Patent Publication Number: US-2006002099-A1

Title: Electromagnetic shield assembly

Description:
BACKGROUND  
      Electronic circuit components may be surrounded by shields, or covers, to suppress dangerous or disruptive electromagnetic (EM) radiation created by electronic circuit components at communication frequencies, including radio frequencies. In some environments, electronic components may be enclosed in some form of conductive cover that is connected to a circuit ground. An EM shield may be a solid metal housing or lid shaped to create a chamber enveloping an electronic circuit. EM shields have been developed for use in compact electronic environments that include numerous electronic components on a substrate. In such compact environments, the electronic components may be difficult to isolate from one another using individual encapsulating EM shields. However, shields interfere with communication between the various shielded electronic circuits. In such compact environments it there is a need to provide both effective EM shielding and proper inter circuit communication while maintaining the compact size that is desired in an increasing number of electronic devices.  
     BRIEF SUMMARY OF THE DISCLOSURE  
      An EM shield assembly may include an EM shield, as well as a conductive assembly capable of conducting electrical current as part of a circuit. The EM shield assembly may include a top and sides shaped to form a chamber capable of enclosing one or more circuit assemblies. The EM shield may be formed of laminate, or created by combining multiple layers of materials. Such a laminate EM shield may include conductive strips and vias extending along one or more dielectric layers so as to form the conductive assembly. Some examples, EM shields may alternatively or additionally include a layer of resistive material, such as a resistive film, to provide damping of electronic resonances.  
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  is a plan view of a circuit structure including an EM shield assembly mounted on a substrate.  
       FIG. 2  is a cross section taken along line  2 - 2  of  FIG. 1 .  
       FIG. 3  is a cross section taken along line  3 - 3  of  FIG. 2 .  
       FIG. 4  is a cross section taken along line  4 - 4  of  FIG. 2   
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS  
       FIGS. 1-4  depict a simplified exemplary circuit structure  8  including an EM shield assembly  10 , or cover having various features illustrated in a single composite embodiment for convenience. These features may have various forms, and may be realized in other shield assemblies individually or in various other combinations. As used herein, an EM shield assembly may house one or more circuit assemblies  12  that may individually or in combination form one or more complete circuits, one or more portions of one or more circuits, one or more combinations of elements or components of a circuit, or any combination of circuits, circuit portions, and circuit components, and may include shared circuit portions or components.  
      In this example, then, EM shield assembly  10  may be mounted to a substrate  14 . EM shield assembly  10 , also referred to as an enclosure, may include an electromagnetically conductive layer forming a shield  16  having a top  20 , and sides  22 , such as sides  22   a,    22   b,    22   c,  and  22   d,  forming an enclosed chamber  26 . As seen in the cross-section of  FIG. 2 , the EM shield assembly may be a laminate, or may be composed of multiple layers. Conductive exterior layer  16  may extend along the entire exterior of the EM shield assembly. Shield  16  may be made of an electromagnetically (including electrically or magnetically) conductive material, such as aluminum, copper or other metal, and may be formed of a combination of materials, at least one of which is conductive, such as a conductive layer with a non-conductive (dielectric) or semi-conductive material. Directly adjacent to the shield  16  may be dielectric body  17 . Dielectric body  17  may be, further arranged in one or more layers.  
      The shield  16  may also include one or more openings or cutouts, such as cutout  24 , to allow the passage of electric current, such as in the form of signals or power, into or out of the chamber  26  without substantial compromise to the shielding. While only one cutout is illustrated in this example it should be appreciated that EM shield  16  may include more than one cutout and that such cutouts may be located on the top or any side of shield assembly  10 . As will be seen, a conductor  33 , shown mounted on substrate  14 , may pass through cutout  24  to electrically connect circuit assembly  12  with other circuit assemblies or components located outside of the EM shield assembly. Conductor  33  may be a wire, a microstrip line, or any other configuration capable of conducting electrical current.  
      Shield  16  may protect circuit assemblies, such as assembly  12 , enclosed in chamber  26  from environmental and electromagnetic influences and/or isolate the enclosed circuit assemblies. While not shown, a shield  16  may further include an interior wall that can separate chamber  26  into more than one sub-chamber that may be capable of isolating two or more circuit assemblies.  
      Circuit assembly  12  may include various components. For purposes of illustration, circuit assembly  12  may include respective circuit elements  30  and  32  connected by a suitable interconnect, such as by a bond wire  34 . Bond wire may be connected to circuit element  30  by connection to a terminal  31  positioned on the circuit element. Circuit element  30  may include a lumped or distributed element, or combination or network of passive and/or active elements, such as transmission lines, resistors, capacitors, inductors, and semiconductor devices, and may be mounted on a circuit chip having a dielectric, semiconductive or conductive substrate. Furthermore, the circuit assembly  12  may include one or a combination of diodes and transistors in an integrated circuit (IC) or chip, including, for example, a monolithic microwave integrated circuit (MMIC), application specific integrated circuit (ASIC), or the like. For purposes of illustration, circuit element  32  may be an electrical conductor for transmitting a signal or power relative to circuit element  30 .  
      As seen in  FIG. 2 , the EM shield  16  may be mounted directly to substrate  14  at one or more points. In this example, substrate  14  is conductive and provides a ground for the shield. Other forms of substrate may be used, such as a dielectric with one or more conductive layers. Accordingly, shield  16  may be directly attached to substrate  14  using a conductive adhesive  36 . Conductive adhesives may include conductive epoxy, conductive pads, solder, brazing material, deformed metal, z-axis conducting elastomer, or any similar conductive or resistive material. Further, there may be one or more type of conductive adhesive used in the mounting of EM shield assembly  10 . It should be noted that shield side  22   c  may not be directly attached to substrate  14  in the area of cutout  24 .  
      Optionally, EM shield  16  may include one or more electrical ground connectors (not here shown) for grounding the shield  16  to the local circuit ground. These ground connectors may be in the form of metal strips extending from one or more portions of the shield into the substrate, into an adjacent EM shield, or to whatever ground connection is available.  
      Conductors  32  and  33  may be mounted onto substrate  14  using insulating layers  38 . The insulating layers may be in the form of an insulating epoxy or other adhesive, or an insulating pad. These insulating layers isolate the conductors from the conductive substrate. Circuit element  30  of circuit assembly  12  may require that it be grounded, in which case the backside of circuit element  30  may be attached to substrate  14  using conductive adhesive  36 .  
      Shield assembly  10  may further include a conductive assembly  39  supported by a dielectric body  17  relative to shield  16 . Conductive assembly  39  may provide a continuous electrical path  41  through the shield assembly. Path  41  may have branches, and multiple paths, whether adapted to carry signals or power. Components of the conductive assembly may extend along a surface of, or be embedded in, dielectric body  17 . Dielectric body  17  may be a single layer of dielectric or a plurality of layers of dielectric, which layers may or may not be separated by one or more layers of non-dielectric material. In this example, dielectric body  17  includes first and second dielectric layers  42  and  46 . The first and second dielectric layers  42  and  46  may be continuous along EM shield  16 , extending along the EM shield top  20  and sides  22   a,    22   b,    22   c,  and  22   d.  Dielectric body  17  may also extend only over one or more portions of shield  16 . In some areas, the first and second dielectric layers  42  and  46  may merge into one layer, unseperated by any non-dielectric material. The second dielectric layer  46  may have an interior face  48  that defines chamber  26 . Although dielectric layers  42  and  46  are shown extending across the top and down the sides of the EM shield assembly, separate dielectric layers may be used, for instance, to form the portion of the dielectric body making up the sides.  
      Conductive assembly  39  may include a conductive strip  44  that may extend through or on dielectric body  17 . In the configuration shown, the conductive strip  44  is sandwiched between first dielectric layer  42  and second dielectric layer  46 . Conductive strip  44  may be composed of any suitable conductive materials including the conductive metals discussed above.  
      The EM shield sides  22   a  and  22   c  may include one or more vias  50 , such as vias  50   a  and  50   c,  that extend between the first and second dielectric layers  42  and  46 . The vias may be formed by drilling, etching, or otherwise creating respective elongated via holes  52 , such as holes  52   a  and  52   c.  Such via holes, or tunnels, may extend between the first and second dielectric layers  42  and  46 , or extend through an individual layer. These via holes may then be filled with electromagnetically conductive material to form the vias.  
      Vias  50   a  and  50   c  may extend the entire height of sides  22   a  and  22   c,  as shown. Shield assembly  10  may have a lower edge  54  in contact with substrate  14 . Vias  50  a and  50   c,  accordingly have lower ends positioned near lower edge positions  54   a  and  54   c  of the shield assembly, and extend up to contact conductive strip  44  at respective intersection points  56   a  and  56   c.  The lower ends of the vias may be connected to signal conductors  32  and  33 , respectively, by any suitable means, such as the use of conductive adhesive  36 .  
      Vias  50   a  and  50   b  may connect with conductive strip  44  to form conductive assembly  39  within the EM shield assembly  10 . Conductive assembly  39  may be capable of being used as part of a circuit. For example, conductive layer  53  may be capable of conducting current or a signal used or generated by circuit element  30  of circuit assembly  12 . An electrical current may be conducted from circuit element  30 , through terminal  31 , bond wire  34 , signal conductor  32 , via  50   a,  conductive strip  44 , via  50   c,  and conductor  33 . Thus, through conductor  33 , a signal or power may flow between circuit assembly  12  and a circuit outside of shielded chamber  26 .  
       FIGS. 1-4  show an embodiment of an EM shield assembly  10  that includes only one conductive strip  44  embedded in shield assembly  10 , and which interfaces with vias  52   a  and  52   c  at intersection points  56   a  and  56   c,  respectively. An EM shield assembly  10  may include more than one conductive strip or conductive assembly, and such strips may further be embedded between more than two isolating dielectric layers or extend on a dielectric layer or body. Similarly, while these figures show vias formed within two sides of the EM shield assembly, such vias may be contained within all or any combination of sides of the shield assembly. Further there may be multiple vias within any one side. Other forms or configurations for realizing a conductive assembly  39  may be used.  
      The interior surface  48  of EM shield assembly  10  may include portions of resistive material  60  that may be effective in damping undesired electrical propagation within chamber  26 . Shielded enclosures such as chamber  26  may have resonances at various frequencies that may interfere with the proper operation of circuit assembly  12  within the chamber  26 . Resistive material  60  may decrease or provide damping to such resonances, and a layer of resistive material  60  may extend along all or part of the interior surface  48  of dielectric layer  46 . Such resistive material  60  may also be positioned in other ways, such as between layers of dielectric material.  
      Resistive material  60  may be resistive ink, film, paint, other resistive coating, or a combination thereof. This material may be applied to the surface  48  by silk-screening, stenciling, spraying, squirting, painting, inkjet printing, lithography, or any other convenient method. This resistive material  60  may be applied to the surface  48  according to a pattern of distribution so as to create multiple areas of resistive material  62  and  64 .  
      As shown in  FIG. 2 , interior shield surface  48  may include a first resistive material area  62  that is uniform and covers a portion of the interior shield surface along both the top  20  and the side  22   a  of shield  10 . This interior surface may also include a second resistive material area  64  that covers only a portion of side  22   c.  The resistive material may be of uniform thickness or the thickness may vary. Some areas of resistive material may be connected to substrate  14  using conductive adhesive  36 .  
      An alternative embodiment is shown in  FIG. 4 . As seen in this cross-section, the interior shield surface may be covered with a resistive material  60  that is uniformly patterned. Such a pattern  70  may have a predetermined ratio of open, insulating areas  66  to covered, resistive material areas  68 . This resistive material pattern  70  may be applied in a sheet or a film that may include a mesh. Such a film may be defined having an average resistivity of between 10 omhs/square and 1000 omhs/square.  
      As has been mentioned, the addition of such a layer of resistive materials may result in damping of resonances and extraneous couplings within chamber  26  over a desired range of frequencies. In order to improve this damping, resistive material  60  may be selected according to resistivity, and the thickness and pattern in which the material may be applied. Additionally, damping may be improved by properly selecting the dielectric material included in the first and the second dielectric layers  42  and  46  of shield  20 , and the thickness this dielectric material The EM shield assembly  10  may be fabricated from circuit board material, including conductive and dielectric material. Fabrication of the EM shield assembly  10  and/or the substrate  14 , or the circuit structure  8  including the EM shield assembly  10 , substrate  14 , and associated circuitry, such as circuit assembly  12 , can be carried out together with the items arrayed on panels. The mass fabrication on panels may then be followed by an operation to singulate the assemblies.  
      Through such an assembly, there may be multiple circuit assemblies  12  within each chamber  26 , and there may be multiple chambers,  26  within each EM shield  10 . Additionally, a given EM shield assembly may include or omit resistive material  60  for damping, and may include or omit conductive assemblies  39 .  
      Accordingly, while embodiments of circuit structures have been particularly shown and described with reference to the foregoing disclosure, many variations may be made therein. Other combinations and sub-combinations of features, functions, elements and/or properties may be used. Such variations, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower or equal in scope, are also regarded as included within the subject matter of the present disclosure. The foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or later applications. The claims, accordingly, define inventions disclosed in the foregoing disclosure. Where the claims recite “a” or “a first” element or the equivalent thereof, such claims include one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.