BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to circuit boards (CBs), e.g., printed circuit boards, imprinted boards, and any other support substrate. More particularly, to an apparatus and method for improving coupling between signals routed across plane discontinuities on CBs.  
           [0003]    2. Description of the Related Art  
           [0004]    CBs are typically manufactured with a plurality of layers, each layer being permanently affixed to an adjoining layer through a structural, non-conductive material. FIG. 1 is an example of a four layer CB  100 . Referring to FIG. 1, the first and fourth layers  112  and  124 , respectively, are signal trace layers on which signal lines are routed. The second and third layers  116  and  120 , respectively, are voltage reference layers, e.g., the second layer  116  is a power layer and the third layer  120  is a ground layer. The power layer  116  and ground layer  120  are affixed to a core  118  comprised of a fiberglass mesh material such as FR4. The first signal layer  112  and the power layer  116  and the second signal layer  124  and the ground layer  120  each sandwich a pre impregnated epoxy material  114 , commonly referred to as pre-preg.  
           [0005]    Signal lines are primarily routed on the first and fourth layers  112  and  124 , respectively. Oftentimes, signal lines must be routed from the first layer  112  to the fourth layer  124  through the reference layers  116  and  120 . When this occurs, the power layer  116  and the ground layer  120  must be split, cutout, or slotted (the result is collectively termed a discontinuity) to avoid a short circuit between the signal lines being cross-routed and the reference layers. Signals routed around the cross-routed signal must necessarily be laid out across the discontinuity.  
           [0006]    [0006]FIG. 2A is a cross sectional view of an exemplary CB  200  with a discontinuity  202 . FIG. 2B is a top view of the CB  200 . Referring to FIGS.  2 A-B, the CB  200  includes a ground plane (or layer)  204  and voltage planes (or layers)  206  and  208 . A discontinuity  202  exists between voltage planes  206  and  208 . A signal  210  bridges or spans the discontinuity  202  as most clearly shown in FIG. 2B.  
           [0007]    High-speed signals that span discontinuities in adjacent reference planes, like signal  210 , generate electromagnetic (EM) radiation because of the electrical break caused by the plane discontinuity. This EM radiation adversely affects electromagnetic containment (EMC) and signal integrity (SI). For one, the discontinuity increases the electrical ground path increasing loop inductance. And the larger inductance might cause signal distortion and phase shifts.  
           [0008]    To avoid these issues, CB designers avoid routing high-speed signals over discontinuities. But these constraints are often difficult to maintain as CB real estate shrinks or as signal density increases. Another way CB designers avoid these problems is to use stitching capacitors across discontinuities, e.g., stitching capacitor  212 . The stitching capacitor  212  electrically couples the plane  208  to the plane  206  through vias  214 . The stitching capacitor  212  provides alternating current (AC) coupling that reduces EM radiation at the discontinuity reducing, in turn, adverse EMC and SI effects. The addition of stitching capacitors, however, is costly. One stitching capacitor is required for each signal crossing a discontinuity. Thus, the CB component count increases, increasing cost. More components require additional CB real estate, also increasing cost.  
           [0009]    Accordingly, a need remains for an apparatus and method of improving coupling across plane discontinuities on circuit boards. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The foregoing and other objects, features, and advantages of the invention will become more readily apparent from the detailed description of an embodiment that references the following drawings.  
         [0011]    [0011]FIG. 1 is a cross sectional view of a CB.  
         [0012]    [0012]FIG. 2A is a cross sectional view of another CB.  
         [0013]    [0013]FIG. 2B a top view of the CB shown in FIG. 2A.  
         [0014]    [0014]FIG. 3 is a cross sectional view of a CB according to the present invention.  
         [0015]    FIGS.  4 A-C are diagrams of the CB discontinuity shown in FIG. 3.  
         [0016]    [0016]FIG. 5 is a simulation graph of the CB shown in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    [0017]FIG. 3 is a cross sectional view of a CB according to the present invention. For simplicity, the CB shown in FIG. 3 includes four layers, a ground layer  320 , a voltage layer  316 , and two signal trace layers  312  and  324 . A person of reasonable skill in the art should recognize that the invention might be embodied in CBs having any number of layers.  
         [0018]    Referring to FIG. 3, a CB  300  includes a core  318  comprised of a non-conductive material, e.g., FR4. The core  318  provides structural strength and rigidity to the CB  30 Q. A person of reasonable skill in the art should recognize a variety of materials for CB cores.  
         [0019]    A reference plane (or layer)  316  is formed on the core  318 . A reference plane  320  is likewise formed on the core  318 . The reference planes  316  and  320  might provide a ground (e.g., GND) or a predetermined power supply voltage (e.g., VCC) to signal traces routed on signal layers  312  and  324 , as explained further below. The reference planes  316  and  320  might comprise 1-ounce copper. A person of reasonable skill in the art should recognize other suitable materials for the reference planes  316  and  320 . A person of reasonable skill in the art should recognize well-known methods for forming the reference planes  316  and  320  on core  318 , e.g., electroless or electroplating processes.  
         [0020]    Discontinuities are formed in the reference planes, e.g., discontinuity  330  formed on voltage reference plane  316 . A person of reasonable skill in the art should recognize well-known methods for forming discontinuities  330  in the reference plane  316 , e.g., standard copper etching processes that chemically etch and define patterns, planes, lines, and the like on conductive layers such as reference plane  316 .  
         [0021]    Referring to FIGS. 3 and 4A-C, one embodiment of the discontinuity  330  is a split  406 , that is, where a first portion  402  is separated from end to end from a second portion  404  of the reference plane  416 . Another embodiment of the discontinuity  330  is as a slot  408  (FIG. 4B). Yet another embodiment of the discontinuity  330  is as a cutout  410  (FIG. 4C).  
         [0022]    Referring to FIG. 3, discontinuities, e.g., discontinuity  330 , in the reference planes  316  and  320  allow a signal to be cross-routed from a first signal layer  312  to a second signal layer  324  without being shorted through the reference planes  316  and  320 . Although the discontinuity  330  is shown only on reference plane  316 , a person of reasonable skill in the art should recognize that any number of discontinuities is possible in any number of reference planes, e.g., planes  316  and  320 .  
         [0023]    A dielectric barrier layer  326  is formed on the reference plane including the discontinuity, e.g., reference plane  316 . The barrier layer  326  prevents electrical shorts across the discontinuity  330 . The dielectric barrier layer  326  comprises a non-conductive epoxy material, e.g., 1060 pre-preg or a liquid curable epoxy. A person of reasonable skill in the art should recognize other suitable materials for the dielectric barrier layer  326 . A person of reasonable skill in the art should recognize well-known methods for forming the dielectric barrier  326  on the reference plane  316 .  
         [0024]    The dielectric barrier layer  326  is opened on one side of the discontinuity  330  to expose a portion of the reference plane  316 . The opening  328  might have a variety of shapes depending, e.g., on the process or type of equipment (e.g., laser) used to create it. The opening might be slotted or created using vias. A person of reasonable skill in the art should recognize a variety of processes and equipment to create the opening  328  in a variety of well-known shapes including drilling blind or buried vias, laser drilling vias or slots, or photodefining openings on dielectric materials such as curable liquid epoxies or solder masks.  
         [0025]    A conductive layer  332  spanning the length and width of the discontinuity  330  is formed on the dielectric layer  326 . The conductive layer  332  includes a first portion  338  connecting the conductive layer  332  to the reference plane  316  through the opening  328 . A second portion  336  bridges or spans the discontinuity  330 . A third portion  334  extends across the reference plane  316  on another side of the discontinuity  330  as shown in FIG. 3. Unlike the first portion  338 , the third portion  334  remains disconnected from reference plane  316 . In other words, the conductive layer  332  is electrically connected to the reference plane  316  at one end through the opening  328 .  
         [0026]    The conductive layer  332  increases AC signal coupling between the signal layers and underlying reference planes. The conductive layer  332 , therefore, improves signal strength over a broad frequency spectrum without requiring additional components, e.g., stitching capacitors. The conductive layer  332  minimizes EMC and SI problems relaxing CB signal routing constraints, improving CB surface area usage, and reducing component cost.  
         [0027]    The conductive layer  332  might be a carbon material, e.g., graphite or carbon black. A person of reasonable skill in the art should recognize other suitable materials for the conductive layer  332  including conductive materials not necessarily including carbon. The conductive layer  332  might be deposited on the CB using a variety of well-known commercial processes, e.g., processes available to deposit carbon materials to enhance adhesion for electroless copper plating of vias. The conductive properties of carbon along with the ability to apply them in very thin layers (e.g., &lt;1 mil thick), lend themselves to improve coupling across plane discontinuities as described herein.  
         [0028]    A pre-preg epoxy layer  314 , the signal layers  312  and  324 , and solder mask  340  and  342  complete the CB stack up. A person of reasonable skill in the art should recognize well-known methods of forming the pre-preg layers  314 , the signal layers  312  and  324 , and the solder masks  340  and  342 .  
         [0029]    [0029]FIG. 5 is a graph of simulation results  500  for the CB shown in FIG. 3. Referring to FIGS. 3 and 5, the graph lines show results of using graphite for the conductive layer  332 . The discontinuity  330  is 20 mils wide by 2300 mils long. The reference plane  316  is made of 1-ounce copper. The reference plane  316  and the ground plane  320  are separated by a core  318  being 1.6 mils wide of 1060 FR4 pre-preg.  
         [0030]    The graph lines in FIG. 5 are bounded on the one end, by simulation results of a signal trace  502  with no underlying discontinuity and, on the other end, by a signal trace  504  with a 20-mil discontinuity but no conductive layer  332  added to the CB. The signals traces  506 ,  508 , and  510  refer to traces over a 20-mil discontinuity with a conductive layer  332  interposed as shown in FIG. 3. Trace  506  refers to a conductive layer  332  having its third portion  334  extend over the discontinuity by 5 mils. Trace  508  refers to a conductive layer  332  having its third portion  334  extend over the discontinuity by 10 mils. And trace  510  refers to a conductive layer  332  having its third portion  334  extend over the discontinuity by 20 mils.  
         [0031]    The simulation results shown in FIG. 5 indicate that a CB including a conductive layer  332  increases the signal strength across the discontinuity by 10-15% over a 100 to 4,000 MHz frequency band. A person of reasonable skill in the art should understand the results shown in FIG. 5 are merely exemplary.  
         [0032]    Having illustrated and described the principles of our invention, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the accompanying claims.
The invention relates to an apparatus and method for improving coupling across plane discontinuities on circuit boards. A circuit board includes a discontinuity, e.g., a split, slot, or cutout, formed on a voltage reference plane. A conductive layer overlies the discontinuity. The conductive layer has a first portion connected to the underlying reference plane and a second portion spanning the discontinuity. The first portion is connected to the reference plane using a slot or vias. And the conductive layer has a third portion extending over the reference plane but remaining disconnected from it. The conductive layer might be graphite or carbon black.