Abstract:
A technique to simultaneously reduce high-frequency insertion loss and cross-talk for a multi-layered add-in card is disclosed. The technique is based on selective removal of ground and power planes beneath the edge fingers. This selective removal of power and ground planes removes excess capacitance at the edge fingers, lowering the insertion loss at high frequencies, while maintaining an impedance match with an associated connector. Simultaneously, the leftover metallic ground/power plane provides electromagnetic shielding and thus reduces the cross-talk between the differential pairs. Optimum performance of the connector with minimized insertion loss and cross-talk can be obtained for high-speed analog and digital applications.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to motherboard add-in cards and, more particularly, to an add-in card used in high-frequency signaling operations.  
         BACKGROUND OF THE INVENTION  
         [0002]    Printed circuit boards (PCBs) are single or multiple-layered platforms upon or within which printed conductors and electrical components are mounted. The components are connected together by the printed conductors that are either embedded within layers of the PCB or etched upon its surface. In addition to providing support, the PCB is a thermal conductive path for heat dissipation and an insulator for the conductors.  
           [0003]    The PCB base or substrate is made from an insulator such as phenolic or fiberglass material. Electrical signals pass through the components along the conductors, which are made from copper or other highly conductive material. Some components are permanently affixed to the PCB, while others are connected using edge connectors, cartridges, and other support assemblies.  
           [0004]    A conventional add-in card is a PCB that inserts into a connector on a receiving PCB, such as a motherboard of a personal computer. A terminating edge of the add-in card is inserted into the connector. One or both sides of the terminating edge include several closely spaced conductive fingers, known as edge fingers, extending transversely along its length. Formed along with the conductors and component mounting pads, the edge fingers may be gold-plated or otherwise coated with a non-corrosive material, such that insertion and removal does not damage the edge fingers. The edge fingers are spaced apart and typically are raised from their respective outer layer slightly.  
           [0005]    The connector likewise includes plated surfaces against which the edge fingers make contact when the add-in card is inserted. The plated surfaces may flex to allow the edge fingers to frictionally engage with the connector. The connector may be permanent or non-permanent, such as to engage a removable add-in card. The terminating edge of the add-in card may be beveled to facilitate insertion into the connector. Like the motherboard, the add-in card may be single-sided, double-sided, or multi-layered.  
           [0006]    The PCB and the support assemblies are designed to ensure that the electrical signals pass from the PCB to the add-on component, and back, without interruption of the signal transmission. Each component has an electrical characteristic, known as impedance, with which signal loss can be predicted. Impedance is the opposition to the flow of electrical current by the component, expressed in Ohms. Ideally, the PCB and its components have impedance characteristics that are similar to the connector.  
           [0007]    Impedance mismatch can occur anywhere along a signal path, such as when the signal transitions between PCBs. Thus, for an add-in card connected to a motherboard, an impedance mismatch can occur when the signal passes from the motherboard to the connector, and again when the signal passes from the connector to the add-in card.  
           [0008]    Insertion loss is the attenuation of signals as they pass through the connector. For a typical 100 Ohm differential bus (50 Ohm single-ended), impedance mismatch causes insertion loss, which is a function of frequency, and which becomes more pronounced at higher frequencies. High-performance connectors have been developed to ensure that signals reaching add-in cards at these speeds avoid failures due to high insertion loss. It is well known, however, that the performance of the connector alone does not prevent such failures. Instead, how the signal path transitions between the motherboard and the add-in card greatly impacts the connector performance.  
           [0009]    In a multi-layered PCB, one or more layers, known collectively as reference layers, can be dedicated to power (power plane) or to ground (ground plane). Conventionally, the ground and power planes extend entirely throughout the add-in card, including the terminating edge. Sometimes, the ground/power planes adjacent to the edge fingers cause too much capacitance at the edge fingers, resulting in a severe impedance mismatch between the add-in card and the connector. The impedance mismatch can cause the signal to reflect back, resulting in signal distortion and possibly even data loss.  
           [0010]    One solution is to completely remove the ground and power planes underneath the edte fingers. This removes too much capacitance, however, and the impedance mismatch remains. Further, cross-talk between the signals may result. Cross-talk occurs when a signal from one input (or channel) crosses with a signal from another input (or channel). Cross-talk is particularly problematic in systems with high insertion loss, where those systems employ differential signaling.  
           [0011]    Differential signals, or differential pairs, are groups of signals that are routed together and are approximately the same length. The signal value on each path is the difference between the individual voltages on each path (in contrast to measuring the voltage against a ground voltage). Differential signaling may be preferred where the signal source is far from the signal receiver or where electromagnetic interference (EMI) is a problem, such as where high-frequency signals are transmitted.  
           [0012]    Add-in cards that transmit differential signals are particularly vulnerable to cross-talk, complicating the insertion loss problem. This is especially so when high-speed signals are transmitted through the add-in card. Thus, there is a continuing need to design an add-in card that minimizes insertion loss and crosstalk, particularly when high frequency signals are passing through the connector. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1A is a perspective view of a system including a motherboard, a connector, and an add-in card according to one embodiment of the invention;  
         [0014]    [0014]FIG. 1B is a detailed view of the edge fingers of the add-in card of FIG. 1A according to one embodiment of the invention;  
         [0015]    [0015]FIG. 2 is a detailed view of the terminating edge of the add-in card of FIG. 1A in which the non-grounded edge fingers are differential pairs according to one embodiment of the invention;  
         [0016]    [0016]FIG. 3 is an exploded view of an add-in card according to the prior art;  
         [0017]    [0017]FIG. 4 is an exploded view of an add-in card in which the ground and power planes are removed at the terminating edge according to the prior art;  
         [0018]    [0018]FIG. 5 is an exploded view of the add-in card of FIG. 1A in which the ground and power planes are selectively removed according to one embodiment of the invention;  
         [0019]    [0019]FIG. 6 is a cross-sectional view of the add-in card of FIG. 5, including ground and power planes, according to one embodiment of the invention;  
         [0020]    [0020]FIG. 7 is a bottom view of the add-in card of FIG. 5 according to one embodiment of the invention; and  
         [0021]    [0021]FIG. 8 is an exploded view of an add-in card in which a single grounded edge finger separates differential pairs, according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    In accordance with the embodiments described herein, a multi-layered add-in card is disclosed in which the ground and/or the power plane are removed under selected edge fingers. The ground and power planes extend to the terminating edge beneath the remaining edge fingers. The selective removal of the ground and power planes reduces excess capacitance at the edge fingers, minimizing the possibility of increased insertion loss at high frequencies while both maintaining an impedance match with an associated connector and simultaneously minimizing cross-talk between the edge fingers. The performance of the connector is thus optimized.  
         [0023]    In the following detailed description, reference is made to the accompanying drawings, which show by way of illustration specific embodiments in which the invention may be practiced. However, it is to be understood that other embodiments will become apparent to those of ordinary skill in the art upon reading this disclosure. The following detailed description is, therefore, not to be construed in a limiting sense, as the scope of the present invention is defined by the claims.  
         [0024]    In FIG. 1, according to one embodiment, a system  100  includes a connector  30 , a motherboard  50 , and an add-in card  20  in which ground and/or power planes have selectively been removed. The system  100  may be part of a typical processor-based system, such as a computer, a communications device, or other PCB-based platform, in which signals transmit between the motherboard and the add-in card through the connector.  
         [0025]    The add-in card  20  and motherboard  50  are conventional multiple layer PCBS. Motherboard, as used herein, refers to any PCB including a connector for receiving another PCB. Add-in card refers to any PCB that is connectable to another PCB by engaging with a connector. These terms are commonly used by those of ordinary skill in the art and are not meant to limit the scope of the claims.  
         [0026]    The motherboard  50 , also known as the main circuit board, generally houses a central processing unit, memory, and other logic, and may be coupled to a power supply, a video display, a mass storage device, and so on. The add-in card comprises a component layer  22 , upon which circuitry may be disposed, a ground plane  24 , a power plane  26 , and an end layer  28 . The ground and power planes are collectively known as reference planes. The end layer may also be a component layer.  
         [0027]    The multi-layered add-in card  20  includes a terminating edge  42 , which is that portion of the add-in card to be received by the connector  30 . Typically, each layer of the add-in card is the same shape and size, coupled in a sandwich arrangement, such that, for any circuit on the component layer  22 , the ground plane  24  is adjacent to the circuit. Likewise, for a circuit on the end layer, the power plane  26  is adjacent to the circuit.  
         [0028]    The add-in card includes edge fingers  40  extending transversely across the terminating edge of the component layer  22  and the end layer  28 . The edge fingers  40  are plated extensions that connect to signal pads and/or circuitry on the add-in card and across which signals transmit between the add-in card and the motherboard. The connector includes plated surfaces  32  for making contact with the edge fingers when the add-in card is inserted.  
         [0029]    A closer examination of the edge fingers  40  (see FIG. 1B) reveals alternating pairs of grounded  44  and non-grounded edge fingers  46 . A pair of grounded edge fingers is next to a pair of non-grounded edge fingers, which is likewise next to a second pair of grounded edge fingers, and so on. The grounded edge fingers are connected to the ground plane  24 , such as by connecting to ground pads on the component layer  22 .  
         [0030]    In one embodiment, the non-grounded edge fingers  46  connect to differential signal pads, as depicted in add-in card  20   a  of FIG. 2. A typical add-in card may have 4, 8, or 16 differential pairs on each outer layer. The differential signals of the component layer  22  are denoted using “+” and “−” in FIG. 2. The edge fingers  40  include alternating pairs of grounded edge fingers  44  and differential pairs  48 . The ground plane  24  is adjacent to (or behind) the component layer and extends to the terminating edge  42 . The power plane  26  is similarly adjacent to the ground plane, followed by the end layer.  
         [0031]    As used herein, a portion of the add-in card or one of its layers is “aligned with” a second portion of the add-in card if a cross-sectional cutout of the add-in card including the one location also includes the other location. Typically, some portion of the ground plane  24  is aligned with a location on one of the other layers, as is some portion of the power plane  26 .  
         [0032]    Although not visible in FIG. 2, the end layer  28  also includes alternating pairs of grounded edge fingers and differential pairs. In one embodiment, the differential pairs of the component layer are not aligned with the differential pairs of the end layer. In FIG. 3, for example, the four layers of an add-in card  80 , depicted separately, illustrate the non-alignment of differential pairs. Differential pairs on the component layer  22  are aligned with grounded pairs on the end layer  28 . Likewise, differential pairs on the end layer are aligned with grounded pairs on the component layer. The ground plane  24  and the power plane  26  extend to the terminating edge of the add-in card.  
         [0033]    In some environments, the ground and/or power planes along the terminating edge of the add-in card cause too much capacitance for the edge fingers. This causes a severe impedance mismatch with the connector  30 . The impedance mismatch may lead to poor insertion loss response at high frequencies. In one embodiment, the measured insertion loss response of a connector with full ground and power planes in the add-in card increased after about 3 GHz.  
         [0034]    One solution is to completely remove the ground and power planes at the terminating edge  42 , i.e., underneath the edge fingers  40  of the add-in card. Instead of having the ground and power planes extending entirely through the add-in card, the ground plane is stopped before the terminating edge, as depicted in the add-in card  82  of FIG. 4.  
         [0035]    The ground plane extends from the top of the add-in card to the terminating edge. Likewise, a power plane  26  extends from the top of the add-in card to the terminating edge. Neither the ground plane nor the power plane is aligned with the edge fingers.  
         [0036]    The removal of the ground and power planes at the terminating edge of the card significantly reduces excess capacitance and thus decreases insertion loss at the connector. However, additional problems arise. For one, removal of the ground planes underneath the edge fingers increases the cross-talk between signals on the opposite sides of the card and connector, particularly where high-speed signals are transmitted.  
         [0037]    Further, by eliminating capacitance on the add-in card, an impedance mismatch between the card and the connector may nevertheless occur. Such mismatches are likely to result in signal reflection, distortion, and even data loss. Only the excess capacitance of the add-in card needs to be removed, no more and no less. Depending on the connector and add-in card stack-up design, completely removing the ground and/or power planes underneath the edge fingers eliminates too much capacitance. This causes an impedance discontinuity because the impedance of the modified add-in card is now higher than the impedance of the associated connector.  
         [0038]    Thus, according to one embodiment, a portion of the ground and power planes are selectively removed from the add-in card  20 , as shown in FIG. 5. Rather than completely removing the ground and power planes underneath the edge fingers, only the ground and power planes underneath the differential pairs are partially removed.  
         [0039]    As in prior examples, the ground plane of add-in card  20  is adjacent to the component layer, the power plane, and the end layer. In FIG. 5, instead of extending across the length of the add-in card, the ground plane  24  has cut or omitted portions  52  that correspond to the differential pairs  48  of the component layer  22 . The ground plane is substantially not aligned with the differential pairs. However, the ground plane remains aligned with the grounded edge fingers  44 .  
         [0040]    Likewise, the power plane  26  includes cut or omitted portions  54  that correspond to the differential pairs  48  of the end layer  28 . The power plane is thus substantially not aligned with the differential pairs along the end layer, but is aligned with the grounded edge fingers thereon. Both the ground and power planes continue to extend across the remainder of the add-in card.  
         [0041]    The ground and power planes service both the component layer and the end layer, which may itself be a component layer, of the add-in card. Ground pads on either the component layer or the end layer can be connected to the ground plane, such as by vias interspersed cross-sectionally through the add-in card. Likewise, signal pads on either outer layer can be connected to the power plane.  
         [0042]    [0042]FIG. 6 depicts a cross-section of the add-in card  20 , as indicated by line  60  of FIG. 5. The cross-section line  60  is taken between grounded edge fingers  44  of the component layer  22  and between differential pairs  48  of the end layer  28 . At the terminating edge  42 , the ground plane  24  extends the entire length of the add-in card, but, because of the differential pairs on the end layer, the power plane stops at the terminating edge.  
         [0043]    [0043]FIG. 7 is a bottom view of the add-in card  20  of FIG. 5. Differential pairs alternate with ground pairs, as shown. The ground plane is removed only where aligned with the differential pairs on the component layer. Where aligned with differential pairs on the end layer, the ground plane remains in place.  
         [0044]    Likewise, the power plane is removed only where aligned with the differential pairs on the end layer. The power plane remains aligned with differential pairs on the component layer. Thus, the ground and power planes on the opposite side of the add-in card adjacent to each differential pair remain intact.  
         [0045]    In one embodiment, the selective removal of the ground and power planes greatly decreases the electromagnetic coupling between the differential pairs on the component layer, particularly as compared to completely removing the ground and power planes at the terminating edge. This improves connector performance, particularly where high-speed signals are transmitted, according to one embodiment. The selective removal of the ground and power planes reduces the edge finger capacitance without increasing the cross-talk between high-speed signals on opposite sides of the card.  
         [0046]    In one embodiment, the size of the ground plane that is removed is variable, according to such factors as the size of the board, the speed of the signaling operations to be performed, the width of the edge fingers, the stack-up design of the add-in card, and other considerations. The ground plane is removed to optimize both the insertion loss and cross-talk, based on the electrical modeling of the connector and add-in card.  
         [0047]    The selective, partial removal of the ground and power planes underneath the edge fingers does not increase the card fabrication cost, in one embodiment. The selective removal technique can be directly applied to the add-in card during manufacture, thus providing a low-cost way to improve the connector performance.  
         [0048]    In FIG. 5, two grounded edge fingers are positioned between each differential signal pair. There are two grounded edge fingers to separate the adjacent differential pairs on each side of the connector and card. A shift of these differential pairs by two edge fingers on the opposite side of the card, such that differential pairs are not directly opposite one another, can be made. A similar adjustment of the connector can likewise be made.  
         [0049]    It is possible, however, to have other variations of the illustrated scheme. For example, two adjacent grounded edge fingers, separating the adjacent differential pairs, may be combined together to form single “fat” grounded edge fingers. Or, as shown in FIG. 8, a single grounded edge finger  44   b  may separate differential pairs  48  rather than a pair of grounded edge fingers. Some of omitted portions  52  of the ground plane are aligned with omitted portions  54  of the power plane. In either variation, according to one embodiment, selective removal of the ground and power planes was effective to improve insertion loss of the connector. Other variations of the described grounded and non-grounded edge finger configuration are possible without departing from the spirit of the invention.  
         [0050]    By selectively removing the ground planes underneath some of the edge fingers in an add-in card, the insertion loss of the connector can be improved. Further, by not removing the ground plane entirely at the terminating edge, the cross-talk performance of the add-in card can be optimized. The selective, partial removal of the ground planes underneath the edge fingers is not likely to increase the card fabrication cost. Since selective removal of the ground planes can be achieved during manufacture of the add-in card, this technique provides a low-cost way to improve the performance of the connector.  
         [0051]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.