CAPACITOR AND METHOD OF PACKAGING IT

Abstract: A metal film parallel plate capacitor 10 comprises a plurality of layers of dielectric material 21 each carrying a metallic film 23 and arranged in a stack with the metallic film 23 of each layer being separated by dielectric material 21 from the metallic film 23 of an adjacent layer. The capacitor is protected from the environment by a packaging system including wrapping means 16, such as adhesive tape extend­ ing around all side faces of the stack of layers. Electrically conductive coatings 14 are applied to and cover end faces of the stack of layers and are connected to the metallic film 23. Preferably electrical leads 18 are connected to the coatings 14 by a plasma impact connection method and this prevents reflow of the electrical lead 18 connections when the capacitor is connected in an electrical circuit.

Description: The present invention relates to a metal film parallel plate capacitor and its packaging. A typical metal film parallel plate capacitor comprises a plurality of layers of dielectric material each coated with a metallic film and arranged in a stack with the metallic film of each layer being separated by the dielectric material from the metallic film of an adjacent layer, a protective means for protecting the stack of layers from the environment, and electrical connection means connected to the metallic films. At present, metal film parallel plate capacitors are packaged in a number of ways. For example, they are inserted within a plastic box and encapsulated within the box by an epoxy resin or a similar material to ensure retention of the capacitor within the box and to protect it from the environment as well as to enhance the pull away strength of the electrical connection between the electrical leads and the capacitor structure. Another common means of packaging such capacitors is to suspend the capacitors within an epoxy resin material and thereby coat the entire capacitor and a small portion of its lead structure with an epoxy resin material. This packaging method also is designed to environmentally protect the capacitor as well as to enhance pull away strength of the electrical lead-to-capacitor bond. The electrical lead-to-capacitor bond is accomplished in such prior art devices as addressed above by providing a spray metal zinc-tin coating to interconnect alternate layers of metallised film and soldering the electrical leads to that zinc-tin coating. There are certain problems attendant to such a lead connection process, chief among which is the problem of possible reflow of the electrical lead-to-capacitor bond caused by heat applied to the electrical leads during the process of attaching the leads to an assembly such as a printed circuit board. Such reflow serves to weaken the physical electrical lead-to-capacitor bond and thereby perhaps even alter the electrical quality of the bond as well. Further, the epoxy resins and similar materials used to encapsulate capacitors presently are not impervious to moisture or other contaminants and incursion of moisture or other contaminants may occur, thereby altering, perhaps significantly, the electrical characteristics of the encapsulated device. GB-A-2011717 describes a method of manufacturing a metal film parallel plate capacitor comprising a plurality of layers of dielectric material each coated with a metallic film and arranged in a stack with the metallic film of each layer being separated by the dielectric material from the metallic film of an adjacent layer, wrapping means extending around all side faces of the stack of layers for protecting the stack of layers from the environment, and electrically conductive coatings applied to and covering end faces of the stack of layers and connected to the metallic films. According to this invention such a capacitor is characterised in that the wrapping means provides mechanical support to increase the resistance to delamination of the stack of layers and comprises a thermally insulative tape wound around the stack of layers, or a thermally insulative sleeve of heat shrinkable material applied around and shrunk onto the stack of layers, whereby the capacitor is mountable directly on to a circuit board in a leadless fashion. A particular example of a device in accordance with this invention will now be described with reference to the accompanying drawings; in which:- Figure 1 is a perspective view of the capacitor prior to application of a sealant with tape partially pulled away to illustrate wrapping the tape around the device; and, Figure 2 is a transverse section through the capacitor. A capacitive device 10 is shown in Figure 1 comprising a plurality of capacitive structures 12 coated at each end with a porous metal conductive material 14 and wrapped with a wrapping means such as adhesive tape 16 to provide additional physical strength to the capacitive device 10 in order that the capacitive device 10 may better resist delamination of its plurality of capacitive strata 12 and, further, to enhance resistance of the capacitive device 10 to incursions of humidity and other contaminants. Of course, other wrapping materials, such as heat shrinkable material, could be used in lieu of tape. In such an embodiment of the present invention, the heat shrinkable material is applied to the capacitive device 10 and subjected to heat to form a sleeve about the capacitive device 10 and conforming to the shape thereof. Electrical leads 18 may be attached to the capacitive device 10 by fusion with the porous metal conductive material 14 through a process described in detail in divisional application EP-A-0355870. In accordance with the present invention the wrapping means comprises thermally insulative tape (or heat shrinkable material which is heat insulative at processing temperatures to which the capacitive device 10 is subjected during mounting to an assembly) thereby allowing the electrical leads 18 to be omitted and enabling mounting of the capacitive device 10 in leadless fashion. In Figure 2 a capacitive device 10 is shown schematically in transverse section revealing alternating plates of a first group of plates 20, each of which plates has a layer of dielectric material 21 with metallic film 23 attached thereto and a second group of plates 22 of similarly formed layers of dielectric material 25 and attached metallic film 27 structure. As is shown in Figure 2 the first group of plates 20 alternates with the second group of plates 22 with each respective group 20, 22 being aligned within itself yet as a group offset from the other group so that, at the right side of Figure 2, the first group of plates 20 is electrically connected in common by a porous metal conductive material 14 and at the left side of Figure 2 the second group of plates 22 is electrically connected in common by porous metal conductive material 14. A sealant is introduced by way of vacuum impregnation both within the interstices of the capacitive device 10 as well as between the capacitive device 10 and the tape or other wrapper 16. A common sealant used for such a purpose is wax. Since wax has a higher dielectric constant than the air it is displacing from the interstices of the capacitive device 10 the dielectric constant of the capacitive device 10 will be improved by this impregnation process. Moreover, the wax used in the preferred embodiment of the present invention is extremely successful at preventing incursions of moisture in humid conditions, a common problem among capacitive devices of the prior art type described above. The preferred embodiment of the present invention employs a single-metal porous metal conductive material 14. Aluminium has been found to be particularly successful in this application. The present invention is designed to provide a capacitive device which will be competitive both in terms of price as well as in terms of electrical performance in today's marketplace wherein capacitive devices of the type contemplated by this invention (i.e. metallized film parallel plate capacitors) have become essentially commodity items and, as such, extremely price competitive. It is essential that a producer of capacitive devices desiring to remain commercially competitive be able to reduce his cost of manufacture as much as possible without reducing the electrical performance of the devices. The present invention fulfills both of those essentials of the marketplace; in fact, the electrical performance of this device is enhanced while its cost of manufacture is significantly reduced.

Claims: A metal film parallel plate capacitor (10) comprising a plurality of layers of dielectric material (21) each coated with a metallic film (23) and arranged in a stack with the metallic film (23) of each layer being separated by the dielectric material (21) from the metallic film of an adjacent layer, wrapping means (16) extending around all side faces of the stack of layers for protecting the stack of layers from the environment, and electrically conductive coatings (14) applied to and covering end faces of the stack of layers and connected to the metallic films (23), characterized in that the wrapping means (16) provides mechanical support to increase the resistance to delamination of the stack of layers and comprises a thermally insulative tape wound around the stack of layers, or a thermally insulative sleeve of heat shrinkable material applied around and shrunk onto the stack of layers, whereby the capacitor is mountable directly on to a circuit board in a leadless fashion. A capacitor according to claim 1, in which sealing means are included impregnated into the stack of layers to displace fluid contaminants, introduced between the outside of the stack and the wrapping means (16) and upon exposed surfaces to seal the capacitor (10) against incursions by humidity or other contaminants. A capacitor according to claim 1 or 2, in the electrically conductive coating (14) is a coating of a single-metal porous metal material such as porous aluminium.

Inventors: RAYBURN, CHARLES C.
Applicants: ILLINOIS TOOL WORKS INC.
Application year: 1984
Main classification: H01G1
IPC codes: H01G4, H01G13, B23K11, B65D65