Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-03-18
2001-01-30
Gandhi, Jayprakash N. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S800000, C174S034000, C206S719000
Reexamination Certificate
active
06181573
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to shielding of electromagnetic radiation and, more particularly, to a device for shielding electric or electronic components on a Printed Circuit Board (PCB).
BACKGROUND OF THE INVENTION
In present telecommunication systems, in particular systems for cellular and cordless radio telecommunications, transceivers operating at frequencies in the GHz Radio Frequency (RF) band are used.
It is known that electric or electronic components or apparatuses and devices in the vicinity of electromagnetic radiation sources can be driven into malfunctioning if subjected to certain levels of electromagnetic radiation.
To avoid as much as possible any unwanted influence of electromagnetic radiation from devices such as oscillators in transceivers for telecommunication or other devices operating in the RF band, it is known to shield such radiation sources by encasing the radiation source or the device such as a transceiver incorporating the radiation source in a suitable housing. Such a housing is usually formed of conductive materials having an ability to concentrate high levels of magnetic flux. Accordingly, the electromagnetic radiation is prevented from leaving the housing.
On the other hand, it is also possible to encase an electric or electronic component which is sensitive to electromagnetic fields, such to prevent that any magnetic, electric or electromagnetic fields can reach the component through its encasing. Housings for shielding electromagnetic radiation are also known as Faraday cages.
U.S. Pat. No. 5,339,222 discloses a two-part electromagnetic radiation shielding housing for receiving a complete PCB.
GB-A-2,285,181 discloses a two-part shielding device for mounting at a PCB such that in use, a component requiring shielding is completely surrounded and enclosed by the shielding device and a signal ground layer of the PCB to which the device is electrically connected. The component can be either an electromagnetic radiation source or a component which is vulnerable to electromagnetic fields.
With the present trend towards using frequencies in the GHz range and due to increasing governmental regulations on electromagnetic stray field levels of devices such as mobile or cordless telephones and (small) base stations for indoor use, the presently available shielding devices are not suitable to meet future demanding ElectroMagnetic Compatibility (EMC) requirements.
SUMMARY OF THE INVENTION
The present invention has for its object to provide an improved electromagnetic radiation shielding device for use with a PCB providing an effective shielding for frequencies up to and above the GHz RF range.
It is a further object of the present invention to provide an electromagnetic radiation shielding device for mounting on a PCB and receiving a component or a number of components requiring shielding and mounted on the PCB.
It is a still further object of the present invention to provide an electromagnetic radiation shielding device which can be manufactured from one piece of sheet metal by common manufacturing techniques such as folding and drawing.
These and other objects and advantages of the present invention are provided by a two-part electromagnetic radiation shielding device for mounting on a Printed Circuit Board (PCB), comprising a substantially open frame member for receiving an electric or electronic component requiring shielding and a closing can or cover member. The frame and cover members are constructed from electrically conductive material and have peripheral sides adapted to grip each other such that, in use, the frame and cover members surround the component mounted on the PCB in a closing arrangement. Following the invention, the peripheral sides of the frame and cover member have a shape such that, in use, a continuous gripping force is exerted to establish and maintain circumferential electrical contact between the frame and cover member at facing edges of the peripheral sides of the members.
With the design of the present invention separate mechanical and electrical contact engagements of the frame and cover members are obtained, such that both can be optimised for its own purpose.
That is, under a continuous mutual mechanical force exerted by the design of the frame and cover part according to the invention, an electrical contact zone over the complete circumference of the frame and cover member is established to maximize its EMC performance. This contact zone is free from any holes or gaps arising from design purposes. Warping of any or both of the frame and cover members is compensated and flattened out by the mechanical force provided.
In the design according to the present invention, the electrical contact between the frame and cover members is primarily provided by the facing edges of the members and secondary by the mechanical contact of the peripheral sides for keeping the members engaged.
This, different from the shielding device disclosed by GB-A-2,285,181 wherein the mechanical and electrical contact points are the same and provided by resilient spaced apart fingers of the cover member which, in use, engage flat peripheral sides of the frame member. Through the openings between the fingers electromagnetic radiation, in particular at frequencies in the GHz RF frequency band, can leak from a radiation source enclosed by the device towards its outside or from the outside of the device towards the component enclosed. Although a secondary electrical contact will be established between the flat top of the cover and inwardly extending flanges of the frame member, this electrical contact is subject to loosening of the engagement of the frame and cover member due to mechanical shocks and warping of the cover, such that a complete circumferential electrical contact is not guaranteed.
With the present invention, a continuous force is exerted to maintain the circumferential electrical contact between the facing edges of the frame and cover members independent of mechanical shocks or warping of one or both of the members. In a preferred embodiment of the device according to the invention, the facing edges of the members comprise contact flanges extending transverse to the engaging peripheral sides. This to reduce as much as possible interruptions in the electrical contact due to small mechanical irregularities from burrs, notches and scratches. In order to achieve an optimum shielding effect, it is preferred to have these flanges, and in general the facing edges, formed as a closed contour from one piece of material, such that, in use, an essentially uninterrupted circumferential electrical contact between the facing edges is established and maintained.
For an optimum shielding effect, it is further preferred to have the electrical contact as close a possible near the PCB. To this end, in an embodiment of the device according to the invention, the facing edges are provided by edges of the frame member of its peripheral sides which, in use, are near the PCB and by the free edges of the peripheral sides of the cover member. Such that, in use, the electrical contact between the facing edges is provided near the PCB. In a preferred embodiment of the invention, the frame member is arranged to receive the cover member.
In another preferred embodiment of the shielding device of the invention, the peripheral sides of one of the members comprise a bevelled step and the peripheral sides of the other member comprise a resilient portion, such that in use the bevelled step and resilient portion engage each other thereby forcing the members in a closing arrangement.
That is, the step and the resilient portion are shaped such that, in use, the resilient portion by its engagement of the bevelled step exerts a force in a direction of closing the members, such to provide and maintain a force contact between the facing edges of the members.
To facilitate the mounting of the frame and cover member, the resilient portion may take the form of a shallow V, such that in use the V portion engages the bevelled step.
Due to the separation of the me
Burns Doane Swecker & Mathis L.L.P.
Gandhi Jayprakash N.
Telefonaktiebolaget LM Ericsson (publ)
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