Abstract:
Embodiments of the invention include a Printed Wiring Board (PWB) having a first via connected to a top-side signal source, a second via connected to a bottom-side signal destination, and a third via connected to the first via on a lower signal layer of the PWB and further connected to the second via on an upper signal layer of the PWB. In embodiments of the invention, the third via is referred to as an S-Turn via. The S-Turn PWB routing configuration advantageously reduces reflections causes by via stubs at Multi-Giga Hertz (MGH) frequencies. Other embodiments are described.

Description:
FIELD 
       [0001]    The invention relates generally to Printed Wiring Board (PWB) technology, and more particularly, but without limitation, to an S-Turn via structure in a double-sided multi-layered PWB. 
       BACKGROUND 
       [0002]    Multi-layered Printed Wiring Boards (PWB&#39;s) are generally known in the art. In the case of a double-sided PWB, such as a mid-plane, the double-sided PWB is configured to receive connector pins or other components on both a top side and a bottom side of the PWB during assembly. A signal having a source on a top side of the PWB and a destination on a bottom side of the PWB is typically routed through a first Plated-Through-Hole (PTH) via, a trace in a routing layer of the PWB, and a second PTH via. 
         [0003]    A signal path that follows such a routing does not use certain portions of the first PTH via and the second PTH via. The unused portions of the first PTH via and the second PTH via are referred to as via stubs. For high-frequency signals, for example Multi-Giga Hertz (MGH) signals, such via stubs can cause reflections at harmonic frequencies of the signal and create an impedance mismatch that results in a loss of signal strength and/or signal distortion. 
         [0004]    A conventional method for eliminating or reducing the effect of via stubs is to remove via stubs by back-drilling. This method has many disadvantages, however. For instance, back-drilling increases the number of PWB fabrication steps, reduces PWB fabrication yield, and increases PWB fabrication cost. Moreover, back-drilling may not be practical for double-sided PWB&#39;s. Improved features and/or routing methods are therefore needed to address the problem presented by via stubs in double-sided PWB&#39;s. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention will be more fully understood from the detailed description below and the accompanying drawings, wherein: 
           [0006]      FIGS. 1A-1D  are cross-section illustrations of multi-layered PWB&#39;s; 
           [0007]      FIG. 2  is a cross-section illustration of a multi-layered PWB, according to an embodiment of the invention; 
           [0008]      FIG. 3  is a flow diagram of a PWB routing process, according to an embodiment of the invention; 
           [0009]      FIG. 4A  is a graph of signal properties in a multi-layered PWB, according to a simulation of a PWB routing in the conventional art; and 
           [0010]      FIG. 4B  is a graph of signal properties in a multi-layered PWB, according to a simulation of a PWB routing that is consistent with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Embodiments of the invention will now be described more fully with reference to the figures, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The illustrated features of PWB&#39;s are not drawn to scale. 
         [0012]      FIGS. 1A-1D  are cross-section illustrations of multi-layered PWB&#39;s. 
         [0013]      FIG. 1A  illustrates a multi-layered PWB  102  having a top side  104 , a bottom side  106 , three upper signal layers  108 , a middle signal layer  110 , and three lower signal layers  112 . Each of the signal layers are separated by an insulation layer  114 . Each of the signal layers may include one or more conductive traces, for example copper traces, which are used as part of a signal path. 
         [0014]    Variations to the multi-layered PWB illustrated in  FIG. 1A  are possible. For instance, a multi-layered PWB may have any number of upper and lower signal layers. 
         [0015]    The PWB configurations illustrated in  FIGS. 1B ,  1 C, and  1 D are consistent with the structure of the PWB illustrated in  FIG. 1A  and described, together with possible variations, above. 
         [0016]      FIG. 1B  illustrates a portion of a multi-layered PWB  116  having a via  118  coupled to a trace  126  and a via  122 . The trace  126  is on an upper signal layer (not shown). A signal path  128  extends from a top portion of via  118  through the trace  126  to a bottom portion of via  122 . Via stub  120  exists in an unused portion of via  118 . Via stub  122  exists in an unused portion of via  122 . Via stub  120  may be sufficiently long to cause undesirable reflections that interfere with a signal on the signal path  128 . 
         [0017]      FIG. 1C  illustrates a portion of a multi-layered PWB  130  having a via  132  coupled to a trace  140  and a via  136 . The trace  140  is on a lower signal layer (not shown). A signal path  142  extends from a top portion of via  132  through the trace  140  to a bottom portion of via  136 . Via stub  134  exists in an unused portion of via  132 . Via stub  138  exists in an unused portion of via  136 . Via stub  138  may be sufficiently long to cause undesirable reflections that interfere with a signal on the signal path  142 . 
         [0018]      FIG. 1D  illustrates a portion of a multi-layered PWB  144  having a via  146  coupled to a trace  154  and a via  150 . The trace  154  is on a middle signal layer (not shown). A signal path  156  extends from a top portion of via  146  through the trace  154  to a bottom portion of via  150 . Via stub  148  exists in an unused portion of via  146 . Via stub  152  exists in an unused portion of via  150 . Via stubs  148  and  152  may be sufficiently long to cause undesirable reflections that interfere with a signal on the signal path  156 . 
         [0019]      FIGS. 1B ,  1 C, and  1 D thus illustrate PWB structures having potentially problematic via stubs. 
         [0020]      FIG. 2  is a cross-section illustration of a multi-layered PWB, according to an embodiment of the invention. The multi-layered PWB illustrated in  FIG. 2  is a double-sided PWB, for instance a mid-plane. 
         [0021]    As illustrated in  FIG. 2 , a PWB  202  includes a top side  204  and a bottom side  206 . The PWB  202  further includes a via  208 , a trace  212 , a via  214 , a trace  220 , and a via  222 . The via  208  is configured to receive a connector pin  228 , for example a Press-Fit Pin (PFP), on the top side  204 . The via  222  is configured to receive a connector pin  230 , for example a PFP, on the bottom side  206 . The trace  212  is on a lower signal layer (not shown). The trace  220  is on an upper signal layer (not shown). 
         [0022]    The connector pin  228  is associated with a signal source, and the connector pin  230  is associated with a signal destination. A signal path  226  extends from the connector pin  228  through the via  208 , the trace  212 , the via  218 , the trace  220 , and the via  222 , terminating at the connector pin  230 . The signal path  226  thus forms an S-Turn in the PWB  202 , and the via  214  may be referred to as an S-Turn via. 
         [0023]    Via stub  210  exists in an unused portion of the via  208 . Via stubs  216  and  218  exist in unused portions of via  214 . Via stub  224  exists in an unused portion of via  222 . Each of the via stubs  210 ,  216 ,  218 , and  224  are sufficiently short so that a signal on the signal path  226  is not substantially attenuated or otherwise distorted by via stub reflections. 
         [0024]    Variations to the PWB configuration illustrated in  FIG. 2  are possible. For instance, the via  208  may be configured to connect to a component other than connector pin  228 . Likewise, the via  222  may be configured to connect to a component other than connector pin  230 . In addition, the via  218  may be a buried via rather than the illustrated PTH via. In a buried via configuration, the buried via may not include via stubs  216  and  218 . 
         [0025]      FIG. 3  is a flow diagram of a PWB routing process, according to an embodiment of the invention. The PWB routing process illustrated in  FIG. 3  and described below is especially applicable to a double-sided multi-layered PWB. 
         [0026]    After starting in step  302 , the process defines a first via associated with a signal source on a top side of a PWB in step  304 . Then, in step  306 , the process defines a second via associated with a signal destination on a bottom side of the PWB. The process defines a third via in step  308 . The process connects the first via to the third via on one of a plurality of bottom signal layers of the PWB in step  310 , and then connects the third via to the second via on one of a plurality of top signal layers of the PWB in step  312  before terminating in step  314 . Connections on signal layers may be accomplished using conductive traces, for example copper traces. 
         [0027]    A result of the routing process illustrated in  FIG. 3  and described above is a signal path having an S-Turn shape. The third via can thus be referred to as the S-Turn via. 
         [0028]    Variations to the process described with reference to  FIG. 3  are possible. For instance, in one embodiment, each of the first, second, and third vias are defined as PTH vias in steps  304 ,  306 , and  308 , respectively. In an alternative embodiment, the third via is defined as a buried via. Moreover, in one embodiment the first via is configured to accept a connector pin, such as a PFP, on the top side of the PWB in step  304 , and the second via is configured to accept a connector pin, such as a PFP, on the bottom side of the PWB in step  306 . But in alternative embodiments, the first via and/or the second via could be configured to accept a component other than a connector pin. 
         [0029]    The routing process illustrated in  FIG. 3  and described above could be performed manually. Alternatively, the routing process illustrated in  FIG. 3  and described above could be automatically or semi-automatically, for example by an automated PWB routing software tool. In the case of automated or semi-automated operation, the process could be implemented using processor-executable code, and the processor-executable code could be stored on processor-readable storage medium, such as a hard drive or Compact Disc (CD). Moreover, the routing process illustrated in  FIG. 3  and described above could be implemented in hardware, or in a combination of hardware and software. 
         [0030]      FIG. 4A  is a graph of signal properties in a multi-layered PWB, according to a simulation of a PWB routing in the conventional art.  FIG. 4A  illustrates the dB magnitude of return loss in curve  405 , the dB magnitude of insertion loss in curve  410 , and the phase in curve  415 .  FIG. 4B  is a graph of signal properties in a multi-layered PWB, according to a simulation of a PWB routing that is consistent with the embodiment illustrated in  FIG. 2 .  FIG. 4B  illustrates the dB magnitude of return loss in curve  420 , the dB magnitude of insertion loss in curve  425 , and the phase in curve  430 . A comparison of the two graphs thus illustrates that a PWB that is constructed in accordance with an embodiment of the invention eliminates a predicted signal attenuation that is centered at approximately 8 GHz. 
         [0031]    It will be apparent to those skilled in the art that modifications and variations can be made without deviating from the spirit or scope of the invention. For example, the PWB structure and method disclosed herein are applicable various configurations of PWB&#39;s having two or more signal routing layers. Thus, it is intended that the present invention cover any such modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.