Abstract:
Lossy materials can be used to suppress EMI transmission. Disclosed are methods for applying lossy materials to EMI shielded enclosures to improve EMI shielding effectiveness and the EMI shielded enclosures so produced. In some embodiments, the EMI shielded enclosure includes a printed-circuit board mountable device. In one embodiment, lossy material can be applied to the interior of an EMI shielded enclosure using an adhesive. In another embodiment, lossy materials can be applied to the exterior of the EMI enclosure to suppress EMI incident upon the EMI shielded enclosure, thereby reducing the susceptibility of electronics contained within the EMI shielded enclosure. In yet another embodiment, lossy materials can be applied to both the interior and exterior of the EMI enclosure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application Serial No. 60/340,343, filed on Dec. 14, 2001, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to generally to electronic component packaging and, more specifically, to electronic component packages that are shielded to protect against electromagnetic interference (EMI).  
         BACKGROUND OF THE INVENTION  
         [0003]    As used herein, the term EMI should be considered to refer generally to both electromagnetic interference and radio frequency interference (RFI) emissions, and the term “electromagnetic” should be considered to refer generally to electromagnetic and radio frequency.  
           [0004]    During normal operation, electronic equipment typically generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can be of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Alternatively, or additionally, susceptors of EMI may be similarly shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain, because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.  
           [0005]    Shields are generally constructed to reduce EMI at a particular wavelength, or range of wavelengths. EMI shields are typically constructed of a highly-conductive material operating to reflect the radiation component of the EMI and to drain to electrical ground the conducted component of the EMI. For example, EMI shields are typically constructed of a metal, such as copper, aluminum, gold, tin, steel, and stainless steel, sheet metal and nickel. EMI shields may also be constructed of combinations of different metals, such as nickel-coated copper, and combinations of a conductive material with an electrical insulator, such as metal-plated plastic. In the abstract, an ideal EMI shield would consist of a completely enclosed housing constructed of an infinitely-conductive material without any apertures, seams, gaps, or vents. Practical applications, however, result in an enclosure constructed of a finitely-conducting material and having some apertures. Generally, reducing the largest dimension (not merely the total area) of any aperture, as well as reducing the total number of apertures, tends to increase the EMI protection or shielding effectiveness of the enclosure. Apertures may be intentional, such as those accommodating air flow for cooling, or unintentional, such as those incident to a method of construction (e.g., seams). Special methods of manufacture may be employed to improve shielding effectiveness by welding or soldering seams, or by milling a cavity. The shielding effectiveness of an EMI enclosure having an aperture is a function of the wavelength of the EMI. Generally, the shielding effectiveness is improved when the largest dimension of the aperture is small compared to the wavelength (i.e., less than one-half the wavelength). As the frequencies of operation increase, however, the associated wavelengths of induced EMI decrease, leading to a reduction in shielding effectiveness for any non-ideal EMI enclosure.  
           [0006]    EMI shielded enclosures are typically constructed of conductive materials that induce resonances of the electromagnetic energy within the cavity. For example, reflections of the electromagnetic field at the boundaries of the cavity can create standing waves within the cavity under certain conditions. Such resonances tend to increase the peak amplitudes of the electromagnetic energy through additive effects of the multiple reflections. These resonance effects, by increasing the peak energy levels within the enclosure, can reduce the apparent shielding effectiveness at the resonant frequencies because the same enclosure is shielding a larger source of EMI—the resonant peak electromagnetic energy.  
           [0007]    EMI protection is particularly important in small, densely packaged, sensitive electronic applications operating at high frequencies. In one application, a communications transceiver, such as a Gigabit Interface Converter (GBIC), converts electrical currents into optical signals suitable for transmission over a fiber-optic cable and optical signals into electrical currents. GBICS are typically employed in fiber-optic telecommunications and networking systems as an interface for high-speed networking. As the name suggests, the data rates of transmission are greater than one gigabit-per-second (Gbps). In some applications GBIC modules are installed within an EMI enclosure. One particular form factor for an EMI cage  50 , or housing, shown in FIGS. 1A and 1B is described in a Multi-source Agreement (MSA) prepared by several cooperating members within the related industry. As shown in FIG. 1, one end  55  of the housing  50  is opened to accommodate the insertion and extraction of a GBIC transceiver (i.e., a transceiver having a form factor compliant with the Small-Form-Factor-Pluggable specifications described in the “Cooperation Agreement for Small Form-Factor Pluggable Transceivers,” dated Sep. 14, 2000, the contents of which are herein incorporated by reference in their entirety). The MSA-recommended EMI cage  50  offers a design level of shielding effectiveness for GBIC operations at 1 Gbps; however, as operating frequencies increase, the shielding effectiveness of the recommended EMI cage, without modification, will be inadequate. For example, emerging applications using the optical carrier protocols described in the Synchronous Optical Network (SONET) standards can operate above 1 Gbps (e.g., the OC-48 protocol supporting data rates of up to 2.5 Gbps and OC-192 protocol supporting data rates of up to 10 Gbps).  
           [0008]    There exist certain methods for providing EMI shielding to electronic components. For example, U.S. Pat. No. 5,639,989 issued to Higgins, III, the disclosure of which is herein incorporated by reference in its entirety. Higgins discloses the use of a housing wherein all interior surfaces are conformally coated with a first EMI material consisting of a polymer containing filler particles. The method disclosed in Higgins applies the first EMI material as a conformal coating. The disclosed method also indicates that selection of different materials for filler particles results in the attenuation of electromagnetic energy within specified frequency ranges.  
         SUMMARY OF INVENTION  
         [0009]    In general, the present invention relates to an EMI shield, such as a highly-conductive metal enclosure or cage, that incorporates an electrically absorbing or lossy material to absorb a portion of the EMI, thereby enhancing the performance of the EMI shield over a range of operational frequencies. The absorbing material may remove a portion of the EMI from the environment by the process of ohmic loss—dissipating a portion of the EMI in the form of thermal heating. The absorbing material, when placed within a conductive cavity may also alter the resonant characteristics of the cavity to reduce resonant “peaks” of the electromagnetic fields within the cavity, and/or to translate in frequency, a resonant peak or cutoff.  
           [0010]    In one aspect, the invention relates to a shielding system for providing shielding from high-frequency, electromagnetic interference. The shielding system includes an electrically-conductive shield adapted for covering at least a portion of a device. The shielding system also includes an electromagnetic absorbing material disposed on a first side of the electrically-conductive shield. The combined electrically-conductive shield and electromagnetic absorbing material attenuate a transfer of electromagnetic energy with respect to the shielded device.  
           [0011]    In one embodiment, the shielding system includes an enclosure defining a cavity suitable for housing a device, such as a board mounted device. In another embodiment, the housing is adapted for enclosing the device on substantially all sides. In yet another embodiment, the electromagnetic absorbing material is applied to at least a portion of an interior surface.  
           [0012]    In one embodiment, the housing is adapted for attachment to a circuit board. In another embodiment, the housing includes at least one portion being removably attached. In another embodiment, the electrically-conductive shield is adapted for housing at least one of a fiber optic transmitter and a fiber optic receiver. In another embodiment, the electrically-conductive shield is adapted for housing a gigabit interface converter (GBIC). In yet another embodiment, the electrically-conductive shield includes a form factor substantially compliant with a small-form-factor-pluggable standard.  
           [0013]    In one embodiment, the electrically-conductive shield is selected from the group consisting of aluminum, copper, nickel, tin, silver, gold, beryllium, phosphor bronze, steel, stainless steel, and combinations thereof. In another embodiment, the electrically-conductive shield includes sheet metal.  
           [0014]    In one embodiment, the energy absorptive material is selected from the group consisting of electrically conductive material, carbonyl iron powder, sendust, ferrite, iron silicide, magnetic alloys, magnetic flakes, and combinations thereof. In another embodiment, the energy absorptive material includes electrically absorbing particles suspended in a matrix. In another embodiment, the energy absorptive material is selected from the group consisting of electrically conductive material, carbon, carbonyl iron powder, sendust, ferrites, iron silicide, magnetic alloys, magnetic flakes, steel wool, carbon-impregnated rubber, ferrite in a plastic stranded carrier, metal foils, metal clad materials including iron, nickel, and iron/nickel compositions, paste composites selected from the group consisting of iron, nickel, copper with epoxy, lacquer binders, and combinations thereof, and combinations thereof. In yet another embodiment, energy absorptive material is attached using a pressure sensitive adhesive.  
           [0015]    In another aspect, the invention relates to a process for attenuating a transfer of high-frequency electromagnetic energy with respect to a device. The process includes the steps of reflecting electromagnetic energy at an electrically-conductive shield adapted for covering at least a portion of a device, altering an electromagnetic resonance associated with the electrically-conductive shield, and absorbing a portion of electromagnetic energy proximate to the electrically-conductive shield. The shield thereby reduces a transfer of electromagnetic energy with respect to the device. In one embodiment, the step of altering the electromagnetic resonance includes reducing a peak amplitude of the electromagnetic resonance.  
           [0016]    In another aspect, the invention relates to a process for attenuating a transfer of high-frequency electromagnetic energy with respect to a device. The process includes the steps of providing an electrically-conductive shield adapted for covering at least a portion of a device, and providing an electromagnetic absorbing material adapted for absorbing at least a portion of electromagnetic energy within a predetermined range of wavelengths. The process also includes the steps of treating at least a portion of the electrically-conductive shield with electromagnetic absorbing material and placing the treated electrically-conductive shield in the immediate vicinity of the device. The electrically-conductive shield reduces a transfer of electromagnetic energy with respect to the device.  
           [0017]    In one embodiment, the step of treating includes applying the electromagnetic absorbing material to a first side of the electrically-conductive shield. In another embodiment, the step of treating is selected from the group including painting, dipping, spraying, vapor depositing, silk screening, mechanically fastening, chemically bonding, and combinations thereof. In another embodiment, the step of treating includes at least one of molding, forming, and forming in place the electromagnetic absorbing material onto the electrically-conductive shield.  
           [0018]    In one embodiment, the step of providing an electromagnetic absorbing material includes forming a sheet of absorbing material having a predetermined thickness, and adapting the sheet of absorbing material for application to a first side of the electrically conductive shield. In another embodiment, the adapting step includes applying a chemical fastener to at least one side of the sheet of absorbing material. In yet another embodiment, the adapting step includes applying a pressure sensitive adhesive to at least one side of the sheet of absorbing material.  
           [0019]    In yet another aspect, the invention relates to a shield for attenuating a transfer of high-frequency electromagnetic energy with respect to a device, the shield including means for reflecting electromagnetic energy adapted for covering at least a portion of the device, means for altering an electromagnetic resonance response associated with the means for reflecting electromagnetic energy, and means for absorbing a portion of electromagnetic energy proximate to the means for reflecting electromagnetic energy. The shield thereby reduces a transfer of electromagnetic energy with respect to the device. In one embodiment, the means for reflecting electromagnetic energy includes an electrically-conductive shield. In another embodiment, the means for altering an electromagnetic resonance includes an electromagnetic absorbing material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention is pointed out with particularity in the appended claims. The advantages of the invention may be better understood by referring to the following description, taken in conjunction with the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1A is a schematic drawings depicting external orthogonal views of a top, side, and end of an electrically-conducting EMI cage for a device;  
         [0022]    [0022]FIG. 1B is a schematic drawing depicting an external view of a bottom of the electrically-conducting EMI cage of FIG. 1A;  
         [0023]    [0023]FIG. 2A is a schematic drawing of one embodiment of the invention, in which a lossy material is applied to an inside of a portion of an EMI enclosure;  
         [0024]    [0024]FIG. 2B is a schematic drawing of an alternative embodiment of the invention, in which a lossy material is applied to an outside of a portion of an EMI enclosure;  
         [0025]    [0025]FIG. 2C is a schematic drawing of another alternative embodiment of the invention, in which a lossy material is applied to both the inside and the outside of a portion of an EMI enclosure;  
         [0026]    [0026]FIG. 3 is a more detailed drawing depicting the embodiment of the invention shown in FIG. 2A;  
         [0027]    [0027]FIG. 4A is a schematic drawing of another alternative embodiment of the invention, in which a lossy material is applied to the outside portion of an EMI enclosure;  
         [0028]    [0028]FIG. 4B is a schematic drawing of yet another alternative embodiment of the invention, in which a lossy material is applied to the inside portion of an EMI enclosure;  
         [0029]    [0029]FIG. 4C is a schematic drawing of still another alternative embodiment of the invention, in which a lossy material is applied to both the inside and the outside of an EMI enclosure;  
         [0030]    [0030]FIG. 5 is a schematic drawing of an embodiment of the invention, in which a lossy material is applied directly to an electronic component;  
         [0031]    [0031]FIG. 6 is a schematic drawing depicting one embodiment of the invention, in which a lossy material is applied to a first side of an EMI enclosure;  
         [0032]    [0032]FIG. 7 is a graph representing test results comparing the radiated emissions performance of one embodiment of the invention to a prior art shield.  
         [0033]    [0033]FIG. 8 is a flow diagram depicting the steps of the embodiment of the invention shown in FIG. 2A;  
         [0034]    [0034]FIG. 9A is a schematic drawing of a circuit board mountable embodiment of the invention;  
         [0035]    [0035]FIG. 9B is a schematic drawing of a tape and reel packaging configuration of the embodiment illustrated in FIG. 9A;  
         [0036]    [0036]FIGS. 10A and 10B are schematic drawings of a two-piece circuit board mountable embodiment of the invention; and  
         [0037]    [0037]FIGS. 10C and 10D are schematic drawing of a tape and reel packaging configuration of the embodiment illustrated in FIGS. 10A and 10B, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    Lossy materials can be used to suppress EMI transmissions, for example, by converting the electromagnetic energy into another form of energy, such as thermal energy. The electromagnetic fields can induce electrical currents to flow within the lossy material having a finite electrical conductivity, resulting in ohmic losses. In one embodiment, the lossy material can be composed of ferrite-like material mixed in an elastomer, such as a resin binder. In other embodiments, the lossy material can be composed of a lossy filler material, such as carbon powder mixed in an open-celled reticulated foam. The lossy materials can be configured in sheet form or in a liquid form for coating a substrate. Some examples of lossy materials are electrically conductive material, carbon, iron, carbonyl iron powder, sendust, ferrites, iron silicide, magnetic alloys, magnetic flakes, steel wool, and combinations thereof. Other examples of lossy materials include carbon-impregnated rubber, ferrite in a plastic stranded carrier, metal foils, metal clad materials including iron, nickel, and iron/nickel compositions, paste composites selected from the group consisting of iron, nickel, copper with epoxy, lacquer binders, and combinations thereof, and in combination with the previous exemplary lossy materials. Other materials used to achieve electromagnetic effects include alumina (Al 2 O 3 ), sapphire, silica (SiO 2 ), titanium oxide (TiO 2 ), and combinations thereof.  
         [0039]    In some embodiments, the lossy material can be combined with other materials to achieve a desired effect. For example, the lossy material can be combined with a fire retardant to meet stringent flammability standards. One such flammability standard is the UL94VO vertical flame test, described in detail in Underwriter Laboratories Standard 94, entitled “Tests for Flammability of Plastic Materials for Parts in Devices and Appliances,” 5 th  Edition, 1996, the disclosure of which is incorporated herein by reference in its entirety. In one embodiment, flame retardant additive is prepared in a particulate form and combined with a lossy material, such as carbonyl iron powder whereby each additive is suspended in a matrix, such as an elastomer, or resin binder.  
         [0040]    Various U.S. patents describe lossy materials and their uses. See, for example, U.S. Pat. No. 4,408,255 issued to Adkins, U.S. Pat. No. 5,689,275 issued to Moore et al., U.S. Pat. No. 5,617,095 issued to Kim et al., and U.S. Pat. No. 5,428,506 issued to Brown et al., the disclosures of which are herein incorporated by reference in their entirety. Co-pending United States provisional patent application serial No. 60/419,873, filed on Oct. 21, 2002, the disclosure of which is incorporated herein by reference in its entirety, also describes lossy materials. Some manufactures of lossy materials are R&amp;F Products of San Marcos, Calif.; ARC Technical Resources, Inc. of San Jose, Calif.; Tokin America, Inc. of Union City, Calif.; Intermark-USA, Inc. of Long Island City, N.Y.; TDK of Mount Prospect, Ill.; Emerson &amp; Cuming Composite Materials, Inc., of Canton, Mass.; Cuming Corp. Microwave Products, of Avon, Mass.; and Capcon of Inwood, N.Y.  
         [0041]    According to the present invention, EMI shielding can be added to newly fabricated or existing packages, or housings, for electronic components by applying a first, high-frequency, absorbing EMI material to a second, reflecting EMI material. The high frequency absorbing material includes a lossy material. In some embodiments that lossy material is broad band in nature, absorbing EMI energy over a broad range of frequencies. The reflecting EMI material can be any of the EMI shielding materials, such as metals, including sheet metals currently used by those skilled in the art.  
         [0042]    In one embodiment, the lossy material can be fabricated in a sheet and applied to at least a portion of a conductive EMI shield, such as a metallic EMI shield. Alternatively, the lossy material can be applied as a sheet, or coating, during the course of manufacture. The lossy materials can be added to the interior, the exterior, or both the interior and exterior surfaces of the EMI shield.  
         [0043]    [0043]FIG. 2A shows one embodiment of an EMI shield configured as an EMI enclosure  99  including an absorbing material  102  applied to a reflecting material  100 . In this embodiment, the absorbing, or lossy, material  102  is applied to at least a portion of the reflecting material  100  using an adhesive  104 . The adhesive can be a curable adhesive, such as an epoxy, or a non-curable adhesive, such as a pressure sensitive adhesive. The adhesive can be a conductive adhesive or a non-conducting adhesive.  
         [0044]    The EMI enclosure  99  can include a highly-conductive reflecting material  100 , such as aluminum, copper, nickel, tin, silver, gold, beryllium, steel, stainless steel, sheet metal, including compounds or combinations of different conducting materials, such as nickel plated copper, phosphor bronze, tin plated steel, etc. The EMI enclosure  99  can also include an insulative material, such as a plastic suitably coated with an electrically conducting, or metallic layer—such metal-coated plastic applications are common in the packaging of small, light-weight electronic devices.  
         [0045]    The EMI enclosure  99  can be a substantially closed container, such as a box, or a partially-open container, such as a box or a cage having fewer than six sides. For example, the EMI enclosure  99  may include only five sides for applications in which the EMI enclosure  99  is installed onto another surface, such as a circuit board, substrate, or conductive enclosure. In some applications, the EMI enclosure  99  can be a plane, such as a plate adapted for fastening to an electronic device. The EMI enclosure  99  can also include one or more apertures  106 . The apertures, for example, can be useful for providing interconnections to any electronic components and/or devices disposed therein. Additionally, apertures can be useful for cooling, or even for cost and/or weight savings for those applications in which a closed surface is unnecessary.  
         [0046]    The lossy material  102  can be selectively applied to the entire internal and/or external surfaces of the EMI enclosure  99 . Alternatively, the lossy material  102  can be selectively applied to a portion of the internal and/or external surfaces. For example, in applications in which the electromagnetic energy has a preferred polarization, or in applications in which only a portion of the EMI enclosure  99  is exposed to the EMI energy, the lossy material  102  can be applied in a limited manner, covering the most-vulnerable (i.e., exposed) surface or surfaces.  
         [0047]    The lossy material  102  can be applied during the manufacturing process, or the lossy material  102  may be applied post-manufacturing. Use of the non-conductive, pressure-sensitive adhesive  104  facilitates application of the lossy material  102  after the manufacturing process. For example, the lossy material  102  may be applied during board-level assembly, either before or after any application of conformal coating, during the system level assembly, or at any time post-installation. In this manner, the lossy material may be applied to any EMI enclosure, at any time. For example, interference due to EMI often first occurs, or is first evident, during, or shortly after, equipment installation or upgrade. It is typically during this time that the electromagnetic environment often changes in unpredictable ways; therefore, it is beneficial to mitigate EMI during a post-installation scenario, by particularly tailoring the application of the lossy material as necessary.  
         [0048]    Referring to FIG. 2B, another embodiment of the EMI enclosure  99  is shown in which the lossy material  102  is applied to the exterior surface of the EMI enclosure  99  using adhesive  104 . An external application attenuates any impinging EMI from other external systems and devices. Additionally, an external application of the lossy material  102  attenuates surface currents residing on the enclosure  99 , as well as any proximate near fields.  
         [0049]    [0049]FIG. 2C shows the EMI enclosure  99  having the lossy material  102  applied using the adhesive  104  to both the interior and the exterior surfaces of the conducting housing  100  to attenuate the EMI on both sides of the EMI enclosure  99 .  
         [0050]    [0050]FIG. 3 shows in orthogonal projection, the exterior top, side, and bottom of an embodiment in which an absorbing material  102  is applied to the EMI cage  50  of FIGS. 1A and 1B. The absorbing material  102  is prepared in a rectangular sheet sized to fit along the interior top surface of the EMI cage  50 . In one embodiment, the sheet of EMI material  102  is formed having a thickness of about 0.03 inch. The absorbing sheet  102  can be secured to the interior top surface using an adhesive. In another embodiment, the absorbing sheet  102  can be removably secured to the interior top surface of the EMI cage  105  relying on mechanical pressure provided by a suitably-sized component when inserted therein.  
         [0051]    [0051]FIG. 4A shows a cross-section view of an EMI enclosure  99  with an external lossy material layer  200  applied to the external surface of the conductive material  100 . In this application, the lossy material  200  is applied without the use of an adhesive. Alternative processes of applying the lossy material  200  to the conductive material  100  include any of a variety of coating processes, such as painting, dipping, spraying, inking, silk-screening, and other processes known to those skilled in the art.  
         [0052]    [0052]FIG. 4B shows a cross-section view of the EMI enclosure  99  with an internal lossy material layer  202  similarly applied to the internal surfaces of the conducting material  100 . FIG. 4C shows a cross-section view of the EMI enclosure  99  having both an external lossy material layer  200  and an internal lossy material layer  202  respectively applied to both the exterior and interior surfaces of the enclosure  99 .  
         [0053]    [0053]FIG. 5 shows another embodiment in which an EMI enclosure  99  is directly applied to an electronic component or device  400 . For example, the EMI enclosure  99  can be selectively applied to at least one surface of an integrated circuit or device case. In one embodiment, the EMI enclosure  99  forms an EMI shield including a highly-conductive layer  100 , an optional adhesive layer  104 , a lossy material layer  102 , and a second adhesive layer  104 . In other embodiments, the EMI enclosure  99  can be fastened to the device  400  without the adhesive layer  104 , using an alternative fastening means, such as a screw, a clip, or a frictional clamp.  
         [0054]    [0054]FIG. 6 shows an exploded view of one embodiment of an EMI enclosure  99  in which substantially the entire exterior surface of an EMI shield  105  is protected with a layer of the lossy material  102 . The lossy material  102  is applied as earlier described with an adhesive  104  and apertures are provided in the lossy material  102  to accommodate intentional apertures of the EMI shield  105 . This technique may be used to coat any or all of the external surfaces of the EMI shield  105 . The lossy material  102  can be pre-cut, or preformed according to predetermined dimensions before being applied to the EMI shield. Alternatively, the lossy material  102  can be fabricated in a compliant embodiment, such that the lossy material  102  can be formed (e.g., folded) about at least a portion of the EMI enclosure. Examples of compliant embodiments include flexible absorbing materials, such as a rubberized absorber (e.g., a polymer, or rubber matrix including absorbing particles). Again, apertures  106  can be provided, as required, to accommodate interconnects, cooling air flow, etc.  
         [0055]    The addition of a lossy material layer  102  onto the EMI enclosure  99  is particularly effective for improving shielding effectiveness at frequencies above 1 GHz. For example, application of a lossy material layer  102  consisting of a silicone elastomer loaded with carbonyl-iron powder (CIP) to the top of the EMI enclosure shown in FIG. 3 provides an improved shielding effectiveness compared to the prior art device  50  shown in FIG. 1. In particular, addition of the lossy material layer  102  improves shielding effectiveness at frequencies found to be problematic in the prior art device  50  shown in FIG. 1. In one embodiment, the lossy material  102  can be formed into a sheet having, for example, a thickness of approximately 0.02 inches. The sheet of lossy material  102  can then be die-cut to shape and applied to a top inside surface of a shielded cage, such as the prior art device  50 , using a non-conductive, pressure-sensitive adhesive  104 . Depending on the particular application, the operating conditions, the lossy material  102 , and how and where it is applied, it is contemplated that an improvement in shielding effectiveness of about 10 dB or more is achievable.  
         [0056]    Referring now to FIG. 7, test results are provided graphically comparing radiated emissions performance of an EMI enclosure similar to the device  50  illustrated in FIG. 1, to radiated emissions performance of the same EMI enclosure having an absorber, or lossy material. Specifically, the absorber  102  tested was fabricated as a 0.02 inch thick, silicone elastomer loaded with carbonyl iron power. The loaded elastomer was die-cut and applied to the interior top of the EMI enclosure. The vertical axis of the graph represents the attenuation, or loss (measured in decibels “dB”) provided by each enclosure and the horizontal axis represents a corresponding frequency (measured in megahertz “MHz”). The results indicate that the shield treated with absorber offers greater attenuation than the untreated shield for all frequencies measured. For example, at approximately 2,000 MHz, the absorber-treated shield provides an improvement, or difference in attenuation (i.e., “A”), or an improvement of at least about 8 dB.  
         [0057]    Referring to FIG. 8, a process for shielding electronic components includes the step of providing an EMI shield, such as a highly-conductive EMI material (step  500 ). Generally, the highly-conductive material, or EMI shield, is formed to enclose one or more electronic components. The EMI shield operates to reflect radiated electromagnetic energy, tending to maintain on one side of the shield electromagnetic energy originating on that side of the shield. Where there are apertures in the shield, however, some EMI will penetrate to the other side. The EMI shield also operates to collect conducted EMI and direct it to an electrical ground. (For example, the EMI shield may be electrically grounded through surface-mount soldering to a substrate, such as a printed-circuit board. The EMI shield may also be grounded through one or more grounding leads or bonding straps, or through one or more mounting pins designed to interconnect to electrical ground. The EMI shield contains within its enclosed cavity a portion of the EMI generated by the contained electronic components, while simultaneously protecting the contained components from susceptibilities of external electromagnetic energy.  
         [0058]    Next, an absorbing material is provided (step  510 ). The absorbing or lossy material converts the EMI energy into another form of energy, such as thermal energy, thus removing that converted portion of the EMI from the environment. The absorbing material may be applied to the interior surface, the exterior surface, or to both surfaces, either partially or wholly. For applications in which the lossy material is applied to the interior portions of an EMI shield forming a cavity, in addition to absorbing a portion of the EMI, the lossy material alters the quality factor of the resonance formed by the cavity. For those applications in which the lossy material operates over a broad range of frequencies, the broad-band features of the lossy material operate to remove resonances over the broad frequency range. Finally, second EMI material (i.e., the lossy material) is bonded, secured, or otherwise applied to the first EMI material (i.e., the EMI shield) (step  520 ).  
         [0059]    In some embodiments, the process optionally includes an additional step (shown in phantom) of securing the absorbing EMI shield to a substrate, such as a circuit board (step  525 ). Accordingly, in some applications, it is advantageous to provide EMI protection at the device, or even the component level. Such targeted shielding can result in cost savings, reduced complexity, and even improved performance. Product shields adapted for housing an electronic device mounted on a circuit board (“board level shield”), such as a printed circuit board, are commercially available. One supplier of board-level EMI shields is BMI, Inc. of Schaumburg, Ill. Exemplary board-level electrically-conducting EMI shields include BMI part numbers BMI-S-101 through BMI-S-107, and BMI-S-201 through BMI-S-207.  
         [0060]    Board-level EMI shields are electrically conducting and generally form a box having five sides, the bottom of side of the box being open. Thus, the open box can be placed on top of a board-mounted electronic device. FIG. 9A illustrates a top, front, and side view of one embodiment of a board level shield  600 . The board-level shield  600  can optionally include one or more apertures  602  on a top conducting surface  604 , as illustrated. Once again, the apertures  602  can accommodate interconnects to and from the shielded device, as well as allowing for cooling air flow. A conducting side surface  605   a  of the board-level shield  600  can also be adapted for mounting to a substrate. For example, the side  605   a  includes mounting pins  610 , or mounting pads for surface mount applications, that can be used to secure the board-level shield  600  to a circuit board by soldering. In some embodiments, the pins  610  extend into and/or through the circuit board such that the mounting edge  615  of the side makes contact with the circuit board. Intimate contact of the sides of the shield  600  with the circuit board is generally desirable to reduce unwanted apertures through which EMI could propagate. An electromagnetic absorber  618  can then be applied to one or more of the conducting sides  604 ,  605   a ,  605   b  of the board-level shield  600 , as previously described. As illustrated, the absorber  618  is configured as a sheet and mounted to the shield&#39;s inside top surface  604 . In some embodiments, the absorber  618  can additionally or alternatively be mounted to the sides  605   a ,  605   b , and/or to any of the external surfaces of the shield  600 .  
         [0061]    [0061]FIG. 9B illustrates one packaging embodiment of the board-level absorbing EMI shield  600 . Several board level shields  600  can be packaged on a tape  620  that can be wound upon a reel. Such a tape and reel packaging embodiment is well adapted for automated devices used to populate printed circuit boards in high-volume, high-throughput applications. Although the illustrations are directed to printed circuit board applications, the invention is scalable and can be adapted for mounting to other surfaces, such as semiconductor substrates.  
         [0062]    [0062]FIGS. 10A through 10D illustrate an alternative embodiment including a two-piece board-level absorbing EMI shield. As illustrated, an electrically conducting frame  700  is constructed similar to the previously-described one-piece board-level shield  600 , except at least one of the surfaces is removable. For example, the frame  700  can include one or more apertures  705 , such that one surface is substantially open, thereby allowing access to any components and/or devices housed therein. Optionally a support member  710  can be provided for structural support and/or to facilitate handling of the frame  700  by automated devices. A separate surface, such as a top or cover  740 , can then be provided, whereby the top can be fastened to the frame  700 . In some embodiments, the cover  740  can be removably fastened using a mechanical fastening means providing for example, a frictional fit, or a tension fit (e.g., spring fingers). In other embodiments, the cover  740  can be removably fastened using a chemical fastener, such as a conductive adhesive or a solder. Again, at least one side, such as the cover  740  can include an absorber  744  and, optionally, apertures  742  for interconnects and/or cooling. As illustrated in FIGS. 10C and 10D, two-piece EMI shields can also be packaged in tape and reel configurations to support high-volume mass production. In one embodiment illustrated in FIG. 10C, frames  700  are mounted on a first tape  750   a . Similarly, as illustrated in FIG. 10D, covers  740  are mounted on a second tape  750   b . In other embodiment, the combined frame  710  and cover  740  can be pre-assembled and packaged on a single tape.  
         [0063]    Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. The various features and configurations shown and equivalents thereof can be used in various combinations and permutations. Accordingly, the invention is to be defined not by the preceding illustrative descriptions, but instead by the following claims.