Abstract:
A sheet metal cover for a printed circuit board (PCB) includes a plurality of legs continuous with a substantially planar elevated section. The legs are attached to the PCB, and electrical connections are provided between the legs and an internal ground plane of the PCB at the attachment locations. The sheet metal cover is thereby grounded, inhibiting the transmission of electromagnetic signals through the sheet metal cover. The elevated section of the sheet metal cover prevents select electronic devices on the PCB from being viewed or probed. Openings through the sheet metal cover allow heat sinks or heat generating electronic devices (e.g., inductors) to be exposed through these openings, thereby facilitating cooling of these elements by airflow. An electrically conductive gasket attached to the underside of the elevated section may contact the heat sinks, further minimizing the radiation of EMI emissions.

Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application 61/585,210, entitled “Printed Circuit Board Cover”, which was filed on Jan. 10, 2012, and is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and structure for reducing radiated emissions emanating from a printed circuit board. The present invention can be adapted to a printed circuit board (that might otherwise require significant re-design) to reduce radiated electro-magnetic emissions. The present invention also restricts probing/viewing the underlying components and circuitry to meet Federal Information Processing Standard (FIPS) requirements. 
     RELATED ART 
     Radiated emissions emanating from individual components on a printed circuit board have previously been reduced by constructing a conductive “equal potential” enclosure (often referred to as a Faraday enclosure) around the component, and bonding the enclosure to the underlying reference ground plane(s). The effectiveness of reducing the radiated emissions depends upon completeness of the enclosure and bond connections to the ground plane(s), which must increase as the emission frequencies of concern increase. 
     An example of this approach has been employed previously by first providing a perimeter ground ring on the component side of the outer surface of the printed circuit board around the radiating component. It is necessary that the ground ring have frequent via stitches to the underlying ground plane(s), so that high frequency currents have a relatively short return path. The number of via stitches required increases, and the spacing between them decreases, as the frequencies of concern increase. In this manner, a conductive enclosure is formed around all surfaces of the component. Most often, the radiating components have an associated heat sink, so a continuous conductive bond between the ground ring and the heat sink must be constructed. 
     As the complexity of printed circuit boards has increased along with higher operating frequencies and smaller electronic packages containing many more connection points, the increased density of traces has made the above-described enclosure technique more challenging to implement, so this technique is not often used today. It would therefore be desirable to have an improved method and structure for reducing radiated emissions from a printed circuit board. 
     The Federal Information Processing Standard (FIPS) defines varying levels of physical security for electronic modules, including for example, a level that includes requirements for evidence of physical tampering, and a level that includes requirements for physical tamper-resistance (making it difficult for attackers to gain access to sensitive information contained in the module). 
     One conventional method for providing FIPS protection includes milling a solid aluminum block to include a large cavity that is shaped to fit over the electronic devices mounted on the upper surface of a printed circuit board. The milled aluminum block is attached to the printed circuit board, wherein a peripheral boundary of the milled aluminum block is placed into electrical contact with a ground trace that is exposed at the upper surface of the printed circuit board. The ground trace typically has a width of at least about ⅛ inch, and is exposed around the entire periphery of the upper surface of the printed circuit board, thereby ensuring good electrical contact with the milled aluminum block. The ground trace must be coupled to all underlying ground planes of the printed circuit board by the previously discussed distributed stitching. By placing the milled aluminum block into contact with the external surface ground traces of the printed circuit board, the milled aluminum block is grounded, thereby providing protection for electromagnetic interference (EMI). However, making the ground trace wide enough to ensure good contact with the milled aluminum block undesirably consumes layout area on the printed circuit board. 
     When attached to the printed circuit board, the milled aluminum block completely encloses all electronic devices on the upper surface of the printed circuit board, such that these electronic devices cannot be viewed or probed by an attacker. The milled aluminum block may also physically cover switches that are mounted on the printed circuit board, thereby providing tamper resistance to these switches. However, the milled aluminum block does not does not allow any airflow to reach these enclosed electronic devices, thereby undesirably limiting the cooling of these devices. In addition, the milled aluminum block is expensive in terms of both material and fabrication costs. 
     It would therefore be desirable to have an improved method and structure for obscuring electronic devices on a printed circuit board, and reducing radiated emissions from the printed circuit board, which overcome the above-described deficiencies of the prior art. 
     SUMMARY 
     Accordingly, the present invention provides a sheet metal cover for a printed circuit board. The sheet metal cover of the present invention can be fabricated inexpensively using conventional sheet metal processing techniques. The sheet metal cover includes a plurality of legs that support a planar elevated section. The legs of the sheet metal cover are attached to the printed circuit board, whereby the planar elevated section is supported over electronic devices mounted on the printed circuit board. The planar elevated section inhibits viewing and probing of the underlying electronic devices. In one embodiment, attachment elements (e.g., screws/bolts) extend through openings in the legs and into mounting holes in the printed circuit board to physically attach the sheet metal cover to the printed circuit board. In this embodiment, the legs of the sheet metal cover are placed in electrical contact with conductive elements that surround the mounting holes and extend from an upper surface of the printed circuit board to the internal ground planes of the printed circuit board. These conductive elements are only required at discrete locations on the printed circuit board (i.e., where the legs of the sheet metal cover contact the printed circuit board), and therefore do not adversely impact the required layout area of the printed circuit board. By grounding the sheet metal cover in the above-described manner, radiated electromagnetic emissions are reduced. 
     In accordance with one embodiment, one or more openings can be formed through the planar elevated section of the sheet metal cover, thereby exposing select elements mounted on the printed circuit board. For example, an opening through the planar elevated section of the sheet metal cover can be provided such that a heat sink or a heat generating electronic component (e.g., an inductor), extends through the opening (without contacting the sheet metal cover). Exposing a heat sink or inductor through the sheet metal cover advantageously allows the heat sink/inductor to be cooled by an airflow introduced over the resulting structure. Note that an exposed heat sink or inductor does not generally provide access that can be exploited by an attacker. 
     In one embodiment, an electrically conductive gasket is attached to the underside of the planar elevated section of the sheet metal cover. The gasket may be attached to the sheet metal cover by an electrically conductive bonding material. In one embodiment, the gasket extends into the openings formed through the planar elevated section of the sheet metal cover, and contacts the associated heat sink(s) (or electronic component(s)). An electrically conductive bonding material may attach the gasket to the associated heat sink (or electronic component). In this manner, the heat sink (or electronic component) is electrically coupled to the grounded sheet metal cover by the gasket material, advantageously reducing radiated electromagnetic emissions from the printed circuit board. In addition, the gasket provides an additional physical barrier that prevents an attacker from viewing/probing in the spaces between the heat sink (or electronic component) and the sheet metal cover. 
     The present invention will be more fully understood in view of the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top view of a printed circuit board module in accordance with one embodiment of the present invention. 
         FIG. 1B  is a cross-sectional view of a mounting hole structure of the printed circuit board module of  FIG. 1A  in accordance with one embodiment of the present invention. 
         FIG. 2  is a top view of a sheet metal cover that is mounted over the printed circuit board module of  FIG. 1A  in accordance with one embodiment of the present invention. 
         FIG. 3  is a top view of an electrically conductive EMI gasket that is attached to the sheet metal cover of  FIG. 2  in accordance with one embodiment of the present invention. 
         FIG. 4  is a top view illustrating the gasket of  FIG. 3  attached to an underside of the sheet metal cover of  FIG. 3  in accordance with one embodiment of the present invention. 
         FIG. 5  is a top view that illustrates the sheet metal cover of  FIG. 2  and the gasket of  FIG. 3  attached to the printed circuit board module of  FIG. 1  and an underlying tray in accordance with one embodiment of the present invention. 
         FIG. 6  is a cross-sectional view taken along section line A-A of  FIG. 5 . 
         FIG. 7  is a top view of an electrically conductive EMI gasket, which can be used to replace the gasket of  FIG. 3  in accordance with an alternate embodiment of the present invention. 
         FIG. 8  is a top view that illustrates sheet metal cover of  FIG. 2  and the gasket of  FIG. 7  attached to the printed circuit board module of  FIG. 1  and an underlying tray in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a top view of a printed circuit board (PCB) module  100  in accordance with one embodiment of the present invention. In the described embodiments, PCB module  100  is a network switching device. However, it is understood that the present invention can be implemented with other types of PCB modules. PCB module  100  includes printed circuit board  101 , rear connector modules  102 - 107 , front connector modules  111 - 119 , top-surface mounted electronic devices  121 - 129 , bottom-surface mounted electronic devices  131 - 133 , internal ground plane  140 , mounting hole structures  141 - 149 , top-surface mounted heat sinks  150 - 155 , and top-surface mounted inductors  161 - 163 . 
     Rear connector modules  102 - 107  provide connections to traces within PCB  101 , and are configured to engage with external connector modules (not shown) at the rear end of PCB  101 . In one embodiment, rear connector modules  102 - 107  facilitate connections to a backplane and to one or more power supplies. Similarly, front connector modules  111 - 119  provide connections with traces within PCB  101 , and are configured to engage with external connector modules (not shown) at the front end of PCB  101 . In accordance with one embodiment, connector module  111  may provide a connection for a user interface, while connector modules  112 - 119  may provide interfaces for engaging Ethernet cables. 
     Electronic devices, such as ASICs, FPGAs and discrete electronic circuit elements, are mounted on the top and bottom surfaces of PCB  101  in a manner well known to those of ordinary skill. Electronic devices mounted on the top surface of PCB  101  are generally illustrated as squares having solid lines in  FIG. 1A , while electronic devices mounted on the bottom surface of PCB  101  are illustrated as squares having dashed lines. Although there are many electronic devices mounted on the top and bottom surfaces of PCB  101 , only top-surface mounted electronic devices  121 - 129  and bottom-surface mounted electronic devices  131 - 133  are labeled in  FIG. 1 . It is understood that heat sinks  150 - 155  are mounted on top of heat generating electronic devices (not shown in  FIG. 1A ), which are mounted on the top surface of PCB  101 . Inductors  161 - 163 , which generate significant amounts of heat during the normal operation of PCB module  100 , are also mounted on the top surface of PCB  101 . 
     Printed circuit board  101  includes internal ground planes  140 , which stabilize a ground supply voltage reference to the various electronic devices mounted on PCB  101  in a manner known to those of ordinary skill in the art. Internal ground plane  140  is located between the upper and lower surfaces of PCB  101  (i.e., is not exposed at the upper/lower surfaces of PCB  101 ). Mounting hole structures  141 - 149  provide electrical connections to internal ground plane  140  in a manner described in more detail below. 
       FIG. 1B  is a cross sectional view of mounting hole structure  141  in accordance with one embodiment of the present invention. It is understood that mounting hole structures  142 - 149  are substantially identical to mounting hole structure  141 . Mounting hole structure  141  includes a mounting hole  141 A that extends through PCB  101 , electrically conductive pads  141 B that surround the mounting hole  141 A on the upper surface of PCB  101 , and electrically conductive traces  141 C that extend through PCB  101  to connect pads  141 B to internal ground plane  140 . 
     As described in more detail below, a connector element (e.g., screw) is inserted through the mounting hole  141 A to attach a sheet metal cover  200  ( FIG. 2 , below) to the upper surface of PCB  101 . In this configuration, the sheet metal cover is placed into electrical contact with electrically conductive pads, thereby grounding the sheet metal cover  200 . 
     Note that mounting hole structures in addition to mounting hole structures  141 - 149  are included on PCB  101 , but are not labeled with reference numbers for reasons of clarity. In accordance with one embodiment, these additional mounting hole structures also provide electrical connections to internal ground plane  140 . 
       FIG. 2  is a top view of a sheet metal cover  200  in accordance with one embodiment of the present invention. Sheet metal cover  200  includes a substantially planar elevated section  201  and legs  211 - 219 . Mounting holes  241 - 249  are formed at the bottoms of legs  211 - 229 , respectively. Openings  250 - 251  and  261 - 263  are formed through elevated section  201  as illustrated. As described in more detail below, mounting holes  241 - 249  are aligned with mounting hole structures  141 - 149 , respectively, and connector elements (e.g., screws/bolts) are inserted to connect sheet metal cover  200  to PCB module  100 . When aligned in this manner, heat sinks  150 - 152  and electronic module  126  are exposed through opening  250 , and heat sinks  153 - 155  are exposed through opening  251 . As described in more detail below, heat sinks  150 - 155  extend through openings  250 - 251  without contacting sheet metal cover  200 . Similarly, inductors  161 - 163  are exposed through openings  261 - 263 , respectively. Electronic device  127  is also exposed through opening  252 . 
       FIG. 3  is a top view of an EMI gasket  300  in accordance with one embodiment of the present invention. Gasket  300  is constructed of a material that is electrically conductive. In one embodiment, gasket  300  is constructed of conventional fabric-over-foam EMI gasketing material (e.g., rip-stop nylon fabric with conductive nickel plated strands within the weave). Gasket  300  may alternately be constructed of other electrically conductive material such as beryllium copper (BeCu), sheet metal, copper tape or Mylar® coated/plated with an electrically conductive material. It is understood that other electrically conductive materials can be used to implement gasket  300  in other embodiments. Gasket  300  is dimensioned to fit under the elevated section  201  of sheet metal cover  200 . Gasket  300  includes openings  328 ,  350 - 352  and  361 - 363 . Generally, gasket  300  is attached to the underside of the elevated section  201  of sheet metal cover  200  by an electrically conductive adhesive (e.g., epoxy), such that openings  350 - 352  of gasket  300  are aligned with openings  250 - 252  of sheet metal cover  200 , respectively. Openings  361 - 363  of gasket  300  are also aligned with openings  261 - 263 , respectively, of sheet metal cover  200 . As described in more detail below, the opening  328  of gasket  300  is aligned with electronic device  128  on PCB  101  when sheet metal cover  200  is mounted on PCB  101 . 
       FIG. 4  is a top view that shows gasket  300  mounted underneath the elevated section  201  of sheet metal cover  200 , in accordance with one embodiment of the present invention. Note that gasket  300  is dimensioned such that portions of gasket  300  extend partially into openings  250 - 251  and  261 - 263 , as illustrated. That is, the openings  350 - 351  and  361 - 363  in gasket  300  are slightly smaller than the corresponding openings  250 - 251  and  262 - 263  in sheet metal cover  200 . 
       FIG. 5  is a top view that illustrates sheet metal cover  200  and gasket  300  attached to PCB module  100  and an underlying tray  500  in accordance with one embodiment of the present invention.  FIG. 6  is a cross-sectional view taken along section line A-A of  FIG. 5 . Note that  FIG. 6  illustrates the electronic device  129  upon which heat sink  154  is mounted. 
     Screws  541 - 549  are inserted through the mounting holes  241 - 249 , respectively, in sheet metal cover  200  and through the mounting hole structures  141 - 149 , respectively, of PCB  101 . Screws  541 - 549  engage with corresponding posts in the underlying tray  500 . For example, as illustrated by  FIG. 6 , screws  543 - 545  engage with posts  503 - 505 , respectively, of tray  500 . As a result, PCB module  100  is suspended over (and attached to) tray  500 . Screws  541 - 549  force sheet metal cover  200  into electrical contact with the electrically conductive pads of the mounting hole structures  141 - 149  of PCB  100  (e.g., pads  141 B of  FIG. 1B ), thereby electrically coupling the sheet metal cover  200  to the internal ground plane  140 . In one embodiment, tray  500  and the associated posts (e.g., posts  503 - 505 ) are electrically conductive (e.g., metal), and are also grounded. In one embodiment, screws  541 - 549  are also electrically conductive. Grounding sheet metal cover  200  in this manner advantageously reduces the radiated emissions exiting the smaller grounded cavities that are formed by the resulting assembly. 
     As illustrated by  FIG. 5 , heat sinks  150 - 152  and electronic device  126  are exposed through opening  250  of sheet metal cover  200  and opening  350  of gasket  300 , and heat sinks  153 - 155  are exposed through opening  251  of sheet metal cover  200  and opening  351  of gasket  300 . In accordance with one embodiment, the edges of gasket  300  that define openings  350  and  351  are placed in physical and electrical contact with associated heat sinks. For example, as illustrated by  FIGS. 5 and 6 , edges of gasket  300  are placed into contact with heat sinks  150 - 154 . An electrically conductive adhesive can be used to attach the edges of gasket  300  to the heat sinks  150 - 154 . As a result, the electrically conductive gasket  300  further reduces the transmission of electromagnetic energy through the cover. 
       FIG. 7  is a top view of an electrically conductive gasket  700 , which can be used to replace gasket  300  in accordance with an alternate embodiment of the present invention. Gasket  700  includes openings  750 - 755 , which are dimensioned and positioned to engage with each of the edges of heat sinks  150 - 155 , respectively. Gasket  700  is attached to the underside of sheet metal cover  200  in the manner described above, such that openings  750 - 752  are exposed through opening  250  in sheet metal cover  200 , and openings  753 - 755  are exposed through opening  251  of sheet metal cover  200 . 
       FIG. 8  is a top view that illustrates sheet metal cover  200  and gasket  700  attached to PCB module  100  and underlying tray  500  in accordance with the present embodiment. Electrically conductive gasket  700  laterally surrounds and electrically contacts each of the edges of heat sinks  150 - 155 , thereby further reducing the transmission of electromagnetic energy through the cover structure. Again, an electrically conductive adhesive can be used to connect gasket  700  to heat sinks  150 - 155 . 
     In the embodiment illustrated by  FIGS. 7 and 8 , gasket material is only present at locations near where the gasket  700  is placed into electrical contact with the heat sinks  150 - 155 . However, it is understood that in other embodiments, the coverage provided by gasket  700  can be extended. For example, the gasket  300  of  FIG. 3  can be modified by replacing the openings  350 - 351  with openings similar to the openings  750 - 755  of gasket  700 . 
     Returning now to  FIGS. 5-6 , inductors  161 - 163  are exposed through openings  261 - 263 , respectively, of sheet metal cover  200  and through openings  361 - 363 , respectively, of gasket  300 . Electronic device  127  is exposed through opening  252  of sheet metal cover  200  and opening  352  of gasket  300 . Electronic device  128  is exposed through opening  328  of the electrically conductive EMI gasket  300 . Exposing heat sinks  150 - 155 , inductors  161 - 163  and electronic devices  126 - 128  in the above-described manner advantageously facilitates the transfer of heat away from these elements. That is, airflow introduced across the resulting structure will advantageously provide direct cooling of heat sinks  150 - 155 , inductors  161 - 163  and electronic devices  126 - 128 . 
     Sheet metal cover  200  and gasket  300  physically cover selected electronic devices (e.g., electronic devices  122 - 125 ) on the upper surface of PCB module  100 , thereby preventing probing/viewing of these electronic devices. In general, devices that emit large quantities of heat, but do not expose sensitive signals (e.g., heat sinks and inductors) are exposed through openings of sheet metal cover  200 , while electronic devices that transmit or receive sensitive/critical signals (e.g., electronic devices  122 - 125 ) are covered by sheet metal cover  200 . 
     Note that while the lower surface of the gasket  300  is close enough to the underlying electronic devices to prevent probing/viewing of these devices, there is a small gap between the gasket  300  and these underlying devices, thereby allowing some airflow to reach (and cool) these electronic devices. 
     Sheet metal cover  200  is inexpensive to fabricate, using conventional sheet metal processing techniques (e.g., stamping and pressing). 
     As mentioned above, grounded sheet metal cover  200  advantageously limits electromagnetic radiation from PCB module  100 . In particular, the electronic devices  131 - 133  mounted on the lower surface of PCB  101  tend to emit electromagnetic signals, from the radiating component(s) on the surface of PCB  101  during normal operation of PCB module  100 . Grounded sheet metal cover  200  and gasket  300  significantly reduce the propagation of these electromagnetic signals, thereby reducing electromagnetic interference (EMI) with nearby devices/modules. 
     Although the present invention has been described in connection with several specific embodiments, it is understood that variations of these embodiments are considered to fall within the scope of the invention. For example, although the present invention has been described in connection with a sheet metal cover/gasket that covers/exposes certain portions of a particular PCB module, it is understood that the present invention can be readily modified to accommodate different PCB modules. Thus, the present invention is limited only by the following claims.