MICROMINIATURE PLANAR SIGNAL TRANSMISSION CABLE

Abstract: A microminiature planar signal transmission cable includes at least one support film having opposite first and second surfaces, and a thickness of less than about 0.003 inch. The cable further includes at least one layer of adhesive disposed on the first surface of the support film. The layer of adhesive preferably has a thickness of less than about 0.001 inch. At least one preformed signal transmission conductor having a widthwise dimension of less than about 0.002 inch is embedded within the layer of adhesive. A package for the signal transmission cable is further disclosed. The package has at least one layer of carrier film having a generally planar surface, and a layer of pressure sensitive adhesive disposed on the planar surface of the carrier film. The carrier film is adhesively affixed to the second surface of the support film of the transmission cable.

Description: TITLE OF THE INVENTION MICROMINIATURE PLANAR SIGNAL TRANSMISSION CABLE FIELD OF THE INVENTION This invention generally relates to a transmission cable and methods for manufacturing the same. More particularly, the present invention relates to a novel microminiature planar signal transmission cable which is less costly and more reliable than other similar cables and which enables preferred methods for connection to other devices. BACKGROUND OF THE INVENTION Microminiature interconnecting cables are well-known in the art of interconnects for electronic storage devices and other electronic assemblies. Such interconnects provide electrical communication between two or more metallic pads of electronic components or other objects of interest. For example, Figure 1 illustrates a flexible printed circuit cable or "flex circuit", generally indicated at 10, which is manufactured pursuant to a photo or chemical etching process. The flex circuit 10 includes one or more dielectric layers, e.g., layer 12, alternating with one or more printed conductive layers, such as copper. After etching, the copper conductive layer is manipulated to have a plurality of conductors 14 disposed on top of the dielectric layer 12. One shortcoming associated with the flex circuit 10 is that it is, contrary to its name, not free to move multi-axially, which is often useful when terminating or routing wires. Also, since the copper conductors are not insulated, they may be susceptible to electrically shorting. Another shortcoming associated with the flex circuit is that its construction is limited to materials which are compatible with chemical etching processes. Yet another shortcoming of the flex circuit 10 is that it is limited to rectangular cross-sectional dimensions due to its fabrication. Chemical etching, for example, results in the formation of generally perpendicular edges 16 with respect to the top surface 18 of the dielectric layer 12. However, preformed conductor shapes, such as round or elliptical conductor shapes, are commonly available through commercial sources. Some of the advantages of these conductor shapes are increased, multi-axis flexibility, improved fatigue life, improved electrical characteristics and compatibility with common bonding methods, such as filleted solder bonding and ultrasonic wire-bonding. Since the flex circuit 10 is limited to the conductor 14 having the perpendicular edge 16, none of the advantages of conductor shapes other than rectangular can be achieved. A further shortcoming of the flex circuit 10 is that it cannot be fabricated in continuous lengths. In addition to the flex circuit 10, it is also well-known to extrude such microminiature cables. Figure 2 illustrates an example of an extruded cable, generally indicated at 20, having parallel conductors, each indicated at 22, in an insulating media 24 which completely encapsulates the conductors 22. The extruded cable 20 can be produced to have any number of conductor cross sections. One shortcoming associated with extruded cables is that they typically require an insulating media removal step to expose the conductors for termination. This removal step is time consuming and costly to perform. A further shortcoming of an extruded cable is that it is limited to insulation materials which can be extruded. It is worth noting that the packaging of microminiature cables presents a significant challenge to the manufacturer of such a cable due to the extremely small size of the cable. The foregoing illustrates limitations known to exist in present microminiature signal transmission cables. Thus, it is apparent that it would be advantageous to provide an improved cable directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. SUMMARY OF THE INVENTION The present invention advances the art of microminiature planar signal transmission cables beyond which is known to date. In one embodiment of the present invention, a microminiature planar signal transmission cable comprises at least one support film having opposite first and second surfaces, and a thickness of less than about 0.003 inch. The cable further comprises at least one layer of adhesive disposed on the first surface of the support film. The layer of adhesive preferably has a thickness of less than about 0.001 inch. At least one preformed signal transmission conductor, having a widthwise dimension of less than about 0.002 inch, is embedded within the layer of adhesive. In another embodiment of the invention, a package for a microminiature planar signal transmission cable as described above comprises at least one layer of a carrier film having a generally planar surface, and a layer of pressure sensitive adhesive disposed on the planar surface of the carrier film. The carrier film is adhesively affixed to the second surface of the support film of the transmission cable. It is, therefore, a purpose of the present invention to provide a microminiature planar signal transmission cable having preformed conductors with any cross-sectional shape for improved signal transmission. Another purpose of the present invention is to provide a microminiature cable which is inexpensive to produce in high-volume production and which can be manufactured in continuous lengths. Yet another purpose of the present invention is to provide a microminiature cable which can be fabricated from any number of suitable materials. A further purpose of the present invention is to provide a microminiature cable having conductors which can be insulated and selectively positioned with respect to a support film by an adhesive. Yet another purpose of the present invention is to provide an electronic interconnection which is compatible with well known methods for attaching conductors to metal pads, such as ultrasonic wire-bonding and soldering. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing summary, as well as the following detailed description of a preferred embodiment of the invention, will be better understood when read in conjunction with the appended drawings. For purposes of illustrating the invention, there is shown in the drawings an embodiment which is presently preferred. It should be understood, however, that the invention is not limited to the precise arrangement and instrumentality shown. In the drawings: Figure 1 is a cross-sectional view of a prior art flexible printed circuit cable or flex circuit; Figure 2 is a cross-sectional view of another prior art extruded cable; Figure 3 is a cross-sectional view of microminiature planar signal transmission cable of the present invention; Figure 4 is a cross-sectional view similar to Figure 3, illustrating the conductors of the cable with insulation material disposed thereon; Figure 5 is a cross-sectional view illustrating a microminiature planar signal transmission cable of another preferred embodiment of the present invention; Figure 6 is a cross-sectional view of a microminiature planar signal transmission cable of yet another preferred embodiment of the present invention; Figure 7 is a plan view of a microminiature planar signal transmission cable of the present invention showing stripped conductor portions; and Figure 8 is a cross-sectional view of a package in accordance with the teachings of the present invention. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, wherein similar reference characters designate corresponding parts throughout the several views, the microminiature planar signal transmission cable of the present invention is generally illustrated at 30 in Figure 3. As shown therein, the cable 30 comprises three principal components, namely, a support film 32, a layer of adhesive 34 applied on the support film 32, and a plurality of conductors 36 embedded within the layer of adhesive 34. It should be understood that the cable 30 of the present invention can have any number of support films and adhesive layers, as required for strength and rigidity, and still fall within the scope of the present invention. In addition, the cable 30 can be fabricated with any number of conductors as well. The support film 32 includes an upwardly facing (first) surface 38 and an opposite, downwardly facing (second) surface 40. As shown in Figure 3, the upwardly facing surface 38 has the layer of adhesive 34 disposed thereon. Preferably, the support film 32 of the cable 30 is fabricated from any suitable polymeric material, such as polyimide, for example. This material is sold commercially on spools or reels, and can be purchased from UBE Industries, Ltd. under the registered trademark UPILEX. The support film 32 gives the cable 30 its required strength and rigidity. Preferably, the support film 32 has a thickness of less than about 0.003 inch. The support film 32 can also be fabricated from polyester or a fluoropolymer material, such as porous polytetrafluoroethylene, polytetrafluoroethylene, fluorinated ethylenepropylene, or perfluoroalkoxy polymer. In this regard, as the term porous polytetrafluoroethylene ("PTFE") is used herein, it shall mean a membrane which may be prepared by any number of known processes, for example, by stretching or drawing processes, by papermaking processes, by processes in which filler materials are incorporated with the PTFE resin and which are subsequently removed to leave a porous structure, or by powder sintering processes. Preferably, the porous PTFE is a porous expanded polytetrafluoroethylene membrane having a microstructure of interconnected nodes and fibrils, as described in U.S. Patent Nos. 3,953,566; 4,187,390; and 4,110,392, which are incorporated herein by reference, and which fully describe the preferred materials and processes for making them. The layer of adhesive 34 is applied onto the first surface 38 of the support film 32 by any suitable apparatus and method. Preferably, the layer of adhesive 34 should have a uniform thickness and density, since voids in the layer can cause inadequate bonding of the conductors 36 to the support film 32. A preferred adhesive 34 is a cross-linked, blended polyester polymer. The layer of adhesive 34 has a thickness of less than about 0.001 inch. One preferred method of applying the layer of adhesive 34 to the support film 32 is to liquify the adhesive by adding a solvent. Thereafter, the liquified adhesive/solvent mix is applied to the first surface 38 of the support film. After applying the mix, the support film 32 is then dried by any suitable method which eliminates the solvent. This results in a layer of adhesive 34 which can be manipulated by a doctor blade, or some other similar apparatus, to level the adhesive layer. During this application process, the support film 32 is removed from its spool or reel and rewound onto another spool after the layer of adhesive 34 has cured. The preformed conductors 36 may be any suitable shape and define a width dimension of less than about 0.002 inch. The conductors 36 are a metal, such as gold, aluminum, or gold plated copper, so that bonding between the conductors and flat metallic pads is enabled by conventional bonding methods. If more than one conductor 36 is required, the conductors are disposed in parallel, co-planar relation at spaced-apart, predetermined intervals. Typically the conductors 36 are spaced at intervals ranging from about 0.0015 inch to about 0.0100 inch. The conductors 36 are embedded within the layer of adhesive 34 by any suitable method that employs either force or temperature. The conductors 36 are aligned and spaced-apart to the desired configuration, and preferably passed through a nip point (as through rollers) where temperature can also be applied to ensure proper bonding between the support film 32 and the conductors 36. After securing the conductors 36 to the support film 32, the cable 30 is cooled and collected in any well-known fashion. Turning to Figure 4, the individual conductors 36 may be insulated with a layer 42 of suitable insulation material, such as, for example, polyurethane, polyimide, porous polytetrafluoroethylene, polytetrafluoroethylene, or fluorinated ethylenepropylene. The individual conductors 36 may be individually insulated by conventional methods, such as by extrusion, tape wrapping, or dip coating, for example. It is also anticipated that a material may be employed for the conductor insulation which has a higher melt temperature than the material used for the support film 32 to facilitate processing in certain instances. Additionally, the conductors 36 may be coated with a conductive material, such as, but not limited to copper, aluminum, gold, silver, a carbon- loaded fluoropolymer, a plated polymer, a plated fluoropolymer, or a plated porous polytetrafluoroethylene, prior to inclusion in the cable 30. Referring now to Figure 5, there is generally indicated at 44 another microminiature cable of another preferred embodiment having a support film 46, a layer of adhesive 48 and a plurality of conductors 50. Cable 44 is substantially identical to cable 30, except that the conductors 50 are nearly completely surrounded by the layer of adhesive 34 for more securely bonding the conductors to the support film 46. Figure 6 illustrates a cable generally indicated at 52 that is substantially identical as cable 30 except that the conductors are spaced-apart at varying intervals. It should be observed that in addition to the foregoing, one or more ground planes (not shown) may be included in any embodiment of the present invention. A suitable ground plane material may be metal foil or another conductive material. Specific embodiments of the present invention are many, so long as the thicknesses of the support film 32 and layer of adhesive 34 are less than 0.003 inch and 0.001 inch, respectively. As described above, one or more support films can be used. Alternately, one or more adhesive layers also may be used either as free films or as coatings on one or more of the support films. Tooling to construct the cable (30, 44 or 52) of the present invention consists of machined drums or rollers, such that the cable components (e.g., support film, adhesive layer and conductors) described above are formed together, by a continuous process, under heat or pressure. Conductors or support films can be guided so that their positions with respect to each other are controlled. Tooling may be machined so as to hold each conductor in position. Tooling may also be heated, or heat may be applied to some or all of the cable components before they enter the tooling. Process temperatures and the time at which the components are held at elevated temperature are chosen such that the conductors are embedded into the adhesive layer. After the cable is formed, it may be necessary to trim the edges to remove excess material and to control the dimensions of the finished cable or the distance from each edge to each conductor. This may be done by any suitable method, such as by rolling blades, by stationary blades, or by laser. As illustrated in Figure 7, the cable of the present invention may be prepared for termination by having a portion of the support film near one end or both ends removed prior to the conductor embedding process. Also, the conductors 36, if insulated, may be stripped by any suitable method, such as by laser ablation techniques. This will allow the free multi-axis movement of the cable for routing onto termination pads, for example. Referring to Figure 8, there is generally indicated at 54 a strip of packaging material ("package") for receiving cable 30. As shown, the package 54 includes two layers, a carrier film 56 and another adhesive layer 58. The package can be constructed in any suitable manner for receiving the cable 30 thereon, and can be purchased from commercial sources, for example, from Minnesota Mining and Manufacturing Company of Minneapolis, Minnesota. The arrangement is such that the cable 30, 44, or 52 is spooled off its reel and positioned so that the downwardly facing, or second, surface 40 of the support film 32 is facing the adhesive layer 58 of the package 54. A nominal amount of force suitably applied to the cable 30 releasably bonds the cable to the package 54. The resulting packaged cable is then wound on another reel for further packaging and shipping. The package 54 is especially suited for the microminiature cable of the present invention since it enables the cable to be removed therefrom by peeling the cable from the adhesive 58 in a controlled and uniform manner without causing damage to the cable. Although a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages which are described herein. Accordingly, all such modifications are intended to be included within the scope of the present invention, as defined by the following claims.

Claims: CLAIMS Having described the invention, what is claimed is: 1. A microminiature planar signal transmission cable comprising: at least one support film having opposite first and second surfaces, and a thickness of less than about 0.003 inch; at least one layer of adhesive disposed on said first surface of said support film, said layer of adhesive having a thickness of less than about 0.001 inch; and at least one preformed signal transmission conductor having a widthwise dimension of less than about 0.002 inch, said conductor being embedded within said layer of adhesive. 2. A cable as set forth in claim 1 , said conductor having an insulating layer disposed therearound. 3. A cable as set forth in claim 1 , further comprising at least one optical fiber. 4. A cable as set forth in claim 1 , said conductor being fabricated from a group consisting of: gold, aluminum, copper, gold plated copper, silver plated copper, nickel plated copper, and tin plated copper. 5. A cable as set forth in claim 2, said insulating layer being fabricated from a group consisting of: polyurethane, polyimide, porous polytetrafluoroethylene, polytetrafluoroethylene, and fluorinated ethylenepropylene. 6. A cable as set forth in claim 1 , said conductor being coated with conductive material. 7. A cable as set forth in claim 6, said conductive material being selected from a group consisting of: copper, aluminum, carbon-filled fluoropolymer, gold, and silver. 8. A cable as set forth in claim 1 , said support film being fabricated from polyimide. 9. A cable as set forth in claim 1 , said support film being fabricated from a group consisting of: polyester, porous polytetrafluoroethylene, polytetrafluoroethylene, and fluorinated ethylenepropylene. 10. A cable as set forth in claim 1 , said layer of adhesive comprising a cross-linked blended polyester polymer. 11. A package for a microminiature planar signal transmission cable of the type comprising at least one support film having opposite first and second surfaces, and a thickness of less than about 0.003 inch, at least one layer of adhesive disposed on said first surface of said support film, said layer of adhesive having a thickness of less than about 0.001 inch, and at least one preformed signal transmission conductor having a widthwise dimension of less than about 0.002 inch, said conductor being embedded within said layer of adhesive, said package comprising: at least one layer of carrier film having a generally planar surface; and a layer of pressure sensitive adhesive disposed on the planar surface of the carrier film, said carrier film being adhesively affixed to the second surface of said support film of the transmission cable. 12. A package as set forth in claim 11 , said conductor of the cable having an insulating layer disposed therearound. 13. A package as set forth in claim 11 , said the cable further comprising at least one optical fiber. 14. A package as set forth in claim 12, said insulating layer being fabricated from a material selected from a group consisting of: polyurethane, polyimide, porous polytetrafluoroethylene, polytetrafluoroethylene, and fluorinated ethylenepropylene. 15. A package as set forth in claim 11 , said conductor of the cable being coated with conductive material. 16. A package as set forth in claim 15, said conductive material being selected from a group consisting of: copper, aluminum, carbon-filled fluoropolymer, gold, and silver. 17. A package as set forth in claim 11 , said support film of the cable being fabricated from polyimide. 18. A package as set forth in claim 11 , said layer of adhesive of the cable comprising a cross-linked blended polyester polymer. 19. A package as set forth in claim 11 , said carrier film of the package being fabricated from polyimide.

Inventors: POLENSKE JIMMIE H JR, REYNOLDS J SCOTT SR, RUSSELL ROBERT A, POLENSKE, JIMMIE, H., JR., REYNOLDS, J., SCOTT, SR., RUSSELL, ROBERT, A.
Applicants: GORE & ASS, W.L. GORE & ASSOCIATES, INC.
Application year: 1997
Main classification: H01B7
IPC codes: H01B5, H01B7