Abstract:
Methods and systems for reducing noise coupling in high-speed digital systems. Exemplary embodiments include a method, including etching a plurality of high speed signal traces onto a core insulating layer, forming trenches on respective sides of the plurality of high speed signal traces, thereby removing insulating material adjacent to the plurality of high speed signal traces and forming pedestals having remaining insulating material, the plurality of high speed signal traces disposed on and coupled to the remaining insulating material, coupling pre-preg material on the high speed signal traces, removing the pre-preg material adjacent the trenches, thereby retaining the pre-preg material aligned with the high speed signal traces, and heating and pressing a core layer to the pre-preg layer, and heating and pressing the pre-preg layer to the core insulating layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/924,416, filed Oct. 25, 2007, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     TRADEMARKS 
       [0002]    IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
       BACKGROUND 
       [0003]    1. Field 
         [0004]    This invention relates to printed circuit board fabrication, and particularly to methods and systems for reducing noise coupling in high-speed digital systems. 
         [0005]    2. Description of Background 
         [0006]      FIG. 1  illustrates a conventional printed circuit board having a core layer  105  having a return current reference layer  106 , a pre-preg layer  110  and a core insulator layer  120  having a return current reference layer  115  and a number of single layer nets  125 . Cross-talk noise coupling on adjacent nets  125  (i.e., victim nets) can result in poor signal integrity in high speed system. The effects of cross-talk noise can be intensified with increased signal speeds. 
         [0007]    Currently, many techniques have been proposed to reduce noise and cross-talk coupling, but often rely on complex digital signal processing (DSP) techniques and filtering algorithms to achieve noise isolation. 
         [0008]    There is currently a need to detect and implement, and at the same time, achieve high noise isolation. 
       SUMMARY 
       [0009]    Exemplary embodiments include a printed circuit board apparatus having reduced noise coupling, the apparatus including a core layer having an upper and lower surface, the upper and lower surface each including a copper sheet layer, a pre-preg layer having an upper surface and a lower surface, the upper surface of the pre-preg layer coupled to the lower surface of the core layer, a core insulating layer having an upper surface and a lower surface, the upper surface of the core insulating layer coupled to the lower surface of the pre-preg layer, a return current reference layer disposed on the lower surface of the core insulator layer and high-speed signal traces disposed on the upper surface of the core insulating layer, each of the high speed signal traces disposed on a pedestal defined by a section of the pre-preg layer and the core insulating layer, each pedestal being separated by an air gap disposed between adjacent pedestals. 
         [0010]    Further exemplary embodiments include a printed circuit board fabrication method, including etching a plurality of high speed signal traces onto a core insulating layer, forming trenches on respective sides of the plurality of high speed signal traces, thereby removing insulating material adjacent to the plurality of high speed signal traces and forming pedestals having remaining insulating material, the plurality of high speed signal traces disposed on and coupled to the remaining insulating material, coupling pre-preg material on the high speed signal traces, removing the pre-preg material adjacent the trenches, thereby retaining the pre-preg material aligned with the high speed signal traces, wherein a width of a trench portion disposed in the pre-preg material is greater than a width of a trench portion disposed in the core insulating layer and heating and pressing a core layer to the pre-preg layer, and heating and pressing the pre-preg layer to the core insulating layer, wherein the plurality of high-speed signal traces disposed on an upper surface of the core insulating layer, each of the plurality of high speed signal traces disposed on a pedestal defined by a section of the pre-preg layer and the core insulating layer, each pedestal being separated by an air gap disposed between adjacent pedestals. 
         [0011]    System and computer program products corresponding to the above-summarized methods are also described and claimed herein. 
         [0012]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
       TECHNICAL EFFECTS 
       [0013]    As a result of the summarized invention, technically we have achieved a solution which cross-talk noise coupling from adjacent nets on PCBs is reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  illustrates a conventional printed circuit board; 
           [0016]      FIG. 2  illustrates a PCB apparatus that reduces cross-talk for single-ended nets in accordance with exemplary embodiments; 
           [0017]      FIG. 3  illustrates a PCB apparatus that reduces cross-talk for differential nets in accordance with exemplary embodiments; 
           [0018]      FIG. 4  illustrates a flowchart of a printed circuit board fabrication method for reducing cross talk in accordance with exemplary embodiments; and 
           [0019]      FIG. 5  illustrates a graph of noise coupling simulation results. 
       
    
    
       [0020]    The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION 
       [0021]    In exemplary embodiments, the systems and methods described herein reduce cross-talk coupling or noise induction created by capacitive and inductive coupling. Capacitive coupling occurs due to the presence of dielectric medium. In exemplary embodiments, the systems and methods described herein include adjacent printed circuit board (PCB) stripline structures that are surrounded with air. A PCB trace on an inner layer is supported in an air cavity by a pedestal of insulating material. The pedestal is created by digging trenches in core laminate and pre-preg. In exemplary embodiments, pre-preg (pre-impregnated) refers to a layer of exemplary structures described herein, of insulation material inserted between the etched cores. In exemplary embodiments, the pre-preg can be a combination of mat, fabric, non-woven material or roving with resin, usually cured to the B-stage, ready for molding. A standard pre-preg contains more resin than is desired in the finished part; excess resin is bled off during cure. A net resin pre-preg contains the same resin content that is desired in the finished part; no resin bleed. Pre-preg containing a chemical thickening agent is called a mold-mat and those in sheet form are called sheet molding compounds. 
         [0022]    In exemplary embodiments, the systems and methods described herein reduce cross-talk noise coupling from adjacent nets on PCBs. In exemplary embodiments, the systems and methods described herein decrease spacing between adjacent traces without any increase in noise coupling. 
         [0023]    Capacitive coupling occurs due to presence of a dielectric medium as discussed above. The higher the value of the dielectric constant, Dk, of the dielectric medium, the stronger the capacitive coupling. In exemplary embodiment, trenches are cut and scoured out on each side of the signal traces with a laser (or any other etching mechanism, such as chemical, etc.). A pedestal of high dielectric constant material supports the trace. The thickness of core and pre-preg determine the distance of the trace from the conducting layer on other side of core. By the proximity rule, the return path is on the conducting layer(s) closest to the signal trace. 
         [0024]    In exemplary embodiments, air is trenched in the dielectric between the adjacent side by side signal traces. Since the relative dielectric of the material between the signal and the reference layers is much greater than air, the result is a structure where the capacitance from signal trace to return path is much greater than the capacitance for signal trace to signal trace. Since the cross talk is define by coupling ratio of coupled capacitance to total capacitance, the trace can be closer in proximity with little coupling and the current will return through the reference planes and does not couple with adjacent traces. 
         [0025]    Exemplary embodiments described herein include systems and methods for both single-ended and differential nets as now described. 
         [0026]    Turning now to the drawings in greater detail,  FIG. 2  illustrates a PCB apparatus  200  that reduces cross-talk for single-ended nets. In exemplary embodiments, the apparatus  200  includes a core layer  205 , a pre-preg layer  210  and a core insulating layer  220  having a return current reference layer  215  and a number of single layer nets  225 . In exemplary embodiments, the core layer  205  and the core insulator layer  220  are an insulating material. In exemplary embodiments, the core layer  205  includes copper sheets  206  bonded on each surface of the core layer  205 , in which the lower layer is a return current reference layer. In exemplary embodiments, an upper copper sheet bonded on an upper surface on the core insulating layer  220  is etched to create the single layer nets  225 . A lower copper sheet bonded to the lower surface of the core insulating layer  220  is retained as the return current reference layer  215 . In exemplary embodiments, the apparatus  200  further includes trenches  230  that have been etched into both the core insulating layer  220  and the pre-preg layer  210 . In exemplary embodiments, the formation of the trenches  230  further defines distinct pedestals  235 , each pedestal  235  supporting a single layer net  225 . As described herein, the trenches  230  are cut and scoured out on each side of the single layer nets  225  (i.e., the signal traces), thereby creating air gaps between respective pedestals  235 . 
         [0027]      FIG. 3  illustrates a PCB apparatus  300  that reduces cross-talk for differential nets in accordance with exemplary embodiments. In exemplary embodiments, the apparatus  300  includes a core layer  305 , a pre-preg layer  310  and a core insulating layer  320  having a return current reference layer  315  and a number of differential nets  325 . In exemplary embodiments, the core layer  305  and the core insulator layer  320  are an insulating material. In exemplary embodiments, the core layer  305  includes copper sheets  306  bonded on each surface of the core layer  305 , in which the lower layer is a return current reference layer. In exemplary embodiments, an upper copper sheet bonded on an upper surface on the core insulating layer  320  is etched to create the differential nets  325 . A lower copper sheet bonded to the lower surface of the core insulating layer  320  is retained as the return current reference layer  315 . In exemplary embodiments, the apparatus  300  further includes trenches  330  that have been etched into both the core insulating layer  320  and the pre-preg layer  310 . In exemplary embodiments, the formation of the trenches  330  further defines distinct pedestals  335 , each pedestal  335  supporting a differential net  325 . As described herein, the trenches  330  are cut and scoured out on each side of the single layer nets  325  (i.e., the signal traces), thereby creating air gaps between respective pedestals  335 . 
         [0028]      FIG. 4  illustrates a flowchart of a printed circuit board fabrication method  400  for reducing cross talk in accordance with exemplary embodiments. At block  405 , traces (i.e., the single layer nets  225  or the differential nets  325 ) are etched onto the core insulator layers  220 ,  320 . At block  410 , the trenches  230 ,  330  are cut on each side the high speed signal trace so that all insulating material in the proximity of the nets  225 ,  325  is removed. The pedestals  235 ,  335  are formed from the remaining insulating material under the nets  225 ,  325 . At block  415 , sections of the pre-preg layers  210 ,  310  are cut, removed, and aligned above the target high speed signal trace (e.g., the nets  225 ,  325 ), which had trenches  230 ,  330  on each and supported by a pedestal  235 ,  335 . In exemplary embodiments, the cut out sections are larger in the pre-preg layers  210 ,  310  than the core insulating layers  220 ,  320  to prevent the pre-preg layers  210 ,  310  from expanding into the trenches  230 ,  330  that are in formation. At block  420 , the stack of core layers  205 ,  305 , core insulating layers  220 ,  320  and the pre-preg layers  210 ,  310  are pressed and heated. 
         [0029]      FIG. 5  illustrates a graph  500  of noise coupling simulation results. The graph  500  plots noise level versus frequency comparing a noise coupling plot  505  in conventional PCBs against a reduced noise coupling plot  510  of a PCB in accordance with exemplary embodiments. 
         [0030]    The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof. 
         [0031]    As one example, one or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately. 
         [0032]    Additionally, at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided. 
         [0033]    The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0034]    While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.