Abstract:
A non-drain differential signal transmission cable includes a pair of signal conductors aligned in parallel, an insulation around the pair of signal conductors, a shield conductor around the insulation, and a ground connecting pin to electrically connect the shield conductor to a ground, the ground connecting pin including a wire. An end portion of the pair of signal conductors is exposed with the insulation and the shield conductor removed. The ground connecting pin includes a winding portion wound around the shield conductor to be electrically connected to the shield conductor, and a pin portion extending from the winding portion and having an elongate shape.

Description:
The present application is based on Japanese patent application Nos. 2011-203521 and 2012-174052 filed on Sep. 16, 2011 and Aug. 6, 2012, respectively, the entire contents of which are incorporated herein by reference, 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a non-drain differential signal transmission cable and a ground connection structure thereof. 
     2. Description of the Related Art 
     In devices to handle high-speed digital signals of several Gbit/s or more, such as a server, a router or a storage device, differential signal transmission is used for signal transmission between devices or substrates (circuit boards) in a device. 
     The differential signal transmission is conducted such that signals with 180° inverted phases are transmitted through two paired signal conductors and a difference between the signals received on the side of a receiver is synthesized and outputted. Since currents flowing in the pair of signal conductors flow in opposite directions to each other, electromagnetic wave radiated from a transmission line is small. In addition, since noise from outside is equally superposed on the pair of signal conductors, the effect of noise can be cancelled by synthesizing and outputting the difference on the side of the receiver. Due to these reasons, the differential signal transmission is often used for the high-speed digital signal transmission. 
     As shown in  FIG. 16  and  FIG. 17  which is a cross sectional view taken on line B-B of  FIG. 16 , a differential signal transmission cable  160  used for the differential signal transmission has a pair of signal conductors  161 , an insulation  162  covering together the pair of signal conductors  161 , a shield conductor  163  provided on an outer periphery of the insulation  162  and a sheath  164  provided on an outer periphery of the shield conductor  163 . 
     The shield conductor  163  may be formed by winding a tape with a conductor (a shielding tape) or is formed by covering with a braided strand. In addition, the sheath  164  may be formed by winding an insulating tape or is formed by extrusion coating of resin. 
     The differential signal transmission cable  160  is a twinax cable which has a pair of signal conductors  161  aligned in parallel and in which a difference in physical length between the pair of signal conductors  161  and attenuation of signal at high frequency are less than a twisted pair cable formed by twisting a pair of signal conductors. In addition, since the shield conductor  163  is provided covering the pair of signal conductors  161 , the characteristic impedance is not unstable even if a metal is placed near the cable, and the noise immunity is also high. Due to such advantages, twinax cables are often used for short-distance signal transmission at relatively high speed. 
     By the way, the differential signal transmission cable  160  does not have a drain wire. Therefore, for connecting the differential signal transmission cable  160  to a substrate  165 , after peeling the differential signal transmission cable  160  in a tiered manner, each of the paired signal conductors  161  is connected to a signal line pad  166  on the substrate  165  using a solder  167  while the shield conductor  163  is directly connected, using the solder  167 , to a ground pad  170  which is connected to an inner ground layer  168  in the substrate  165  via a through-hole  169 . 
     The related art may include JP-A-2011-90959. 
     SUMMARY OF THE INVENTION 
     As described above, since the shield conductor  163  is directly soldered to the ground pad  170 , heat can be necessarily conducted from the tip of a soldering iron to the shield conductor  163  and the insulation  162  during the soldering work. 
     Therefore, if the shield conductor  163  is melted or evaporated and the insulation  162  is deformed or melted by the heat applied during the soldering work (e.g., about 230 to 280° C.), the impedance mismatch may occur at a connecting portion between the differential signal transmission cable  160  and the substrate  165  (a cable connecting portion) to impair the electrical characteristics of the differential signal transmission cable  160 . 
     In addition, since a solder fillet needs to be formed in a solder layer in order to ensure an appropriate (highly reliable) solder-connected state of the shield conductor  163 , the ground pad  170  needs to have such a large width (or area) that the solder fillet can be formed therein. 
     Therefore, when the plural differential signal transmission cables  160  are mounted, the package density is limited since the arrangement interval between the plural differential signal transmission cables  160  depends on the width of the ground pad  170 . 
     Accordingly, it is an object of the invention to provide a non-drain differential signal transmission cable that can prevent the thermal load applied to the shield conductor/insulation during the soldering work and improve the package density, and a ground connection structure thereof. 
     (1) According to one embodiment of the invention, a non-drain differential signal transmission cable comprises:
         a pair of signal conductors aligned in parallel;   an insulation around the pair of signal conductors;   a shield conductor around the insulation; and   a ground connecting pin to electrically connect the shield conductor to a ground, the ground connecting pin comprising a wire,   wherein an end portion of the pair of signal conductors is exposed with the insulation and the shield conductor removed, and   wherein the ground connecting pin comprises a winding portion wound around the shield conductor to be electrically connected to the shield conductor, and a pin portion extending from the winding portion and having an elongate shape.       

     In the above embodiment (1) of the invention, the following modifications and changes can be made. 
     (i) The pin portion is formed by twisting together both end portions of the wire. 
     (ii) The ground connecting pin comprises a pin member comprising a spiral portion formed by shaping a portion of the wire into a spiral shape and the pin portion formed by shaping the end portion of the wire into a pin shape, the pin member being preliminarily made, and wherein the spiral portion of the pin member is attached as the winding portion around the shield conductor to form the ground connecting pin. 
     (iii) The winding portion is formed by winding a portion of the wire twice or more around the shield conductor. 
     (iv) The winding portion is solder-connected to the shield conductor. 
     (v) The wire comprises a copper wire and silver- or tin-plating applied to the copper wire. 
     (vi) The pin portion is disposed parallel to the pair of signal conductors. 
     (vii) The pin portion is disposed so as to cross a center line that passes through a center of the pair of signal conductors. 
     (viii) Two of the pin portion are provided. 
     (ix) The two pin portions are provided line-symmetrically with respect to a line orthogonally passing the center of a line segment connecting the centers of the pair of signal conductors. 
     (2) According to one embodiment of the invention, a ground connection structure of a non-drain differential signal transmission cable comprises:
         the non-drain differential signal transmission cable according to the above embodiment (1); and   a substrate on which a signal line pad for connecting the pair of signal conductors and a ground pad for connecting the shield conductor are formed;   wherein the pair of exposed signal conductors is solder-connected to the signal line pad and the shield conductor is electrically connected to the ground pad via the pin portion.       

     In the above embodiment (2) of the invention, the following modifications and changes can be made. 
     (x) The signal line pad is formed at an edge of the substrate so as to be orthogonal to one side of the edge at an interval equal to that of the signal conductors. 
     (xi) The ground pad is formed parallel to the signal line pad. 
     (xii) The signal line pad and the ground pad are formed at a distance from the edge of the substrate. 
     (xiii) The non-drain differential signal transmission cable is arranged at the edge of the substrate so that only the pair of signal conductors and the pin portion are located on the substrate. 
     (xiv) The signal line pad and the ground pad are formed on both sides of the substrate, and the non-drain differential signal transmission cables are attached to the both sides of the substrate. 
     (xv) The ground pad is formed symmetrically on both sides of the signal line pads. 
     Effects of the Invention 
     According to one embodiment of the invention, provided are a non-drain differential signal transmission cable that can prevent the thermal load applied to the shield conductor/insulation during the soldering work and improve the package density, and a ground connection structure thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein: 
         FIG. 1  is a perspective view showing a non-drain differential signal transmission cable in a first embodiment of the present invention; 
         FIG. 2  is a perspective view showing a non-drain differential signal transmission cable in a modification of the first embodiment of the invention; 
         FIG. 3  is a cross sectional view showing an example of a cable structure to which the invention is applicable; 
         FIG. 4  is a cross sectional view showing an example of a cable structure to which the invention is applicable; 
         FIG. 5  is a cross sectional view showing an example of a cable structure to which the invention is applicable; 
         FIG. 6  is a cross sectional view showing an example of a cable structure to which the invention is applicable; 
         FIG. 7  is a perspective view showing a non-drain differential signal transmission cable in a second embodiment of the invention; 
         FIG. 8  is a perspective view showing a non-drain differential signal transmission cable in a modification of the second embodiment of the invention; 
         FIG. 9  is a perspective view showing a pin member; 
         FIG. 10  is a perspective view showing a ground connection structure of a non-drain differential signal transmission cable in an embodiment of the invention; 
         FIG. 11  is an explanatory diagram illustrating a procedure in which the non-drain differential signal transmission cable shown in  FIG. 7  is connected to a substrate to make a ground connection structure of a non-drain differential signal transmission cable; 
         FIG. 12  is a perspective view showing a ground connection structure of a non-drain differential signal transmission cable in a modification of the invention; 
         FIG. 13  is a cross sectional view taken on line A-A, showing the ground connection structure of a non-drain differential signal transmission cable shown in  FIG. 12 ; 
         FIG. 14  is a perspective view showing a ground connection structure of a non-drain differential signal transmission cable in a modification of the invention; 
         FIG. 15  is a frequency distribution graph showing an evaluation result of impedance distribution at a cable connecting portion of the ground connection structure of a non-drain differential signal transmission cable in  FIG. 14 ; 
         FIG. 16  is a perspective view showing a ground connection structure of a non-drain differential signal transmission cable in a conventional art; and 
         FIG. 17  is a cross sectional view taken on line B-B, showing the ground connection structure of a non-drain differential signal transmission cable shown in  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be described below in conjunction with the appended drawings. 
     Firstly, a non-drain differential signal transmission cable in a first embodiment will be described. 
     As shown in  FIG. 1 , a non-drain differential signal transmission cable  10  in the first embodiment is provided with a pair of signal conductors  11  arranged side by side, an insulation  12  provided around the pair of signal conductors  11 , a shield conductor  13  provided around the insulation  12  and a ground connecting pin  14  formed of a wire  15  for solder connection of the shield conductor  13  to a ground (which is, e.g., a below-described ground pad or may be a terminal, etc.), wherein end portions of the pair of signal conductors  11  are exposed from the insulation  12  and the shield conductor  13 , and the ground connecting pin  14  is provided with a winding portion  14   a  as a portion of the wire  15  wound around the shield conductor  13  and a pin portion  14   b  as a pin-shaped end portion of the wire  15 . 
     The non-drain differential signal transmission cable means a differential signal transmission cable which does not have a drain wire. 
     For making the ground connecting pin  14 , the wire  15  is wound around the shield conductor  13  and an end portion of the wound wire  15  is formed into a pin shape. 
     For forming the winding portion  14   a , a portion of the wire  15  is wound twice or more around the shield conductor  13 . This allows the winding portion  14   a  to be tightly in contact throughout the entire circumference of the shield conductor  13 , and accordingly, effect on electric field distribution in the vicinity of the wound portion of the wire  15  caused by a gap generated between the shield conductor  13  and the winding portion  14   a  is eliminated and impedance mismatch caused thereby can be eliminated. 
     The winding portion  14   a  is soldered to the shield conductor  13 . As a result, it is possible to reliably ensure a contact state between the shield conductor  13  and the winding portion  14   a.    
     Since the shield conductor  13  is connected to a ground via the pin portion  14   b , heat is applied to the shield conductor  13  only at the time of soldering the winding portion  14   a  to the shield conductor  13 . In addition, an amount of solder used for soldering the winding portion  14   a  to the shield conductor  13  is smaller than an amount of solder used for soldering the shield conductor  13  directly to the ground. This is because the latter requires only a small area for solder connection. Therefore, an amount of heat applied at the time of soldering the winding portion  14   a  to the shield conductor  13  is smaller than an amount of heat applied when soldering the shield conductor  13  directly to the ground, and does not cause melting or evaporation of the shield conductor  13  and deformation or melting of the insulation  12 . 
     The wire  15  is composed of a copper wire and silver- or tin-plating applied to the copper wire. The copper wire is excellent in conductivity and is also cheap, hence, it is possible to reduce the price of the non-drain differential signal transmission cable  10 . In addition, it is possible to improve solder wettability by applying silver- or tin-plating, which allows a good connecting condition to be ensured when the winding portion  14   a  formed of a portion of the wire  15  is soldered to the shield conductor  13  and when the pin portion  14   b  formed of a portion of the wire  15  is soldered to a ground. 
     The pin portion  14   b  is provided in parallel to the pair of signal conductors  11 . Accordingly, a distance between the pair of signal conductors  11  and the pin portion  14   b  can be kept constant, and thus, impedance mismatch caused by variation in the distance between the pair of signal conductors  11  and the pin portion  14   b  can be reduced. 
     The pin portion  14   b  is provided so as to cross a center line X which passes through the centers of the paired signal conductors  11 . Therefore, it is not necessary to bend the pair of signal conductors  11  or the pin portion  14   b  at the time of connecting the non-drain differential signal transmission cable  10  to the substrate (the detail will be described later) and it is possible to respectively solder the pair of signal conductors  11  and the pin portion  14   b  to the ground in a state of being arranged in parallel to each other and in a state that a distance therebetween is kept constant, hence impedance mismatch is less likely to occur. 
     Two pin portions  14   b  may be provided as is in a non-drain differential signal transmission cable  10 ′ shown in  FIG. 2 . In this case, the pin portions  14   b  are provided line-symmetrically with respect to a line Y which is orthogonal passing the center of a line segment S connecting the centers of the paired signal conductors  11 . Accordingly, electric field distribution for the pair of signal conductors  11  can be balanced, and impedance mismatch caused by an asymmetry property of the electric field distribution can be reduced. 
     Cable structures to which the invention is applicable include a cable structure  30  having a pair of signal conductors  11 , the insulation  12  covering around the pair of signal conductors  11  all together, the shield conductor  13  provided on an outer periphery of the insulation  12  and a sheath  17  provided on an outer periphery of the shield conductor  13 , as shown in  FIG. 3 . The cable structure  30  is used in the non-drain differential signal transmission cables  10  and  10 ′ shown in  FIGS. 1 and 2 . 
     The invention is also applicable to any cable structures as long as a drain wire is not included, e.g., applicable to LAN cable, etc. Referring to  FIGS. 4 to 6 , the invention is applicable to, e.g., a cable structure  40  using a foamed insulation  18  instead of the insulation  12  (see  FIG. 4 ), a cable structure  50  having two longitudinally arranged wires  21  each formed by covering the signal conductor  11  with an inner skin layer  19 , the foamed insulation  18  and an outer skin layer  20  (see  FIG. 5 ) and a cable structure  60  in which the two longitudinally arranged wires  21  are fused and bonded together (see  FIG. 6 ). Meanwhile, the cable structures  50  and  60  of  FIGS. 5 and 6  have a gap  22  between the wires  21  and the shield conductor  13 . 
     Next, a non-drain differential signal transmission cable in a second embodiment will be described. 
     As shown in  FIG. 7 , a non-drain differential signal transmission cable  70  in the second embodiment is different from the non-drain differential signal transmission cable  10  in the first embodiment only in that the pin portion  14   b  is formed by twisting together the both end portions of the wire  15 . 
     In addition, two pin portions  14   b  may be provided as is in a non-drain differential signal transmission cable  70 ′ shown in  FIG. 8  in the same manner as the non-drain differential signal transmission cable  10 ′ in the modification of the first embodiment. 
     It should be noted that other structures are the same as those of the non-drain differential signal transmission cables  10  and  10 ′ and the explanation thereof will be omitted. 
     Although it has been explained that the ground connecting pin  14  in the non-drain differential signal transmission cables  10 ,  10 ′,  70  and  70 ′ is formed by winding the wire  15  around the shield conductor  13  and then forming the end portion of the wound wire  15  into a pin shape, it is not limited thereto. 
     For example, to form the ground connecting pin  14 , a pre-made pin member  90  provided with a spiral portion  91  formed by shaping a portion of the wire  15  into a spiral shape and the pin portion  14   b  formed by shaping the end portion of the wire  15  into a pin shape as shown in  FIG. 9  may be attached around the shield conductor  13  using the spiral portion  91  of the pin member  90  as the winding portion  14   a.    
     At this time, the spiral portion  91  is formed to have an inner diameter which is about several μm larger than the outer diameter of the shield conductor  13  so as to facilitate attachment to the shield conductor  13 . 
     When attaching the pin member  90  to the shield conductor  13 , the spiral portion  91  is attached around the shield conductor  13  by soldered connection. 
     Although the pin member  90  to be the ground connecting pin  14  of the non-drain differential signal transmission cable  10  in the first embodiment is illustrated as an example in  FIG. 9 , it is possible to use a pin member in the non-drain differential signal transmission cables  10 ′,  70  and  70 ′ in the same manner. 
     Next, a ground connection structure of a non-drain differential signal transmission cable in the present embodiment will be described. An example using the non-drain differential signal transmission cable  70  will be described here. 
     As shown in  FIG. 10 , a ground connection structure of a non-drain differential signal transmission cable (hereinafter, simply referred to as “ground connection structure”)  100  in the present embodiment is provided with the non-drain differential signal transmission cable  70  and a substrate  25  on which signal line pads  23  for connecting the pair of signal conductors  11  and a ground pad  24  for connecting the shield conductor  13  are formed, and the ground connection structure  100  is characterized in that the pair of exposed signal conductors  11  is soldered to the signal line pads  23  using a solder  16  and also the shield conductor  13  is soldered to the ground pad  24  via the pin portion  14   b  using the solder  16 . 
     The signal line pads  23  are formed at an edge of the substrate  25  so as to be perpendicular to a side  26  of the edge at an interval equal to that of the signal conductors  11 . Accordingly, it is possible to respectively solder the signal conductors  11  to the signal line pads  23  in a state that a distance therebetween is kept constant, and impedance mismatch at a cable connecting portion is thus less likely to occur. 
     In addition, the signal line pads  23  are connected to signal lines  27  formed on the substrate  25  and signals are transmitted through the signal lines  27 . 
     The ground pad  24  is formed on one side of the signal line pad  23  so as to be parallel thereto. This is to align with the pin portion  14   b  which is provided in parallel to the signal conductors  11 . As a result, the distance between the signal conductors  11  and the pin portion  14   b  can be kept constant and it is thus possible to reduce impedance mismatch at the cable connecting portion. 
     In addition, the ground pad  24  is connected to an inner ground layer  29  in the substrate  25  via a through-hole  28 . Alternatively, the ground layer may be formed as a surface layer. A technique such as coplanar wiring is used when formed as a surface layer. 
     The signal line pads  23 , the ground pad  24  and the inner ground layer  29  are formed at a distance d from the edge of the substrate  25 . As a result, it is possible to prevent the shield conductor  13  of the non-drain differential signal transmission cable  70  from contacting with the signal line pads  23 , the ground pad  24  and the inner ground layer  29  when the non-drain differential signal transmission cable  70  is connected to the substrate  25 . Contact of the shield conductor  13  with the ground pad  24  or the inner ground layer  29  does not cause a problem of signal transmission even though there is a problem of impedance mismatch. However, when the shield conductor  13  contacts with the signal line pads  23 , a short circuit occurs and signals cannot be transmitted. The structure described above is to avoid such a problem. 
     The signal line pads  23 , the signal lines  27 , the ground pad  24  and a non-illustrated circuit pattern are simultaneously formed on the substrate  25 . 
     The non-drain differential signal transmission cable  70  is arranged at the edge of the substrate  25  so that only the pair of signal conductors  11  and the pin portion  14   b  are located on the substrate  25 . The reason is as follows. 
     Conventionally, a terminal portion of the differential signal transmission cable  160  is placed on the substrate  165  such that the shield conductor  163  is connected to the ground pad  170  and, in this state, the signal conductors  161  are soldered to the signal line pads  166 . Therefore, the signal conductors  161  need to be bent by a size equivalent to about half of the height of the insulation  162  so that the signal conductors  161  come into contact with the signal line pads  166  (see  FIGS. 16 and 17 ). At this time, the insulation  162  may be deformed by an external force acting thereon, which causes impedance mismatch at the cable connecting portion and deterioration of electrical characteristics of the differential signal transmission cable  160 . 
     On the other hand, due to the arrangement in which only the pair of signal conductors  11  and the pin portion  14   b  are located on the substrate  25 , the pair of signal conductors  11  and the pin portion  14   b  can be soldered to the signal line pads  23  and the ground pad  24  without being bent. As a result, impedance mismatch caused by deformation of the insulation  12  can be prevented and it is also possible to prevent deterioration in electrical characteristics of the non-drain differential signal transmission cable  70 . Furthermore, the height of the ground connection structure  100  per se can be reduced by the size equivalent to about half of the height of the insulation  12  and it is thus possible to downsize the ground connection structure  100 . 
     As shown in  FIG. 11 , it is possible to make the ground connection structure  100  by connecting the non-drain differential signal transmission cable  70  to the substrate  25 . In detail, the pair of signal conductors  11  are placed on the signal line pads  23  and, at the same time, the pin portion  14   b  is placed on the ground pad  24 , and then, solder connection is performed using the solder  16 . At this time, the pair of signal conductors  11  and the pin portion  14   b  are soldered in a state that a distance therebetween is kept constant without bending. As a result, the ground connection. structure  100  with reduced impedance mismatch at the cable connecting portion is obtained. 
     In the ground connection structure  100 , since the shield conductor  13  and the ground pad  24  are connected via the pin portion  14   b , the ground pad  24  only needs to have a width (or area) which allows solder connection of the pin portion  14   b . In other words, the width (or area) of the ground pad  24  of the ground connection structure  100  can be smaller than the case of directly soldering the shield conductor  13 . Therefore, in the ground connection structure  100 , since a width (or area) on the substrate occupied by the ground pad  24  is smaller than a conventional art, it is possible to improve a package density of the non-drain differential signal transmission cable  70  compared to the conventional art. 
     Alternatively, the signal line pads  23  and the ground pads  24  may be formed at the same positions on both sides of the substrate  25  as shown in  FIG. 12  and  FIG. 13  which is a cross sectional view taken on line A-A of  FIG. 12 , and in this case, the non-drain differential signal transmission cables  70  are attached to the same positions on the both sides of the substrate  25 . In other words, the non-drain differential signal transmission cables  70  are longitudinally placed in a state that the pin portions  14   b  thereof are placed in reversed positions to each other, and each non-drain differential signal transmission cable  70  is attached by respectively connecting the pair of signal conductors  11  and the pin portion  14   b  thereof to the signal line pads  23  and the ground pad  24  which are formed on the connecting surface. As a result, it is possible to further improve the package density of the non-drain differential signal transmission cable  70  on one substrate  25 . 
     Alternatively, the ground pads  24  may be formed symmetrically with respect to a longitudinal extension line E of the non-drain differential signal transmission cable  70  so as to sandwich two signal line pad  23  from both sides, as is in a ground connection structure  100 ′ shown in  FIG. 14 . In this case, one of the ground pads  24  is a dummy ground pad to which the pin portion  14   b  is not soldered. This allows electric field distribution for the pair of signal conductors  11  around the cable connecting portion to be further balanced, and it is possible to further ensure impedance match at the cable connecting portion. 
     When a sample is actually made based on the ground connection structure  100 ′ shown in  FIG. 14  and impedance distribution at a cable connecting portion is evaluated, frequency distribution graph as show in  FIG. 15  is obtained. 
     According to the result, impedance at the cable connecting portion in the ground connection structure  100 ′ is 95 to 102Ω and it is understood that characteristics sufficient for a system with impedance of 100Ω is obtained. A strict technical specification of about 100±5Ω is required especially for high-speed application, and the ground connection structure  100 ′ meets this requirement. 
     As described above, according to the invention, it is possible to prevent thermal load from being applied to the shield conductor/the insulation during soldering work, i.e., to prevent melting or evaporation of the shield conductor and deformation or melting of the insulation during the soldering work, and also possible to improve a package density. 
     Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be therefore limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.