other than nickel, such as copper or steel.

#An assembly of the body and novel cathode lead is shown in cross-section in FIG. 3. The body 10 is comprised of a porous valve-metal pellet 12, having a valve-metal-oxide film illustrated as 13 being formed over all the surfaces of the porous pellet. The valve-metal is preferably tantalum. A solid electrolyte 14 overlies the formed pellet and is in intimate contact with the dielectric film 13. The solid electrolyte is preferably manganese dioxide. Overlying the solid electrolyte is a carbon layer 15, more particularly a layer of graphite particles, as is conventional in the art. Next, a metal-containing solderable coating 16 is applied over the graphite. This coating is preferably made by applying in a conventional manner a silver bearing paste over the graphite by dipping and heating the paste to drive of the volatile binder materials therein and/or to cure the organic polymer constituents of the baste biner system.

#The body 10 is now prepared for having the cathode lead attached thereto. It is well-known in the large scale manufacture of solid electrolytic capacitors to execute each of the above noted steps simultaneously for a multiplicity of capacitor bodies. This is typically accomplished by attaching a plurality of equally spaced pellets by their anode lead wires to a metal bar. A typical conventional practice is to then prepare a set of tinned copper or nickel lead wires sandwiched between a cardboard strip and an adhesive tape. The parallel wires extend away from the cardboard strip and are arranged in pairs, each pair being spaced from the adjacent pairs by a distance corresponding to the spacing of the capacitor pellets on the metal bar. The pellets are then clamped into a fixture and the anode wires are severed from the bar, the spacing of the pellets being retained by the clamping fixture. The card of lead wires is registered with the pellets in the clamp so that a short wire in each wire pair lies adjacent to an anode wire of a pellet, and a weld is made to join the short wire and anode wire. A long wire of each pair should now be registered adjacent to the cathode coating of each pellet and a dip soldering step joins the long wire and the cathode coating of the pellets.