Methods and apparatus for EMI shielding

Abstract

Disclosed are methods and apparatus for improving the resiliency and airflow through a honeycomb air vent filter while providing EMI shielding. In one embodiment, the honeycomb can be manufactured from a dielectric (e.g., plastic) substrate to provide improved resistance to deformation as compared to conventional aluminum honeycomb. The dielectric honeycomb substrate is metallized to provide EMI shielding capability. The metallized honeycomb substrate is cut slightly oversize to fit an opening in an electronic enclosure, which results in elastic deformation of resilient perimeter spring fingers that are used to hold the metallized dielectric honeycomb in place and provide electrical conductivity between the metallized dielectric substrate and the enclosure, thereby eliminating the use of a frame. In another embodiment, additional conductive layers can be added to the metallized dielectric honeycomb. In yet another embodiment, the metallized dielectric honeycomb substrate can be utilized in a framed configuration to provide improved durability.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

of from 0.002 inch to 0.05 inch, but are not limited to this range. For applications where a more rugged dielectric honeycomb substrate 52 is required, a higher density dielectric honeycomb substrate 52 or a different honeycomb geometry can be selected.

To manufacture a vent panel according to the invention, in one embodiment referring now to FIG. 5, a dielectric vent panel, such as the honeycomb substrate 52 described above, is provided (step 60). The honeycomb substrate 52 can be prepared by extrusion or by molding (e.g., injection molding) as an integral element. Such molding techniques are well adapted for polymer substrates. Alternatively, the honeycomb substrate 52 can also be manufactured by bonding or otherwise attaching together a plurality of corrugated strips. Such bonding techniques are well adapted to substrates formed from a fibrous material, such as paper, as well as polymer substrates. Alternatively, a number of tubes, each tube forming one of the cells 53, can be fastened together in a planar array, such that a longitudinal axis of each of the tubes is generally parallel with the axes of its neighboring tubes. The fastening could be achieved using a chemical bond, such as a glue, a thermal weld, or a mechanical bond, such as a crimp. The honeycomb substrate 52 can also be manufactured by other methods, such as machining a sheet of the substrate material, for example by boring each of the cells 53 using a drill, or cutting each using a die.

Next, the dielectric honeycomb substrate 52 can be shaped into any desired configuration (step 62). For example, a planar dielectric substrate 52 can be configured in any desired planar shape, such as a square, a rectangle, a circle, etc., having predetermined dimensions to conform to an intended aperture. Such overall shaping can be performed during the manufacturing stage of the substrate 52, for example by selectively altering the shape of a mold, or extruder. The shaping can also be performed post-manufacturing. For example, the substrate 52 can be cut using a knife, a saw, shears, a laser, or a die. Additionally, certain dielectric substrates, such as polymers, lend themselves to a variety of machining techniques. For example, a dielectric honeycomb substrate 52 can be machined to shape one or more of its edges along its perimeter to include a bevel, or a rabbet. Still further, the dielectric substrate 52 can be shaped to include a convex or concave surface or indentation over a portion of either or both of its planar surfaces. Such a planar surface deformation may be desired, to accommodate a mechanical fit.

In order to provide EMI shielding, a conductive layer 54 is applied to the dielectric honeycomb substrate 52, resulting in the metallized dielectric honeycomb filter 50. In one method, a first conductive layer is applied to the dielectric honeycomb substrate 52 (step 64). The first conductive layer can be applied using a variety of techniques known to those skilled in the art, such as electroless plating or physical vapor deposition. See, for example, U.S. Pat. No. 5,275,861 issued by Vaughn and U.S. Pat. No. 5,489,489 issued to Swirbel et al., the disclosures of which are herein incorporated by reference in their entirety. For example, a conductor, such as copper, can be applied using an electroless bath as taught by Vaughn.

The electroless bath method is particularly well suited for a class of polymers known as plateless plastics. This class of plastics includes acrylonitrile-butadiene-styrene (ABS) and polycarbonates, along with other polymer compounds, such as polysulfones, polyamides, polypropylenes, polyethylene, and polyvinyl chloride (PVC). Generally, the dielectric honeycomb substrate 52 should be pretreated to remove any impurities (e.g., dirt, and oil). Depending on the type of material, the substrate 52 may be treated still further to enhance its adhesion properties with the initial conductive layer. For example, the surface can be abraded by mechanical means (e.g., sanding or sandblasting) or by a chemical means (e.g., by using a solvent for softening or an acid for etching). A chemical pretreatment can also be added to alter the chemistry of the surface, further enhancing its ability to chemically bond to the first layer.

Other methods of applying the first conductive layer include applying a conductive paint, such as a lacquer or shellac impregnated with particulate conductors, such as copper, silver, or bronze. Still other methods of applying the first conductive layer include physical deposition, such as evaporation, non-thermal vaporization process (e.g., sputtering), and chemical vapor deposition. Sputtering techniques include radio-frequency (RF) diode, direct-current (DC) diode, triode, and magnetron sputtering. Physical vapor deposition includes such techniques as vacuum deposition, reactive evaporation, and gas evaporation.

Depending on the desired thickness and/or coverage, the step of applying the first conductive layer can be optionally repeated (step 66a), such that one or more additional conductive layers, being made of substantially the same conductor, are applied to the previously-treated substrate 52, thereby increasing the thickness of the layer. In repeating the application of the conducting layer, generally the same method of plating can be used; however, a different method can also be used.

Generally, any conductive material can be used for the conductive layer 54. Some examples of metals that can be used for the conductive layer 54 are copper, nickel, tin, aluminum, silver, graphite, bronze, gold, lead, palladium, cadmium, zinc and combinations or alloys thereof, such as lead-tin and gold-palladium. The conductive layer 54 can also be applied directly as a conductive compound. For example, the substrate 52 can be treated with a single electroless bath having both copper and nickel. The resulting conductive layer 54 is a compound of both copper and nickel.

Optionally, more than one type of conductive layer can be applied to the honeycomb substrate 52 (step 68). For example, after the initial conductive layer 54 has been applied, one or more additional conductive layers of the same, or different material, can be applied using electroless plating, electrolytic plating, physical vapor deposition, or other methods known to those skilled in the art (step 70). Electrolytic plating wold generally be available for applying subsequent conducting layers, as the initial conducting layer would provide the requisite surface conductivity.

In one embodiment, a second conductive layer of nickel is applied over a first conductive layer of copper, the copper providing a relatively high electrical conductivity and the nickel providing a corrosion resistant top coat. As with the initial conductive layer 54, and for similar reasons, the second type of conductive coating can be optionally reapplied until a desired thickness is achieved.

Additional layers of coating or treatment of still other different types of conductive, or even non-conductive materials can be optionally applied to the metallized dielectric honeycomb filter 50 (step 72). For example a fire retardant, a mildew inhibitor, or an anti-corrosion treatment can be applied to the metallized dielectric honeycomb filter 50. These coatings can be selectively applied either covering the entire surface, or any portion thereof. For example, the metallized dielectric honeycomb filter 50 can be completely immersed in a fire retardant, or selectively treated with a corrosion inhibitor, using a masking technique such that a perimeter of the filter 50 remains untreated, thereby avoiding any reduction in the quality of the achievable electrical contact.

Further, the metallized, treated filter 50 can again be shaped, as required, by any of the previously disclosed techniques (step 74). Also, an edge treatment can be optionally applied to the perimeter or mounting surface of the filter 50 (step 76). Particular edge treatments include commercially available EMI gaskets, including metallized spring fingers, conductive fabrics, conductive elastomers, wire mesh, conductive foam, and conductive fabric coated elastomers.

FIG. 6A shows another embodiment of metallized dielectric honeycomb filter 50. In this embodiment, the dielectric honeycomb substrate 52′ is formed by a plurality of tubes 55, which can be co-extruded together. In another embodiment, the plurality of tubes 55 can be bonded together. This type of structure could also be produced by injection molding or similar plastic manufacturing processes. The dielectric honeycomb substrate 52′ is then metallized as previously described.

FIG. 6B is an exploded view of a cell 53′ from FIG. 6A, which shows the conductive layer 54 on the dielectric honeycomb substrate 52′ of cell 53′. Again, the conductive layer 54 can be applied by any of the techniques previously described.

In order to improve airflow, reduce costs, and simplify manufacture, the metallized dielectric substrate 50, referring again to FIG. 4, does not have a frame, so that a larger percentage of the surface area of the metallized dielectric substrate 50 can accommodate airflow through the opening 18 in the enclosure 20. The metallized dielectric honeycomb filter 50 can be easily cut to fit the size of the opening 18 in the enclosure 20. Alternatively, the dielectric honeycomb substrate 52 can be cut to size prior to adding the conductive layer 54. Cutting the metallized dielectric honeycomb filter 50 through the cells 53 results in partial, open sided honeycomb cells 86 bounded in part by cell walls forming resilient spring fingers 88. The resilient spring fingers 88 elastically deform when the filter 50 is installed so as to both ensure electrical contact with the enclosure 20 and hold the metallized dielectric honeycomb filter 50 firmly in place. Thus, the spring fingers 88 form a conductive edge extending substantially about the perimeter of the filter 50 for placing the filter 50 into electrical communication with the enclosure 20.

The cylindrical tubes 55 that make up the cells 53 of the metallized dielectric honeycomb filter 50, as shown in FIG. 6A, can also be made very flexible so a to elastically deform when the metallized dielectric honeycomb filter 50 is installed and thereby ensure electrical contact with the enclosure 20. Electrical contact is ensured with the resilient spring fingers 88′ formed by cutting cells 53′ on the perimeter of the metallized honeycomb filter 50. In addition, by eliminating the conventional frame, the costs associated with manufacturing the filter are reduced.

The cells 53 can be cut along their diameter, leaving an approximately semicircular cell portion, as shown. Alternatively, the cells 53 can be cut leaving either a greater or lesser amount of the cell wall to form a spring.

FIG. 7A shows how the metallized dielectric honeycomb filter 50 would be installed in a channel 91 located in an electronics cabinet in a horizontal mounting configuration. A door member or final cap 92 encloses the metallized dielectric honeycomb filter 50 in the channel 91. In this configuration, all of the mounting surfaces of the cabinet and vent panel are orthogonal. The metallized dielectric honeycomb filter 50 can be sized such that the resilient spring fingers 88 elastically deform and fit snugly in the channel 91 to ensure a tight fit and good electrical contact. By using the channel 91 integrally formed in the enclosure 20, the need for a separate EMI gasket between the filter 50 and enclosure 20 is eliminated. FIG. 7B is a cross-section of the metallized dielectric honeycomb filter 50 installed horizontally in the opening in an enclosure for electronic equipment taken along section 7B—7B of FIG. 7A.

In yet other embodiments, shown in FIG. 7C-7I, the opening 18 can be tapered either vertically (having different surface areas comparing from top to bottom), or horizontally (having different width measurements comparing the front and rear edges). In a vertical configuration, as shown in FIG. 7C, the frameless metallized dielectric honeycomb filter 50 will have a taper along its thickness, which would be similar to the taper in the cabinet wall. The metallized dielectric honeycomb filter 50 would be inserted into the tapered cabinet opening (alternatively, the cabinet can include non-tapered, or straight edges) at about a 90 degree angle to the plane of the opening until an intimate compression fit (similar to a cork) is achieved. Stops can optionally be place above and/or below the vent panel to keep it in place during usage. FIG. 7D illustrates a cross-section of the metallized dielectric honeycomb filter 50, angled in the thickness direction, installed vertically in an opening in an enclosure for electronic equipment taken along section 7D—7D of FIG. 7C. FIG. 7E illustrates an alternative embodiment in which the perimeter of the filter 50′ is shaped to provide a rabbet edge 94 to accommodate a suitable mating surface 91′. Again, stops can optionally be placed above the vent panel to keep it in place during usage. FIG. 7F shows the metallized dielectric honeycomb filter 50, with taper or a rabbet along its thickness, installed on the top surface 20A, and a wall 20B of an electronic equipment enclosure 20.

FIG. 7G shows the horizontal configuration, where the frameless metallized dielectric honeycomb filter 50 will have a taper along its length, which is similar to a taper in a cabinet channel 91 adapted for receiving a tapered filter 50. As illustrated, the filter 50 has a first width W₁ along a front edge, and a different second width, W₂, along a rear edge. The metallization dielectric honeycomb filter 50 is inserted into the cabinet channel 91 in the plane of the filter and along the channel axis until the metallized dielectric honeycomb filter 50 is snug along both side walls 93′ and rear wall 93″. A final cap or door member 92 can be clamped or otherwise attached over the metallized dielectric honeycomb filter 50 to seal the final side. FIG. 7H illustrates a cross-section of the tapered metallized dielectric honeycomb filter 50 installed horizontally in a opening in an enclosure for electronic equipment taken along section 7H—7H of FIG. 7G. FIG. 7I illustrates a metallized dielectric honeycomb filter 50 with a taper along its length installed in an electronic equipment enclosure 20.

In other embodiments, shown in FIGS. 8A—8M, a band frame or slim profile frame 96′,96″,96′″,96″″, generally 96, is added to the metallized dielectric honeycomb filter 50. In the band frame configuration, as shown in FIGS. 9A-8M, a flat metal strip 98′,98″,98′″,98″″, generally 98, with numerous spring fingers 100′, 100″, generally 100, (FIGS. 8A-8E) or dimples 101′, 101″, generally 101, (FIGS. 8F-8J) along its length is wrapped tightly around the perimeter of the metallized dielectric honeycomb filter 50 on its thickness side forming a band 96′,96″,96′″,96″″ generally 96. The band 96 on its interior, flat side, compress the metallized dielectric honeycomb filter 50 along its thickness to create good electrical contact, while the spring fingers 100/dimples 101 on the opposite, exterior side of the band 96 make good electrical contact with the cabinet (generally attain a resistance value below some predetermined desirable threshold value). The features on the band 96 are oriented appropriately depending on whether the metallized dielectric honeycomb filter 50 is inserted vertically (FIGS. 8D and 8E) or horizontally (FIGS. 8B and 8C) with respect to a cabinet opening. The benefit of this band frame 96 is that it leaves the airflow surface of the metallized dielectric honeycomb filter 50 virtually unblocked while increasing the flexibility of the vent panel.

FIG. 8B is a schematic drawing illustrating a front view of one side of the band frame 96 96′ of FIG. 8A, where the band frame has horizontal spring fingers 100′. FIG. 8C is a schematic drawing illustrating a side view of one side of the band frame 96′ of FIG. 8A. Similarly, FIG. 8D is a schematic drawing illustrating a front view of a band frame 96″ (not shown with metallized dielectric honeycomb filter 50), where the band frame has vertical spring fingers 100″, attached, for example, to a band 98 at one end and oriented for vertical insertion. FIG. 8E is a schematic drawing illustrating a side view of the band frame 96′ of FIG. 8D.

FIG. 8G is a schematic drawing illustrating a front view of the band frame 96′″ of FIG. 8F, where the band frame 96′″ has elongated conductive protrusions, or dimples 101′, extending outward from the perimeter. FIG. 8H is a schematic drawing illustrating a side view of the band frame 96′″ of FIG. 8H in which the band frame 96′″ has elongated dimples 101′. Similarly, FIG. 8I is a schematic drawing illustrating a front view of a band frame 96″″ (not shown with metallized dielectric honeycomb filter 50), where the band frame 96″″ has circular dimples 101″ formed, for example, within a band 98″″. FIG. 8H is a schematic drawing illustrating a side view of the band frame 96″″ of FIG. 8I.

In yet another embodiment, in a slim frame configuration, shown in FIG. 8K, the band 99 98 uses a band frame 96 and incorporates narrow features or tabs 102 that wrap around the metallized dielectric honeycomb filter 50 on its top and/or bottom surface a small amount, such as less than about 0.25 inch. Additionally, the tabs 102 can be cut away in substantial areas, such that they only wrap around portions of the metallized dielectric honeycomb filter 50 on its top and/or bottom surface. This embodiment provides additional support for the metallized dielectric honeycomb filter 50 while only reducing the airflow surface by a small amount.

FIG. 8L is a schematic drawing illustrating a front view of one side of the band frame 96 of FIG. 8K, where the band frame has tabs 102 that are fashioned to bend such that when bent inward 90 degrees about either surface of the filter 50, the tabs secure the band frame 96 to the filter 50. FIG. 8M is a schematic drawing illustrating a side view of one side of the band frame 96 of FIG. 8L.

In yet further embodiments, the band frame 96 may be constructed from any conductive material that maintains maximum air flow area through the dielectric honeycomb filter 50, but in addition to being electrically conductive, can also help to accommodate variations in dimensional tolerances during insertion of the dielectric honeycomb filter 50 onto the cabinet 20. Dimensional tolerances between the dielectric honeycomb filter 50 and the enclosure 20 can be accommodated, for example, by using conductive foam, conductive fabric, or conductive fabric wrapped foam from the band material. These band materials create good electrical contact just like a metal band, but unlike a metal band, these band materials have a much lower compression force and are more complaint allowing them to readily accommodate tolerance variations between the metallized dielectric honeycomb filter 50 and the cabinet 20. Conductive fabric wrapped foams and conductive foams can be obtained from Laird Technologies, Inc., located in Delaware Water Gap, Pa.

In one embodiment, illustrated in FIG. 9, the conductive foam or the conductive fabric wrapped foams 104 for use as a band 98 can be slit or manufactured into strips that are approximately as wide or wider as the metallized dielectric honeycomb filter 50 is thick. The strips of these band materials 104 can then be applied to the perimeter of the metallized dielectric honeycomb filter 50 by using an adhesive attachment method, such as previously sensitive adhesive or glue 106, to form a complete band around the periphery of the metallized dielectric honeycomb filter 50. The thicknesses of these band materials can vary from about 0.5 millimeter to about 10 millimeter, or as needed to fill the gaps between the metallized dielectric honeycomb filter 50 and the enclosure 20. These band materials 104 have the advantages of being electrically conductive, flexible and easily compressed, making them useful as an EMI seal/gasket between the metallized dielectric honeycomb filter 50 and the cabinet 20. This allows the EMI radiation induced electrical currents to flow readily from the metallized dielectric honeycomb filter 50 through the band material, to the cabinet 20, and then finally to ground. The compressible foam material fills the gaps and maintains a good compression fill between the metallized dielectric honeycomb filter 50 and the cabinet 20, while sealing any surface discontinuities, seams, and gaps that could act as EMI leakage points.

As readily understood by those skilled in the art, many different configurations can be used to contain the metallized dielectric honeycomb filter 50 in the enclosure 20.

The metallized dielectric honeycomb filter 50 provides improved airflow while meeting stringent flammability standards. Once One such inflammability standard is the UL94 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. The metallized dielectric vent panels 50 according to the invention are able to achieve V0 flame rating, as well as V1 and V2 vertical ratings described in the standard.

EMI shielding effectiveness and airflow test data for a metallized dielectric honeycomb filter in accordance with certain embodiments of the invention are shown respectively in FIGS. 10 and 11. The filter tested for shielding effectiveness is made of a polycarbonate polymer with a plating of nickel over copper. The test panel is about 0.5 inches thick, with a cell size of about 0.125 inches, and a density of about 4 lb/ft³. The nickel layer is about 5-10 micro inches thick and was applied by electroless plating. The copper layer is about 20-50 micro inches thick and was applied by electroless plating. FIG. 10 shows that in the frameless configuration, the metallized dielectric honeycomb filter provides a range of about 80-90 dB of shielding effectiveness up to 1 GHz. In the framed configuration, the nickel over copper metallized dielectric honeycomb filter provides a range of about 40-60 dB of shielding effectiveness up to 1 GHz. FIG. 10 also provides test results for traditional aluminum honeycomb vent panels with different finishes (bare and chromated). The aluminum vent panels with no plating and with chromate finish provide only 30-40 dB of shielding effectiveness up to 1 GHz.

The filters tested for airflow effectiveness are the standard aluminum honeycomb and two different polycarbonate polymer honeycombs with a plating of nickel over copper in accordance with the invention. The test panels were about 0.5 inch thick with a cell size of about 0.125 inch. One of the dielectric honeycomb panels had a density of about 4 lb/ft³ and the other dielectric honeycomb panel had a density of about 10 lb/ft³. FIG. 11 shows that there is substantially no difference in airflow characteristics between standard aluminum honeycomb of the same thickness and cell size and the 4 or 10 lb/ft³ density of the metallized dielectric honeycomb. All of the air flow testing was conducted on the honeycomb without the presence of a frame, so that the results represent the air flow performance of the honeycomb materials.

Accordingly, vent panels produced in accordance with the invention can yield significantly improved shielding effectiveness for the same airflow characteristics as conventional metal vent panels.

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 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.

Claims

1. A frameless vent panel adapted to shield against electromagnetic interference (EMI) comprising:

a dielectric panel having a thickness defined by a first side and a second side, and defining a plurality of apertures, said dielectric panel further having a perimeter;

a first electrically conductive layer applied to the dielectric pane panel, wherein the conductively coated dielectric panel attenuates a transfer of electromagnetic energy from the first side to the second side of the substrate ; and

a strip of compressible conductive foam material, said strip having a width substantially equal to the thickness of said dielectric panel and being wrapped around the outer perimeter thereof and substantially covering said thickness between said first side and said second side, whereby the frameless vent panel is configured to be mounted within an opening in a manner such that the strip of compressible conductive foam material around the outer perimeter of the dielectric panel is compressed.

2. The frameless vent panel of claim 1, wherein the dielectric panel is of a polymer.

3. The frameless vent panel of claim 1, wherein the dielectric panel is of a material selected from the group consisting of polycarbonate, polypropylene polypropyle, acrylonitrile-butadiene-styrene (ABS), polyethylene, polyvinyl chloride (PVC), carbon, fiberglass, paper and combinations thereof.

4. The frameless vent panel of claim 1, wherein the dielectric panel comprises a plurality of tubes bonded together.

5. The frameless vent panel of claim 1, wherein the dielectric panel comprises a plurality of tubes co-extruded together.

6. The frameless vent panel of claim 1, wherein the dielectric panel is produced using an injection molding process.

7. The frameless vent panel of claim 1, wherein the dielectric panel comprises a plurality of corrugated dielectric sheets bonded together, wherein the bonded corrugated dielectric sheets define the plurality of apertures.

8. The frameless vent panel of claim 1, wherein the electrically conductive layer is of a material selected from the group consisting of copper, nickel, tin, aluminum, silver, gold, graphite, lead, palladium, cadmium, zinc and combination thereof.

9. The frameless vent panel of claim 1, further comprising a second electrically conductive layer in electrical communication with the first electrically conductive layer.

10. The frameless vent panel of claim 1, wherein the plurality of apertures is configured as a two-dimensional array of like apertures.

11. The frameless vent panel of claim 10, wherein a cross-sectional shape of each of the like apertures is a shape selected from the group consisting of circular, elliptical, hexagonal, square, rectangular, triangular, rhomboidal, and combinations thereof.

12. The frameless vent panel of claim 1, wherein a cross-sectional diameter of each of the like apertures is selected to be between about 0.06 inches and 1 inch.

13. The frameless vent panel of claim 1, wherein the dielectric panel is selected to have has a density of between about 2 lb/ft³ and about 20 lb/ft³.

14. The frameless vent panel of claim 1, wherein the vent panel provides at least about 20 dB of attenuation to EMI at 10⁹ MHz.

15. The frameless vent panel of claim 1, wherein said strip of compressible conductive foam is secured to said dielectric panel with adhesive.

16. The frameless vent panel of claim 1, wherein said strip of compressible conductive foam has a thickness in a range from about 0.5 millimeter to about 10 millimeters.

17. The frameless vent panel of claim 1, wherein the strip of compressible conductive foam material has a thickness of between about 0.5 millimeter to about 10 millimeter that is compressed to fill the gap between the outer perimeter of the dielectric panel and the opening in which the frameless vent panel is mounted, which allows EM radiation induced electrical currents to be conducted from the dielectric panel through the compressible conductive foam material.