Abstract:
A circuit board design is disclosed that is useful in high-speed differential signal applications uses either a via arrangement or a circuit trace exit structure. In the via arrangement, sets of differential signal pair vias and an associated ground are arranged adjacent to each other in a repeating pattern. The differential signal vias of each pair are spaced closer to their associated ground via than the spacing between the adjacent differential signal pair associated ground so that differential signal vias exhibit a preference for electrically coupling to their associated ground vias. The circuit trace exit structure involves the exit portions of the circuit traces of the differential signal vias to follow a path where they meet with and join to the transmission lines portions of the traces.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of prior U.S. Provisional Patent Application No. 60/544,522, filed Feb. 13, 2004.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to circuit board arrangements, and more particularly to via arrangements that are used on printed circuit boards for high-speed electrical transmission applications.  
         [0003]     In the field of data communication, data transfer speeds have steadily increased over the years. This increase in speed has required the development of high-speed electronic components for use in the telecommunications field, such as Internet use and use in data transfer and storage applications. In order to obtain an increase in the speed at which electrical signals are transmitted, it is known to use differential signals.  
         [0004]     Twisted pair wires are commonly used to transmit differential signals and are most commonly used in electrical cables. These signal cables have one or more twisted pairs of wires that are twisted together along the length of the cable, with each such twisted pair being encircled by an associated grounding shield. These twisted pairs typically receive complimentary signal voltages, i.e., one wire of the twisted pair will carry a +1.0 volt signal, while the other wire of the twisted pair will carry a −1.0 volt signal. The wire pairs are twisted together along the axis of the cable so that each of the wires extends in a helical path along the cable and the wires are spaced apart from each other the same distance along this helical path for the length of the cable.  
         [0005]     As the signal cables are routed on a path to an electronic device, they may pass by or near other electronic devices that emit their own electric field. These devices have the potential to create electromagnetic interference in the transmission lines formed by the signal cables. However, the twisted pair construction of the cables minimizes or diminishes any induced electrical fields by maintaining the two wires in a desired orientation so that they will capacitively couple to each other and to an associated grounding shield or drain wire, and this construction thereby substantially prevents electromagnetic interference from occurring in the cable and affecting the transmission of data signals through the cable.  
         [0006]     In order to maintain electrical performance integrity from such a transmission line to the circuitry of an associated electronic device, it is desirable to obtain a substantially constant impedance throughout the transmission line, from circuit to circuit and to avoid large discontinuities in the impedance of the transmission line. Large discontinuties in the impedance of the transmission line can lead to the generation of undesireable crosstalk between the signal paths of the transmission line or elecrical “noise”. Both this type of noise and crosstalk adversely affect the integrity of electrically transmitted signals at high frequencies (or data transfer speeds). The “transmission line” between electronic devices not only includes cables and connectors that interconnect two devices together, but also includes the printed circuit boards of the devices.  
         [0007]     The impedance of twisted pair transmission cables may be controlled because it is easy to maintain a specific geometry or physical arrangement of the signal conductors and the grounding shield, an impedance change will usually encountered in the area where a cable is mated to a connector, where the connector is mounted to a printed circuit board and where the connector is mounted to a circuit board. This last area is referred to in the art as the “launch” area” where signals are launched from the transmission lines on (or in) the circuit board into a connector mounted thereto. Likewise, the signals may be launched from the connector into the circuit board and this area is commonly also referred to as an “exit” area. These areas are the same but may have different terms depending on the orientation and direction of the signal path, either from the circuit board to the connector or from the connector to the circuit board. The present invention is directed to improved structures used in these circuit board launch or exit areas.  
         [0008]     Circuit boards are made up of multiple layers of conductive and nonconductive material. Each layer may be considered as defining one of multiple planes of the circuit board. A nonconductive layer may be used as a base of the circuit board and a surface or surfaces thereof may be coated with a conductive material such as a copper foil or plating. Portions of this are removed to form conductive extents on the surfaces of the board which are typically referred to in the art as “traces”. These traces define circuit paths on the board base layer. A subsequent nonconductive layer is then applied onto the surfaces of the base layer and another conductive coating is applied to that layer and etched into a pattern. A third nonconductive layer is applied over this second conductive layer and the process is repeated until a multi-layer circuit board is formed. The different conductive layers are typically connected together by what are known in the art as “vias”. A via is a hole that is drilled through the circuit board and the inner surface of which is plated. This plating interconnects the various conductive layers. The traces on the circuit board may lead to a via location when it is desired to connect the traces to other traces. Similarly, the vias may also be used to receive through-hole mounting pins or other mounting pins of connectors.  
         [0009]     Pairs of traces may be formed in a circuit board layer to carry a pair of differential signals and each pair will define a differential signal transmission line of the circuit board. Each circuit board layer or plane, may support one or more such differential signal transmission lines. It is important to control the impedance of these transmission lines to minimize crosstalk and electrical interference during operation of the devices without unduly complicating the circuit board design and the circuit layouts on the circuit board.  
         [0010]     The present invention is therefore directed to a circuit board design, utilizing circuit board vias and exits of conductive traces from the vias tat cooperatively define an electrical signal transmission line, to provide a high level of operational performance and which maintains the desired electrical characteristics, such as the impedance of the circuit board signal transmission lines.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, it is a general object of the present invention to provide a circuit board structure for use in high speed signal transmission wherein a ground plane is provided for a differential signal transmission line on the circuit board and is positioned in a preferential location with respect to where the differential signal traces connect to a via on the circuit board so that each differential signal trace and its corresponding via pair engages in electrical coupling with the ground rather than with a nearby differential signal transmission line, which is made up of a pair of conductive traces and vias.  
         [0012]     Another general object of the present invention is to provide an improved circuit board structure in which the configuration of a pair of conductive differential signal traces leading to or away from a via is specifically configured to control the impedance of the conductive traces that make up a differential signal transmission line on the circuit board.  
         [0013]     Another object of the present invention is to provide a printed circuit board structure that may be used as either a “launch” or an “exit” area for mating with electronic components, such as electrical connectors, in which the structures include a pair of differential signal traces mated to through hole vias in the circuit board, and wherein the traces have a particular structure in the area where they exit from the vias so as to affect the impedance of the differential signal system  
         [0014]     A further object of the present invention is to provide an improved circuit board construction wherein a pair of differential signal vias are positioned proximate to an associated ground via, the circuit board having at least one ground plane layer formed therein, and the ground plane having an anti-pad formed therein that encompasses the two differential signal vias and which is connected to the associated ground via and another ground via associated with another pair of differential signal vias, and another anti-pad that is positioned adjacent to the one anti-pad and encompassing a second, adjacent pair of differential signal vias, but contacting a second ground via associated with the adjacent pair of differential signal vias.  
         [0015]     Still a further object of the present invention is to provide a circuit board with a new exit pattern for conductive traces leading from a pair of differential signal vias, the exit pattern including a bend in each of the exit portions of the traces, one bend of one of the trace exit portions lying inside of a bending radius of the other, outer trace exit portion, so that one of the trace exit portions is generally spaced apart from each other a similar and consistent distance from the body of the transmission line which they define to the position where one of the traces exits from an associated via.  
         [0016]     Yet another object of the present invention is to provide an pattern for a pair of conductive circuit board traces exiting a pair of respective differential signal vias and leading to a differential signal transmission line on the circuit board, each of the traces including a conductive collar portion that encircles and contacts a corresponding via, an exit portion extending from the collar portion and terminating in the signal transmission portion, the exit portion including an increased width portion, the signal transmission portion extending lengthwise along an extent of the circuit board that is spaced apart from the pair of differential signal vias and which does not intersect the vias, the exit portions including at least one change of direction in order to meet with the signal transmission line.  
         [0017]     Still yet a further object of the present invention is to provide a circuit board having the differential signal via trace exit pattern described above, and the circuit board including a plurality of ground plane layers, each of the ground plane layers having an anti-pad, the perimeter of which encompasses the collar and exit portions of the pair of differential signal traces.  
         [0018]     The present invention provides these objects, advantages and benefits by way of its structure. In one principal aspect of the present invention, four vias are provided on a circuit board. Two of the vias are designated as differential signal vias and as such, they include conductive traces that lead away from the differential signal vias within or on a layer of the circuit board and these traces define a differential signal transmission line of the circuit board layer. The remaining two vias are designated a ground vias and as such, they are connected to a ground reference plane, which is preferably in a plane or layer of the circuit board other than the plane or layer in which the differential signal transmission line extends. The ground reference plane is formed in a manner so that it has an opening formed therein that encompasses the pair of two differential signal vias. The ground reference plane is connected to both of the ground vias. The four vias are arranged at the corners of an imaginary four-sided figure, such as a square, rectangle, rhombus of the like and the ground reference plane may be solid and planar, or it may have a grid, or lattice-like, structure.  
         [0019]     In another principal aspect of the present invention, a new launch, or exit, pattern for conductive traces leading from a pair of differential signal vias is provided. The exit pattern includes a pair of conductive traces that extend in a plane or layer of the circuit board from a pair of associated vias, preferably a pair of differential signal vias and each of the traces includes a bend within its launch or exit portion of the trace. One bend of one of the trace exit portions is disposed inside of a bending radius of the other, (and outer) trace exit portion, so that the spacing of the pair of trace exit portions from each other is generally a similar and consistent distance from an associated via to the body of the transmission line.  
         [0020]     In yet another principal aspect of the present invention, a pattern for a pair of conductive circuit board traces exiting (or entering) a pair of respective differential signal vias and leading to a differential signal transmission line on the circuit board is provided. Each of the traces includes a conductive collar portion that encircles and contacts a corresponding via, and it further includes an exit portion that extends from the collar portion and joins to or terminates at the signal transmission line. The exit portion includes an increased width portion, and in one embodiment, this increased width portion may begin at near the centerline that runs from the center of one differential signal via to the other differential signal via. This increased width portion extends and may traverse at least one bend in its path to the signal transmission line, where it terminates by reducing down in width to that of the signal transmission line to which it is joined. In another embodiment, the increased width portions have a configuration of a “flag”, when viewed from above or from a direction normal to the plane of the conductive trace. The increased width portions also approach each other in a close spacing for coupling purposes. The increased width portions shall usually traverse at least one bend, or change in direction along their path from their vias to the signal transmission lines.  
         [0021]     These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     In the course of this detailed description, the reference will be frequently made to the attached drawings in which:  
         [0023]      FIG. 1  is a schematic view of the environment in which the present invention is used, namely, in a backplane environment for high-speed signal and data transfer applications;  
         [0024]      FIG. 2  is a plan view of a known circuit board structure with two vias formed therein;  
         [0025]      FIG. 3  is a perspective view of a via opening on to the surface of a circuit board;  
         [0026]      FIG. 3A  is a diagrammatic, detailed view of a known printed circuit board with a via formed in place in the body of the circuit board and extending completely through the circuit board, and the circuit board having multiple ground planes arranged as layers within the body or between other layers of the circuit board;  
         [0027]      FIG. 4  is a plan view of another known circuit board arrangement for differential signal applications and illustrating two differential signal vias of the circuit board being surround by an non-conductive area that is formed in a conductive ground plane that surrounds pairs of the vias;  
         [0028]      FIG. 5  is a plan view of yet another known circuit board arrangement with two vias formed therein, similar to that shown in  FIG. 4 , and wherein the ends of the nonconductive areas surrounding the vias are enlarged with respect to the remainder of the non-conductive area, to give the open area a “dogbone” or “dumbbell” shape;  
         [0029]      FIG. 6  is a perspective view of a circuit board illustrating a 5-die pattern of vias that may be used for differential signal applications;  
         [0030]      FIG. 7  is a plan view of a circuit board via arrangement constructed in accordance with the principles of the present invention, illustrating a preferential ground arrangement;  
         [0031]      FIG. 8  is the same view as  FIG. 7 , but with a wide ground plane layer in place on top of the circuit board for clarity purposes and connected to two ground vias and illustrating the dimensional arrangement of the open area surrounding the pair of differential signal vias;  
         [0032]      FIG. 9  is a perspective view of the via arrangement similar to that of  FIG. 8  showing the points of interconnection between the ground plane, which is illustrated on the top surface of the circuit board section and two of the ground vias, the ground plane having a grid or lattice-like configuration, and having an open area with a perimeter that encompasses a pair of differential signal vias;  
         [0033]      FIG. 10  is a perspective view of the arrangement of vias in  FIG. 9 , but illustrating additional ground plane layers as part of the overall circuit board construction, with the open areas encompassing the pair of differential signal vias through the height, or depth, of the circuit board, with the ground planes being selectively connected to the ground vias of the arrangement;  
         [0034]      FIG. 10A  is a top plan view of the via and ground plane arrangement of  FIG. 10 , further illustrating a pair of differential signal transmission line traces exiting from a pair of associated differential signal vias;  
         [0035]      FIG. 11  is a top plan view, taken at a slight angle, illustrating a pair of differential signal vias and a pair of conductive traces exiting, or “launching” or “breaking out” from the vias and meeting a differential signal transmission line;  
         [0036]      FIG. 11A  is a top plan view of a structure similar to that shown in  FIG. 11 , but where the exit portion has a flag configuration and not an increased width portion;  
         [0037]      FIG. 12  is the same view as  FIG. 11 , but oriented 90 degrees and slightly more in perspective and illustrating the depth of the vias and the signal trace breakouts connected to the signal vias;  
         [0038]      FIG. 13  is a perspective view of the arrangement of  FIG. 11 , taken from an end thereof at a different angle, and illustrating how the circuit traces exit from their two associated vias and illustrating the increased width portions of the conductive traces;  
         [0039]      FIG. 13A  is a top plan view of another conductive trace exit pattern constructed in accordance with the principles of the present invention;  
         [0040]      FIG. 14  is a plan view of a known differential signal via arrangement and a pair of circuit traces exiting therefrom;  
         [0041]      FIG. 15  is a plan view of another known differential signal via arrangement, with a pair of traces exiting therefrom and forming a signal transmission line of the circuit board;  
         [0042]      FIG. 16  is a perspective view of another embodiment of a the differential signal trace exit pattern; and,  
         [0043]      FIG. 16A  is a top plan view of the differential signal trace exit pattern of  FIG. 16  with a ground reference plane superimposed over the trace pattern.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]      FIG. 1  is a perspective view of a backplane assembly  100  in which a printed circuit board, referred to herein as a “motherboard”  101  is joined to a secondary circuit board  102  by way of one or more connectors  103 . The connectors  103 , as are known in the art, connect conductive circuits  104  which utilize conductive traces  105  disposed on a surface of the motherboard  101 , to similar circuits  106  disposed on the secondary circuit board  102 . These circuits  104 ,  106  typically lead to electronic components  110  that are mounted to the circuit board.  
         [0045]     Cables may be used to connect the assembly  100  of  FIG. 1  to another electronic assembly and these cables are but one form of an electronic signal transmission line. Other forms of such transmission lines may be incorporated in the circuit boards  104 ,  106  of the assembly and one such form may take the form of a plurality of conductive traces disposed on or within a plane, or layer, of the circuit board. An example of such a transmission line is shown in  FIG. 2  and is representative of the circuit board structure that is used in the electronics industry today. [0045] In  FIG. 2 , a circuit board  120  is shown having a plurality of vias  121  arranged in a pattern for receiving corresponding conductive tails of an electronic component that is mounted to the circuit board  120  and which is not shown. The vias  121  typically include a hole  122  that extends through the entire thickness of the circuit board  120 . The vias  121  are plated along their interior surfaces  128 , and the vias  121  typically include a small annular ring of plating material  123  that can collect at the intersection of the hole and the surfaces of the circuit board  120 . A pair of conductive traces  124 ,  125  are shown extending away from the vias  121  and, in differential signal applications, two of the traces  124 ,  125  will cooperatively define a differential signal transmission line “ST”that leads to a connector, electronic component or the like.  
         [0046]     The vias  121  are used not only to mount connectors and components to the circuit board  120 , but are also used to interconnect various circuit of the board together. As stated above, a circuit board is typically made up on a series of layers of a fiberglass resin or similar compound. A plating layer is applied to one of these layers and is etched to form conductive traces on the surface of the layer. Another layer of fiberglass or resin is applied to the first layer, circuit traces are formed and so on until a multi-layer circuit board is formed with a plurality of circuits extending through the board on the different layers thereof. The vias are formed by drilling holes into the circuit board and exposing the conductive layers and then the inner surfaces of the vias are plated, thereby connecting together, all of the layers that touch the hole edge.  
         [0047]      FIG. 3  illustrates in enlarged detail, a layer of the circuit board  120  that contains a via  121 . The via is plated and includes an interior coating  128  of plating material that surrounds the hole  122 . A gap G may be formed on the board layer and this gaps provides separation between the via plating  128  and a ground reference plane conductive layer  129  that surrounds the via  121 . This gap G is provided to provide protection against shorting and it has been discovered that the ground plane layer may detrimentally influence the transmission of differential signals from a pair of differential signal vias. However, with this structure, the gap G that occurs between the via and the edge of reference plane causes the via to as a capacitor toward the reference plane. This effect is especially pronounced in structures where there are multiple ground planes with gaps or openings that surround a single via it can cause signal reflection. This reflection takes energy out of the overall transmission line system.  
         [0048]      FIG. 3A  illustrates in a schematic manner, the different layers  129   a ,  129   b  and  129   c  of the circuit board  129  and how the via hole  122  extends through all of the layers  129   a - c  in order to mate with surface trace  124   a  and inner layer traces  124   b  and  124   c.    
         [0049]     One manner of improving the performance of differential signal vias on a circuit board is that which is illustrated in  FIG. 4  and which described in U.S. Pat. No. 6,607,402, issued Aug. 19, 2003 and assigned to Teradyne, Inc. In this patent, a circuit board  120  is shown having a plurality of vias  121  formed therein. The vias  121  are arranged in pairs for differential signal transmission, and the circuit board  120  contains a ground reference plane  129 . A portion  130  of the underlying ground plane area which encircles the a pair of differential signal vias is removed to form an opening. This removed area, or opening  130 , is commonly referred to in the art as an “anti-pad”. The &#39;402 patent explains that the anti-pad  130  should encircle the two vias  121 . This structure has certain disadvantages associated with it. For example, the vias  121  both act as capacitors in multiple places across the gap between the vias  121  and edge of the ground plane opening. This capacitor effect tends to take energy out of any signal transmission line that may be joined to the vias  121 . The use of this small-sized via anti-pad is an attempt to loosely couple the two signal vias  121  together electrically, but the proximity of the surrounding ground pad or plane inhibits true strong differential coupling between the two differential signal vias  121 .  
         [0050]      FIG. 5  illustrates another known modification to circuit board vias, in which an anti-pad  131  is narrowed in its center portion  133  between the two vias  121  to adopt an overall “dogbone” or “dumbell” appearance. With this appearance, the anti-pad  131  is large in the area  135  surrounding the pair of vias  121 , but it then narrows down a bit between in the area  136  between the two vias. This narrowing results in a recapture of some of the system energy that is normally lost in operation, but the small area of the ground plane anti-pad inhibits proper performance. This structure represents an attempt to balance the capacitance of the system and to loosely couple the two signal vias together while still keeping the affinity of the two signal vias for their surrounding ground plane.  
         [0000]     Assymmetrical Peferential Via Positioning  
         [0051]      FIG. 6  illustrates another circuit board  200  have what is referred to herein as a “5-die” via pattern formed therein. This pattern includes two pairs of differential signal vias  202 ,  204  positioned on opposite sides of a single intervening ground via  205 . Each such pair of differential signal vias includes two distinct vias  202   a ,  202   b  and  204   a ,  204   b . The two vias of each such differential pair are typically aligned together along a first axis L 1  (shown extending from lower left to upper right in  FIG. 7 ). This pattern is repeated along a direction that is transverse to the first axis L 1 . The differential signal vias  202   a - b,    204   a - b  typically have conductive traces leading from them to another destination on the circuit board  200 , while the ground via  205  is typically connected to a ground plane layer disposed within the circuit board  205  on an inner surface thereof and not sown in  FIG. 6 .  
         [0052]     In this type of via pattern, two pairs of differential signal vias each share the single ground via in the center of the pattern. It has been discovered by us that this 5-die pattern creates crosstalk and it is difficult to very finely control the impedance of such a system. The grouping of one of the differential via pairs  202 ,  204  and the center ground via is preferably triangular in configuration with the three vias being located at the vertices of imaginary triangles represented by the bold lines T in  FIG. 6 .  
         [0053]      FIG. 7  is a top plan view of a circuit board  300  with a via layout constructed in accordance with the principles of the present invention in which the spacing of the vias is staggered so that one pair of differential signal vias “AA” is located closer to their associated ground via  302  (shown in approximately the center of the pattern) than are a second pair “BB” of differential signal vias. Multiple vias  301  are formed in the circuit board  300  and an associated ground via  302  is provided in association with and preferably aligned with a pair of differential signal vias  303 . The two differential signal vias  303  are preferably aligned along a first axis L 1  to form a pair of differential signal vias, and the associated ground via  302  is spaced apart from the first axis, but located between the two signal vias when viewed in a direction transverse to the first axis L 1 .  
         [0054]     We refer to this structure as a “preferential ground” via layout because the spacing W 1  between one differential signal via pair AA and its associated ground via  302  is less than the spacing W 2  between the one differential signal via pair AA and another, adjacent pair BB of differential signal vias  306 . In this manner, the one pair of differential signal vias AA is biased in its coupling toward its associated ground  302  and not toward either the other, adjacent differential signal via pair BB or the ground via  302   b  associated with that differential signal via pair BB.  
         [0055]      FIGS. 8-10  illustrate another embodiment of the present invention in which one or more ground reference planes of the circuit board are provided with a specially configured anti-pad that encompasses the two differential signal vias that make up a differential signal via pair. The dimensional relationship of these arrangements is first shown in  FIG. 8 , where reference number  400  indicates the circuit board, which includes a plurality of signal vias  401 , two of which are combined to form a pair  402  of differential signal vias. A large ground plane  405  is present either on the surface of the circuit board or on an interior layer thereof. The ground plane  405  has a large anti-pad  410  formed in it, and as can be seen in  FIG. 8 , the anti-pad  410  is generally rectangular in shape, having dimensions B and H as shown. It is preferred that the opening have an aspect ratio AR of from about 1.2 to 1.5, which is obtained by the equation: 
   AR=H/B.    
         [0056]     The ground plane  405  surrounding the pair  402  of differential signal vias  401  may be a large ground plane, as illustrated. In this manner, the likelihood that the differential signal pair will be split into multiple single-ended signals is reduced. The differential signal vias  401  are seen to penetrate the top metal ground plane layer  405  of the circuit board  400  and have a separation spacing (center-to-center) that is less than either B or H, the outer dimensions of the opening, or anti-pad  410 . In this manner, the anti-pad  410  is effectively decoupled from the differential signal pair and common mode coupling is minimized, while differential mode coupling between the two differential signal vias is increased.  
         [0057]     Additionally, one via  404  of the two ground vias  403 ,  404  is defined as a preferential ground, meaning that it is placed closer to the differential pair  402  than the other and is therefore designated as a primary ground reference. With this assymettrical relationship, the common mode coupling of the pair of differential signal vias is minimized and is defined for subsequent tuning of the impedance of the system, i.e., along its extent through the circuit board. The ground plane  405  is connected to both ground vias on the top and bottom surfaces of the circuit board as illustrated in  FIG. 9 , if ground planes are used in that fashion and as illustrated in  FIG. 10 , it is preferred that the inner ground plane layers are selectively connected to the ground vias. In  FIG. 9 , it will be noted that the ground plane  405  takes the form more of a grid or lattice like structure, rather than a large solid ground plane layer. Such a grid or lattice is indicated for use for areas of circuit boards that have a high density of pairs of differential signal vias.  
         [0058]     In  FIG. 10 , a multiple layer, or plane, circuit board is shown, with the resin or other insulative material removed for clarity. The ground planes  405   a ,  405   b  are disposed on opposing top and bottom surfaces of the circuit board and they are connected to both of the ground vias  403  &amp;  404 . In the inner ground reference plane layers  405   c  &amp;  405   d , there is no connection between the ground planes and either of the two vias  403 ,  404 . A pair of signal traces  420  are shown exiting from the differential signal via pair  420  between ground plane layers  405   e  and  405   f.  In order to optimize the via performance through the circuit board  400  and its stack of layers, the two ground planes that flank the signal traces  420  are connected to the ground vias  403 ,  404 .  
         [0059]     The exit paths that the conductive signal traces  420  take between the three vias  401 - 403  is shown best in  FIG. 10A .  FIG. 10A  is a top plan view of the via and ground plane structure of  FIG. 10 , illustrating the ground plane on the top surface of the circuit board (with the board structure removed for clarity) and illustrating the connection of two inner signal traces to the differential signal via pair. This also illustrates the path which the signal traces  420  take in their route out, or exit from the differential signal vias.  
         [0000]     Signal Trace Breakout From Vias  
         [0060]     It is also desirable to control the impedance of the transmission lines in the area in which the traces exit from the vias and continue their transmission path on the circuit board. Problems arise in these exit areas. Previously it was known to attempt to maintain the spacing of the conductive trace pair in symmetrical arrangements around a center line running between the differential signal via pair. This is shown in  FIG. 14 , where two vias  501 ,  502  of a pair of differential signal vias are spaced apart from each other by a distance D. A pair of conductive traces  503  are connected to the vias  501 ,  502  and exit therefrom. Their exit path extends initially out at an angle along exit portions  504  of the traces toward a centerline C that separates the two vias  501  until the traces are separated by a uniform spacing DD. These exit portions  503  have a short length and do not intersect each other in their extent, but they join corresponding elongated portions  505  that extend parallel to each other on opposite sides of the centerline C. The two vias  501 ,  502  and their associated traces  503  define a signal transmission line of the circuit board  500  supporting them. With a single pair of differential signal vias, the needed spacing, geometry and the length of the two traces may be kept symmetrical so that any variances in the exit are kept to an absolute minimum. By maintaining the geometry and symmetry of the circuit trace, the impedance can be controlled in this area. However, it is not always possible to route out traces from vias in a symmetrical pattern, especially in areas of the circuit board where there is a high density, or closely spaced pairs of differential signal vias.  
         [0061]     Problems will arise when the conductive traces leading from a pair of differential signal vias are staggered so that the traces are either not of equal length, or are not symmetrical in their pattern as a pair. Such a problematic arrangement is illustrated in  FIG. 15  where a circuit board  500  is illustrated as having an array of vias  501 ,  502  arranged in pairs in two lines. Two of the vias  501 ,  502  form a differential signal pair and two conductive traces  505 ,  506  are shown leading from the vias to a signal transmission line  507 . The one trace  506  has a short exit portion  510 , while the other trace  505  has a longer exit portion  511  to account for the spacing between the two vias  501 ,  502 . The signal transmission line  507  portion of the traces extends between the two rows of vias. In order to ensure that the impedance of the signal transmission line will maintain a desired value, it becomes necessary to equalize the length of the transmission line portion  507  to take into account the difference in the lengths and angles of the two exit portions  510 ,  511  of the traces. This is done by inserting a compensating portion  512 , shown as a partial loop, which increases the overall length of the trace  506  without unduly increasing the lateral length. However, the use of such a compensating portion  512  takes up valuable space on the circuit board which otherwise could be used for additional circuitry and therefore this solution to controlling the impedance of a circuit board signal transmission line is undesireable.  
         [0062]      FIGS. 11-13  &amp;  11 A illustrate one embodiment of a circuit board  600  having a circuit trace pattern  601  that provides desirable impedance characteristics of a signal transmission line  610  that exits from a pair  609  of differential signal vias  608   a ,  608   b . With this arrangement, it has been discovered by us that it is possible to “tune” the performance of the transmission system from the vias  608   a ,  608   b  all the way their associated signal transmission line  612 , which as shown, is formed from two conductive traces  613   a ,  613   b . The circuit trace pattern shown in these Figures is one that is typically found on an inner layer of the circuit board  600  and the two traces  613   a ,  613   b  mate with the differential signal vias  608   a ,  608   b  along their plated body portions  604 . ( FIG. 12 .) In using patterns of the invention, we have found that it is possible to launch the energy of the system as the traces break away or “out” from the differential signal vias. These structures serve to return energy to the system. In this manner the invention can provide a continuously coupled differential trace pair from a point that is nominally between the via pair.  
         [0063]     As stated above, a large concentration of energy occurs at the pair of vias  609 , and in order to recapture this energy, the via exit portions  620  have enlarged width portions, or areas,  621  which are joined to the vias by way of annular collar portions  622 . The enlarged width portions  620  are further joined to the via plating  622  with what we describe as “flag” portions  623 . These flag portions  623  , and in part, the enlarged width portions  621  present more metal plate area to increase the capacitance in the area between the vias where the electrical energy is concentrated. The flag portions  623  give a good 90 degree centerline exit to the beginning of the exit portions.  
         [0064]     As shown best in  FIG. 11 , the two pairs of vias disposed in the circuit board  600  are arranged along a first axis L 1 . The lower pair of vias in the Figure are a pair of differential signal vias, and the conductive trace exit portions  620  are of enlarged width and extend first along that first axis toward each other, and then at an angle outwardly from that first axis along a second axis, designated AX 2  in  FIG. 11 , which preferably extends transverse to first axis L 1  as illustrated. They then turn along a pair of bends  680 ,  681  that have a radius so that one trace  613   a  fits inside of the other trace  613   b  and continue along a third axis AX 3 , that is generally parallel to axis L 1  and which is generally transverse to axis AX 2 . In this manner, a constant spacing EE may be obtained between the two traces from the area XX where the flag portions  623  extend out toward each other to the area where the exit portions join the signal transmission area ST. This is to provide continuous coupling of the differential signal traces.  
         [0065]      FIG. 11 A  is a top plan view of the exit portions of a pair of traces. In this embodiment, the two differential signal vias are surrounded by a dogbone-style opening  690  similar to that shown in  FIG. 5 . As mentioned above, and as illustrated in this embodiment, the exit portions  723  of the traces take the form of flag-type structures, which are plate-type areas in lieu of thin traces leading away from the vias. These plate areas increase the capacitive coupling between the traces at the via area and also lower the inductance. The flag portions also approach each other (extend along a first axis) to maintain the desired separation distance between the traces in their exit from the vias, and subsequently, the exit portions extend out from the flag portions along a second axis which intersects the first axis. It can be seen that the traces follow three different paths, first along the axis L 1 , then secondly along the axis AX 2  and then lastly along the axis AX 3 . Axes L 1  and AX 2  intersect, as do axes AX 2  and AX 3 .  
         [0066]      FIG. 13A  is a top plan view of another embodiment of the invention showing the exit path of a pair of conductive traces  550  from a pair of vias  551  until they join to a signal transmission line  552 . The traces  550  include flag portions  555  as part of their exit portions with enlarged plate areas that exit from the vias toward each other along an axis L 1 . One of the traces  550   a  lies inside of the other trace  550   b  as it curves back upon itself to the signal transmission line portions  552  that extends generally transverse to the axis L 1 . The exit portions further traverse a path that has approximately five distinct bends in it, with each bend of the structure of  FIG. 13A  being identified by the lines B-B and each bend representing an occurrence when the trace exit portions change direction.  
         [0067]     A ground reference plane  590  is shown superimposed above the trace exit pattern. In this layer of the circuit board, the reference plane  590  and the and the annular collar portions  591  are found. They are shown as located in a layer above the trace exit pattern, but they could also be located in a layer beneath the trace exit pattern. There are two ground vias  593  that are interconnected to the ground plane  590  and they are located at edges of the opening  594  formed in the ground plane that encompasses the two differential signal vias  551 . One of the ground vias  593   a  is the primary ground via that is associated with the pair of differential signal vias  551 , and the other ground via  593   b  is one that is associated with the pair of differential signal vias that is to the left and not shown in  FIG. 13A . The pair of differential signal vias  551  are located closer to their associated ground via  593   a , being spaced away therefrom a distance W 1 , which is shorted than the distance W 2  the pair is spaced from the ground via  593   b . The annular collar portions  595  of these ground vias have been removed as shown on the right half of the ground vias in  FIG. 13A  so that they do not extend along a circular path of 360 degrees. Rather, it is preferred that these type of annular collar portions have a curved extent of about 150 to 200 degrees, with about 180 degrees being preferred. This is done to reduce capacitive coupling between the signal traces exit portions and the non-associated ground via  593   b.    
         [0068]      FIG. 16  illustrates another style of circuit trace exit or breakout pattern constructed in accordance with the principles of the present invention. In this arrangement, two conductive traces  450   a ,  450   b  exit from a pair of associated vias  401 ,  402 . The exit portions of these traces  450   a ,  450   b  include one trace portion  471  with a tight bend radius that is nested within the other trace bend portion  470 . This inner trace  471  may be considered as bending back upon itself as it initially extends from the via  401  toward its other paired via  402  and then turns upon itself. The significant part of this structure may be found in the initial portion that extends out from the via  401  to the other paired via  402 . The trace then continues with a curved portion that is spaced close to the exit portion  471  of the outer via. In this manner, both the closeness of the two traces is maintained as well as similar path lengths.  
         [0069]      FIG. 16A  is a top plan view of  FIG. 16 , and it illustrates, in a manner similar to  FIG. 13A , a ground reference plane superimposed either above or beneath the trace exit pattern. In this ground reference plane, the associated ground via is spaced closer tot he pair of differential signal vias than is the non-associated ground via. This Figure shows best how the exit portion  473  first extend out from its via  401  toward the other via  402  of the via pair in order to establish the separation distance. It then loops back upon itself at  474  at a point where it may follow the interior of the outer via at a desired separation distance.  
         [0070]     While the preferred embodiment of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.