Lead attachment method

Abstract: A method of attaching leads (18) to a porous metal material (14) comprises the steps of: (a) mechanically and electrically contacting the porous metal material (14) with a first terminal means (24) for applying an electrical potential to the porous metal material (14); (b) providing adjacent the porous metal material (14) a lead (26) to be attached which is attached to a second terminal means for applying an electrical potential to the lead (26); (c) applying an electrical potential across the first terminal means (24) and the second terminal means; and, (d) applying a driving force (F) to the lead (26) sufficient to impact and continue to press against the porous metal material (14) sufficiently to complete an electrical circuit between the first terminal means (24) and the lead (26) through the porous metal material (14). The electrical potential and the driving force are sufficient to create a plasma (36) of the porous metal material adjacent the lead (26) and to fuse the lead (26) with the porous metal material (14). This technique is particularly useful in the production of capacitors (10).

Description: 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. In GB-A-826,286 a method is disclosed for joining a wire lead to a capacitor by creating a discharge between the lead and the metal end surface of the capacitor, and bringing the lead into contact with the metal end surface during the discharge to cause localized heating to weld the lead to the capacitor. According to this invention a method of attaching leads to a porous metal material comprises the steps of: (a) mechanically and electrically contacting the porous metal material with a first terminal means for applying an electrical potential to the porous metal material; (b) providing adjacent the porous metal material a lead to be attached which is attached to a second terminal means for applying an electrical potential to the lead; (c) applying an electrical potential across the first terminal means and the second terminal means; and, (d) applying a driving force to the lead sufficient to impact and continue to press against the porous metal material sufficiently to complete an electrical circuit between the first terminal means and the lead through the porous metal material, characterised in that the electrical potential and the driving force being sufficient to create a plasma of the porous metal material adjacent the lead and to fuse the lead with the porous metal material, forming pure metal fibres in the form of tenacula embedded within the porous metal material. Preferably the porous metal material forms an end face of a capacitor and the method is used to apply leads to the capacitor. The method preferably also includes the step of severing the lead at a distance from the porous metal material to form an electrical lead. A particular example of a method 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; Figure 2 is a transverse section through the capacitor and a diagrammatic representation of the initial set up for attaching electrical leads to it; Figure 3 is a drawing similar to Figure 2 illustrating an intermediate phase in the attachment of an electrical lead; Figure 4 is a drawing similar to Figure 2 illustrating the final phase in the attachment of an electrical lead. 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 are attached to the capacitive device 10 by fusion with the porous metal conductive material 14 through a process to be described in more detail hereinafter. 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. Prior art devices involve a plurality of capacitive strata with spray metal porous conductive material applied to the ends of the capacitor to electrically connect alternate layers of capacitive structure, and attachment of leads to each end through the porous metal conductive material with a final step of encapsulation of the device in some manner. One common means of encapsulation in today's marketplace is insertion of the capacitive device into a plastic box and filling the plastic box with epoxy or a similar material. Another common means of encapsulation is to dip the capacitive device in epoxy or similar material and thereby coat the capacitive device and a portion of its lead structure with the material. Prior art devices also use a zinc-tin porous metal conductive material on the edges of the device for electrical commonality of alternating layers producing the capacitive relationship within the device in order to facilitate soldering of electrical leads to the zinc-tin material. Two disadvantages of this soldering means of attachment are that, first, the bond is physically not as strong as is desired; and, second, the electrical lead-to-capacitor bond may be subject to reflow caused by heat conducted by the electrical leads to the join during attachment of the electrical leads within an electrical assembly, such as a printed circuit board. The present invention overcomes both of these problems as will be explained in more detail hereinafter. Figure 2 illustrates, schematically, initial set up for plasma fusion of electrical leads to a capacitive device according to the preferred embodiment of the present invention. 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. (For purposes of clarity, similar elements will receive similar reference numerals throughout the various figures). Continuing to refer to Figure 2, a first electrical lead 24 is placed in electrical contact with the porous metal conductive material 14 at one end to the capacitor device 10. A second electrical terminal 26, which electrical terminal 26 will ultimately form one of the electrical leads 18 of Figure 1, is suspended adjacent the porous metal conductive material 14. Electrical circuitry 28 is used to apply a potential difference between the first electrical lead 24 and the second electrical lead 26 by connections 30 and 32. Electrical circuit 28 is of the type well known in the art providing for charging of capacitors in order that a large potential may be maintained across its connections 30 and 32 without necessitating power supply requirements beyond those normally found in an industrial environment. Suspended adjacent the second electrical terminal 26 is a plunger mechanism 34 to which a force F will be applied as hereinafter described. Referring now to Figure 3, a subsequent step in attachment of the lead 26 to the porous metal conductive material 14 is shown. At this point the force F has been applied to the plunger mechanism 34 to drive the second electrical terminal 26 against the porous metal conductive material 14, thereby completing an electrical circuit between the second electrical terminal 26 and the first electrical terminal 24 through the porous metal conductive material 14. The potential produced by the electrical circuit 28 in the preferred embodiment of the invention is sufficient, in combination with the amount of force F applied to the plunger mechanism 34 and the resultant impact of the second electrical terminal 26 against the porous metal conductive material 14, to form a plasma 36 within the porous metal conductive material 14 in the vicinity of the second electrical terminal 26. Force F continues to be applied to the plunger mechanism 34 to maintain physical and electrical contact between the second electrical terminal 26 and the porous metal conductive material 14 with the result that, with the presence of the plasma 36 in the vicinity of the second electrical terminal 26, the force F, through the plunger mechanism 34, drives the second electrical terminal 26 into the plasma 36 of the porous metal conductive material 14. As shown in Figure 4, which figure depicts the final phase of lead attachment according to the preferred embodiment of the present invention, the force F, through the plunger mechanism 34, drives the second electrical terminal 26 into the porous metal conductive material 14 a distance approximately equal to one-half the diameter of the second electrical terminal 26. As the second electrical terminal 26 is driven into the porous metal conductive material 14, the plasma 36 begins to cool and revert to pure metal fibres as at 38 in Figure 4. Figure 4 illustrates the final result of the lead attachment process of the present invention. This final result is particularly advantageous in that the pure metal fibres 38 are in the form of tenacula embedded within the porous metal conductive material 14 and provide an extremely strong physical as well as electrical bond between the second electrical terminal 26 and the capacitive device 10 through the porous metal conductive material 14. The same procedure as above described for lead attachment is applied to the remaining side of the capacitive device 10, and, in subsequent processing, 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 in order that bi-metal reaction enhancement of oxidation will be eliminated; aluminium has been found to be particularly successful in this application. Since aluminium has a significantly higher melting point than common zinc-tin materials used as connecting material for prior art capacitors, the problem of reflow during subsequent soldering operations in attaching the capacitor within a subassembly, such as a printed circuit board, is obviated. The inducement of a plasma through the application of high potential across the first electrical terminal 24 and the second electrical terminal 26 as well as the impact of the plunger means 34 against the second electrical terminal 26 in driving the second terminal 26 against the porous metal conductive material 14 results in an extremely fast process, sufficiently fast to insure an excellent physical and electrical bond between the electrical terminal 26 and the capacitive device 10 through the porous metal conductive material 14, without any heat damage such as melting or vaporization of the metallized films 23, 27 carried upon the dielectric substrates 21, 25 within the capacitive device 10, which metallized films 23, 27 actually form the capacitor plates of the capacitive device 10. Generally, the fusion process described above for electrical lead attachment occurs in an elapsed time on the order of one millisecond.

Claims: A method of attaching leads (18) to a porous metal material (14) comprising the steps of: (a) mechanically and electrically contacting the porous metal material (14) with a first terminal means (24) for applying an electrical potential to the porous metal material (14); (b) providing adjacent the porous metal material (14) a lead (26) to be attached which is attached to a second terminal means for applying an electrical potential to the lead (26); (c) applying an electrical potential across the first terminal means (24) and the second terminal means; and, (d) applying a driving force (F) to the lead (26) sufficient to impact and continue to press against the porous metal material (14) sufficiently to complete an electrical circuit between the first terminal means (24) and the lead (26) through the porous metal material (14), characterised in that the electrical potential and the driving force being sufficient to create a plasma (36) of the porous metal material adjacent the lead (26) and to fuse the lead (26) with the porous metal material (14), forming pure metal fibres in the form of tenacula (38) embedded within the porous metal material (14). A method according to claim 1, wherein the method further comprises the step of severing the lead (26) at a distance from the porous metal material (14) to form an electrical lead (18). A method according to claim 1 or 2, wherein the porous metal material (14) forms an end face of a capacitor (10) and the method is used to apply leads (18) to the capacitor (10).

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