Abstract:
A printed circuit board with improved signal integrity for one or more differential signal pairs incorporates one or more conductive regions. In an exemplary embodiment, via structures for the differential pair that interconnect signal traces are isolated from the conductive region by an antipad area around the via structures and a conductive bridge. In alternate embodiment, an antipad area around the via structures includes a bridge between the via structures. The antipad area may comprise, by way of non-limiting example, a clipped circular aperture or a modified rectangular aperture. The bridge may, by non-limiting examples, comprise a portion of the conductive region to permit impedance tailoring of the differential pair with respect to the conductive region.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This divisional application claims the benefit of U.S. patent application Ser. No. 11/283,558, filed Nov. 17, 2005 and U.S. Provisional Application 60/643,050, filed Jan. 10, 2005, which are both herein incorporated by reference in their entirety. 
     
    
     BACKGROUND  
       [0002]     Exemplary embodiments disclosed herein pertain to electronic printed circuit boards which provide support and interconnectivity for electronic components to form electronic circuit apparatus. More particularly, exemplary embodiments disclosed herein pertain to printed circuit boards provided with differential signal pairs.  
         [0003]     Today&#39;s electronic products, including computers, telecommunication equipment, and networking systems, are being designed to operate at ever increasing data transmission rates. Printed circuit boards, backplanes, midplanes, printed wiring boards, flex circuits, rigid flex-circuits, multi-chip modules (MCM), interposers and the like (herein referred to collectively as “PCBs”) have traditionally been considered as merely passive interconnects, but increasingly, PCBs are recognized to have potentially deleterious effects on integrity or fidelity of signals carried by the PCB.  
         [0004]     One method for improving signal integrity in a PCB includes the use of differential signal pairs. Generally, two conductors of a differential signal pair are designed to run in the PCB close to each other, so that a source of electrical noise coupled to the differential pair represents “common mode noise” that is relatively easily minimized or eliminated. The separation distance between the conductors defining a differential pair influences the characteristic impedance of the differential pair.  
         [0005]     A via structure typically provides a conductive path between conductive layers in the z-axis direction (i.e. orthogonal to the x-y plane of a PCB) Via holes are formed by a variety of techniques including but not limited to laser drilling, mechanical drilling, and techniques based on photo definition. They are subsequently partially or wholly filled or coated with a conductive material, usually metal. These via structures may be blind, buried, through-hole and may or may not include pads on the conductive layers, as is well known to those skilled in the art of PCB design.  
         [0006]     PCB via structures used to route differential pair signals between conductive layers of a PCB can reduce the characteristic impedance of the PCB to an unacceptably low level. Within the PCB via structure, the size and shape of the pad (if present) and antipad impact the characteristic impedance of the differential pair as it is routed through the via structure.  
         [0007]     One limitation of the prior art is the lack of impedance control with respect to differential pairs in that the pad (if present) and antipad shape are predominantly circular and size limited by the design considerations of the PCB. This, as noted above, may result in unacceptably low impedances.  
         [0008]     U.S. Pat. No. 6,607,402, incorporated herein by reference, describes a configuration for increasing the characteristic impedance of a differential pair as it is routed through a set of PCB vias by removing conductive material between the two via structures used to route the differential pairs between conductive plane layers so that the antipad regions from the two via structures comprising the differential pair form one large antipad region.  
         [0009]     One exemplary limitation of the prior art is a lack of a provision for impedance tailoring with respect to the differential pairs. This results in reduced system performance due to a lack of optimum impedance matching with the signals on the differential pairs.  
         [0010]     These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.  
       SUMMARY OF EXEMPLARY EMBODIMENTS  
       [0011]     In certain exemplary embodiments, a printed circuit board (PCB) with improved signal integrity for one or more differential signal pairs incorporates one or more conductive regions. Vias for a differential pair that interconnect two or more signal trace layers, in certain embodiments, are isolated from the conductive region by an “antipad” defined as a nonconductive area around the via structures. In some instances, an antipad area for the vias may include a conductive “bridge” between the via structures.  
         [0012]     An embodiment, by way of non-limiting example, includes a first via structure configured as a first portion of a circuit for a differential pair, a second via structure configured as a second portion of the circuit for the differential pair, a first antipad area surrounding the first via structure isolating the first via structure from a conductive region on the same layer of the printed circuit board as the antipad, a second antipad area similar in shape and function to the first antipad area surrounding the second via structure, and a conductive bridge placed between the antipad regions associated with the first and second via structure. The ends of the conductive bridge may be connected to the conductive region that surrounds the entire set of via structures and antipads.  
         [0013]     One advantage that the bridge provides is the ability to tailor the impedance of the differential pairs with respect to the conductive region, resulting in improved signal integrity and enhanced performance of the electronic circuit device.  
         [0014]     These and other embodiments and advantages of the bridge and other features disclosed herein will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Several exemplary embodiments will now be described with reference to the drawings, wherein like components are provided with like reference numerals. The exemplary embodiments are intended to illustrate, but not to limit, the invention. The drawings include the following figures:  
         [0016]      FIG. 1  illustrates a cross-sectional view of a multilayer PCB, in an exemplary embodiment;  
         [0017]      FIG. 2  illustrates a portion of the conductive region of  FIG. 1 , showing further detail of exemplary antipad regions and a conductive bridge; and  
         [0018]      FIG. 3  illustrates a portion of the conductive region  110   c  of  FIG. 1 , showing further detail on the exemplary antipad regions  130  and  131  and the conductive bridge  132  of an alternate exemplary embodiment.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0019]      FIG. 1  is a cross-sectional view of a multilayer PCB  100  embodiment, in accordance with a non-limiting example. The PCB  100  includes a number of dielectric layers  105  separating a number of conductive layers  110 . A differential signal pair includes a pair of signal traces  110   a  on the first layer, and a pair of signal traces  110   b  on the second layer. The differential pair comprises a positive going signal (e.g., carried by the signal traces  110   a  and  110   b ) and a negative going signal (e.g., carried by the signal traces  110   a ′ and  110   b ′). It will be appreciated that these signals can also be reversed. Furthermore, although depicted as being on adjacent layers, it will be appreciated that one or both of the signal traces  110   a  and  110   b  maybe formed on other layers of the PCB  100 .  
         [0020]     As used herein, a “conductive region” is part or all of a plane or layer of a PCB made from a conductive material such as copper. As used herein, a “via structure” includes, but is not limited to a via barrel formed within a via hole that extends between two or more layers of a PCB. A via structure may optionally include a via pad or the like. A “via barrel” refers to the conductive material within a via hole, which is often a hollow cylinder but which can be other shapes such as a solid cylinder, a partial cylinder, strips, and other shapes. A “via pad” is typically made from a conductive or partially conductive material which is in electrical contact with the via barrel. A “via antipad” is a nonconductive region surrounding or partially surrounding a via pad. An antipad may comprise an insulating material, or can simply be a void provided around the via.  
         [0021]     As also used herein, a “conductive region” refers to either a partial or complete ground plane or a power plane. The conductive region may cover a complete layer or part of a layer. The conductive region therefore is typically fairly extensive, e.g., at least about 1 cm 2  or sometimes hundreds of centimeters square or larger as opposed to conductive traces which tend to be quite fine. That is, the area of the conductive region is greater than the area of conductive traces in the proximity of the vias.  
         [0022]     A conductive via structure  120  couples one of the signal traces  110   a  of the differential pair to a respective signal trace  110   b . A conductive via structure  121  couples the other of the signal traces  110   a ′ of the differential pair to the other respective signal trace  110   b ′. A first antipad region  130  is configured to isolate the via structure  120  from a conductive region  110   c  of the PCB  100 . A second antipad region  131  is configured to isolate the via structure  121  from the conductive region  110   c . A bridge  132  of the conductive region that normally surrounds the via structure  110   c  preferably separates the first antipad region  130  and the second antipad region  131 .  
         [0023]      FIG. 2  illustrates a portion of the conductive region  110   c  of  FIG. 1 , showing further detail on exemplary antipad regions  130  and  131  and the bridge  132 , in one embodiment in accordance with the present invention. In this embodiment, the via structure  120  and the via structure  121  are surrounded by the antipad regions  130  and  131  respectively. This isolates the via structure  120  and the via structure  121  from the conductive region  110   c.    
         [0024]     The via structure  120 , in an exemplary embodiment, comprises a conductive via structure of radius R 1 , and similarly, the via structure  121  may comprise a conductive radius R 1 . The centers of the via structures  120  and  121 , in this example, are separated by a distance D 1 . The antipad regions  130  and  131  comprise, for example, circular or elliptical geometries with radii R 2  and R 3 , the centers of which are separated from each other by a distance D 2 . Other configurations comprise alternate embodiments.  
         [0025]     By way of non limiting example and depending on connectors or other components to be coupled to the vias  120  and  121 , R 2  may be equal to, greater than, or less than R 3 ; however, both R 2  and R 3  are larger than R 1 . D 1  maybe equal to or less than D 2 . In an exemplary embodiment, D 3  is less than R 2  and R 3 .  
         [0026]     In this example, antipad regions  130  and  131  represent clipped circular shaped nonconductive regions of, or apertures etched into, the conductive region  110   c , with left-right symmetry about the center of the bridge  132 , and with top-bottom symmetry about a line between centers of the pads for the vias  120  and  121 . It will be appreciated that symmetry provides balanced impedances for the differential signals with the accompanying performance enhancement of the electronic device made with the PCB.  
         [0027]     In some exemplary embodiments where D 1 =D 2 , the antipad regions  130  and  131  are such that a distance between the via structures  120  and  121  and the conductive region  110   c  is greater than a distance between the via structures  120  and  121  and the bridge  132 . In one embodiment, the distance between the via structures  120  and  121  and the conductive region  110   c  is 0.008 inches, and the distance between the via structures  120  and  121  and the bridge  132  is 0.0055 inches. In this embodiment, the bridge  132  is 0.005 inches wide, formed by well known methods of etching the antipad into the conductive region  110   c.    
         [0028]      FIG. 3  illustrates the portion of the conductive region  110   c  of  FIG. 1 , showing further detail on the antipad regions  130  and  131  and the bridge  132 , in another exemplary embodiment. In such embodiments, the antipad regions  130  and  131  comprise rectangular or modified rectangular shapes. Other shapes comprise additional embodiments. A distance D 4  from the via structure  120  to the conductive region  110   c  may be greater than, equal to, or less than a distance D 5  from the via structure  120  to the conductive region  110   c . In this exemplary embodiment, D 3  is less then D 4  and D 5 . The antipad regions  130  and  131 , in this example, are symmetrical left-to-right about the center of the bridge  132 , and from top-to-bottom about a line between the pads for the vias  120  and  121 . It will be appreciated that symmetry again advantageously provides balanced impedance for the differential signals. In an exemplary embodiment, D 3  is less than D 4  and D 5 .  
         [0029]     In some embodiments, the antipad regions  130  and  131  comprise modified rectangular shapes with rounded corners. In some embodiments, the antipad regions  130  and  131  comprise modified rectangular shapes with 45 degree (i.e., mitered or beveled) corners. It will be appreciated that the modified rectangular shapes of these embodiments can improve processing (manufacturing) of the PCB, for example by improving reliability of etching of the conductive region  110   c  to form the antipad regions  130  and  131 . Again, the suggested geometric shapes are by way of non limiting example, and various closed and open geometric shapes, including polygonal, curved, and compound shapes comprise various alternate embodiments.  
         [0030]     In some embodiments, the bridge  132  represents a minimal line width of conductive material (e.g., copper) that still allows the pads of the vias  120  and  121  to be physically close to each other yet separate and isolated from each other. By way of non-limiting example, embodiments of the bridge width might be in the range of approximately 0.1 mils to approximately 5 mils. By way of additional non-limiting example, embodiments of the bridge the width might be in the range of about 2 mils to 4 mils. A particular non-limiting example of an embodiment would be, for example, 4 mils. In other non-limiting examples of alternate embodiments, the bridge width may be less than 0.1 mils or greater than 5 mils.  
         [0031]     Determining preferred values for D 1  through D 5  can be accomplished using commonly available 3D numerical modeling and simulation tools including those based on FEM, FDTD, TLM, MOM. Those skilled in the use of these modeling and simulation tools can determine these dimensions utilizing standard modeling techniques.  
         [0032]     In some embodiments, set forth by way of further non-limiting examples, the bridge  132  between the via structures  120  and  121  of the differential pair comprises a conductive material (e.g., 2 oz. copper) that is the same conductive material as the conductive region  110   c . In alternative embodiments, the bridge  132  comprises a range of copper weight ranging from ¼ ounce to 4 ounces. In other embodiments, other materials than copper can be used, and other weights for the copper or other materials.  
         [0033]     Although various embodiments have been described using specific terms and devices, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention, which is set forth in the following claims. In addition, it should be understood that aspects of various other embodiments may be interchanged either in whole or in part. It is therefore intended that the claims be interpreted in accordance with the true spirit and scope of the invention without limitation or estoppel.