EMI/RFI and vibration resistant electronics enclosure

Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition

Reexamination Certificate

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Details

C428S611000, C361S816000, C361S818000

Reexamination Certificate

active

06252159

ABSTRACT:

BACKGROUND OF THE INVENTION
Sensitive electronics are often prone to radio frequency interference (RFI) as well as electromagnetic interference (EMI) at frequencies ranging up to several megahertz. The electrical performance of an analog or digital circuit can be impaired significantly by such types of interference. In addition, vibrational energy impinging upon an enclosure containing such circuits and transmitted into the electronic components themselves can impair circuit performance to a degree dependent upon the microphonic sensitivity of the electronic components and the internal wiring used. Sources of the RFI, EMI, and vibrational energy are assumed to be external to the enclosure and may include other high frequency electronics (especially digital circuitry), electrical motors, transformers, AC power lines (and cables), electrical switches, electrical lighting, loudspeaker output, etc. In addition to preventing RF, magnetic and vibrational energy from entering the circuitry contained within the enclosure, it is desirable to prevent any RF or magnetic energy generated by the contained circuitry from leaving the enclosure, where it could affect other sensitive electrical circuits.
Electronics are usually housed within an enclosure formed of either aluminum, steel, or plastic. Plastic enclosures offer no protection from RFI or EMI unless they are sprayed with a highly conductive coating (usually a metal) and properly grounded. This can provide a degree of RFI protection, but offers very little protection from magnetic interference, except at very high frequencies (>100 kHz). Steel enclosures offer some protection from magnetically generated interference, but sacrifice a significant degree of RFI protection due to the fairly poor electrical conductivity of steel at very high frequencies. This can be remedied somewhat by plating the steel with a very conductive metal such as copper. However, steel is still not entirely effective in preventing low frequency (e.g., 60 Hz) magnetic interference, especially if the field strength is high, because of the marginal magnetic permeability of the steel materials typically used. An aluminum enclosure offers good protection from RFI and very high frequency EMI (assuming correct grounding guidelines are followed) due to its high degree of electrical conductivity. Aluminum enclosures, however, offer very little protection from EMI at frequencies below 100 kHz since they are not magnetically permeable, i.e., they are transparent to low frequency magnetic fields. It is apparent that none of the commonly used materials for electronics enclosures offers a high degree of both RFI and EMI protection, particularly EMI protection at low frequencies. In addition, immunity from vibrational energy is rarely addressed correctly in most enclosure designs. Often, materials with dubious damping qualities are used. In most cases, even when the appropriate materials are utilized, the surface area coverage is inadequate to provide any significant amount of vibrational energy damping, particularly at lower frequencies, below 200 Hz. It should be emphasized that usually, immunity from vibrational energy is not addressed at all.
It is therefore an object of the present invention to prevent RF, magnetic and vibrational energy from entering the circuitry contained within an enclosure. It is a further object of the invention to provide an enclosure that prevents entry of such energy, while preventing any RF or magnetic energy generated by the contained circuitry from leaving the enclosure, where it could affect other sensitive electrical circuits. It is yet another object of the invention to provide an enclosure that does not have the detrimental qualities of enclosures formed solely of either aluminum, steel, or plastic.
SUMMARY OF THE INVENTION
The above objects and others are accomplished by a panel for an enclosure for housing at least one electronic component, which includes an electrically conductive layer, a separate, magnetically permeable layer connected to the electrically conductive layer, and a separate, extensionally damped layer covering at least 80 percent of a surface of the panel and attached to one or more of the electrically conductive layer and the magnetically permeable layer.
The electrically conductive layer may be formed of a material conductive at frequencies greater than 100 kHz and include at least one of aluminum, copper-plated steel, plastic which is sprayed with a conductive coating, and plastic which is vapor coated with a conductive coating. The magnetically permeable layer comprises at least one of CO-NETIC® and NETIC® magnetic shielding alloy and is a foil of at least about 0.1 mm in thickness, preferably at least about 0.25 mm. The CO-NETIC® magnetic shielding alloy can be CO-NETIC AA foil, and said NETIC® magnetic shielding alloy can be NETIC S3-6 foil. The magnetic shielding alloy should be able to protect the housed components from low frequency EMI, specifically that below 100 kHz, and magnetic interference below 60 Hz. The magnetic shielding should be hydrogen annealed, especially when it will be subject to bending and other forming.
The extensionally damped layer may include a single pass extruded, thermoplastic alloy damping material, such as ISODAMP® CN, with a thickness of at least about 1.5 mm. Preferably, the thickness of the ISODAMP® CN layer is at least about 3.1 mm. The extensionally damped layer may also be SORBOTHANE®, with a thickness of at least about 3.2 mm. The extensionally damped layer is able to protect the housed components from vibrational energy at high and low frequencies, especially very low frequencies below 200 Hz.
Each of the electrically conductive layer, magnetically permeable layer, and extensionally damped layer are preferably connected using an adhesive material.
The above objects and others are further accomplished by an enclosure for housing at least one electronic component, which comprises at least one panel having properties as detailed above.
The electrically conductive layer is preferably an outermost layer, and the extensionally damped layer is preferably an innermost layer. Electrical and magnetic conductivity must be maintained between all panels of the enclosure, and a ground is provided between circuitry of the electrical component and the electrically conductive layer. A connector, which connects each of the panels together, can provide continuity of electrical conductivity throughout the enclosure.


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CO-NETIC & NITIC Magnetic Shielding Alloys, Brochure.
SORBOTHANE Inc., Brochure Jun. 1995.

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