Abstract:
Vias are used in multilayer printed circuit boards to route electrical interconnects between layers. Some via constructions embodiments result in the formation of a via-stub section. Via stub sections can distort signals passing through the interconnect and decrease the usable bandwidth of the interconnect. To minimize distortion and increase bandwidth, one or more terminating elements can be attached to the unterminated end of the via-stub section. The impedance terminating element may include, by way of non-limiting example, one or more resistors, capacitors, and/or inductors between the via stub and a ground layer. The impedance terminating element may be formed internally to the PCB or mounted to the PCB surface.

Description:
BACKGROUND  
       [0001]     Embodiments disclosed herein relate generally to printed circuit board (PCB) production and design, and more particularly to high-performance multi-layer PCB production and design.  
         [0002]     Printed circuit boards, backplanes, midplanes, printed wiring boards, flex circuits, rigid flex-circuits, multi-chip-modules (MCM), and the like (herein referred to collectively as PCBs) have traditionally been used to provide an interconnect (a set of electrical circuit connections) for digital, analog and RF (radio frequency) signals that need to be routed between different locations within electronic devices including computers, networking systems and telecommunication equipment.  
         [0003]      FIG. 1  is a cross-sectional view of a typical multilayer PCB  100  in accordance with the prior art. The PCB  100  includes multiple signal traces and/or planar conductive layers  110   a - f  separated by rigid or flexible planar insulating dielectric layers  115   a - e  bonded together. When routing an interconnect through a PCB, it often becomes necessary to move the interconnect between planar layers  110   a - f . An electrical connection path used to accomplish this function is a via. Thru-hole vias  120  pass completely through the body of PCB  100 , blind vias  125  only have one end of the via extend to the surface of the PCB  100 , and buried vias  130  have neither end extend to the surface of the PCB  100 .  
         [0004]     Small conductive regions called pads may be directly attached to the via on one or more of the conductive layers. Pads that are a functional part of the interconnect are called functional pads  135   a - f . Pads that are not needed in order to complete the interconnect as it is routed between layers are called non-functional pads  140 . Pads may be electrically isolated from the rest of the layer by an non-conductive anti-pad region  145 .  
         [0005]     In  FIG. 1 , signal traces  150   a  and  150   b  on layers  110   a  and  110   c  respectively are electrically connected together through via  120  and functional pads  135   a  and  135   b . Alternate interconnect connection schemes including press-fit connector pins  160  and discrete/planar passive components and circuits attached directly to a via pad can also be used in lieu of the signal trace  150   a.    
         [0006]     The series portion of the via that is necessary in order to provide an electrical connection between the desired layers is called the via-thru section  155   a - c . The via may also include a non-functional portion called a via-stub section  165   a   165   b   165   c that extends beyond the functional pads  135   b    135   d    135   f . The end of the via-stub section that is not connected to the functional part of the interconnect  170   a - c  is the unterminated end of the via-stub section.  
         [0007]     The presence of one or more via-stub sections in an interconnect can significantly reduce its useable bandwidth and detrimentally distort signals that propagate through the interconnect. One technique that has been used to mitigate the interconnect bandwidth and signal distortion limitations in the prior art includes back-drilling as shown in detail  180  of  FIG. 1 . A drill whose diameter  185  is wider than the finished hole-size is used to remove a substantial portion  190   a  of the via-stub section, leaving a short residual stub  190   b  that no longer has a significant impact on the interconnect&#39;s performance.  
         [0008]     However, back-drilling is time consuming, expensive, and errors may irreparably damage the PCB  100 . Further, back-drilling cannot be applied to via structures whose via-stub sections  165   b - c  do not extend all the way to the surface of the PCB unless provisions are taken to keep essential interconnects from being routed through the PCB regions  195   a - b  that will be removed during the backdrilling operation.  
       SUMMARY  
       [0009]     An embodiment, by way of non-limiting example, includes a multi-layer board structure comprising a number of conductive layers separated by a number of non-conductive layers, the multi-layer board structure being provided with at least one via extending from at least a first conductive layer beyond a second conductive layer. An electrically conductive material disposed within the via to electrically interconnect the at least two conductive layers, the electrically conductive material having an stub portion which extends beyond the second conductive layer. A terminating element electrically connected to the stub portion.  
         [0010]     Another embodiment, by way of non-limiting example, includes forming a via in a multi-layer printed circuit board structure comprising a number of conductive layers separated by a number of non-conductive layers; providing an electrically conductive material within the via to electrically connect at least two conductive layers of the conductive layers; and providing a terminating element to electrically terminate a stub section of the electrically conductive material.  
         [0011]     Another embodiment, by way of non-limiting example, includes a number of conductive layers; a number of conductive vias configured to route signals between the conductive layers; and a number of terminating elements electrically connected to via-stub elements of the number of conductive vias.  
         [0012]     Another embodiment, by way of non-limiting example, includes a first conductive trace configured as a first portion of a circuit for carrying an electrical signal from a source to a destination coupled to the printed circuit board, the first conductive trace on a first layer of the printed circuit board; a second conductive trace configured as a second portion of the circuit, the second conductive trace on a second layer of the printed circuit board; a via including an electrically conductive material configured to couple the first conductive trace and the second conductive trace; and a terminating element configured to couple energy diverted from the circuit by the via to a ground layer of the printed circuit board.  
         [0013]     An advantage of certain embodiments is that it increases the useable bandwidth of an interconnect without resorting to expensive backdrilling. Another advantages is that the terminating elements can be used with all types of vias, including through hole vias, blind vias and buried vias.  
         [0014]     Terminating elements can be arranged to decrease distortion and increase useable bandwidth of a variety of PCB based transmission line interconnect configurations containing via-stub sections including single ended microstrips, single-ended striplines, edge-coupled differential-pair microstrips, edge-coupled differential-pair striplines, broadside-coupled differential-pair microstrips, broadside-coupled differential pair striplines, and slot-lines.  
         [0015]     Terminating elements can be comprised of: one or more discrete components such as resistors, capacitors and inductors mounted on the surface of the PCB; one or more planar resistive, capacitive and inductive elements incorporated directly into the PCB during the manufacturing process; or a combination of discrete components and planar passive elements. Planar elements incorporated into the PCB manufacturing process can be constructed from a variety of materials including polymer thick film materials, thin film materials, metallic alloys, and organic materials.  
         [0016]     In exemplary embodiments, terminating elements can comprise circuits. In other exemplary embodiments, terminating elements can be active components. These and other embodiments and advantages will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a cross section of a printed circuit board showing an exemplary prior art multi-layer printed circuit board including vias.  
         [0018]      FIG. 2  is a cross section of a printed circuit board graphically showing how a plurality of individual via-stub sections are combined into a circuit-stub section.  
         [0019]      FIG. 3  is a cross section of a printed circuit board showing several embodiments of via-stub sections terminated with discrete components.  
         [0020]      FIG. 4  is a cross section of a printed circuit board showing several embodiments of via-stub sections terminated with co-planar elements located on a conductive layer in a printed circuit board.  
         [0021]      FIG. 5  is a plan view showing via stub termination scheme for reducing signal distortions.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The embodiments discussed herein are illustrative of some examples of structures and methods. As these embodiments are described with reference to illustrations, various modifications or adaptations of the methods and/or specific structures described will become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the exemplary embodiments, and through which these teachings have advanced the art, are considered to be within the scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is understood that the present invention is in no way limited to only the exemplary embodiments and combinations illustrated herein.  
         [0023]     A typical PCB interconnect is comprised of more than one series conductive path. When routing an interconnect between PCB layers, a separate via must be used for each series conductive path.  FIG. 2  shows several embodiments.  FIG. 2  shows a cross section of a portion of a PCB located directly underneath a press-fit connector (only the pins  160  of the connector are shown for clarity). Interconnects designed to be routed as a single-ended microstrip or stripline transmission lines inside the PCB generally require a minimum of two vias, one for the signal path  210   a  and one for the signal return path  210   b . Interconnects configured as differential-pair microstrip or stripline transmission lines normally require at least three vias, two for the two differential signal paths  215   a - b  and one for the common mode return path, either  215   c  or  215   d . Often both vias  215   c - d  are used for the common mode return path of a differential pair in order to preserve electrical symmetry between the common mode and differential modes of the differential pair interconnect. It is also common practice to directly connect vias associated with return signals, for example  215   c - d , to all layers  220  that contain ground planes  205   a - d.    
         [0024]     The collection of via-stub sections associated with the plurality of vias used to route an interconnect between PCB layers forms a circuit-stub. Circuit-stub  225   a  is associated with the routing of an isolated (signal return not connected to all PCB layers containing ground planes) single ended interconnect  210   a - b  configured as a stripline transmission line using layers  100   a - 110   c . Circuit-stub  225   b  is associated with the routing of an edge-coupled differential pair  215   a - d  using layers  110   a - c  when the common mode return paths  215   c - d  are connected to all PCB layers  220  containing ground planes  205   a - d . For the embodiments shown in  FIG. 2 , the unterminated end of the circuit-stub sections occurs on layer  110   f.    
         [0025]      FIG. 3  shows how the circuit-stubs of  225   a - b  of  FIG. 2  can be terminated using discrete components mounted on the side where the unterminated end of the circuit-stub is located. For the embodiments shown, this is the PCB side which contains layer  110   f . Depending on the impedance looking into different portions of the circuit-stub  300 ,  305 ,  310 ,  315 ,  320 , and  325 , discrete terminating elements  330   a - f  are connected as required. For complex circuit-stubs consisting of a plurality of individual via stubs, an electromagnetic simulation program may have to be used to quantify the impedances looking into the different portions of the circuit-stub  300 ,  305 ,  310 ,  315 ,  320 , and  325 . For most commonly used interconnect geometries, a significant reduction in signal distortion and increase in useable bandwidth only require discrete resistors in locations  330   b  and  330   e . If additional performance improvements are required, then terminating elements in all locations  330   a - f  may be required. Additional performance improvements can also be obtained by substituting substantially resistive elements with a more complex circuit comprising of one or more discrete resistors, capacitors and inductors. Commonly used PCB component mounting and connection techniques including direct attachment to a via-stub section pad (shown) and the use of short traces to connect the components to the via-stub section pads (not shown) may be employed.  
         [0026]      FIG. 4  shows the preferred circuit-stub embodiment using planar passive terminations that are built into the PCB during manufacturing. The same general principles outlined for the discrete embodiment described in the previous paragraph also apply with discrete terminating elements  330   a - f  being replaced by planar passive terminating elements located at  400   a - f  on layer  110   f.    
         [0027]     The planar terminating elements do not have to be at the end of the unterminated circuit-stub end as long as the residual circuit-stub  410  is short enough so its impact on the interconnect&#39;s performance is minimal. For example, noticeable improvement in interconnect performance can still be attained if the terminating elements  405   a - f  are located on layer  110   e.    
         [0028]      FIG. 5  illustrates a magnified top view of a portion of a ground layer (for example, layers  110   e  and  110   f ) incorporating one of the impedance terminating elements  400   a - f  and  405   a - f  of  FIG. 4  as an annular resistor  510 , in one embodiment in accordance with the present invention. The annular resistor  510  is described in detail in U.S. Pat. Nos. 5,447,258, 5,604,847, and 5,108,569, assigned to Hadco Santa Clara, Inc., a wholly owned subsidiary of Sanmina-SCI Corporation; the disclosures of which patents are incorporated herein. The via  505   a  is at least partially surrounded by and coupled to an inner circumference of the annular resistor  510 . At least a portion of the outer circumference of the annular resistor  510  is coupled to the ground layer  500 . In some embodiments, the annular resistor  510  comprises a resistive material such as a polymer thick film material, a resistive thin film metal, a resistive metallic alloy, and/or a resistive organic material to form a resistor electrically coupled to both the ground layer  500  and the via  505   a.    
         [0029]      FIG. 5  also illustrates a terminating element  400   a - f  and  405   a - f  configured as a rectangular resistor  515 , in an alternative embodiment in accordance with the present invention. The rectangular resistor  515  is an alternative embodiment of the annular resistor  510 , as disclosed in the aforementioned patents incorporated by reference. The via  505   b  is at least partially surrounded by and coupled to an inner circumference of the rectangular resistor  520 . At least a portion of the outer periphery of the rectangular resistor  515  is coupled to the ground layer  500 . The rectangular resistor  515  comprises a resistive material such as a polymer thick film material, a resistive thin film metal, a resistive metallic alloy, and/or a resistive organic material to form a resistor electrically coupled to both the ground layer  500  and the via  505   b.    
         [0030]     As will be appreciated by those skilled in the art, terminating elements can be provided in various shapes to add in the manufacturing process and to tune their operating characteristics. By way of non-limiting example, a passive element can be a conic section such as a circle or an ellipse. By way of further non-limiting example, a passive element can be a polygon such as a square, rectangle, or octagon.  
         [0031]     In another embodiment the terminating element may comprise an area of resistive material  520  connecting two via pads together. Such a configuration would be used, for example for elements  400   b    400   e    405   b  and  405   e  in  FIG. 4 . The planar passive elements  510 ,  515  and  520  may be combined into one structure to create, for example, the terminating structures  400   a - c ,  400   d - f ,  405   a - c  and  400   d - f.    
         [0032]     Those skilled in the art will understand that there are numerous other shapes for the impedance terminating elements  510 ,  515  and  520 . For example, the terminating element may be oval or rounded rectangular. The rounded rectangular shape may improve processing of the layer such as etching of the layer  500  and deposition of the terminating element  510 ,  515 , or  520  in the etched portion of the layer  500 .  
         [0033]     While the exemplary embodiments described above include a single component as the terminating element, as used herein the “terminating element” also includes terminating circuitry. For example, a terminating element may include an RC circuit, an LC circuit, and RL circuit, an RLC circuit, etc. Furthermore, while the exemplary embodiments described above have passive components such as resistors, capacitors and inductors used in as the terminating element, other embodiments may include active components such as diodes or transistors.  
         [0034]     Although a preferred embodiment of the invention has 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 both in whole or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred version contained herein.