Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1997-10-29
2003-05-27
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S034000, C361S816000
Reexamination Certificate
active
06570085
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to shielding apparatus for containing high frequency electromagnetic radiation within a personal computer, cellular telephone, or other electronic instrument.
Electromagnetic compatibility (EMC) is a broad term used to describe electromagnetic interference (EMI), radio frequency interference (RFI) and electrostatic discharge (ESD), and the above terms are often used interchangeably.
The fact that electronic devices are both sources and receptors of EMI creates a two-fold problem. Since electromagnetic radiation penetrating the device may cause electronic failure, manufacturers need to protect the operational integrity of their products. Secondly, manufacturers must comply with the regulations aimed at reducing electromagnetic radiation emitted into the atmosphere. Proper design is necessary to prevent the device's function from being disrupted by emissions from external sources and to minimize its system's emissions.
Today, plastics are replacing metals as the material for electronic enclosures since plastics offer increased design flexibility and productivity with decreased cost. The switch from metal to plastics as a housing material for electronic equipment has contributed to concern over EMI shielding. Plastics are insulators, so EMI waves pass freely through unshielded plastic without substantial impedance or resistance. Additionally, ever increasing device miniaturization and the increase in clock speeds of microprocessors used in computing devices makes it more difficult to handle the EMI pollution these faster computers generate. So a variety of technologies using metal/polymer combinations and composites are being used as a shielding material in electronic devices.
Current methods for shielding of electromagnetic interference (EMI) include the use of metal housings, metal, filled polymer housings, metal liners for housings, and conductive coatings for the interior of rigid polymer or composite housings.
Metal coatings for rigid plastic housings are applied through use of conductive paints or through application of metal platings applied by chemical plating (electroless plating), by electroplating, or through vacuum metallization. In addition, metal foils with adhesive backings may be applied to the inside of plastic cases for electronic instruments to achieve shielding requirements. Zinc Arc spray techniques are also available to apply a metal coating to a plastic housing.
Another shielding material is provided through the use of metal fibers sintered onto a polymeric substrate as is taught in U.S. Pat. No. 5,226,210, and commercially produced as #M 610D Thermoformable EMI-shielding material by the Minnesota Mining and Manufacturing Company of St. Paul, Minn.
Each of the current shielding methods have shortcomings. The major disadvantages of plating are its high cost, complex process cycles, and its application is limited to only certain polymer resins. Metal-filled resins for injection molding suffer from poor conductivity compared to metals. The conductive polymer resin is very expensive and complex shape molding is difficult from flow and uniformity perspectives.
Three general types of conductive metal-bearing paints are in general use. Silver paints have the bast electrical properties but, they are extremely expensive. Nickel paints are used for relatively low attenuation applications and are limited by high resistance and poor stability. Passivated copper paints have moderate cost and lower resistivity, but also lack stability. All paint applications have difficulties with coating uniformly, blow back in tight areas and recesses depending on part complexity, and application problems which can lead to flaking. Paints also fail ESD testing over 10 KVA.
EMI shielding through the use of metal cases for the personal computer or other electronic device may not always be desired because of concerns about weight and aesthetics, with weight being a serious concern for laptop computers or portable and handheld devices of any types. The use of a metal shroud to line a plastic case improves over the metal case in aesthetic and design concerns for the outside of the housing but results in an increased assembly step and little weight minimization. Metal also lacks the ability to be formed into complex shapes often taking up unnecessary room adjacent to the circuitry and assembled electrical components.
The use of coated plastic housings for electronic devices, including microcomputer and cellular telephones, may not provide a suitable solution when one considers that personal computers currently offered may operate at clock speeds of 100 MHz which gives rise to opportunities for EMI generation not previously confronted in the personal computer industry. Further, the ever increasing clock speeds of the personal computers being offered makes effective shielding more and more challenging since any breach in the shield which has one dimension in excess of 0.23 inch may allow substantial EMI leakage, causing the unit to fail United States Federal Communication Commission standards.
The use of metallic coatings on plastic housings presents certain manufacturing and service concerns. A slipped tool used during assembly or a repair can cause a scratch in the metal coating of sufficient size to cause a slot antenna, thereby making the case totally useless, and thereby leading to a costly item being discarded with little feasibility for successful recycling.
The seams of a metal plated plastic housing will act like slot antennae unless the housing sections are conductively joined by the use of overlapping joints, conductive gaskets, or conductive tape. When the housing must be opened for a repair or retrofit, it can be understood that some of the conductive interconnection may be degraded by the activity of disassembly.
Further background on prior art methods and characteristics of shielding methods may be examined in ‘EMI/RFI Shielding Guide” published by the GE Plastics Division of the General Electric Company, and in “the Designer's Guide to Electromagnetic Compatibility” by Gerke & Kimmel, Supplement to EDN Magazine, Volume 39, No. 2, (Jan. 20, 1994) to both of which the reader is directed.
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. A thermoformable polymeric sheet is formed into an enclosure sized and shaped to enclose an EMI emitting subsystem or component. The thermoformed polymeric enclosure is then metallized on all or selected surfaces by vacuum metallizing techniques where the thermoformed enclosure is placed in a vacuum chamber, treated with an ionized gas, and then metallized by the use of aluminum or other metal being vaporized by use, for example, of a current-passing tungsten filament, or other vaporization techniques. The enclosure is rotated within the chamber to allow metallization of all desired surfaces. Masking may be employed when certain regions or surfaces are preferred not to be metallized. The enclosure 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 enclosure electrically conductive. The enclosure is formed in the shape and size necessary to house and shield the EMI emitter; for example in the case of a personal computer, the enclosure may serve as a thin-walled case within the rigid outer housing of the computer. Alternatively, the enclosure may be formed to fit as an insert within a device's housing as a substitute for a metal insert shield, or the enclosure may be shaped and sized to contain only certain components which are emitters of, or susceptible to, EMI. Gangs of metallized enclosures may be devised with electrical isolation as desired provided by gaps in the metallization layers. Different electronic devices will require varying degrees of attenuation or shielding effectivene
Harms Allan L.
Shielding for Electronics, Inc.
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