Patent Number: 
Section: description

Referring now to FIG. 1, an electronic enclosure assembly 20 of a cellular phone is a typical clamshell enclosure design and is shown in the assembled configuration, as it would be used. FIG. 2 shows an exploded view of electronic enclosure assembly 20 including a bottom enclosure housing 10 and a top enclosure housing 12. Bottom enclosure housing 10 contains a network of ribs 11, and a plurality of screw bosses 14. Electronic enclosure assembly 20 is fastened together with a plurality of screws 18, and a plurality of screw bosses 14. This fastening method is well known in the art of electronic enclosure design and the details have been omitted so that the focus may be on the present invention. Electronic enclosure assembly 20 also includes an EMI/RFI containment form assembly 24, comprising an EMI/RFI containment form 21 coated with a conductive coating 22, preferably aluminum applied by vacuum metallization techniques, a printed circuit board 32, a plurality of electronic components 36, and a liquid crystal display 44. As shown in FIGS. 2 and 3, printed circuit board 32 is populated by a plurality of electronic components 36 electrically connected to it, and also has an internal ground plane 50 and an EMI/RFI ground trace 46 that is plated and exposed, on its surface facing the form 21. The shape of EMI/RFI ground trace 46 corresponds exactly to the shape of the top surface of EMI/RFI containment form 21, the shape of which in turn corresponds exactly to the shape of ribs 11. Other details of the design such as other active and passive circuit components, speakers, buttons, switches, antennae, wires, batteries, and corresponding holes and features in both bottom enclosure housing 10 and top enclosure housing 12, would be included in a functional design but have been omitted so as not to obscure the present invention. Referring now to FIGS. 2 and 3, EMI/RFI containment form assembly 24 comprises an EMI/RFI containment form 21, a conductive coating 22 on EM/RFI containment form 21, and a gap-filling gasket 25. EMI/RFI containment form 21 is constructed out of either polyester or impact modified syndiocratic polystyrene thin film sheet, with a thickness of 0.003 inches to 0.020 inches depending on application requirements. An example of such a material is VALOX(trademark), manufactured by General Electric Plastics of Pittsfield, Mass., or QUESTRA(trademark), manufactured by Dow Corporation of Midland, Mich. This sheet material is formed into the shape of EMI/RFI containment form 21 by a variety of forming processes that are well known in the industry, such as vacuum forming, pressure forming, vacuum pressure forming, embossing, and injection molding among others. The shape of the compartments 23 in EMI/RFI containment form 21 are dictated by the shape of the cavities 13 in bottom enclosure housing 10, that is, EMI/RFI containment form 21 closely fits into the cavities created by ribs 11 in bottom enclosure housing 10. Containment form 21 includes a peripheral lip 27 which surrounds compartment 23 and extends laterally from outer sidewalls 29 of containment form 21. Compartments 23 are separated by narrow hollow walls 31 which receive ribs 11 of lower housing 10. Ribs 11 and outer wall 33 of lower housing 10 define cavities 13. Lip 27 of containment form 21 overlies ribs 11 or outer wall 33 of lower housing when containment assembly 24 is assembled. Gasket 25 is interposed between ribs 11 and hollow walls 31 and between lip 27 and ribs 11 on outer sidewall 33. Conductive coating 22 is applied to EMI/RFI containment form 21 by either a vacuum deposition or conductive painting process that is well known in the art. Conductive coating is preferably applied to the containment form 21 by the vacuum metalization techniques described in U.S. Pat. No. 5,811,050 to Gabower. Referring now to FIGS. 5 and 6, gap-filling gasket 25 consists of NUVA SIL(trademark), a liquid elastomer material product manufactured by Loctite Corporation. Gap-filling gasket 25 material is applied as a liquid within the recesses of hollow walls 31 of EMI/RFI containment form 21, and cures to an elastomeric state. Referring now to FIGS. 2 and 4, when electronic enclosure assembly 20 is fastened together for use, EMI/RFI containment form assembly 24 is constrained by bottom enclosure housing 10 and top enclosure housing 12. EMI/RFI containment form 21 is compressed between printed circuit board and ribs 11. In an unassembled state, gap-filling gasket 25 is of a thickness that is larger than the actual distance between the top of ribs 11 and the corresponding bottom area of EMI/RFI containment form 21. Because gap-filling gasket 25 is a compliant elastomer, ribs 11 compresses gap-filling gasket 25 which in turn forces EMI/RFI containment form 21 firmly against EMI/RFI ground trace 46 on printed circuit board 32. This firm, conductive connection between EMI/RFI containment form 21 and EMI/RFI ground trace 46 on the printed circuit board 32 creates the necessary contact resistance for an effective EMI/RFI shielding seam within the given areas to be shielded in the electronic enclosure 20. The compliance of gap-filling gasket 25 also acts to fill tolerance gaps or slight misalignments between printed-circuit board 32 and EMI/RFI containment form 21. When electronic enclosure assembly 20 is powered and being used, the flow of electricity through the electronic circuit created by printed-circuit board 32 and electronic components 36 causes EMI or RFI to propagate away from the device. The electromagnetic energy is contained and prevented from propagating outside of electronic enclosure assembly 20 by the continuous conductive enclosure created by the combination of ground plane 50, EMI/RFI ground trace 46, and EMI/RFI containment form assembly 24, which effectively constitutes a sealed Faraday cage. The Faraday cage is a well-known concept in the field of electromagnetics. Referring now to FIG. 7, an alternative embodiment shows that a plurality of gap-filling punctures 28 may be used in place of gap-filling gasket 25. Gap-filling punctures 28 are created by a die-cutting process whereby a die with a plurality of discrete blades punctures through the top surface of lip 27 and hollow walls 31 of EMI/RFI containment form 21. The die is in the exact shape of the top-most surface of EMI/RFI containment form 21. When the blades puncture the polyester material, they deform the material around the puncture slightly up and away from the top surface. Gap-filling punctures 28 are formed into EMI/RFI containment form 21 before conductive coating 22 is applied. When assembled as described above, gap-filling punctures are forced compliantly against EMI/RFI ground trace 46 by ribs 11 and outer wall 33. Since gap-filling punctures 28 are covered with conductive coating 22, a continuous, conductive shield is maintained that prohibits the EMI/RFI that is radiated by electronic components 36 from propagating outside of electronic enclosure assembly 24. The spacing between punctures 28 is chosen to be less than one-half wavelength of the EMI radiation anticipated in order to prevent leaking of EMI. FIG. 8 discloses a close up view of a portion of lip 27 which has been modified with gap-filling bent tabs 52 creating upstanding flaps closely and evenly spaced apart on lip 27 with the spaces between neighboring gap-filling bent tabs 52 being less than one-half wavelength of the frequency to be contained. Such gap-filling bent tabs 52 may also be formed in hollow walls 31 of form 21. The gap-filling bent tabs 52 are forced against ground trace 46 by ribs 11 and outer wall 33 of housing 10 when form 21 and circuit board 32 are mounted in housing 10. Referring now to FIG. 9, another alternative embodiment shows that a plurality of gap-filling dimples 60 may be used in place of gap-filling gasket 25. Gap-filling dimples 60 are created by a forming process whereby small semi-circles are formed along the top surface of EMI/RFI containment form 21. Gap-filling dimples 60 protrude in the direction of printed circuit board 32. Gap-filling dimples 60 are formed into EMI/RFI containment form 21 before conductive coating 22 is applied. When assembled as described above, gap-filling dimples 60 are forced compliantly against EMI/RFI ground trace 46 by ribs 11. Since gap-filling dimples are covered with conductive coating 22, a continuous, conductive shield is maintained that prohibits the EMI/RFI that is radiated by electronic components 36 from propagating outside of electronic enclosure assembly 24. Although the description above contains many specificities, these should not be construed as limiting the scope of the invention, but merely providing illustration of some of the presently preferred embodiments of this invention. EMI/RFI containment form 21 could be manufactured out of a variety of different plastics. Gap-filling gasket 25 could be constructed out of a variety of different compliant materials. For example, gap-filling gasket 25 could be die-cut out of elastomeric sheet material. Other molded-in gap-filling features could be included other than gap-filling dimples 60. For example, gap-filling bent tabs 52 could be molded and die-cut into EMI/RFI containment form 21, as shown in FIG. 8. Although the description of this invention shows a cellular phone, this invention could also be used for RFI shielding such as may be required in radios, portable computers, PDAs (Personal Digital Assistants), or other devices that must be prevented from emitting EMI.