Multilayer ceramic dielectric capacitors.

0001This invention relates to electrical components such as multilayer ceramic dielectric capacitors and in particular, but not exclusively, to metal-impregnated- electrode multilayer-ceramic-dielectric capacitors.

0002According to one aspect of the present invention there is provided a method of manufacturing an electrical solderable connection termination in an electrical component comprising forming on an exposed surface of the component a very porous pseudo termination in which the pores are lead wettable, and impregnating the pseudo termination with lead or a lead alloy to produce the solderable connection termination.

0003According to another aspect of the present invention there is provided a method of manufacturing an electrical solderable connection termination in an electrical component comprising forming a porous pseudo termination on the component substrate by firing a frit containing glass and a coarse ceramic powder thereon, the ceramic powder having a coating of a first metal, and impregnating the pseudo termination with a second metal whereby to provide a connection termination comprised by the second metal held in place by the fired-on ceramic powder,

0004According to a further aspect of the present invention there is provided a method of manufacturing a multilayer ceramic dielectric capacitor including the steps of applying a frit containing glass and a coarse ceramic powder, having a coating of a first metal, to end faces of the capacitor to which electrodes of the capacitor extend, firing the frit to provide pseudo end terminations and impregnating the pseudo end terminations with a second metal to provide end terminations in contact with the capacitor electrodes and comprised by the second metal held in place by the fired-on ceramic powder.

0005Embodiments of the present invention will now be described with reference to the accompanying drawing which shows a perspective schematic view of a basic multilayer ceramic dielectric capacitor.structure.

0006The basic capacitor comprises a stack of ceramic dielectric layers 1 with electrodes 2a and 2b therebetween. Alternate electrodes 2a terminate at the right hand face 3 and do not extend completely to the left hand face 4, whereas alternate electrodes 2b extend to the left hand face 4, but do not reach the right hand face 3. Thus the electrodes 2b can be electrically interconnected at the end face 4 (end terminated) and the electrodes 2a can be likewise electrically interconnected at the end face 3, to form two electrically insulated sets of electrodes 2a and 2h. In order to facilitate preparation of the drawing, which is only schematic, the electrode sets are shown as extending to the lateral sides of the stack. However, in practice neither set of electrodes extend to the lateral sides, which in the finished device are all ceramic. Instead of the internal electrodes 2a and 2b being rectangular and stepped sideways in successive layers as shown, they may alternatively, for example, have elongate portions which cross cne another, as described in our co-pending Application No. 41677/78 (Serial No. 2032683) (A.L.Qliver-G.Mills 1-1), or instead of being rectangular the electrodes may be comprised of pluralities of parallel conductive lines, as described in our co-pending British Appllcation No.82Q4777 (Serial No. ) (J.H.Alexander 21).

0007The manufacture of multilayer-ceramic-dielectric capacitors with the basic structure shown in the drawing comprises the following steps. Green (unfired) ceramic sheets of barium titanate, for example, are prepared in a conventional manner, for example tape casting, tape drawing or printing. In order to manufacture capacitors in quantity a screen-printing technique is employed to print a large number of electrodes side by side on one relatively large sheet of green ceramic, although alternatively a single electrode may be provided on a suitably sized sheet of green ceramic. The electrodes may be printed by means of a conductive ink or, in the case of metal-impregnated electrodes, the printed electrodes on the green ceramic comprise fugitive electrodes printed by means of a fugitive electrode ink comprising a charcoal powder with a binder and solvent such as to make it compatible with the green ceramic. Further sheets are similarly printed. A number of such sheets, the number corresponding to the number of electrodes in each c-apacitor, are stacked one on top of the other with the electrodes staggered and partially overlying one another as illustrated in the drawing. A blank green ceramic sheet is applied to the top of the stack. Extra blank green ceramic sheets may be applied to the top and bottom of the stack in order to protect the outer electrodes and to give adequate strength to the finished device. The block _Qf stacked green ceramic sheets thus formed is cut to make individual green capacitor elements. The electrodes do not extend up to the lateral cut faces as previously mentioned.

0008Subsequently the capacitors are fired. In the case of fugitive electrode devices the green capacitor elements are heated at for example, 1 C/min to 350°C, to bake out the binders in the fugitive ink and the green ceramic, before firing at, for example, 200°C/hour to 1120°C for 2 hours in a conventional manner, during which firing the charcoal of the fugitive ink burns, providing spaces between the dielectric layers comprised by the fired ceramic sheets which become sealed together during the firing process.

0009The fired capacitor elements are then end terminated at faces 3 and 4 whereby to connect the alternate electrodes 2a and 2b of the drawing, or in the case of fugitive electrodes, to connect the alternate spaces between the dielectric layers. After firing the electrodes 2a, or spaces corresponding thereto, are exposed at end face 3, whereas only the electrodes 2b, or spaces corresponding thereto, are exposed at end face 4, the illustrated gaps at the end faces 3 and 4 provided by the staggered overlying arrangement of the electrodes being closed by ceramic of the dielectric layers 1 during the manufacturing process. Likewise the lateral faces are closed by ceramic. Thus end terminations applied to end faces 3 and 4 can only connect a respective one set of electrodes, or spaces.

0010The end terminations of multilayer ceramic capacitors, in particular fugitive electrode (lead-impregnated) capacitors, are conventionally formed from precious metals such as silver or silver/palladium mixtures. These metals are used as fine powders and are bonded to the ceramic by firing on together with glass frits. Such terminations are expensive, in view of the precious metal content, and subject to silver leaching by the lead of lead impregnated capacitors. Whilst we have successfully used aluminium and aluminium/silver terminations in an attempt to solve the dissolution problem and provide cheaper end terminations, as is described for example in our co-pending British Application No. 8203644 (Serial No. 2,104,291 ) (J.H.Alexander 17X), these terminations are not always readily solderable and require further treatment to render them so. We have also found that silver/glass frit terminations can be treated such as to overcome the dissolution problem and solderability problem in one step. This is accomplished by lightly sulphiding the silver of silver/glass frit terminations as is described in our co-pending European Application No. (Serial No. ) (J.H.Alexander 22). In the case of lead impregnated capacitors, the silver sulphide layer formed on the silver provides a barrier between the lead and the silver which prevents silver leaching by the lead, and the lead wets the silver sulphide layer to provide a lead layer thereon which renders the termination readily solderable. These sulphided terminations still include a high proportion of silver and are thus expensive to produce.

0011We have now found that it is possible to replace almost all of the precious metal content of bonded silver terminations with cheaper materials and produce an end termination which is readily solderable and leach resistant.

0012The terminations of the present invention are made by lightly coating the surfaces of ceramic powders, such as alumina Al₂O₃, silica, SiO₂, for example, with silver. The silver-coated ceramic powder is then mixed with a glass frit and made into a termination paste which is applied to a capacitor, or other suitable electrical component, such as film circuits, in a conventional manner and fired to bond it thereto at a temperature of the order of, for example, 700°C. The firing temperature is dependent on the glass frit used and may be in the range 500-800°C. The end terminations thus formed contain so little silver that they do not conduct enough to give a good contact with the electrodes of the capacitor, and are thus termed "pseudo end terminations".

0013The exposed silver coating on the pseudo end terminations is then lightly sulphided. This may be accomplished by heating the capacitors to 350-400°C in a chamber that can be evacuated to remove air from the voids in the pseudo end terminations and the capacitors and subsequently admitting small quantities of hydrogen sulphide or sulphur vapour, for example, into the evacuated chamber where it forms silver sulphide Ag₂S on contact with the silver. The capacitors are then transferred to lead impregnation equipment where air is evacuated from voids in the capacitors, which are then immersed in molten lead that is forced into the pseudo end terminations by nitrogen under pressure, whereby the terminations become filled with and coated by lead. In the case of fugitive electrode capacitors impregnation of the lead into the spaces in the capacitor will be simultaneously achieved. The end terminations thus formed are comprised substantially by lead which is held in place by the fritted ceramic powder. The silver and silver sulphide coating processes serve to ensure that the lead is held to the ceramic powder by virtue of the lead wetting the silver sulphide layer. In view of the preponderance of lead, the end terminations thus formed are readily solderable, whereby to facilitate, for example, the soldering of connection wires thereto. There are no silver leaching problems resulting in lack of end terminations, in view of the low proportion of silver employed and the barrier effect provided by the silver sulphide layer.

0014The ceramic powder employed are necessarily of a relatively coarse nature such that the pseudo end terminations are very porous in order to facilitate the sulphiding and lead impregnation steps. The ceramic powders may be coated with silver in solution. Typically the silver coating is of the order of O<l/-O.3µm thick, and the ceramic powders are of 20-50µm particle size. The sulphiding agents may alternatively comprise organic materials such as CH₃SH, C₂H₅SH, for example.