Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-03-21
2002-11-19
Gandhi, Jayprakash N. (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S752000, C361S800000, C361S816000, C174S034000, C174S034000, C252S500000, C455S300000
Reexamination Certificate
active
06483719
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to electronic components shielded from electromagnetic interference through the use of conforming shield enclosures. Conforming shield enclosures are flexible metalized thermoformed thin-wall polycarbonate polymer film substrates used to shield a radiation source. The present invention relates to conforming shielded forms for electronic component assemblies and specifically to electronic component assemblies which are shielded to protect against electromagnetic and radio frequency interference. Specifically, the shielded electronic component assembly comprises (a) a semiconductor device to be shielded from electromagnetic frequencies; (b) a reference potential source; (c) a housing enclosing the semiconductor device within the assembly; and (d) a conforming shield enclosure electrically connected to the reference potential source. The conforming shield enclosure comprises a flexible, metalized thermoformable polymer having dimensions conforming to the inside of the housing and enclosing and thereby shielding the semiconductor device from electromagnetic frequencies. The conforming shield enclosure is prepared by paint metalization.
DESCRIPTION OF THE BACKGROUND
Propagating electrical signals have an electrical field component and a magnetic field component. Electrical signals cause circuit components to radiate a portion of the spectral energy of the propagating signal causing electromagnetic interference. Electromagnetic interference is the generation of undesired electrical signals in electronic system circuitry because of the unintentional coupling of impinging electromagnetic field energy. Circuit elements are effective in radiating spectral components which have wavelengths similar to the radiating element dimensions. Long circuit elements will be more effective in radiating low frequency noise and short circuit elements will be more effective in radiating high frequency noise. These circuit elements behave like antennae which are designed for the transmission of the radiating wavelengths.
Integrated circuits which have output drivers that create pulses with high amounts of spectral energy are more likely than low power drivers to cause electromagnetic interference because of a mismatch between the driver and the line impedance, and the resistance to instantaneous signal propagation imposed by the parasitics of the conductor. These disturbances in the electromagnetic field result in reflections of the signal energy at the points where the variation occurred. If the signal is not absorbed by the load at the end of the conductor length because of unmatched impedances or lack of line termination, the unabsorbed energy will be reflected back towards the source giving rise to radiated emissions. Proper termination and controlled impedance interconnections can reduce radiated noise.
The coupling of signal energy from an active signal net onto another signal net is called crosstalk. Crosstalk is within-system electromagnetic interference as opposed to electromagnetic interference from a distant source. Crosstalk is proportional to the length of the net parallelism and the characteristic impedance level, and inversely proportional to the spacing between signal nets. Proper interconnect layout design can reduce the incidence of crosstalk. Strong sources of low impedance, magnetic field rich electromagnetic interference are relatively high current and relatively low voltage components. Magnetic fields possessing high intensity can induce spurious current flow in other system components. Noise radiated from within a system can interfere with system performance by coupling with other system elements, not just adjacent conductor nets, as another form of within system electromagnetic interference.
Because electronic systems are becoming smaller and the density of electrical components increasing, the dimensions of the average circuit element is decreasing favoring the radiation of higher frequency signals. The increasing operating frequency in these electrical systems results in increasing high frequency electromagnetic interference. Electromagnetic interference can come from electrical systems distant from a receiving circuit, or the source of the noise can come from a circuit within the same system (crosstalk or near source radiated emission coupling). The effect of all these sources of noise is to degrade the performance or to induce errors in the systems. The prevalence of high frequency systems and portable electronics is creating a very complex spectral environment for the operation of sensitive electrical systems.
The electromagnetic interference shielding of electronic component assemblies has taken many forms. Sensitive or radiating devices may be covered with a lid and/or enclosure which is connected to ground potential in the process of securing the cover in place. Shielding close to the source, where the field intensity is the highest, requires greater shield efficiency to contain the field. It is common to shield the sensitive, electromagnetic interference receiving component or even the entire circuit board. Polymer thick film conductor materials, such as a screen-printable copper filled epoxy paste, are sometimes used to form a shield. Individual ferrite components may be placed on device pins or in series within a circuit to attenuate unwanted noise. A ferrite component may be used with a capacitor in order to form a low frequency inductance-capacitance band pass filter. Many enclosed systems powered by external alternating current are shielded from electromagnetic interference by the incorporation of internal shields which are connected to ground potential. A metal cabinet housing which encloses the system may be designed to function as a shield. Disadvantages of metal housings are that they are often expensive, heavy, and difficult to make in complex shapes. The inside of a molded plastic housing may be coated with a thin metal film through vacuum metalization but this process often yields a brittle less flexible shield. Another method is to coat the enclosure with a thin film of a conductor using a metal-filled paint. A metal-filled plastic may also be used to form the housing.
U.S. Pat. No. 4,012,089 (Ward) discloses an electronic instrument enclosure using a molded, laminated plastic enclosure having an outer shell made of a thermoplastic composition which has desirable physical and electrical characteristics (see FIG.
1
). An inner plastic shell is nested in the outer shell with a stiffening filler material sandwiched between. Heat pipes may be imbedded in the shells to provide cooling for the enclosed electrical components and inserts for the mounting of slides and guidepins can be molded the inner shell for ease in mounting hardware for slidable drawers. The inside walls of the inner shell may be vacuum metalized or plated to provide radio frequency shielding.
U.S. Pat. No. 4,227,037 (Layton) discloses a container having complementary upper and lower portions (see FIG.
1
). Each portion is adapted to mate with and engage to define an enclosed inner chamber. The upper and lower portions each have laminated outer casings. A non-metallic, electrically conductive, inner layer is integrally molded with and bonded between the outer non-metallic reinforcing layers to shield the inner chamber from electromagnetic and radio frequency interference.
U.S. Pat. No. 4,678,716 (Tzeng) discloses an electrically conductive particle for use as a conductive filler in a resin matrix suitable for electromagnetic shielding use in gaskets (see FIG.
1
). The particle comprises an inner core of an aluminum silicon alloy having from 5% to 20% by weight of silicon, an intermediate layer of a metal selected from the group consisting of mercury, palladium, copper, chromium, platinum, gold, nickel, tin, and zinc, and an outer layer of a highly electrically conductive metal.
U.S. Pat. No. 4,739,453 (Kurokawa) discloses a shielding apparatus for shielding electric circuitry mounted on a printed circuit board against interfering elect
Gandhi Jayprakash N.
Muccino Richard R.
Spraylat Corporation
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