Abstract:
A connector system includes a connector mounted on a circuit board. The circuit board has deeper backdrilled vias and the connector has modified signal terminal that can mate with the backdrilled vias so as to provide a surprising increase in the performance of signal traces provided in the top layers of the circuit board.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/129,414, filed Mar. 6, 2015, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to the field of connectors, more specifically to the field of press-fit connectors suitable for high data rates. 
       DESCRIPTION OF RELATED ART 
       [0003]      FIG. 1  illustrates a state of the art backplane/connector interface for a press-fit connector and shall be used as the basis for Config #1 in the charts provided in  FIGS. 5-9 . As depicted, the ground terminal tails  59  and signal terminal tails  58  are configured to extend into a circuit board  60  about 1.3 mm. Making the terminals tails this small has proven beneficial from a performance standpoint in spite of the challenges that result from the assembly process. To improve performance the via can be backdrilled so that the resulting via extends about 1.0 mm into the circuit board  60 . This provides improved performance compared to prior designs but as data rates have increased from 25 to 40 Gbps there is a desire to further improve the connector design. One issue that exists with the move to 40 Gbps channel (which relies on 20 Ghz signaling in an NRZ encoding system) is that the vias and terminals create a substantial stub if signal traces  62  are attempted to be used in the second or third layer of the circuit board, thus it is common to not use the second or third layer as a high data rate capable channel and instead place several extra layers on top of uppermost high data rate capable signal layer. Due to the desire to have symmetrical construction in the circuit board  50  (otherwise the circuit board tends to warp) this tends to require the addition of four or six additional layers on the circuit board (the layers added on top are also added on the bottom), which can increase the cost of the circuit board  60 . Thus, certain individuals would appreciate further improvements to connector and circuit boards to enable lower cost solutions and improved performance. 
       SUMMARY 
       [0004]    A system provides a connector mounted on a circuit board. The circuit board includes a top surface and includes a plurality of ground vias and signal vias. The signal vias can be backdrilled so that the signal via doesn&#39;t extends more than about 0.3 mm past the trace connected to the signal via and in an embodiment the signal via can extend down 0.5 mm from the top surface. The connector includes ground terminal tails positioned in the ground via and signal terminal tails positioned in the signal vias. The signal terminal tails include an enlarged portion that extends into the board not more than 0.5 mm and thus can engage the shorter signal via but the total length of the signal terminal tail can still extend into the circuit board as far as the ground terminal tail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
           [0006]      FIG. 1  illustrates a simplified cross section of a prior art design of a connector system. 
           [0007]      FIG. 2  illustrates a simplified cross section of exemplary embodiment of a connector system with a very short via and terminal configuration. 
           [0008]      FIG. 3  illustrates a simplified cross section of another exemplary embodiment of a connector system with short vias and longer terminals. 
           [0009]      FIG. 4  illustrates an perspective view of a cross section of a circuit board showing two via and terminal configurations. 
           [0010]      FIG. 5  illustrates a graph of insertion loss performance of the configurations depicted in  FIG. 1 ,  FIG. 2  and  FIG. 3 . 
           [0011]      FIG. 6  illustrates a graph of return loss performance of the configurations depicted in  FIG. 1 ,  FIG. 2  and  FIG. 3 . 
           [0012]      FIG. 7  illustrates a graph of near-end cross talk of the configurations depicted in  FIG. 1 ,  FIG. 2  and  FIG. 3 . 
           [0013]      FIG. 8  illustrates a graph of far-end cross talk of the configurations depicted in  FIG. 1 ,  FIG. 2  and  FIG. 3 . 
           [0014]      FIG. 9  illustrates a graph of impedance response of the configurations depicted in  FIG. 1 ,  FIG. 2  and  FIG. 3 . 
           [0015]      FIG. 10  illustrates a simplified cross section of exemplary embodiment of a connector system with a via connected to a trace in a L3 layer. 
           [0016]      FIG. 11  illustrates a simplified cross section of exemplary embodiment of a connector system with a via connected to a trace in a L5 layer. 
           [0017]      FIG. 12  illustrates a simplified cross section of exemplary embodiment of a connector system with a via connected to a trace in a L7 layer. 
           [0018]      FIG. 13  illustrates a graph of insertion loss performance of the configurations depicted in  FIG. 10 ,  FIG. 11  and  FIG. 12 . 
           [0019]      FIG. 14  illustrates a graph of return loss performance of the configurations depicted in  FIG. 10 ,  FIG. 11  and  FIG. 12 . 
           [0020]      FIG. 15  illustrates a graph of near-end cross talk performance of the configurations depicted in  FIG. 10 ,  FIG. 11  and  FIG. 12 . 
           [0021]      FIG. 16  illustrates a graph of far-end cross talk performance of the configurations depicted in  FIG. 10 ,  FIG. 11  and  FIG. 12 . 
           [0022]      FIG. 17  illustrates a graph of impedance response of the configurations depicted in  FIG. 10 ,  FIG. 11  and  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The detailed description that follows describes exemplary embodiments and is not intended to be limited to the expressly disclosed combination(s). Therefore, unless otherwise noted, features disclosed herein may be combined together to form additional combinations that were not otherwise shown for purposes of brevity. 
         [0024]    It should be noted that the details of the connector are not shown herein. The disclosure provided herein is suitable for use with a variety of connector types that are configured to provide high data-rate capable connectors and one potential application is a backplane connector such as the MOLEX IMPEL connector family. Another potential application is a standard input/output (I/O) style connector such as the MOLEX zQSFP stacked connectors. In each case the connector could be modified to include signal tails as described herein to obtain benefits at higher signaling frequencies and data rates. 
         [0025]    As noted above, the current state of the art is problematic at 20 GHz signaling frequencies for signal rows in the top two or three layers. One potential solution is to just make the terminal tails very short, such as is illustrated in  FIG. 2 . Applicants have determined that a signal tail that does not extend into the board more than 0.5 mm, in combination with a via barrel that is about 0.5 mm long, will have very good performance in a configuration where the top three layers include the high data-rate capable signal traces and this theoretical solution is the basis for Config. #3. 
         [0026]    The solution in Config. #3, however, is problematic from an assembly process as it becomes very difficult to align the terminal tails with the vias once the tails become so short. In addition, the short tails tend to have much lower retention force, thus making the resulting solution undesirable from a robustness standpoint, at least without additional retention features. Consequentially persons of skill have been dissuaded from attempting to use an eye-of-the-needle style tail shorter than about 1 mm length and the depicted designs use terminals that are configured to extend into the circuit board between 1 and 1.4 mm in length, with the terminals in  FIG. 1-3  being configured to extend into the circuit board about 1.3 mm. 
         [0027]      FIGS. 3-4  illustrate an alternative embodiment of a connector system that surprisingly has been found to work much better than expected. A circuit board  160  with a top surface  161  and a bottom surface  162  is provided with a plurality of signal vias  170  and ground vias  180 . Ground terminal tails  158  are positioned in the ground vias  180  while signal terminal tails  158  are positioned in the signal vias. 
         [0028]    As depicted, the ground terminals  159  are configured so that the enlarged portion  159   a  (e.g., the eye in an “eye of the needle” design) is positioned in the circuit board a substantial distance so as to provide good retention and in the depicted embodiments the top of the enlarged portion is more than 0.2 mm into the circuit board  160 . This helps provide good retention of the ground terminal tails  159  in the circuit board  160  and improves the robustness of the system. To aid in installation the ground terminal tails are configured to extend about 1.3 mm into the circuit board because, as noted above, shorter tails are difficult to position in the vias. 
         [0029]    The signal terminal tails  158  are also configured to extend about 1.3 mm into the circuit board  160  but are configured so that the enlarged portion  158   a  extends into the circuit board not more than 0.5 mm. In addition, the signal vias are backdrilled so that the via  170  only extends about 0.3 mm past the trace, which could be about 0.5 mm into the circuit board  160  for traces near the top surface of the circuit board  160  (in that regard, the signal vias are similar to the configuration depicted in  FIG. 2 ). This provides an embodiment where the via  170 , once backdrilled, does to not extend more than 0.30 mm past the signal trace  162  and (for traces near the top surface  161 ) also does not extend more than 0.5 mm below the top surface  161 . In other words, the signal terminal tails  158  are configured so that the lowest part of the enlarged portion  158   a  does not extend more than about 0.5 mm into the circuit board  160  while the tail still extends further into the board and preferably extends more than 1.0 mm into the board. 
         [0030]    As can be appreciated, therefore, the signal terminal tails  158  can extend more than 0.5 mm past the backdrilled via  170  and in the depicted embodiment extend 0.8 mm past the via barrel  170 . The signal tails could be shortened somewhat to provide further improvements but shorter tails provides diminishing returns that must be balanced with the desire for reliable assembly. In that regard it should be noted that having the signal terminal tails substantially the same length as the ground terminal tails is beneficial in insuring all the terminals are properly seated in their respective vias before the connector is pressed onto the board. And for retention purposes it is desirable to have the ground terminals with an enlarged portion that is reliably below the top surface so that a reliable engagement between the connector and the circuit board. 
         [0031]    The depicted configuration also allows the signal terminal tails  158  to be inserted into their respective vias prior to having the enlarged portion  159   a  of the ground terminal tails  159  start to be compressed. This helps provide better tactile feedback and reduces the chance that the signal tails could be misaligned and inadvertently damaged/crushed due to the difficulty in perceiving the misalignment because of the higher efforts associated with compressing the enlarged portions  159   a . Another advantage of the depicted system is that the maximum insertion force of the tails can be reduced due to the fact that the enlarged portions  159   a  are finished being compressed before the enlarged portions  158   a  start to get compressed. In other words, the enlarged portions of the terminals are compressed in a sequenced manner with the ground terminal tails being compressed first and the signal terminals tails being compressed second. This configuration is Config. #2 and as can be appreciated, the performance of this system is close to the performance of the theoretical design illustrated in  FIG. 2 , especially at 20 GHz. 
         [0032]    It should be noted that the backdrill is shown as being 0.7 mm in diameter and can also be as large as about 1.0 mm in diameter. While it is beneficial to increase the backdrill diameter from about 0.5 mm to about 0.7 mm, Applicants have determined that there are decreasing returns as the backdrill diameter is increased beyond about 0.7 mm. Therefore for most solutions it is expect that it will be more desirable to use a backdrill diameter of about 0.7 mm. 
         [0033]    Turning to  FIGS. 5-9 , the performance of signal terminals tails in the Config. #2 configuration is illustrated. As can be appreciated, compared to standard terminals, the Config. #2 design provides a substantial improvement that would readily support 20 GHz signaling while providing at channel that has almost 15 dB of signal between the insertion loss and return loss. In contrast, the prior design only had about 11 dB of signal at 15 GHz and, therefore, in the prior design the signal traces in the top layers would not have been suitable to support a 40 Gbps channel. Thus the improved design enables 40 Gbps performance in layers that previously were not functional at such data rates and provides the possibility of reduced costs. 
         [0034]    It should be noted that the depicted designs are connectors with terminals configured to engage vias that are about 0.40 mm in diameter. The features described herein are also effective for slightly larger tails and vias, such as a system where the tails are configured to engage vias that have about a 0.45 mm diameter. 
         [0035]      FIGS. 10-17  illustrate the benefit of a connector with the modified signal tails.  FIGS. 10-12  illustrate embodiments where the tails extend into the circuit board about 1.2-1.3 mm.  FIG. 10  illustrates the configuration associated with Config #2, L3 trace;  FIG. 11  illustrates the configuration associated with Config #2, L5 trace and  FIG. 12  illustrates the configuration associated with Config #2, L7 trace. As can be appreciated, the signal terminal tail is the same in each configuration (e.g., the enlarged portion extends less than 0.5 mm in to the circuit board) but the board is backdrilled so that the distance V1 from the respective trace to the end of the via is kept constant at about 0.3 mm. As can be appreciated, this means that distance P1 is greater than distance P2 and distance P2 is greater than distance P3. In addition, the length of 270c is greater than the length of 270b, which in turn is great than the length of 270a (which is about 0.5 mm). 
         [0036]    As the charts in  FIGS. 13-17  illustrate, the new signal terminal design, in combination with a circuit board that is backdrilled so that the via extends about 0.3 mm past the trace, provides improved insertion loss and return loss as the via length increases but provides slightly worse cross talk as the via length increases. Thus the new signal tails can be utilized in a variety of configurations and in each situation the new signal tails provide desirable performance. 
         [0037]    The disclosure provided herein describes features in terms of preferred and exemplary embodiments thereof. Numerous other embodiments, modifications and variations within the scope and spirit of the appended claims will occur to persons of ordinary skill in the art from a review of this disclosure.