Abstract:
In a flat flex cable, signal lines are surrounded by logic ground planes above and below which are viaed together left and right. The ground planes coupled with the flex cable dielectric determine characteristic the impedance and attenuation of the cable and provide differential signal EMI shielding. All signal layers and logic ground planes are enclosed within the two outermost shield layers which are viaed together left and right and around the connectors to enclose both signal layers and logic ground planes to provide common mode EMI shielding.

Description:
FELD OF THE INVENTION  
       [0001]     The invention pertains to flat flex cables and more particularly to EMC (electromagnetic compatibility) shielded and grounded flex cables for use externally between devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electronic systems composed of multiple devices housed in separate enclosures commonly require external signal interconnects between devices. These systems require that integrated EMC shielding be provided in the flexible cabling extending between enclosures. It is not unusual to find that the external connecting cables are among the largest structures in the system, with the result that common mode currents on these cables are almost always the source of an EMI (electromagnetic interference) problem. To control both differential mode and common mode signals it is necessary to provide both logic ground shielding about the signal lines and secondary frame ground shielding about the logic ground shielding.  
         [0003]     High performance double shielded coaxial cables are a solution, but are too bulky and require large radius bends and as a result are not suitable for use in current state of the art devices that are continuously attempting to achieve smaller, more compact device sizes.  
       SUMMARY OF THE INVENTION  
       [0004]     This invention provides integrated EMC shielding and grounding for flex cables allowing them to be used externally of device enclosures to afford interconnection. The high density flexible interconnecting cable includes a combination of high performance signals enclosed within and referencing surrounding logic ground planes, surrounded by frame ground shield layers which are viaed together at the external boundaries of the cable throughout the cable&#39;s entire length, thus forming an external cable shield. These outer frame ground layers are then physically connected to the frame or chassis ground of the connected device at both the source and termination locations of the cable. This combination of surrounding the signals with both logic ground and then frame ground controls both the differential mode and common mode EMI, necessary for high performance EMI shielding, as well as providing the controlled impedance necessary for signal integrity in a high speed interconnect environment. The advantages of this flat flex cable design is high performance EMI shielding of a high density pin count interconnect that can be used where a small bend radius is required while still retaining enough flexibility for concurrent maintenance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  shows a flex cable incorporating the present invention with one plate member surrounding a connector site removed to illustrate the frame ground via sequence.  
         [0006]      FIG. 2  is a schematic view of the cable structure taken along line  2 - 2  of  FIG. 1 .  
         [0007]      FIG. 3  is an exploded schematic view of the dielectric layers of  FIG. 2  including the copper and adhesive layers prior to assembly. 
     
    
     DETAILED DESCRIPTION  
       [0008]      FIG. 1  illustrates an example of the shielded flat flexible cable  10  of the present invention as an external cable assembly for interconnecting signal lines of two devices that connect to the cable at the high density pin connector sites  12  and  13 . The layered structure of the cable is shown in  FIG. 2  taken along line  2 - 2  of  FIG. 1 , with an exploded view of  FIG. 2  shown in  FIG. 3  wherein the individual layers of dielectric material and the coatings of etched copper and adhesive are identified.  
         [0009]     As seen in the exploded view of  FIG. 3 , the flexible cable  10  is formed of a series of dielectric layers which separate the various etched copper layers. A typical dielectric used for these layers is polyimide film. The dielectric layers  14  and  15  each have an upper copper coating which is etched to form a series of conductive traces  17  and carries on the lower surface a copper coating  18  which extends laterally beyond the conductive traces  18 , but is etched away from the outer edge of cable  10  to form a logic ground plane. The conductive traces  17  are principally signal lines, but may also include logic ground lines which cooperate the logic ground planes and dielectric material characteristics to optimize signal line performance or reduce signal line emissions. As shown, cable  10  contains two layers of conductive traces  17 , but the cable could be designed with a single layer or three or more layers of conductive traces to meet the requirements of the devices that are to be interconnected. The polyimide dielectric layer  21  carries adhesive coatings  22  on each side and functions to separate the ground plane  18  from the conductive traces  17 . Dielectric layers  24  and  25  (similarly coated on each side with adhesive coatings  26 ) provide electrical isolation of the conductive trace layers from the adjacent conductive layers.  
         [0010]     Dielectric layer  29  has a logic ground plane copper surface  30  at the lower surface which, like logic ground planes  18 , extends over conductive traces  17 , but is etched away at the cable edge. Thus, copper layers  18  and  30  function to provide a logic ground plane immediately above and below each layer of conductive traces  17 . The upper surfaces of dielectric layers  29  and  33  are respectively coated with copper layers  34  and  35 , which are the principal conductive surfaces forming the frame ground to provide the common mode EMI shielding. Each of the copper layers  34  and  35  extend to the edge or near to the edge of the polyimide dielectric layer upon which they are formed and thereby to the edge or near to the edge of the cable assembly  10 . The copper layers  34  and  35  are etched to form generally circular voids  37  which are aligned with the margins of the logic ground planes  18  and  30  to enable vias to be formed subsequently that connect the logic ground planes, but are electrically isolated from the frame ground planes  34  and  35 . A final pad cap layer  40  is provided at the upper surface of the cable assembly  10  as a copper layer on polyimide dielectric layer  41 . Dielectric layer  41  has an adhesive layer  42  on the lower surface to enable attachment to the frame ground layer  37 . A similar copper pad cap layer  45  is presented at the lower surface of the dielectric layer  33 . The pad cap layers  40  and  45  are subsequently etched to remove the copper from all, but the pad caps which are utilized to enable via interconnects for the ground planes as described below and for signal line interconnecting vias which are not further described herein.  
         [0011]      FIG. 2  is a section of the cable edge showing the assembled condition of the cable  10 .  FIGS. 2 and 3  are schematic views for purposes of illustration, wherein the layer thicknesses are not to scale. In  FIG. 2  the thickness of the assembly adjacent the outer edge would actually be modestly reduced as a consequence of the copper layers that do not extend to the cable edge. With the dielectric layers compressed together and bonded by the adhesive coatings, two series of holes  48  and  49  are drilled through the assembly to enable plated through vias to be formed. Along the entire perimeter of the cable a series of vias  54  are formed, each extending through a drilled hole  48  between a pad  51  at the upper surface  52  and a pad  53  at the lower surface  56 . The outermost row of vias  54  each electrically connects the upper ground frame layer  34  to the lower ground frame layer  35  and together form a sequence of electrical connections. An inner row of vias  55 , each extending from an upper pad cap  57  to a lower pad cap  58  and through openings  37  in the frame ground layers  34  and  35 , interconnect the logic ground plane layers  18  and  30  disposed above and below each of the conductive trace layers  17 . The inner row of vias  55  form a sequence of periodic electrical connecting means which extends along the length of the cable assembly, but are not present in the interconnect areas about the high density interconnect sites  12  and  13  of the cable assembly.  
         [0012]     The spacing between the outer row of frame ground vias  54  is selected such that the space between adjoining vias is smaller than the wave length of the highest frequency signal encountered on the signal lines. The sequence of inner row logic ground vias are longitudinally staggered with respect to the outer row vias  54  so that the vias of one sequence are aligned with the spaces separating vias of the other sequence. The staggered via rows thereby maximize the optical coverage as viewed from the cable edge to maximize the shielding effectiveness of the overall shielding system.  
         [0013]     As seen in  FIG. 1 , the series of pad caps  51  and the sequence of vias connecting the marginal edge portions of the frame ground planes to which they are connected extend about the entire periphery of the cable  10  including the terminal end portions  60  and  61 . The terminal end portions include a metal plate member  63  which overlies the cable and surrounds the connector site such as site  13  of terminal end  61 . Terminal end  60  is shown with the plate removed to reveal the continuous row of via pad caps  51  about the edge of the cable assembly. The plate  63 , as shown installed on terminal end  61 , is bonded to the cable and electrically connected to the underlying pad caps  51  to afford a positive frame ground connection as the plate  63  engages the frame of the device attached at the connector site  13 . The frame ground planes of the cable  10  that, in cooperation with vias  54 , surround the logic ground structure and signal lines are thus connected to the device frame ground at each end of the cable assembly.  
         [0014]     While a particular embodiment of the invention has been illustrated and described, it would be obvious to those skilled in the art that various other combinations and modifications can be made without departing from the scope of the invention. For example, other techniques than plated through vias may be employed to electrically “stitch” together the margins of the frame ground planes and the logic ground planes to effect the common mode and differential mode shielding for EMI control while maintaining the flex cable characteristics for optimum external connection of electronic devices. It is therefore intended to cover in the appended claims all such combinations and modifications that are within the scope of this invention.