Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to circuit packs having components using electromagnetic interference (EMI) shielding and heat-dissipation enhancement. 
         [0003]    2. Description of the Related Art 
         [0004]    A circuit pack generally comprises a plurality of variously interconnected and physically proximate electronic components that are generally soldered to a base circuit board. As used herein, “circuit pack” refers to any configuration of one or more electronic components connected on a common substrate. These electronic components may include integrated circuits, analog devices, digital devices, and radio-frequency (RF) components. One or more electronic components may require an electromagnetic interference (EMI) shield, also referred to as an RF shield or a can. An EMI shield is useful for reducing electromagnetic interference caused by the shielded component and/or that can affect the shielded component. Thus, an EMI shield can be used to protect other components from EMI generated by the shielded component, and an EMI shield can be used to protect a component from externally-generated EMI. A simple EMI shield is typically made in the shape of an open-bottomed metal enclosure placed over the shielded component and attached to the base circuit board. 
         [0005]    As would be appreciated by one of ordinary skill in the art, there are many ways to form an EMI shield enclosure. For example, the EMI shield can be a unitary piece attached to a circuit board, as disclosed in U.S. Pat. No. 5,530,202 to Dais et al., incorporated herein by reference in its entirety. The EMI shield can also be formed by attaching a lid to a walled enclosure that was previously attached to a circuit board, as disclosed in U.S. Pat. Nos. 7,095,624 B2 to Daoud et al. and 7,113,410 B2 to Pawlenko et al., incorporated herein by reference in their entirety. 
         [0006]    The shielded component may get hot in operation and consequently may benefit from heat-dissipation enhancement to prevent overheating of the shielded component. The phrase “heat-dissipation enhancement,” as used herein, unless otherwise indicated, describes any means whose use increases, or is intended to increase, the heat-loss, or cooling, rate of a component. One way to cool the shielded component is with an airflow provided by a fan associated with the circuit pack. Circuit packs typically use one or more fans to provide cooling air for circuit pack components. In order for the cooling air to reach the shielded component, the EMI shield may be perforated. EMI-shield perforations whose diameters are at least about an order of magnitude smaller than the wavelengths of the EMI of concern generally do not degrade the shielding performance of the EMI shield. 
         [0007]      FIG. 1  shows prior-art EMI shield  102  on a partial circuit board  101 . EMI shield  102  is perforated to allow cooling air to reach a shielded component (not shown). Another way to provide thermal dissipation for the shielded component is to interpose a metal coil spring connected between the shielded component and the EMI shield. The metal spring conducts heat from the shielded component to the surface of the EMI shield from where it may be more easily dissipated. The spring is useful for maintaining contact between the shielded component and the EMI shield while allowing for physical-dimension variations due to (i) component dimensional tolerances and/or (ii) temperature-related expansion and/or contraction. 
         [0008]      FIG. 2  shows a cutaway view of exemplary prior-art heat-dissipating EMI-shielding box  200 . Box  200  comprises EMI shield  201 , which includes top section  202 . EMI shield  201  has perforations to allow for easier circulation of air within it. Top section  202  can be a lid that is attached after the rest of EMI shield  201  is mounted onto a circuit board (not shown), or top section  202  can be an integral part of EMI shield  201 , i.e., formed together with the rest of EMI shield  201  prior to mounting on a circuit board. Box  200  further comprises copper coil spring  203 , which is attached to copper slug  204 . Copper slug  204  is attached to top section  202 , for example, with a screw (not shown). When box  200  is in place in a circuit pack, then copper coil spring  203  is in contact with a shielded component (not shown) and is therefore compressed to an extent determined by, among other factors, the length of copper coil spring  203 , and the heights of the shielded component and of EMI shield  201 . Heat can therefore be conductively dissipated from the shielded component via copper coil spring  203 , copper slug  204 , and EMI shield  201 . 
         [0009]    As the operating frequencies of components keep increasing, their operating temperatures increase and novel means of heat dissipation for EMI-shielded components may be useful. 
       SUMMARY OF THE INVENTION 
       [0010]    In one embodiment, the invention can be an apparatus comprising a bristle pad having an anchor end and a free end, wherein the bristle pad comprises a plurality of flexible bristles. The bristle pad is adapted to be placed between an electronic component and an electromagnetic interference (EMI) shield for the electronic component, wherein the EMI shield provides EMI shielding for the electronic component. The plurality of bristles are adapted to provide a conductive thermal path between the electronic component and the EMI shield, and one or more of the flexible bristles are adapted to flex when the EMI shield is attached to a circuit pack. 
         [0011]    In another embodiment, the invention can be a method for providing a conductive thermal path between an electronic component and an electromagnetic interference (EMI) shield. The method comprises (i) placing a bristle pad between the electronic component and the EMI shield, wherein the bristle pad comprises an anchor end and a free end, the bristle pad comprises a plurality of flexible bristles, the plurality of flexible bristles provide a conductive thermal path between the electronic component and the EMI shield, and the EMI shield is adapted to provide EMI shielding for the electronic component, and (ii) attaching the EMI shield to a circuit pack, whereupon one or more of the flexible bristles flex. 
         [0012]    In yet another embodiment, the invention can be a method for operating a circuit pack comprising an electronic component. The method comprises (i) providing electromagnetic interference (EMI) shielding for the electronic component by an EMI shield attached to the circuit pack, and (ii) conducting thermal energy between the electronic component and the EMI shield using a bristle pad, wherein the bristle pad comprises an anchor end and a free end, the bristle pad comprises a plurality of flexible bristles, the plurality of flexible bristles provide a conductive thermal path between the electronic component and the EMI shield, and one or more of the flexible bristles are adapted to flex when the EMI shield is attached to the circuit pack. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. 
           [0014]      FIG. 1  shows an exemplary prior-art EMI shield on a circuit board. 
           [0015]      FIG. 2  shows a cutaway perspective view of an exemplary prior-art EMI shield with a copper coil spring. 
           [0016]      FIG. 3  shows a cross-sectional view of part of an exemplary circuit pack in accordance with an embodiment of the current invention. 
           [0017]      FIG. 4  shows a perspective view of part of an exemplary circuit pack in accordance with another embodiment of the current invention, wherein the circuit pack is shown in a pre-assembled stage. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Effective heat-dissipation enhancement may be provided for an EMI-shielded component by adhering a pad of metallic bristles to the shielded component, wherein the metallic bristles are long enough to stay in contact with the EMI shield while accounting for component tolerances and/or temperature-related expansion and/or contraction. Advantages of using a bristle pad may include the additional heat dissipation by the bristles themselves and compensation for variations in the gap between the EMI shield and the shielded component, and for potential minor variations in component surfaces. 
         [0019]      FIG. 3  shows a cross-sectional view of part of exemplary circuit pack  300  in accordance with an embodiment of the current invention. Circuit pack  300  comprises circuit board  301 , various components including component  303 , which requires EMI shielding, and EMI shield  302 . Circuit pack  300  further comprises heat-conductive bristle pad  304 , which is connected between shielded component  303  and EMI shield  302  so as to conduct thermal energy therebetween. 
         [0020]    Component  303  may require EMI shielding from externally-generated EMI, or component  303  may require EMI shielding to protect other components from EMI that component  303  generates, or component  303  may require EMI shielding for both purposes. Component  303  can, for example, be a transformer, a coil, an inductor, an integrated circuit, an amplifier, or a transistor. EMI shield  302  may enclose additional components requiring EMI shielding (not shown), as well as additional components that do not require EMI shielding (not shown). 
         [0021]    EMI shield  302  is made of a material, such as a metal, that is thermally conductive and provides EMI shielding. EMI shield  302  may be attached to circuit board  301  by soldering (e.g. using solder reflow), clasping, gluing, welding, adhering, bolting, using screws, or by any other suitable attachment means. EMI shield  302  may be of virtually any shape that substantially creates an enclosure when placed over shielded component  303 . A typical shape is substantially an open-bottomed rectangular box. EMI shield  302  may be perforated so as to allow cooling air to flow through the space inside EMI shield  302  and cool component  303  and/or bristle pad  304 . EMI shield  302  may also have additional perforations for other purposes and such perforations may be smaller or larger than the air-flow perforations and may even degrade the effectiveness of the EMI shielding. 
         [0022]    Bristle pad  304  has an anchor end and a free end. The anchor end attaches to component  303  with heat-conducting adhesive or thermal grease while the free end contacts EMI shield  302 . The free end of bristle pad  304  may also use thermal grease or heat-conductive adhesive. In one alternative embodiment, no thermal grease or adhesive is used, and instead, bristle pad  304  is held in place by other means, such as compressive pressure from component  303  and EMI shield  302 . In one alternative embodiment, the anchor end attaches to the EMI shield and the free end contacts the shielded component(s). 
         [0023]    Bristle pad  304  comprises a plurality of bristles, each bristle having an anchor end at the anchor end of bristle pad  304  and a free end at the free end of bristle pad  304 . Bristle pad  304  also comprises a support structure that holds together the bristles. The support structure may be a copper slug or a thinner conductive sheet at the anchor end of bristle pad  304 . Alternatively, the support structure may be of a thermal-insulating material placed towards, but not at, the anchor end of bristle pad  304 . The footprint of bristle pad  304  may approximate the top-surface shape of component  303 , or may have any other convenient or suitable shape. In one alternative embodiment, bristle pad  304  does not include a support structure to hold together the bristles; instead, the bristles are individually anchored at their anchor ends. 
         [0024]    The bristles of bristle pad  304  are made from a heat-conducting and elastic material, such as copper. When freestanding, such as prior to enclosure between component  303  and EMI shield  302 , the bristles of bristle pad  304  are substantially parallel to each other and are substantially similar to each other. In one alternative embodiment, the free end of bristle pad  304  is shaped so that the bristles are not substantially the same length, for example, so that bristles towards the middle of bristle pad  304  are shorter than bristles towards the periphery of bristle pad  304 . Once enclosed between component  303  and EMI shield  302 , the bristles, which are flexible, will flex to an extent determined by their attributes (e.g., composition, shape, length, cross-section) and the distance between component  303  and EMI shield  302 . Each bristle may be a cylinder whose cross-section may be circular, oval, or other geometric curve. In one alternative embodiment, each bristle may be frustum-like with the wider end as the anchor end and the narrower end as the free end. 
         [0025]    The free end of each bristle may be deformed or shaped for increased contact surface area with EMI shield  302  or to avoid plugging up perforations in EMI shield  302 . For example, the free ends of each bristle may be flattened. The length of each bristle is sufficient to maintain thermo-conductive contact between component  303  and EMI shield  302  despite variations due to physical-dimension tolerances and thermal-related expansions and contractions. In the alternative embodiment wherein bristle pad  304  is shaped so that the bristles are not substantially the same length, some of the bristles might not maintain contact between component  303  and EMI shield  302 . If, as in an alternative embodiment, bristle pad  304  is anchored to EMI shield  302 , then the length of the bristles should not be so long so as to potentially damage components or electrically short leads or traces on circuit board  301 . The diameter of each bristle should be small enough to allow relatively easy bending of the bristle when EMI shield  302  is placed over component  303  with bristle pad  304  in between. Typical diameters for copper-wire bristles may be in the 0.001-0.005 inch range. 
         [0026]      FIG. 4  shows a perspective view of part of exemplary circuit pack  400  in accordance with another embodiment of the current invention, wherein circuit pack  400  is shown in a pre-assembled stage. Circuit pack  400  comprises circuit board  401 , open EMI enclosure  402 , shielded component  403 , EMI shield lid  405 , and bristle pad  404 . Open EMI enclosure  402  encloses several components attached to circuit board  401 , including shielded component  403 . EMI shield lid  405  is shown prior to its installation atop open EMI enclosure  402  and shielded component  403 . EMI shield lid  405  is shown with its bottom side up to expose bristle pad  404 , the view of which would be substantially occluded after the installation of EMI shield lid  405 . Open EMI enclosure  402  and EMI shield lid  405  have perforations to allow the easier circulation of air within the area of EMI enclosure  402 . Bristle pad  404 , which is similar to bristle pad  304  of  FIG. 3 , is attached to EMI shield lid  405  at the anchor end of bristle pad  404 . When EMI shield lid  405  is installed atop open EMI enclosure  402  and shielded component  403 , then the free end of bristle pad  404  will be in contact with the top of shielded component  403 , thereby enhancing the dissipation of heat from shielded component  403 . 
         [0027]    The area, other dimensions, and location on EMI shield lid  405  of bristle pad  404  can be customized for the dimensions and location of shielded component  403 . Additional bristle pads (not shown) may be attached to EMI shield lid  405  to accommodate additional shielded components within open EMI enclosure  402 . In one alternative embodiment, the area of bristle pad  404  can allow for a “one size fits all” fit wherein the area of bristle pad  404  is sufficiently large to accommodate a variety of shielded components having a variety of areas, sizes, and/or locations. Thus, EMI shield lid  405  with “one size fits all” bristle pad  404  can be effectively used with multiple configurations of one or more shielded components within open EMI enclosure  402 . 
         [0028]    In one embodiment, an EMI shield, a shielded component, and a bristle pad are combined into an integrated shielded component wherein the integrated shielded component is attached as a single unit as part of a circuit pack. Such integration may provide enhanced EMI shielding as it may allow for shielding on the bottom of the component. However, such integration may also increase the complexity of the circuit pack as additional steps may be required for effective integration of the EMI shield, shielded component, and bristle pad into the integrated shielded component. 
         [0029]    It will be understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
         [0030]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0031]    Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. As used in this application, unless otherwise explicitly indicated, the term “connected” is intended to cover both direct and indirect connections between elements. 
         [0032]    The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures. Furthermore, the use of particular terms and phrases herein is for the purpose of facilitating the description of the embodiments presented and should not be regarded as limiting. 
         [0033]    References in descriptions of alternative embodiments to particular figures or previously-described embodiments do not limit the alternatives to those particular shown or previously-described embodiments. Alternative embodiments described can generally be combined with any one or more of the other alternative embodiments shown or described. 
         [0034]    Although the steps in the following method claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.

Technology Category: 5