WO2002052916A1

WO2002052916A1 - Emi and rfi containment enclosure for electronic devices

Info

Publication number: WO2002052916A1
Application number: PCT/US2000/035168
Authority: WO
Inventors: Craig M. Janik; Lou Hughes; John F. Gabower
Original assignee: Shielding for Electronics Inc
Current assignee: Shielding for Electronics Inc
Priority date: 2000-12-21
Filing date: 2000-12-21
Publication date: 2002-07-04
Anticipated expiration: 2003-06-21

Abstract

An EMI shield system for containing EMI emissions in electronic devices includes a thermo-formable shield component (26) maintained in contact with a ground trace (46) on a printed-circuit board (32). A rib system (11) integral with an injection-molded housing component (10) forces the thermo-formed shield component against the ground trace (46). An elastomeric gasket (25) between the rib system (11) and the thermo-formed component insures a firm, compliant seal against the ground trace.

Description

EMI and RFI Containment Enclosure for Electronic Devices

FIELD OF THE INVENTION This invention pertains to the field of shielding apparatus for containing unwanted high frequency electromagnetic radiation within an electronic device enclosure such as those included in cellular or other wireless phones, and computer equipment such as portable computers and the like.

DEFINITIONS

Electromagnetic Interference (EMI) and Radio-frequency Interference (RFI) are terms that are often used interchangeably, and are defined as the presence of unwanted electromagnetic emissions.

BACKGROUND OF THE INVENTION

Most electronic devices, and especially digital devices, inherently operate by the controlled switching of the flow of electrical energy in conductors and semiconductors. A byproduct of this switching is that electromagnetic fields are created, and electromagnetic energy is propagated. The frequency and power of the fields are determined by the electrical characteristics of the device. Depending on the device, some propagation of energy is functional, such as in cellular phones, where the changing electromagnetic field is used to transmit information. Yet wireless communication devices such as these, as well as other products such as computers, emit unwanted electromagnetic energy, often referred to as EMI. EMI is undesirable for functional, economic, and health reasons.

EMI is functionally undesirable because its presence can cause the malfunction of the electrical system subjected to the EMI. EMI fields that are emitted from one device may induce current flow or other disruptive electrical effects in nearby devices. In order to prevent rampant disruption of devices due to EMI, the United States Federal Communications Commission (FCC) has developed limit standards for various types and classifications of products. That is, the FCC has set a limit for the amount of EMI energy that can be emitted from a product. Every electronic product design must be tested by a certified testing organization and cannot be distributed if the EMI emissions are above a certain level. These regulations are stringently enforced. Therefore, the emission of EMI from a company's product that does not comply with government regulations can result in an inability to ship that product, thereby bringing economic hardship to that company.

Cellular phones, or other wide-area wireless personal communication devices have also come under scrutiny due to the fact that usage of these products often puts them in close proximity to the user's brain tissue. Although to date there are no conclusive studies that show a link between cellular phone usage and brain or other tissue maladies, there is an increasing public sensitivity to the amount of EMI and electromagnetic energy that is emitted by phones. In fact, cellular phone manufacturers will soon be required by law to publish the amount of electromagnetic energy that is emitted by their phones. As stated earlier, since some electromagnetic energy emission is inherent to the function of a cellular phone, it is the best interests of both the phone manufacturer and phone users, to limit as much as possible the nonfunctional EMI emissions from the device. Unfortunately, EMI containment in electronic devices is increasingly more difficult due to the faster speeds at which the circuits in microprocessors and memory are switched. As electrical switching frequency increases, the wavelength of the electromagnetic energy that is emitted decreases, a relationship described by basic electromagnetic physics. As electromagnetic waves propagate and the smaller the wavelength, the greater the portion of energy that can escape through a given size gap in or between shield components. Thus, smaller wavelengths require smaller gaps in any shielding solution.

US Patent 5,811,050 to Gabower shows a method for fabricating EMI shielding enclosures out of thermo-formed thin film plastic which is then vacuum metallized, that is, coated with a thin layer of metal. The manufacturing and shielding efficacies of metallized thermo-formed film components are well documented in the Gabower patent. Thermo-formed film enclosures with conductive coatings provide highly effective EMI shielding when the enclosure is well-sealed and well-connected to the ground plane in an electronic device. It is the effective sealing of the shielding components that is the subject of the art disclosed herein. Gabower shows a system where EMI emitting devices are completely enclosed by a design with two rectilinear concave thermo-forms attached with an integral film hinge. The two concave forms are made to clamshell around the EMI emitting device, and attach with a variety of formed-in fastening features.

However, in practice there are many applications where a two-sided hinged box cannot be used. For example, in many cellular phones, the EMI emitting components may be positioned on only one side of a printed-circuit board, thus obviating the need for a second concave feature. Space requirements also may limit the ability to completely enclose components or a printed-circuit board, due to the added perimeter dimension required for the formed-in snap fastening features, as shown in Gabower, FIG 5. There are also designs that require separation of various EMI emitting components from each other, on one side of a printed-circuit board. In these cases, the formed film component can be used with great space efficiency if it can be placed directly on the printed-circuit board.

One method for attaching the formed film shielding components to a printed- circuit board would be to use an adhesive. In this case, the adhesive would have to be conductive, which increases its cost. The adhesive would also have to be aligned on the thin edge of the thermo-formed component and make contact with a ground trace on the printed-circuit board all the way around to insure that a tight EMI seal was provided. Adhesives are prone to degradation over time and temperature cycling. If part of the adhesive failed, a section of the thermo-formed film shield component could lift up off of the printed-circuit board and allow EMI to leak. This failure of the thermo-formed film shield component is all the more likely due to inconsistencies in the topography of printed-circuit boards. Many printed-circuit boards are not absolutely flat. They may be warped due to temperature cycling during the manufacturing process. Another problem with using an adhesive is that the thermo-formed shield component is not cleanly or easily removable. In fact, the more aggressive the adhesive, the more difficult it is to remove the thermo-formed shield component. It is important to be able to easily and cleanly remove shielding components because rework of the electrical components is common after initial testing, or for repair of the device.

What is required is a method for insuring the reliable, positive pressure, compliant connection of a thermo-formed shielding component to a prmted-circuit board.

SUMMARY OF THE INVENTION

This invention pertains to EMI shielding for personal computers, cellular telephones and other electronic devices which are subject to Part 15 of the FCC rules. There are four components included in the system: a thermo-formed metallized concave shielding component; a compliant gasket; a printed-circuit board; and a two- piece molded plastic enclosure where at least one of the parts has a rib system.

A thermo-formable polymeric sheet is formed into a concave enclosure sized and shaped to fit over and enclose one or more EMI emitting components on a printed-circuit board. The thermo-formed polymeric enclosure is then metallized on all or selected surfaces by vacuum metallizing techniques where the thermo-formed enclosure is placed in a vacuum chamber, treated with ionized gas, and then metallized by the use of the deposition of vaporized aluminum or other vaporized metal. The enclosure may be rotated within the chamber to allow for metallization of all desired surfaces. The concave thermo-form is thereby provided with walls having a polymeric substrate provided on desired surfaces with a vacuum metallized layer. The vacuum metallized layers are of sufficient thickness to make the surfaces of the concave thermo-form electrically conductive. The thermo-form may have a variety of cavities separated by thermo-formed wall features. The perimeter of the concave thermo-form includes a flange that is a right angle return of the thermo-form material. The perimeter flange and all of the surfaces that connect the interior cavity wall surfaces are co-planar, and correspond in shape to a ground trace, an exposed copper metallic circuit, on the corresponding printed-circuit board. The thermo-formed shield component is placed on the printed-circuit board so that the perimeter flange and all of the interior surfaces at the same level as the outer perimeter are in contact with the corresponding ground trace on the printed circuit board. A compliant, elastic gasket material is placed along the outer (convex) side of the perimeter flange, and along the outer surfaces of the thermo-form that are at the same level as the perimeter flange.

The molded plastic housing has a rib system that corresponds in shape to the perimeter flange and interior flange-level features. The rib system is sized so that when the molded plastic housings are fastened together, the rib system forces the gasket material against the underside of the thermo-formed shield component, which in turn is forced against the ground trace on the printed-circuit board. Thus, consistent, compliant, gapless EMI seal is produced. It is accordingly an object of the invention to provide a gapless and thus EMI leak-proof connection between a thin-film shielding component and a printed-circuit board containing EMI omitting devices.

It is a further object of the invention to provide a reliable and gapless EMI shielding solution that will not degrade over time and temperature cycling. It is also an object of the invention to provide an EMI shielding solution that may be cleanly and easily disassembled and reassembled without harming any of the components.

It is a further object of the invention to provide a method for attaching a thin film EMI shield component so it can compliantly conform to the slight contour irregularities inherent in printed-circuit boards.

It is a further object of the invention to provide a method for attaching a thin film EMI shield component to a printed-circuit board with a compliant gasket that conforms to slight irregularities in the molded rib system.

It is also an object of the invention to provide a reliable EMI solution that is easy to assemble.

It is also an object of the invention to provide a reliable EMI solution that is inexpensive.

And it a further an object of the invention to provide an EMI solution that is effective without depending on tight tolerances of the components. In one aspect of the present invention, an electronic enclosure assembly includes a printed circuit board and an electrically conductive containment shell. The printed circuit board includes electrical components attached to a first side of the printed circuit board, a conductive ground plane, and a ground trace formed on the first side in a pattern that encircles the electrical components. The ground trace is electrically connected to the ground plane. The electrically conductive containment shell covers at least a portion of the first side encircled by the pattern and the electrical components attached thereto. The shell has an opening defined by a conductive ridge formed in the shape of the pattern that is engaged with the ground trace. The ground plane and the conductive shell form a grounded EMI/RFI shield around the electrical components.

In another aspect of the present invention, the electronic enclosure assembly includes a housing having a first portion and a second portion that mounts to the first portion, a printed circuit board disposed in the housing, and an electrically conductive containment shell disposed in the housing. The printed circuit includes electrical components attached to a first side of the printed circuit board, a conductive ground plane, and a ground trace formed on the first side in a pattern that encircles the electrical components. The ground trace is electrically connected to the ground plane. An electrically conductive containment shell disposed in the housing for covering at least a portion of the first side encircled by the pattern and the electrical components attached thereto. The shell has an opening defined by a conductive ridge formed in the shape of the pattern. A rib system extends from an interior surface of the first portion of the housing. The rib system is formed in the shape of the pattern. When the first and second housing portions are mounted together, the rib system presses the conductive ridge against the ground trace for forming an electrical contact therebetween so that the ground plane and the conductive shell form a grounded EMI/RFI shield around the electrical components. These and other objects of the invention will become understood from a review of the detailed description of the invention which follows.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of electronic enclosure assembly 20. FIG. 2 is an exploded perspective view of electronic enclosure assembly 20.

FIG. 3 is a perspective view of the printed circuit board 32, EMI/RFI containment form 52 and the gap-filling gasket 25. FIG. 4 is a detail cross-section view 1-1 of the electronic assembly 20. FIG. 5 is a bottom view of EMI/RFI containment form shell 26. FIG. 6 is a bottom perspective view of EMI/RFI containment form shell 26 with modifications. FIG. 7 is a detail cut-away view of gap-filling puncture in EMI/RFI containment form shell 26.

FIG. 8 is a detail cut-away view of a gap-filling tab. FIG. 9 is a detail cut-away view of gap filling dimple.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1, an electronic enclosure assembly 20 of a cellular phone has an outer 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 rib system 11, and a plurality of screw boss cavities that correspond with a plurality of screw bosses (not shown) in top enclosure housing 12. Bottom enclosure housing 10 and top enclosure housing 12 are manufactured by injection-molding plastic. 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.

As shown in Figs. 2-3, electronic enclosure assembly 20 includes an EMI/RFI containment form assembly 24, which is comprised of an EMI RFI containment thermo-form shell 26 coated with a conductive coating 22, and a printed-circuit board 32 with a plurality of electronic components 36 and an LCD 44. Printed-circuit board 32 is populated on its bottom side 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 top side. The shape of EMI/RFI ground trace 46 corresponds exactly to the shape of the top surface of EMI/RFI containment form shell 26, the shape of which in turn corresponds exactly to the shape of rib system 11 integral to bottom enclosure housing 10. 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 FIG. 3, EMI/RFI containment form assembly 24 includes the printed circuit board 32, the EMI/RFI containment form shell 26 with the conductive coating 22 thereon, and a gap-filling gasket 25. EMI/RFI containment form shell 26 is preferably constructed out of either polyester or impact modified syndiocratic polystyrene thin film sheet, with a thickness of 0.003" to 0.020" depending on application requirements. An example of such a material is Nalox, manufactured by General Electric Plastics of Pittsfield, MA, or Questra, manufactured by Dow

Corporation of Midland, MI. This sheet material is formed into the shape of EMI/RFI containment form shell 26 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 cavities in EMI/RFI containment form shell 26 are dictated by the shape of the cavities in bottom enclosure housing 10, that is, EMI/RFI containment form shell 26 closely fits into the cavities created by rib system 11 in bottom enclosure housing 10. Conductive coating 22 is applied to EMI/RFI containment form shell 26 by either a vacuum deposition or conductive painting process that is well known in the art. Referring now to FIGS. 5 and 6, gap-filling gasket 25 consists of Νuva Sil, a liquid elastomer material product manufactured by Loctite Corporation. Preferably the gap-filling gasket 25 material is applied as a liquid along the recessed forms of EMI/RFI containment form shell 26, and cures to an elastomeric state.

Referring now to FIG. 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 shell 26 is compressed between printed-circuit board 32 and rib system 11. In an unassembled state, gap-filling gasket 25 is of a thickness that is larger than the actual distance between the top of rib system 11 and the corresponding bottom area of EMI/RFI containment form shell 26. Because gap-filling gasket 25 is a compliant elastomer, rib system 11 compresses gap-filling gasket 25 which in turn forces EMI/RFI containment form shell 26 firmly against EMI/RFI ground trace 46 on printed-circuit board 32. This firm, conductive connection between EMI/RFI containment form shell 26 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 mis-alignments between printed-circuit board 32 and EMI/RFI containment form shell 26.

When electronic enclosure assembly 20 is powered on 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 electromagnetism. 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 EMI/RFI containment form shell 26. The die is in the exact shape of the top-most surface of EMI/RFI containment form shell 26. 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 shell 26 before conductive coating 22 is applied. When assembled as described above, gap-filling punctures are forced compliantly against EMI/RFI ground trace 46 by rib system 11. 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.

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 shell 26. Gap-filling dimples 60 protrude in the direction of printed circuit board 32. Gap-filling dimples 60 are formed into EMI/RFI containment form shell 26 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 rib system 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 shell 26 could be manufactured out of a variety of different plastics. Gap-filling gasket 25 could be constructed out of a variety of different complaint 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 shell 26, 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. This method could also be used with other shield forms, such as sheet metal or injection-molded plastic forms.

The above system for maintaining a conductive connection between containment form shell 26 and printed-circuit board 32 ground trace can also be used with containment forms 20 that have been injection-molded. Injection-molding is a very low cost high- volume manufacturing technique and will not be described here because a designer skilled the art of electronics packaging will be familiar with it.

Effective containment form shell 26 may be molded with sufficiently thin walls as to allow the compliance required to fill in gaps in the system disclosed here.

The above system may also be used with containment form shell 26 that are coated with conductive paint. Conductive paint systems are well known in the EMI shielding field. Thermo-formed film or injection-molded containment forms 20 may be painted with paint containing silver particles, on the concave side of the containment fomi shell 26, so that the conductive paint material is forced into contact with ground trace 46 on the printed-circuit board 32.

Claims

What is claimed is:

1. An electronic enclosure assembly, comprising: a printed circuit board that includes: electrical components attached to a first side of the printed circuit board, a conductive ground plane, and a ground trace formed on the first side in a pattern that encircles the electrical components, the ground trace is electrically connected to the ground plane; and an electrically conductive containment shell that covers at least a portion of the first side encircled by the pattern and the electrical components attached thereto, the shell having an opening defined by a conductive ridge formed in the shape of the pattern that is engaged with the ground trace; wherein the ground plane and the conductive shell form a grounded EMI/RFI shield around the electrical components.

2. The electronic enclosure assembly of claim 1, further comprising: a housing in which the printed circuit board and the containment shell are disposed; a rib system that extends from an interior surface of the housing, the rib system formed in the shape of the pattern for exerting pressure on the conductive ridge against the ground trace.

3. The electronic enclosure assembly of claim 2, further comprising: a gasket formed in the shape of the pattern and disposed between the rib system and the conductive ridge.

4. The electronic enclosure assembly of claim 2, further comprising: a plurality of punctures formed in the conductive ridge for engaging the ground trace.

5. The electronic enclosure assembly of claim 2, further comprising: a plurality of tabs formed in the conductive ridge for engaging the ground trace.

6. The electronic enclosure assembly of claim 2, further comprising: a plurality of dimples formed in the conductive ridge for engaging the ground trace.

7. The electronic enclosure assembly of claim 2, wherein the containment shell includes: a thin sheet of non-conductive material; and a conductive coating applied to the sheet of non-conductive material.

8. An electronic enclosure assembly, comprising: a housing having a first portion and a second portion that mounts to the first portion; a printed circuit board disposed in the housing and including: electrical components attached to a first side of the printed circuit board, a conductive ground plane, and a ground trace formed on the first side in a pattern that encircles the electrical components, the ground trace is electrically connected to the ground plane; and an electrically conductive containment shell disposed in the housing for covering at least a portion of the first side encircled by the pattern and the electrical components attached thereto, the shell having an opening defined by a conductive ridge formed in the shape of the pattern; a rib system that extends from an interior surface of the first portion of the housing, the rib system formed in the shape of the pattern; wherein when the first and second housing portions are mounted together, the rib system presses the conductive ridge against the ground trace for forming an electrical contact therebetween so that the ground plane and the conductive shell form a grounded EMI/RFI shield around the electrical components.

9. The electronic enclosure assembly of claim 8, further comprising: a gasket formed in the shape of the pattern and disposed between the rib system and the conductive ridge.

10. The electronic enclosure assembly of claim 9, further comprising: a plurality of punctures formed in the conductive ridge for engaging the ground trace.

11. The electronic enclosure assembly of claim 9, further comprising: a plurality of tabs formed in the conductive ridge for engaging the ground trace.

12. The electronic enclosure assembly of claim 9, further comprising: a plurality of dimples formed in the conductive ridge for engaging the ground trace.

13. The electronic enclosure assembly of claim 9, wherein the containment shell includes: a thin sheet of non-conductive material; and a conductive coating applied to the sheet of non-conductive material.