Abstract:
A paddle card assembly is disclosed for use in providing a high speed transmission line for connecting electronic devices together. The paddle card takes the form of a circuit board that has two distinct portions, a base portion to which wires of a cable are terminated, and an extension portion that extends rearwardly toward the cable wire in order to extend between sets of wires. The extension portion has one or more ground plane layers formed therewith and supported thereby and as such, the extension portion places a ground plane in the termination area of the connector, rear of the trailing edge of the paddle card to provide shielding between pairs of wires on opposite sides of the paddle card where the cable shields are cut back.

Description:
BACKGROUND OF THE PRESENT DISCLOSURE 
       [0001]    The Present Disclosure relates generally to cable interconnection systems, and, more particularly, to improved cable assemblies for use in high speed data transmission applications. 
         [0002]    Conventional cable interconnection systems are found in electronic devices such as routers, servers and the like, and are used to form signal transmission lines. These transmission lines may extend between chip members and connectors, connectors in two different devices, and between devices themselves. Often, differential signal wires are used for each such transmission line in a cable and although it is easy to maintain a desired impedance profile along the length of the cable due to the cable geometry, it is difficult to maintain such a profile at the termination ends of the wires. In some instances, these terminations occur at circuit board that takes the form of an edge, or paddle, card. The wires are terminated to contact pads along the trailing edge of the circuit board. In such a situation, the exterior insulation is stripped back and the bare conductors are terminated to solder pads or the like. Removing the exterior insulation also requires removing the outer shield of the cable wires so that the termination area is left poorly grounded. This ungrounded area has been known to contribute to and increase the crosstalk between the wires in high speed applications. It is desirable to therefore have a cable termination with a structure that lessens the discontinuities in impedance profiles. 
         [0003]    The Present Disclosure is therefore directed to a cable assembly particularly suitable for high speed data transmission applications. 
       SUMMARY OF THE PRESENT DISCLOSURE 
       [0004]    Accordingly, there is provided an improved high speed cable assembly having an improved termination structure suitable for beneficial termination in high speed data transmission applications. 
         [0005]    In accordance with an embodiment described in the Present Disclosure, a cable assembly is disclosed that utilizes a specially configured circuit board, or paddle card, to which the wires of the cable are terminated. The circuit board is formed with two distinct sections, the first of which may be considered a base portion on which a majority of the circuit board circuitry resides, including the front contact pads which engage terminals of an opposing mating connector, as well as the contact pads to which cable wires are terminated. The second portion may be considered an extension of the first portion but it has a different and lesser thickness than the circuit board base portion. It extends rearwardly of the base portion and the contact pads. 
         [0006]    One or more ground plane layers are preferably extended into the area of the circuit board extension portion, rearwardly of the contact pads, and these ground plane layers may be the only circuitry of the circuit board that is supported by the extension portion. Ordinarily the cable wires have their insulation covering and exterior shields stripped from the ends thereof in order to expose free ends of the cable wire conductors, so that the conductors may be easily terminated to the circuit board along the rearmost set of contact pads thereof. In stripping the cable wires, the conductors have a bare extent, and the insulative or dielectric covering of the wires also has a given extent that extends between the leading edge of the insulative covering and the leading edge of the exterior shield. Ordinarily in dense cable connectors, differential signal pairs are arrayed in rows along the top and bottom surfaces of the circuit board. The conductors of the wire pairs are terminated to contact pads on opposite sides of the circuit board and the ends of the insulation of the wire pairs are disposed rearwardly of the circuit board trailing edge. The ends of the exterior shielding are likewise disposed rearwardly of the circuit board trailing edge and rearwardly of the ends of the cable wire insulation and as such, a gap occurs between the wire pairs attached to the top and bottom surfaces of the circuit board. This area is prone to increasing crosstalk and negatively influencing discontinuities in the impedance profile of the cable assembly. 
         [0007]    The extension portion of the circuit board extends rearwardly into this gap between the leading edge of the shield and the trailing edge of the circuit board. It fills the intervening space between top and bottom pairs of cable wires, and because the extension portion supports at least one ground plane layer, it provides shielding between aligned pairs in the vertical direction of the cable assembly. This shielding reduces crosstalk in the termination area, without the need for additional, separate shielding components. The extension portion has a thickness that is less than the thickness of the circuit board base portion so that the circuit board has a stepped configuration when it is viewed from the side. The thickness of this extension portion may be chosen to provide a spacing template for the cable wires as it will preferably fit snugly in the horizontal gap that exists in the vertical direction between wires on opposite surfaces of the circuit board. The reduction in crosstalk between the vertically aligned wires without utilizing additional components, offers a cost saving in manufacturing of cable assemblies of the Present Disclosure. Moreover, the stepped profile of the circuit board provides for an intervening element that can assist in providing strain relief to the cable assembly when the circuit board is overmolded with an insulative material at least in the termination area. Suitable overmolding materials include plastics and/or epoxies. 
         [0008]    These and other objects, features and advantages of the Present Disclosure will be clearly understood through a consideration of the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]    The organization and manner of the structure and operation of the Present Disclosure, together with further objects and advantages thereof, may best be understood by reference to the following Detailed Description, taken in connection with the accompanying Figures, wherein like reference numerals identify like elements, and in which: 
           [0010]      FIG. 1  illustrates a sectional view taken through the termination area of a conventional cable-circuit board assembly; 
           [0011]      FIG. 2  is a perspective view of typical cable connector housing in which cable-circuit board assemblies of the type illustrated in  FIG. 1  are housed; 
           [0012]      FIG. 3  is a perspective view of the cable-circuit board assembly of  FIG. 1 ; 
           [0013]      FIG. 4  is a top plane view of the cable-circuit board assembly of  FIG. 1 ; 
           [0014]      FIG. 5  is a sectional view of a cable-circuit board assembly constructed in accordance with the principles of the Present Disclosure; 
           [0015]      FIG. 6  is a top plan of the cable-circuit board assembly of  FIG. 5 ; 
           [0016]      FIG. 7  is a sectional view of a cable connector with the cable-circuit board assembly of  FIG. 6  housed thereby and with an overmolded body portion; and 
           [0017]      FIG. 8  is an elevational view of an alternate construction of a circuit board used in cable-circuit board assemblies of the Present Disclosure. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    While the Present Disclosure may be susceptible to embodiment in different forms, there is shown in the Figures, and will be described herein in detail, specific embodiments, with the understanding that the Present Disclosure is to be considered an exemplification of the principles of the Present Disclosure, and is not intended to limit the Present Disclosure to that as illustrated. 
         [0019]    As such, references to a feature or aspect are intended to describe a feature or aspect of an example of the Present Disclosure, not to imply that every embodiment thereof must have the described feature or aspect. Furthermore, it should be noted that the description illustrates a number of features. While certain features have been combined together to illustrate potential system designs, those features may also be used in other combinations not expressly disclosed. Thus, the depicted combinations are not intended to be limiting, unless otherwise noted. 
         [0020]    In the embodiments illustrated in the Figures, representations of directions such as up, down, left, right, front and rear, used for explaining the structure and movement of the various elements of the Present Disclosure, are not absolute, but relative. These representations are appropriate when the elements are in the position shown in the Figures. If the description of the position of the elements changes, however, these representations are to be changed accordingly. 
         [0021]      FIGS. 1-4  illustrate a conventional cable connector  10  that has a protective outer connector housing  11  with a hollow termination end  13  that receives an end of a multiple-wire cable  14  and which is connected to a smaller mating end  12 . The mating end  12  of the connector holds a mating blade, shown as a circuit board, or paddle card,  15  in an orientation that is suitable for mating with an opposing, mating receptacle connector (not shown) that has a slot which receives the forward end of the circuit board  15 . In order to provide a means for enuring engagement with the opposing connector after mating with it, the connector  10  is preferably provided with an elongated latch member  17  with engagement tabs at its forward end that are received in openings formed in the opposing connector. The latch member  17  is actutaed by the manipulation of an actuator  18 , shown as a pull tab. 
         [0022]      FIG. 3  is a perspective view of a conventional termination structure used in to connect individual wires  25  of the cable  14  to circuits on the circuit board  15 . As shown in  FIG. 3  the cable  14  encloses a plurality of wires  24 . The wires illustrated are of the twin-ax construction, meaning that they have a pair of conductors  27  running along their lengths and in a spaced apart fashion. The conductors  27  are held in place by an outer insulative and dielectric covering  26 . The dielectric covering  26  is itself enclosed by an outer shield member  29 . The shield member  29  shown is a braided wire, conductive shield, and outer conductive materials are used, such as copper foil and the like. 
         [0023]    Turning to  FIG. 4 , which is a plan view of the termination structure shown in  FIG. 3 , it can be seen that the circuit board  15  takes the general form of a rectangle and has a leading edge  20  and a trailing edge  22 . The leading edge  20  is the forwardmost edge of the circuit board  25  and is that portion of the circuit board that is inserted into the card-receiving slot of an opposing, mating connector. In that regard, the circuit board  15  is typically formed with an array of conductive contact pads  21  that mate with terminals of the opposing connector. Similarly, the trailing edge  22  of the circuit board  15  defines an area where the free ends of the cable wire conductors  27  are terminated to the circuit board  15 . As such, the circuit board  15  has an array of contact pads  23  arranged in a pattern proximate to the trailing edge  22  of the circuit board  15 . 
         [0024]    In termination, the free ends of the cable wire conductors  27  are exposed by removing a given length of their outer covering  26 , and the outer shield member  29  also has a portion of it removed. This removal defines respective leading edges  28 ,  31  of both the wire insulation  26  and the shield member  29 , both of which are spaced apart from the ends of the cable wire conductors. These leading edges  28 ,  31 , as best shown in  FIG. 4 , also are spaced rearwardly of the circuit board contact pads  23  and the trailing edge  22  of the circuit board  15 . This difference defines a gap “G” through which the cable wire conductors  27  extend, both in their bare condition and in their insulated, but unshielded transmission. Drain wires  30  associated with each twin-ax pair may be provided and they are separately attached to the circuit board either by direct attachment or by way of a cradle  30   a  and connected to an internal ground plane layer of the circuit board  15 . 
         [0025]    In this gap area G, the cable wire shields  29  are removed and when the cable wires  25  are terminated to the circuit board  15 , they are typically aligned with each other in pairs, vertically. That is, some pairs of the cable wires  25  are attached to the top surface of the circuit board  15 , while a like amount of cable wires  25  are attached to the bottom surface of the circuit board  15 . The pairs are arranged both side by side in two, generally horizontal planes, but the pairs in these planes are separated from each other by a vertical spacing that is at least equal to the thickness of the circuit board. In the gap area, G, where the cable wire shielding has been removed the signal wires of the cable are closely spaced apart from each other, and any ground plane that may be utilized in the circuit board construction stops near the trailing edge of the circuit board  15 . Hence, there is no shielding in this gap area between the vertically spaced apart wire pairs. Even though the gap distance is relatively small, at high data transfer speeds, such as 10 Gigabits per second (Gbps) crosstalk occurs and rises to a level that is deleterious to efficient signal transmission and may lead to discontinuities in the impedance profile of the cable assembly. This crosstalk is even greater at data transfer speeds of 25 Gbps. 
         [0026]      FIGS. 5-6  illustrate our solution to this problem. The circuit board  15  has been reconfigured and now has two distinct portions. The first portion may be considered as a base portion  15   a  which supports the front and rear contact pads  21 ,  23  and circuitry interconnecting them together, while the second portion may be considered as an extension portion  35  that extends past the circuit board trailing edge  22  and through the gap area G for a given length “EL” that terminates at the end  38  of the extension portion  35 . As used in the Present Disclosure, the term trailing edge” refers to the rearmost edge of the circuit board base portion along which the rear contact pads  23  are arranged, and it will be understood that the extension portion extends past this trailing edge. The rear edge of the extension portion  35  is not the trailing edge of the circuit board  15 . 
         [0027]    As best shown in  FIG. 5 , the two circuit card portions  15 ,  35  have different thicknesses, such that when viewed from the side, the circuit board  15  has a stepped profile. The circuit board base portion  15   a  has a first thickness and the circuit board extension portion  35  has a second thickness, which is less than the first thickness. The circuit board extension portions  35  includes at least one ground plane layer  36  that preferably extends from the base portion  15  into the extension portion  35 . 
         [0028]    Two such ground plane layers  36   a,    36   b  are illustrated in  FIG. 5  as the most preferred structure and it can be seen that the ground plane layers  36   a,    36   b  are spaced apart vertically. The circuit board extension portion  35  and its associated ground plane layer(s)  36  extend rearwardly, for a given distance, past the leading edges  28 ,  31  of the wire insulation  26  and shields  29  filling the gap area G complete with an associated ground to which the exposed portions of the wire conductors may couple, rather than with each other. The ground plane layers  36   a,    36   b  may be slightly smaller than the circuit board extension portion width, as shown in  FIG. 6  so that a margin or setback area  37  is defined to prevent contact between the connector housing  40 , which often is conductive, and the ground plane layers  36  if that is desirable in the connector construction. Alternatively, there may be application where the ground plane layers  36  are desired to contact the connector housing and thus no margin area  37  would be needed. 
         [0029]    Such a structure reduces the crosstalk that occurs in this area, especially at high data transfer speeds of 10 Gbs up to 25 Gbps and above. The use of the circuit board extension portion  35  to solve this problem does so without increasing the complexity of assembly and manufacturing costs as it accomplishes a reduction in crosstalk without attaching any extra component. Crosstalk has been able to be reduced up to 15 dB between the adjacent top and bottom rows of twin-ax wires without adding any extra components to the cable assembly. 
         [0030]    This new development also provides the user with the ability to integrate a strain relief aspect into the termination area. This may be done by forming a body portion utilizing a suitable material such as a plastic or an epoxy. As shown in  FIG. 7 , the body portion  42  extends over the termination area and the free ends of the wire conductors  27  and the leading edge  28  of the wire insulation and leading edge  31  of the wire shields  29 , as well as the entire circuit board extension portion  35 . This overmolded body portion  42  may be configured to contact the inner walls of the connector housing  40 . Additionally the thickness of the circuit board extension portion  35  may be set so that the cable wires  25  lie flat thereupon and their center conductors  27  extend at a level where they also lie flat on the top and bottom surfaces of the circuit board base portion  15  thereby dispensing with the need to bend the conductors down so that they will touch the rear contact pads  23 . 
         [0031]    Preferably, the circuit boards of the Present Disclosure are integrally formed as one piece as shown in  FIG. 5 . Alternatively, it is contemplated that the extension portion  35   a  may be formed separately and inserted into the circuit board base portion  15  as shown in  FIG. 8 . The circuit board base portion  15  preferably is formed with a slot  34  that receives a tongue or blade portion of the extension portion  35 . The ground plane layers  36   a,    36   b  are preferably formed on opposing surfaces of the extension portion  35   a  and the mating blade for contacting ground plane connections within the circuit board base portion slot  34 . 
         [0032]    Although the Present Disclosure has described our new development in terms of twin-ax wires, it will be understood that the principles thereof apply equally to pairs of differential signal wires used in cables where each wire had a center conductor surrounded by an insulative covering and the two wires are enclosed within an outer shielding member. In such an instance, the wires are arranged to align with corresponding contact pads and the insulation on each wire is trimmed back as is the outer shielding member for each such pair of wires, so that the leading edges of the wire insulation and the outer shielding members are disposed rearwardly of the circuit board base portion trailing edge so that the circuit board extension portion may be positioned therebetween in the manner described above. 
         [0033]    Finally, while a preferred embodiment of the Present Disclosure is shown and described, it is envisioned that those skilled in the art may devise various modifications without departing from the spirit and scope of the foregoing Description and the appended Claims.