Abstract:
A shielded riser card assembly for reducing electromagnetic radiation from a computer enclosure is disclosed. The riser card assembly comprises a four-layer riser card having a connector adjacent its lower edge, such as a connector for an NLX system board. A cable connection socket is on the riser card a first distance away from the connector, and a plurality of traces on a surface of the riser card run between the cable connection socket and the connector. A sheet of conductive material covers the plurality of traces and is spaced a second distance apart from the surface of the riser card. At least one fastener connected is to the sheet and attached to the riser card. The fastener conductively connects the sheet of conductive material to a ground layer of the riser card. A plurality of non-conductive spacers are disposed between and in contact with both of the sheet and the riser card, for maintaining the sheet a predetermined distance away from the riser card.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to reducing electromagnetic radiation emanating from computer enclosures, and particularly, from enclosures for relatively inexpensive, mass-produced computer systems employing printed circuit board (“PCB”) technology. 
     2. Description of Related Art 
     As a by-product of normal operation, electronic equipment frequently emits undesirable electromagnetic radiation, often referred to as electromagnetic interference or “EMI.” At the same time, equipment specifications require a maximum acceptable level of EMI outside of an electronics enclosure, to comply with government regulations and other performance requirements. For computer systems, EMI requirements are generally complied with by enclosing the computer system in an enclosure made of metal or other conductive material. Openings in the enclosure may be covered with a metallic grill or mesh screen, and the enclosure as a whole constitutes a “Faraday cage.” To maintain a perfect Faraday cage over a wide bandwidth, no apertures above a specified size may exist in the shield. The higher the frequency of the EMI, the smaller the shield apertures should be. 
     In practice, computer enclosures contain some apertures that are not covered by grills or screen, because of penetrations for connectors, openings for insertions of items such as floppy disks, and assembly joints, among other things. Despite openings such as these, EMI requirements for most computer systems have been easily complied with in the past using relatively simple, low-cost enclosures. But the advent of modern computer systems has made compliance with EMI requirements more difficult, particularly for lower-end computer systems that are sold in an extremely cost-competitive market. 
     That is, it is generally more difficult to meet EMI requirements for modern systems without undesirable cost increases in the enclosures or other system components. Various changes in the industry underlie these new difficulties. For one thing, the increasingly high frequency of commonly available microprocessors, especially above about 500 megahertz, means that computer enclosures must be better sealed against transmission of RF radiation. The higher-frequency microprocessors emit EMI at higher frequencies, which, in turn, can emanate from an enclosure through smaller openings than EMI at lower frequencies. And as frequencies and edge rates in high-speed digital designs continue to increase, EMI as a result of radiation through slots, apertures, and seams in shielding enclosures is becoming increasingly problematic. There is sufficient energy at low-order clock harmonics to cause EMI problems above a few hundred MHz as a result of exciting cavity modes of the enclosure, and efficiently driving even small length slots and apertures that are unavoidable in a practical design. 
     Consolidation among manufacturers, and the drive towards cost reduction in the computer industry in general, also plays a role. To build a computer system at a competitive cost, a computer manufacturer typically will include certain components that are only available as stock items from a limited number of manufacturers. Peripheral DVD drives are an example of a typical stock item made by relatively few manufacturers. The computer manufacturer often has no direct control over the particular configuration of these stock items. Consequently, a particular stock component that is otherwise desirable may provide a pathway for EMI, particularly at high frequencies, to radiate from a computer enclosure. This pathway may be blocked by customization of the component, but customization of a stock item can add substantially to the system cost. 
     In particular, certain peripheral components, such as CD and/or DVD drives, essentially include a rectangular-tubular metallic enclosure, much like a rectangular cross-section waveguide, that is not shielded from transmission of EMI at one or both ends, and are generally mounted inside computer system enclosures with one unshielded end passing through the computer enclosure. For example, in disk drives such as CD, DVD, and floppy disk drives, the disk door on the exterior of the computer enclosure is frequently made of a nonconductive plastic material that does not block transmission of EMI. At the same time, EMI may enter the opposite end of the peripheral enclosure via a cable connection or opening. Hence, the peripheral enclosure can form an efficient waveguide for transmission of EMI at certain frequencies to the exterior of the computer enclosure. For example, certain modern CD/DVD peripherals provide an efficient waveguide for transmission of EMI at frequencies in the range of about 800-1000 MHz. Other components and compartments within computer enclosures may also act as waveguides at these and other frequencies, depending on the details of the component and its relationship to other components of the computer system. 
     One approach for eliminating the waveguide effect of peripherals such as disk drives is to construct the opening door of a metallic or conductive material. This approach is likely to add to the cost of the component, and is not effective when the peripheral door is open. Another approach is to employ multiple ground points for the peripheral component, but this approach may increase assembly cost, and multiple ground points are subject to being disrupted during repair or replacement of the peripheral component. 
     A computer enclosure may contain other EMI openings which, for one reason or another, are difficult to block in a reliable, relatively permanent, and low-cost manner. It is desired, therefore, to provide an alternative method and apparatus for preventing EMI transmission through such openings in computer enclosures, including but not limited to openings created by disk drive peripheral components, that overcomes the limitations of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a shielded riser card assembly for reducing electromagnetic radiation from a computer enclosure, that overcomes the limitations of the prior art. The riser card assembly comprises a four-layer riser card having a connector adjacent its lower edge, such as a connector for an NLX system board. A cable connection socket is on the riser card a first distance away from the connector, and a plurality of traces on a surface of the riser card run between the cable connection socket and the connector. A sheet of conductive material covers the plurality of traces and is spaced a second distance apart from the surface of the riser card. At least one fastener connected is to the sheet and attached to the riser card. The fastener conductively connects the sheet of conductive material to a ground layer (i.e., a signal return layer) of the riser card. A plurality of non-conductive spacers are disposed between and in contact with both of the sheet and the riser card, for maintaining the sheet a predetermined distance away from the riser card. The sheet and spacers may be attached together to form a separate auxiliary shield assembly when detached from the riser card. 
     The riser card and auxiliary -shield assembly may be adapted for application to a computer enclosure enclosing at least one source of electromagnetic radiation of a computer system, where the enclosure includes an unshielded opening in the computer enclosure through which an undesirable amount of electromagnetic radiation from the source is capable of passing to an exterior of the computer enclosure. 
     Generally, the riser card and auxiliary shield assembly is for computer systems having a system board mounted to a frame of the computer enclosure, and a riser card connected to the system board within the computer enclosure. The riser card is a PCB having at least one ground layer and a signal layer, and extends transversely from the system board. The riser card divides an interior space of the computer enclosure into two compartments, a source compartment containing the source of electromagnetic radiation, and an unshielded compartment having the unshielded opening in it. The ground layer of the riser card is positioned towards a side of the riser card facing the unshielded compartment. The riser card and auxiliary shield assembly may be used in cooperation with a sheet metal barrier attached to the frame of the computer enclosure and covering an area around a perimeter of the ground layer. The riser card and auxiliary shield assembly and the sheet metal barrier-together form an EMI shield between the source compartment and the unshielded compartment, whereby the undesirable amount of electromagnetic radiation from the source is prevented from passing to an exterior of the computer enclosure. Compartmentalization by the riser card may be most conveniently accomplished in computer systems constructed according to the NLX form factor, in which the prior art frame components and riser card provide a partial but nearly complete physical barrier between two internal compartments of the system. 
     In some configurations, the riser card may have traces on a side facing the source compartment. These traces may act as antennae for receiving electromagnetic radiation from the source compartment and conducting it to a cable socket on the side of the riser card facing the unshielded compartment. A cable connected to the socket may then transmit the radiation to a device such as a CD/DVD drive having an unshielded opening to the exterior of the enclosure. Accordingly, in an embodiment of the invention, the traces on the source side of the riser card are covered by interposing an auxiliary sheet metal shield between the riser card and the source compartment. The auxiliary shield may be attached to the riser card and connected to its ground layer by fasteners, while being spaced a short distance apart from it by a plurality of spacers. The shield prevents the traces from receiving electromagnetic radiation from the source compartment. 
     In an alternate embodiment, the riser card itself is configured to prevent the traces from receiving radiation from the source compartment. A second ground layer is provided in a layer of the riser card, interposed between the at least one signal layer and the source compartment. The riser card may contain six or more layers. The second ground layer covers the traces and thereby prevents them from receiving source radiation. 
     A more complete understanding of the shielded riser card assembly for reducing electromagnetic radiation emanating from a computer enclosure will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the invention. Reference will be made to the appended sheets of drawings which will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified perspective view of an exemplary computer system as adapted according to the invention, with the top cover removed. 
     FIG. 2 is a plan view of the system shown in FIG.  1 . 
     FIG. 3 is a simplified perspective view of an exemplary frame for a computer enclosure for use with the invention, showing the relationship of respective frame members. 
     FIG. 4 is a plan view of an exemplary riser card for use with the invention. 
     FIG. 5 is schematic diagram showing selected components of a riser card for use with the invention. 
     FIG. 6 is a schematic diagram showing selected components of a riser card according to an alternative embodiment of the invention. 
     FIG. 7 is a schematic diagram representing the interior of a computer enclosure for an exemplary system configured according to the invention. 
     FIG. 8 is an schematic diagram representing the interior of a computer enclosure for an exemplary system configured according to an alternative embodiment of the invention. 
     FIG. 9A is a perspective view of a riser card shield according to an embodiment of the invention. 
     FIG. 9B is a perspective view showing a reverse side of the shield shown in FIG.  11 A. 
     FIG. 10 is a perspective view of a riser card shield according to an alternative embodiment of the invention. 
     FIG. 11 is a simplified perspective view showing exemplary interior components of a system configured according to the invention. 
     FIG. 12 is a simplified perspective view showing exemplary interior components of a system configured according to an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention satisfies the need to reduce electromagnetic radiation emanating from computer enclosures without requiring expensive redesign and remanufacture of conventional components. The invention is especially suitable for use with modern, relatively inexpensive computer systems employing high-frequency (&gt;500 MHz) microprocessors, for which compliance with governmental EMI regulations is required. The invention can be applied to computer systems and enclosures commonly in use today for a relatively insignificant additional cost. In the detailed description that follows, like element numerals are used to describe like elements shown in one or more of the drawings. 
     In general, the invention is useful for partitioning the interior space of a computer enclosure into two primary compartments that are electromagnetically isolated at the frequencies of interest. Partitioning can provide several benefits. Primarily, it can isolate a noise source from electronics in one compartment from an exterior aperture in another that could transmit EMI from the enclosure. Secondarily, because the resonant frequency of a cavity is proportional to the dimensions of the cavity, each of the smaller compartments will have a higher resonant frequency than the unpartitioned interior space. The compartment resonance may thus be moved beyond certain desired test frequencies. Third, because the stored energy/losses ratio is proportional with the volume/surface ratio of the cavity, each of the smaller compartments will have a lower quality factor, and the maximum field inside will be smaller. 
     In the present invention, one of the compartments, the source compartment, contains the primary source or sources of electromagnetic radiation. The other compartment, the unshielded compartment, contains any components that are difficult to completely isolate from the exterior of the computer enclosure, such as removable disk drive peripherals. It should be appreciated that the term “unshielded compartment” is intended to include compartments that are partially unshielded from the exterior of the computer enclosure. In practice, partial shielding of the unshielded compartment is still desirable. 
     An exemplary low-cost computer system  100  for use with the invention is shown in FIG.  1 . Certain details have been omitted for illustrative clarity. A plan view of system  100  is shown in FIG.  2 . The computer enclosure  102  includes any suitable metal cover (not shown) as known in the art. Typically, a suitable metal cover is configured to slide over the sides and top of the enclosure frame  104 , but any other suitable cover may be used. Enclosure  102  is shown with the cover removed to reveal its interior, but it should be appreciated that reduction of EMI from system  100  requires that a suitable cover be in place on frame  104 . Frame  104  holds the components of system  100  together and provides support for the metal cover. 
     Faceplate  112 , typically made of a molded plastic material, is attached to the front end of frame  104  and may include one or more apertures that lead into the enclosure  102 . For example, apertures for disk drives may be protected by doors  166 ,  164  for dust control, but these doors are typically made of a plastic material that does not provide any EMI abatement. Hence, at certain frequencies EMI may radiate from the enclosure  102  via doors  164  and/or  166 . 
     System  100 , as exemplified in FIGS. 1 and 2, is laid out according to the NLX form factor, as known in the art. NLX is a recognized layout specification for computer systems that is often adopted for modern low-cost, mass-produced systems. One of the primary features of the NLX form factor is a dockable motherboard (system board) that slides into the enclosure  102  and docks with the riser card  132  through a side access port in side  106 a of the enclosure (see FIG.  3 ). The riser card  132  is affixed in a demountable but less conveniently accessed manner to the frame  104 , by attachment to the interior midframe  114 . Riser card  132  extends transversely from the docked system board  128 , and carries sockets for peripheral cards that may be added to system  100 , as well as connectors for peripheral components such as disk drives. In essence, the NLX form factor reverses the older practice of designing systems in which the system board is the component that is least easily removed from the enclosure, and the peripheral riser cards are plugged into sockets on the system board. 
     The interior of enclosure  102  includes two primary compartments  160 ,  162  divided by middle frame member (“midframe”)  114  and riser card  132 . Source compartment  160  contains the system board  128 , which, in turn, carries the microprocessor  130  and other semiconductor devices that may emit electromagnetic radiation into compartment  160 . Source compartment  160  is configured as a Faraday cage that is sufficiently sealed to prevent excessive EMI from source compartment  160  from reaching the exterior of enclosure  102 . For example, source compartment may be configured to comply with United States Federal Communications Commission (“FCC”) regulations for EMI, using techniques as known in the art for selection of enclosure materials and maximum allowable aperture size in the exterior walls of compartment  160 . FCC specifications currently apply up to the frequency of the 5 th  harmonic of the highest clock speed. For example, if the highest test frequency is 2 GHz, this corresponds to a recommended maximum aperture dimension “L” of 0.75 cm, by application of the {fraction (1/20)} th  rule (i.e., L=λ/20). In the alternative, the source compartment  160  may be similarly configured to comply with any other applicable standard or specification for EMI abatement. 
     Another feature of the NLX form factor is the location of all drive bays for disk drives, such as removable disk drive  140 , on a side of the riser card opposite from the side into which the system board  128  is docked. The space between the riser card  132  and the side wall  106   b  of the enclosure forms a compartment  162 . Conveniently, this location in compartment  162  permits the riser card  132  to serve as a partial EMI shield between the system board  128  and the disk drives, such as drives  140 , but the NLX specification does not describe a method for a completely effective EMI barrier between the source compartment  160  and compartment  162 . In conventional NLX systems, any shielding provided by the riser card  132  is incidental, and is insufficient to prevent EMI at high frequencies from escaping through the disk drive apertures or other openings in compartment  162 . 
     As described above, complete EMI shielding of compartment  162  is hindered by the use of stock disk drive components, such as drive  140 , that cannot easily be altered to provide adequate EMI shielding. Hence, compartment  162  is described herein as an unshielded compartment. Compartment  162  may contain other components, such as a power supply  150  and a sealed hard disk drive (not shown). It does not contain any significant sources of EMI except such as may be contained in, and shielded by, components within the compartment  162 , such as power supply  150  and disk drive  140 . 
     In conventional NLX systems, the midframe  114  is configured primarily as a structural member of frame  104  and as a mountable support for the riser card  132 . According to the invention, the midframe  114  may be additionally provided with a sheet metal shield  118  (shown in FIG. 3) that surrounds a perimeter of a defined portion or portions of the riser card  132 . The sheet metal shield  118  may be formed integrally with midframe  114 . An exemplary configuration of such a sheet metal shield  118  is described later in the specification. The riser card  132  and midframe  114  may thus be positioned relative to one another such that the conductive ground layer of the riser card and midframe  114  cooperate to close all apertures of concern between compartments  160 ,  162 . For example, those apertures larger than the maximum recommended size may be closed by positioning a properly configured midframe and riser card. The ground layer of riser card  132  may be oriented towards compartment  162 . 
     The NLX standard further provides for riser card  132  to have peripheral port sockets on its side facing the unshielded compartment  162 . A suitable cable, such as ribbon cable  156 , may be plugged into the riser card and connected to a corresponding peripheral component, such as removable disk drive  140 , in the unshielded compartment  162 . The peripheral port sockets on the riser card  132  are connected to traces on the side of riser card facing the source compartment  160  by vias that pass through the riser card. Undesirably, these traces may then act as antennae to receive electromagnetic radiation from the source compartment, and to conduct the EMI signal through cable  156  directly into drive  140 . Drive  140 , in turn, may serve as a waveguide to efficiently drive certain EMI frequencies through door  164 . To prevent this phenomena, the invention provides that the traces on riser card  132  may be shielded from the source compartment  160  by a suitable auxiliary shield  200  that is mounted to riser card  132 . Exemplary auxiliary shields for riser cards are described later in the specification. 
     Additionally, midframe  114  may be provided with an EMI gasket  194  along its top edge to ensure that the EMI barrier provided by the riser card  132 , midframe  114 , and auxiliary shield  200  is preserved. Various suitable EMI gaskets are commonly available, comprising a row of regularly spaced resilient conductive fingers  196  that extend uniformly from the gasket. The EMI gasket  194  ensures that conductive contact and a known maximum aperture size is established between the top cover (not shown) of the enclosure and the midframe  114 . Without EMI gasket  194 , dimensional tolerance buildup or flexure of the top cover may open up apertures between the midframe  114  and the top cover that exceed the maximum recommended aperture size, thereby degrading the EMI shielding effect provided by the midframe  114 /riser card  132  assembly. 
     Because of the location and configuration of riser card  132 , the invention is conveniently applied to NLX systems, but is not limited to such systems. The invention may be applied to any system in which a transverse riser card is available between the EMI sources on the system board and a portion of the enclosure that contains unshielded openings. 
     An exemplary frame  104  for a computer enclosure according to the invention is shown in FIG.  3 . Details of the frame may vary within the scope of the invention. The midframe  114  extends between the back frame  110  and the front frame  111 . The front frame  111  contains openings  154   a ,  154   b  for drive bays on one side of the midframe  114 , and may contain one or more screened ventilation ports  152  on the other side of the midframe. Back frame  110  contains a back panel opening for the back panel of the system board, which may mount to rails  124  on bottom  108 . Side frames  106   a ,  106   b  connect the back frame  110  to the front frame  111  at the opposite sides of frame  108  and support the corners of the enclosure cover. 
     The shape of the midframe  114  is of particular importance. Generally, the midframe has at least a primary opening  120  that is slightly smaller than the riser card that will be mounted to it. Mounting holes  116  correspond to mounting holes  142  in the riser card  132  (shown in FIG.  4 ). Sheet metal shield  118  generally fills the remainder of the area in the plane of midframe  114 , providing an EMI barrier between the two compartments. However, the sheet metal shield may contain other relatively small openings, such as opening  122 , for ventilation or cabling feed-through. The size of opening  122  should be below the maximum recommended aperture size. In the alternative, opening  122  may be located in a position that, because of the configuration of interior components, does not lead to transmission of EMI from the unshielded compartment. Other details of frame  104  may be as known in the art. 
     An exemplary riser card  132  for use with an NLX system is shown in FIG.  4 . Generally, the features of the riser card may be as known in the art. Four mounting holes  142  are in the four corners of the card for mounting to midframe  114 . Riser connector  134  is for connecting to the system board. Peripheral port sockets  138   a-c  are on the reverse side of the riser card, and are connected by traces  136  on the system side of the card to the riser connector  134 . 
     Certain features of the prior art riser card  132 , such as ventilation holes  144  and a notched top edge  146 , may be undesirable. Ventilation holes and other holes, if present, should be below the maximum recommended aperture size. A straight upper edge  148 , corresponding to the top edge of opening  120  in midframe  114 , may be more suitable than a notched edge. Generally, it may be desirable to separately ventilate the two compartments of the system, instead of relying on openings in the riser card and/or midframe to provide cross-ventilation. 
     The EMI-shielding properties of the riser card  132  arise from its ground layer that is generally coextensive with the perimeter  168  of riser card  132 . FIG. 5 shows the ground layer  174  and other components of a conventional four-layer riser card  170  in an exploded schematic cross-sectional view. It should be appreciated that the riser card may include other layers, such as resin layers, that are not shown. The view is taken through the location of the peripheral port sockets  138   a-c  to show an exemplary configuration of the traces  136  of a signal layer. Ground layer  174  may be positioned between other layers  172 ,  176 , such a power layer and/or a signal layer, which may be in any order as known in the art. Traces  136  face the source compartment and pass through vias in the riser card  170  to connect the peripheral port sockets  138   a-c  in the unshielded compartment to the riser connector  134 . Hence, traces  136  may receive electromagnetic radiation from the source- compartment and transmit it to the ports  138   a-c  in the unshielded compartment. 
     To prevent this transmission, a six-layer riser card  180  may be provided, as shown in FIG.  6 . The sockets  138   a-c , layers  172 ,  176 , ground layer  174 , and traces  136 , may be as previously described, and other layers such as resin layers are not shown for purposes of simplicity. An additional layer  178  may be disposed adjacent to the signal layer (which includes traces  136 ), and a second ground layer  182  is disposed over the traces  136 . Traces  136  may pass through vias in the layers  172 - 178  and ground layer  182  to connect to riser connector  134 , and are generally shielded from the source compartment. Layers  172 - 178  may be of any type, such as signal, power, ground, or combination, and may be in any desired order. Additionally, the signal layer containing traces  136  need not be immediately adjacent to ground layer  182 , nor is ground layer  182  required to be an outermost layer. Ground layer  182  covers a sufficient portion of any traces (such as traces  136 ) that may be connected to ports  138   a-c  or any other device on the unshielded side of card  180  to prevent transmission of undesired EMI to the unshielded compartment. For example, ground layer  182  may cover substantially all of such traces, or a majority of such traces, depending on the application requirements. Whatever the specific arrangement of layers, ground layer  182  is positioned to be interposed between the source compartment and any such traces that may be connected to any device on the unshielded side of riser card  180  when it is mounted in its intended position in a computer enclosure. Accordingly, riser card  180  will no longer transmit EMI to the unshielded compartment, and no local auxiliary shield is needed. A further advantage of the second ground layer  182  is that transmission of EMI to other components that may be connected to the riser card may also be avoided, because all of the signal layer or layers may be readily shielded from the source compartment. 
     The interior of a computer enclosure  102  for an exemplary NLX system configured according to the invention is shown in FIG.  7 . Midframe  114 , including sheet metal shield  118 , and riser card  132  divide the interior space into two primary compartments  160 ,  162 . Source compartment  160  contains system board  128  and microprocessor (source)  184 . Unshielded compartment  162  contains a peripheral drive  140  connected by a cable to socket  138   a . An auxiliary shield  200  is attached to riser card  132  over the traces connecting riser connector  134  to the sockets, such as socket  138 a, in the unshielded compartment  162 . In the alternative, no auxiliary shield  200  is used, and the riser card is a six-layer card like riser card  170  described above. 
     The invention is not limited to use with NLX systems. FIG. 8 shows an exemplary non-NLX system configured according to the invention. A PCI-type riser card  186  and other riser cards plug into sockets  192  on system board  128 . A cable  156  connects drive  140  to a socket on card  186 . A sheet metal barrier  188  may be placed adjacent to riser card  186  to isolate an unshielded compartment  162  from source compartment  160 . Barrier  188  may be designed to shield in cooperation with the riser card  186 , i.e., it may have an primary opening somewhat smaller than the area of the riser card, similar to the sheet metal barrier.  118  described above. In the alternative, barrier  188  may have no openings except as are needed for passing cables or for ventilation. Such openings should be smaller than the maximum recommended aperture size. Barrier  188  may also include an EMI gasket (not shown) as previously described for midframe  114 . Traces on card  186  that are exposed to source compartment  160  may be covered with a local auxiliary shield  190  similar to shield  200  described herein. 
     Barrier  188  can also be used for systems in which peripheral drives are connected to sockets on the system board, such as by cable  156 ′. These systems may required local shielding of traces on the system board  128  that lead to sockets for the drive cables. A shield similar to shields  190 ,  200  may be used to shield critical areas of a system board, preventing transmission of EMI through cable  156 ′. 
     An exemplary local auxiliary shield  200  for a riser card is shown in FIGS. 9A and 9B. Selection of shield materials and design of shield thickness may be as known in the art, and shield  200  may be inexpensively stamped from any suitable stock material. More highly conductive metals, such as copper or aluminum, may be especially suitable to shield against high frequency EMI from the system microprocessor. The sheet metal  202  of shield  200  is shaped according to the particular configuration of the riser card that it is designed for. For example, the shield may include openings or notches  208   a-c  to accommodate sockets or other protruding features of the riser card. The shape of shield  200  is otherwise determined by the shape of the area on the riser card that is to be covered. Shield  200  need not be larger than needed to cover all of the traces on the source side of the riser card leading to the peripheral drive sockets. Little or no benefit is derived from an oversized shield. On the other hand, it may not be necessary to completely cover the traces on the riser card, depending on particular system characteristics. 
     Shield  200  may include through holes  207 , each for one of fasteners  206  for attaching the shield to a riser card. Fasteners  206  may be threaded fasteners such as machine screws designed for threaded inserts (not shown) in the riser card. The threaded inserts in the riser card may be made of a conductive material and mounted in the riser card such that they are in contact with the ground layer of the riser card. Shield  200  may then be conveniently grounded by attaching it to the riser card using metallic screws. In the alternative, and less preferably, the shield may be grounded by connecting one or more wire grounds to the shield and to suitable ground points on the system board. 
     Shield  200  may include spacers  204  to set it off from the riser card. Each spacer  204  may be pressed into a mounting hole or otherwise affixed to shield  200 . When shield  200  is attached to a riser card, spacers  204  maintain a uniform spacing between the shield and the riser card, and prevent the shield from contacting traces or other components on the riser card. The spacers should not be too high (such as less than about 0.5 cm) so as to hold the shield close to the riser card. Spacers  204  may be made of a non-conductive or a conductive material, if desired. If a spacer is positioned where it may contact a trace or other element of the riser card (besides a ground layer), it should be made of a non-conductive material. If a spacer is positioned where it may contact a ground layer of the riser card, it may be useful to make the spacer of a conductive material, to provide another grounding point. Both conductive and nonconductive spacers may be used on a single shield  200 . 
     The auxiliary shield need not be flat, and can be bent to accommodate contours of the riser card or midframe. FIG. 10 shows one such riser card shield  210 , according to an alternative embodiment of the invention. Shield  210  is similar to shield  200  previously described, and additionally includes a lip  212  designed to fit over an upper edge of the riser card. The top surface  194  of the lip  212  may include an EMI gasket having a row of resilient conductive fingers  196  positioned to be level with a similar EMI gasket on an upper surface of the midframe. The lip  212  may hang over the midframe and help to stabilize the riser card prior to attaching the riser card to the midframe. Lip  212  may also provide improved shielding of the peripheral port sockets adjacent to a top edge of the riser card. 
     An exemplary application of riser card  180  in a system  220  without a local auxiliary shield is shown in FIG.  11 . Most of the enclosure  102  and frame  104  have been removed to reveal the interior components. Source  130  (microprocessors) are mounted on system board  128  in source compartment  160 . System board  128  is connected to six-layer riser card  180 , having two ground layers on either side of a signal layer. Riser card  180  extends transversely from system board  128  and together with midframe  114  and its sheet metal barrier  118  blocks electromagnetic radiation from entering unshielded compartment  162 . An EMI gasket  194  with a row of resilient fingers  196  is disposed along a top edge of the midframe  114  ensure conductive contact between a cover of the enclosure and the midframe  114  at intervals no greater than the maximum recommended aperture size. Drive  140  in compartment  162  is connected to riser card  180  via cable  156 . Traces  136  connected to cable  156  are covered by the second ground layer  182 , and thereby cannot transmit EMI to drive  140 . 
     An exemplary application of a riser card  170  in an EMI barrier using a local auxiliary shield  210  is shown in FIG.  12 . Riser card  170  is a conventional four-layer type card with traces exposed to the source compartment. Local shield  210  is fastened against the riser card and covers the traces. Other details of system  230  are similar to system  220  shown in FIG.  11 . 
     Having thus described an embodiment of the shielded riser card assembly for reducing electromagnetic radiation from a computer system, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. For example, embodiments have been described in systems having the NLX form factor, but the invention is not limited to NLX systems. For further example, specific shapes of local auxiliary shields have been illustrated, but it should be apparent that the auxiliary shield may be formed in a great variety of other shapes and configurations without departing from the scope of the invention. The invention is further defined by the following claims.