Abstract:
A circuit board includes a first circuit board section which is configured to support a set of light emitting diodes, a second circuit board section which is configured to support high-speed electronic circuitry, and a boundary section which defines an EMI boundary between the first circuit board section and the second circuit board section. The boundary section includes a set of outer surface elongated ground strips configured to form an EMI seal onto which a set of EMI shields is capable of mounting. In some arrangements, the set of outer surface elongated ground strips take the form of metallic bands (e.g., layers of copper). Such bands can be integrated into the circuit board itself (e.g., during PCB fabrication), or added onto one or more of the circuit board surfaces (e.g., after PCB fabrication).

Description:
BACKGROUND 
   A typical printed circuit board (PCB) includes multiple conductive and non-conductive layers which are sandwiched together into a rigid structure which is planar in shape. This rigid plane-shaped structure provides a dense network of electrical pathways which connect a variety of circuit board components together. Such components may include, among other things, integrated circuit (IC) devices (e.g., high-speed area array packages, field programmable gate arrays, serializer/de-serializer interfaces, processors, memories, etc.), discrete components (e.g., diodes, resistors, capacitors, light emitting diodes, etc.), connectors, and so on. 
   There are other items that interface with PCBs as well. For example, a light pipe is an elongated device having one end adjacent a light emitting diode (LED) and another end at a distal location from the LED which may be several inches away (e.g., at a front panel of a cabinet, at a display on an electronic housing, etc.). Such a light pipe is typically made of clear plastic or a similar material. During operation, the light pipe is configured to carry light from the LED to the distal location. 
   Another PCB item is an electromagnetic interference (EMI) shield. An EMI shield is a device which is configured to prevent passage of EMI from one location to another. One conventional EMI shield is in the form of a metallic chassis which completely surrounds a circuit board. Another conventional EMI shield is in the form of a metallic module cage which works, in combination with a circuit board, to enclose an individual circuit board component (e.g., a high-speed optical transducer). 
   SUMMARY 
   Unfortunately, there are deficiencies to the above-described conventional EMI shields. For example, in connection with the above-described conventional metallic chassis which surrounds the circuit board entirely, light from any LEDs on the circuit board will have difficulty being seen by a user. That is, air holes defined by the metallic chassis are typically too small to enable the user to reliability view the LEDs. Furthermore, if the manufacturer were to provide larger light pipe holes within the metallic chassis to enable light pipes to convey the LED light from the LEDs to external locations, the larger light pipe holes would impair the EMI shielding operation of the metallic chassis. 
   Additionally, in connection with the above-described conventional metallic module cage which protects an individual circuit board component, the metallic module cage consumes a significant amount of circuit board real estate compared to the amount of EMI shielding the cage provides. Moreover, mounting holes for such a module cage require additional space (e.g., the actual holes, keep out regions around the actual holes, etc.). Often the positioning of such holes is in critical areas such as where there is a high-density of signal traces leading to and from the circuit board component. Accordingly, if EMI protection for many components is required, the metallic module cage is an ineffective EMI solution. 
   In contrast to the above-described conventional EMI shielding approaches, an improved EMI shielding technique involves utilization of a boundary section of a circuit board which defines an EMI boundary between two other circuit board sections, e.g., one section that supports LEDs and another section that supports high-speed electronic circuitry in the form of multiple circuit board components. The boundary section includes a set of outer surface elongated ground strips (e.g., embedded outer surface metallic bands) configured to form an EMI seal onto which a set of EMI shields is capable of mounting. For such a situation, the set of EMI shields in combination with the boundary section will be able to provide reliable EMI shielding for circuitry requiring EMI protection (e.g., the high-speed electronic circuitry) while enabling other circuits to remain exposed (e.g., the LEDs) for more convenient access. 
   One embodiment is directed to a circuit board including a first circuit board section which is configured to support a set of light emitting diodes, a second circuit board section which is configured to support high-speed electronic circuitry, and a boundary section which defines an EMI boundary between the first circuit board section and the second circuit board section. The boundary section includes a set of outer surface elongated ground strips configured to form an EMI seal onto which a set of EMI shields is capable of mounting. In some arrangements, the set of outer surface elongated ground strips take the form of metallic bands (e.g., bands of exposed copper). Such bands can be integrated into the circuit board itself (e.g., during PCB fabrication), or added onto one or more of the circuit board surfaces (e.g., after PCB fabrication). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a perspective view of an electronic system which employs a circuit board having an EMI boundary section between two other circuit board sections. 
       FIG. 2  is a perspective view of a circuit board of the electronic system of  FIG. 1 . 
       FIG. 3  is a perspective view of the electronic system of  FIG. 1  from a rear angle with a top cover of an enclosure removed and with a row of light pipes of the enclosure remaining. 
       FIG. 4  is a detailed perspective view of a portion of the electronic system of  FIG. 1  from a front angle with the enclosure completely removed. 
   

   DETAILED DESCRIPTION 
   An improved EMI shielding technique involves utilization of a boundary section of a circuit board which defines an EMI boundary between two other circuit board sections, e.g., one section that supports LEDs and another section that supports high-speed electronic circuitry in the form of multiple circuit board components. The boundary section includes a set of outer surface elongated ground strips (e.g., outer surface metallic bands) configured to form an EMI seal onto which a set of EMI shields is capable of mounting. For such a situation, the set of EMI shields in combination with the boundary section will be able to provide EMI shielding for circuitry requiring EMI protection (e.g., the high-speed electronic circuitry) while enabling other circuits to remain outside the EMI barrier (e.g., the LEDs) for more convenient access. 
     FIG. 1  shows an electronic system  20  having an enclosure  22 , a circuit board module  24  (illustrated in  FIG. 1  generally by the arrow  24 ) and a set of EMI shields  26 . As shown, the enclosure  22  includes a base  28 , a cover  30 , and an assembly of light pipes  32 . The cover  30  has a transparent portion  34  (e.g., a user display) and an opaque portion  36 . As shown by the angle of  FIG. 1 , the assembly of light pipes  32  fastens to the cover  30 . In particular, the assembly of light pipes  32  is disposed adjacent the transparent portion  34  of the cover  30 . In some arrangements, the light pipes  32  are rigidly attached to the transparent portion  34  of the cover  30  (e.g., snap fit in place, fused using heat, etc.). 
   As will be explained in further detail shortly, the circuit board module  24  includes a circuit board  38  (illustrated by the arrow  38  in  FIG. 1 ), a set of LEDs  40  (illustrated generally by the arrow  40  in  FIG. 1 ) and high-speed circuitry  42  (illustrated generally by the arrow  42  in  FIG. 1 ). It should be understood that the high-speed circuitry  42  multiple packaged components (e.g., ASICs, processors, FPGAs, etc.), other electrical structures that potentially operate as antennae for high-speed electrical signals (wire leads, pins, connectors, etc.), and so on. Advantageously, however, the circuit board  38  includes an EMI boundary section between two other circuit board sections which forms a reliable EMI seal for the high-speed circuitry  42 . 
   During operation of the high-speed circuitry  42 , the set of LEDs  40  provides visual information to a user. In particular, the light pipe assembly  32  conveys light from the LEDs  40  of the circuit board module  24  to the transparent portion  34  of the cover  30 , and the transparent portion  34  permits light from the LEDs  40  to pass therethrough for visual detection by the user. In some arrangements, the assembly of light pipes  32  includes multiple light pipe rows  32 ( 1 ),  32 ( 2 ), . . . , and the circuit board module  24  includes multiple LED rows  40 ( 1 ),  40 ( 2 ), . . . corresponding to the light pipe rows  32 ( 1 ),  32 ( 2 ), . . . Such a configuration provides a configuration which enables the user to quickly and effectively determine the operating status of the electronic system  20 . Further details will now be provided with reference to  FIG. 2 . 
     FIG. 2  is a perspective view of the circuit board module  24 . As shown in  FIG. 2 , the circuit board  38  of the circuit board module  24  includes a circuit board section  50  which is configured to support the set of LEDs  40 , a second circuit board section  52  which is configured to support the high-speed electronic circuitry  42 , and a boundary section  54  which defines an electromagnetic interference boundary  56  between the circuit board sections  50 ,  52  along the Y-direction. The circuit board section  50  defines conductive circuit board structures  58  such as mounting locations for mounting the LEDs  40 , and signal traces and power supply planes for operating the LEDs  40 . Similarly, the circuit board section  52  defines conductive circuit board structures  60  such as mounting locations for mounting the high-speed circuitry  42 , and signal traces and power supply planes for operating the high-speed circuitry  42 . The boundary section  54  includes a set of outer surface elongated ground strips  62  configured to form an EMI seal. 
   The LED circuit board section  50  does not include any signal traces that purposefully carry high-speed signals of the high-speed circuitry  42 . That is, the LED circuit board section  50  excludes conductive structures carrying high-speed signals for the high-speed electronic circuitry  42  thus enabling the LED circuit board section  50  to be a minimal source, if any, of EMI. Of course, the LED circuit board section  50  does include signal traces and power and ground planes to operate the LEDs  42  but the signals through such conductive structures do not provide disruptive EMI. 
   As further shown in  FIG. 2 , the circuit board  38  is a substantially flat structure (i.e., the circuit board  38  extends in the X-Y plane) and has a top outer surface  64 (T) and a bottom outer surface  64 (B) (collectively, surfaces  64 ). The outer surface elongated ground strips  62  include a top outer surface metallic band  66 (T) disposed along the top outer surface  64 (T), and a bottom outer surface metallic band  66 (B) disposed along the bottom outer surface  64 (B) (collectively, metallic bands  66 ). Such metallic bands  66  are preferably formed by circuit board material (e.g., copper) embedded as circuit board layers within the circuit board  38 . 
   In addition to the surfaces  64 , the circuit board  38  has a front edge  68 (F), a right edge  68 (R), a left edge  68 (L) and a back edge  68 (B) (collectively, circuit board edges  68 ). The front edge  68 (F) extends along the circuit board section  50  which supports the LEDs  40 . The back edge  68 (B) extends along the circuit board section  52  which supports the high-speed circuitry  42 . A long axis  70  of the boundary section  54  runs substantially parallel to the front and back edges  68 (F),  68 (B) (in the Y-direction), and substantially perpendicular to the right and left edges  68 (R),  68 (L). In some arrangements, one or more rows of plated-through-holes (PTHs)  72  runs along the long axis  70  for convenient connection of the EMI shields  26  ( FIG. 1 ) to the circuit board  38 . Further details will now be provided with reference to  FIGS. 3 and 4 . 
     FIG. 3  is a perspective view of the electronic system  20  from a rear angle with the top cover  30  removed but with a row of light pipes  32 ( 1 ) remaining to illustrate the positioning of the light pipes relative to the circuit board module  24 .  FIG. 4  is a detailed perspective view from a front angle of the electronic system  20  with the enclosure  22  completely removed to illustrate further details of the set of EMI shields  26 . 
   As shown in  FIG. 3 , when the circuit board module  24  is installed within the base  28  of the enclosure  22 , the front edge  68 (F) is adjacent the location for the light pipe assembly  32  (e.g., see the light pipe row  32 ( 1 )) and also adjacent the location for the transparent portion  34  of the cover  30  (also see  FIG. 1 ). 
   The set of EMI shields  26  ( FIG. 1 ) includes a top EMI shield  26 (T) and a bottom EMI shield  26 (B). Both EMI shields  26 (T),  26 (B) mount to the circuit board  38  with direct physical and electrical contact (e.g., using screws, in a pin-in-hole press-fit manner, etc.). In particular, the top EMI shield  26 (T) connects to the top metallic band  66 (T) and conceals high-speed circuitry  42  at the top of the circuit board section  52 . Similarly, the bottom EMI shield  26 (B) connects to the bottom metallic band  66 (B) and conceals high-speed circuitry  42  at the bottom of the circuit board section  52 . 
   Although, the EMI shields  26  have air holes for ventilation, the air holes are sized to minimize escape of EMI from the high-speed circuitry  42 . In particular, the EMI shields  26  in combination with the boundary section  54  work together to form a Faraday cage around the high-speed circuitry  42  and the circuit board section  52 . Accordingly, when the EMI shields  26  are in operating positions, a robust EMI barrier exists between the two circuit board sections  50 ,  52 . Preferably, the metallic bands  66  extend edge-to-edge completely from the right edge  68 (R) to the left edge  68 (L) for a tight EMI seal (also see  FIG. 2 ). Nevertheless, the EMI shields  26  do not completely surround the circuit board  38  (i.e., the circuit board section  50  is not within the EMI shield) and the LEDs  40  remain outside of the EMI barrier thus enabling the light pipe assembly  32  to conveniently convey light from the LEDs  40  mounted to the circuit board section  50  to the transparent portion  34  of the cover  30  without degrading the operation of the EMI shields  26 . 
     FIG. 4  shows that the EMI shields  26 (T),  26 (B) are configured to engage each other and the circuit board section  50  to form a tight EMI barrier. Nevertheless, multiple rows  40 ( 1 ),  40 ( 2 ) of LEDs  40  disposed on respective axes  80 ( 1 ),  80 ( 2 ) which are substantially parallel to each other operate freely in an environment outside the EMI barrier. As a result, there is no need for any light pipe holes within the EMI shields  26 (T),  26 (B). Furthermore, the EMI shields  26 (T),  26 (B) in combination with the metallic bands  66  of the circuit board section  50  operate to provide substantial EMI isolation for the high-speed circuitry  42 . Such a situation is well-suited for a variety of electronic applications such as a data communications devices (e.g., a router performing high-speed routing and/or switching operations). 
   As described above, an improved EMI shielding technique involves utilization of a boundary section  54  of a circuit board  38  which defines an EMI boundary between two other circuit board sections  50 ,  52 , e.g., one section that supports LEDs  40  and another section that supports high-speed electronic circuitry  42  in the form of multiple circuit board components. The boundary section  54  includes a set of outer surface elongated ground strips  66  (e.g., outer surface metallic bands) configured to form an EMI seal onto which a set of EMI shields  26  is capable of mounting. For such a situation, the set of EMI shields  26  in combination with the boundary section  54  will be able to provide EMI shielding for circuitry requiring EMI protection (e.g., the high-speed electronic circuitry  42 ) while enabling other circuits (e.g., the LEDs  40 ) to remain exposed for more convenient access. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
   For example, it should be understood that the above-described techniques are capable of being implemented in an electronic system having a chassis with other EMI sealing features. In particular, U.S. patent application Ser. No. 11/252,116 entitled “CHASSIS WITH MULTI-CANTILEVER SPRING FINGERS FOR EMI SHIELDING AND ESD PROTECTION OF ELECTRONIC DEVICES”, the teachings of which are hereby incorporated by reference in their entirety, describes additional EMI protection features which are suitable for use with the electronic system  20  described above.