Electrical capacitors made of thin film dielectric material which is metallized on both sides are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. U.S. Pat. No. 2,470,826 issued on May 24, 1949 to W. McMahon illustrates a capacitor in which a double sided metallized dielectric layer is folded an odd number of times in a longitudinal direction. The folded dielectric material may then be wound along its longitudinal dimension and end terminations may then be applied to it by spraying of metal against the edges of the wound capacitor segment. The type of capacitor described by McMahon patent is extremely desirable because substantially all of the dielectric layer of the capacitor is in the electric field and the capacitor may be made without the insertion of an additional dielectric layer in the main body of the capacitor, although a short initial unmetallized dielectric strip and a protective terminating dielectric strip may be employed with the capacitor, if desired.
The type of capacitor construction that is envisioned by McMahon patent has a number of decided manufacturing and electrical advantages. For example, the problem of masking the dielectric layer during electrode evaporation when the electrode areas are applied to the film is substantially reduced. In addition, no slitting is required as it is during the manufacture of conventional wound film capacitors. Furthermore, the capacitance per unit dielectric area is maximized because of the substantially full use of the dielectric layer in the electric field, and, thus, shorter winding lengths per unit capacitance are required, thereby reducing the winding labor. Additionally, the capacitance lengths can be calibrated and pre-cut to produce a capacitor of more accurate values. Another advantage of this type of capacitor is that there are no air layers between the metallized electrodes and the pleated film surfaces and this increases the voltage at which corona discharge starts.
The McMahon patent suggested that leads could be attached to his pleated and wound capacitor by the well-known Schoop process. Actually, however, the application of high velocity metallic spray coatings to the edges of the capacitor described by McMahon is not commercially practical because metallic spray will penetrate through the dielectric area at a pleat and thereby short one electrode to the other. If the temperature and pressure of the spray are lowered, penetration of the dielectric layers may be reduced but the adhesion of the metallic spray will generally be very poor. The type of multiple leads that were suggested in the McMahon patent that were wound into his capacitor also is not satisfactory since these leads tend to tear the dielectric material and they make the capacitor bulky and inductive.
In order to solve the above mentioned spray shorting problem at a pleated edge, it has been suggested in Australian Pat. No. 159,958, patented on Nov. 24, 1954 to provide unmetallized dielectric strips behind all of the pleats of a pleated metallized capacitor. While this solution may provide a technically feasible method of attaching leads to a pleated metallized dielectric layer capacitor, the number of unmetallized strips that are required on the dielectric layer greatly complicates the manufacturing process for both the dielectric layer itself and the pleated capacitor product. For example, in a capacitor having five pleats, there must be, in addition to the two unmetallized margin areas, five unmetallized strips that run along the entire length of the dielectric material, with three of these strips being on one side of the dielectric layer and two of these strips being on the opposite side of the dielectric layer. It is apparent that the masking problems and the cost of manufacturing such a specialized item make the solution of the Australian patent commercially unattractive. However, it is feasible to manufacture capacitors with unmetallized strips behind each pleat; and moreover leads may be secured to pleated double-sided metallized capacitors by other known methods, for example, by use of conductive epoxy.
The configuration of the capacitor of the present invention provides a product which has all of the advantages of pleated, or pleated and wound, capacitors and also has the added advantages that the distortion of the electric field in the electrode areas is minimized, mechanical stress and the resulting damage is reduced and the capacitor does not assume a "bar-bell" shape with rings of a larger diameter than the rest of the capacitors forming near the ends of the capacitor. In addition to the use of the present invention to manufacture wound film capacitors, it is also possible to utilize the invention to produce ceramic capacitors in which the capacitor dielectric material is folded while the ceramic material is still in a "green" state, in a manner similar to that disclosed in U.S. Pat. No. 3,223,494 issued Dec. 14, 1965 to J. E. Crownover.