Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1999-08-31
2001-08-07
Berhane, Adolf Deneke (Department: 2838)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S034000, C361S817000
Reexamination Certificate
active
06271465
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to electromagnetic interference (EMI) and radio frequency interference (RFI) shielding techniques and, more particularly, to methods and apparatus for providing EMI and/or RFI shielding for electronic and electrical circuitry disposed on a printed circuit board (PCB) and contained within a housing or enclosure.
BACKGROUND OF THE INVENTION
One result of the miniaturization of circuitry is that many more circuits, and different types of circuits, can be placed in close proximity to one another. For example, in some consumer goods, in particular handheld radiotelephones such as cellular telephones and personal communicators, one may find a microprocessor, a digital signal processor, a memory and sensitive RF transmit and receive circuitry all located upon a single PCB within an area of but a few square inches. It can be appreciated that to ensure reliable operation it is important to reduce or eliminate cross-coupling and interference between these various circuits, as well as to prevent externally generated interference from coupling through the radiotelephone housing into the circuits and, vice versa, to prevent signals from the circuits from coupling through the housing to external circuitry.
It is well known in the art to provide a plastic housing having a metalized inner surface to reduce the passage of undesirable signals through a housing. However, metalized housings are expensive to fabricate. Furthermore, a metalized plastic housing is not as easily recycled as an unmetalized, plastic-only housing.
It is also well known in the art to provide metal enclosures for placing over certain sensitive circuitry to prevent signals from coupling into and/or out of the circuitry. For example, sensitive receiver circuits may have a metal shield (or “can”) placed over them, with the can being held in place on the PCB with fasteners (e.g., screws) or soldered into place. Some means is then also provided for electrically coupling the metal shield to a common potential, such as circuit ground or RF ground.
Referring to
FIG. 1
, one prior art metal shield is placed over a sensitive integrated circuit (IC)
1
, and includes an inner sheet metal shield forming a sort of wall around the IC
1
. The inner shield
2
has a plurality of feet or flanges
3
that are held in place with solder
4
to a conductive trace
5
A (e.g., a circuit ground trace) that runs over a surface of a PCB
5
B. An outer sheet metal shield
6
or cover, which may be perforated, fits over the inner shield
2
and is connected to the inner shield
2
using some type of fasteners
7
, such as spot welds or rivets. For ease of handling, the inner and outer shields can be connected together before being placed on the PCB
5
B and soldered (e.g., using surface mount technology) to the trace(s)
5
A.
As can be appreciated, the use of this type of metal shield (which may have dimensions of only two or three centimeters along a side) has a number of disadvantages, including high cost and weight (relative to a metalized cover), as well a requirement that expensive tooling be provided.
Furthermore, the use of sheet metal places restrictions on the shapes that the metal shield can assume. Also, the use of the metal shield may have an impact on the soldering operation, due at least to the relatively larger thermal mass of the shield as compared to the surface mount solder leads found on conventional ICs and other components.
Another conventional shielding technique uses a conductive compression gasket, which may be dispensed, die-cut or metal. However, this approach is also expensive, as well as being environmentally unfriendly and logistically complex, as well as having a high tooling cost.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is a first object and advantage of this invention to provide an improved EMI/RFI shielding technique that overcomes the foregoing and other problems.
It is another object and advantage of this invention to provide an electrically conductive EMI/RFI shield that is substantially lower in cost, simpler in construction, and lighter in weight than the conventional metal shield.
It is another object and advantage of this invention to provide an electrically conductive EMI/RFI shield that does not require a soldering operation or some manually intensive operation to attach to a PCB.
It is a further object and advantage of this invention to provide an electrically conductive EMI/RFI shield of simple, low cost construction that removes a requirement to metalize an inner cover of a housing or enclosure.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. In one aspect this invention provides an electrical shield having a shell comprised of a metalized polymeric material with a shape that is predetermined to form within at least one shielded cavity. The shell has sidewalls that terminate in a flange extending at an angle to the sidewalls. The flange has a lower surface for electrically coupling the shell to a predetermined electrical potential and an upper surface for engaging a structure, preferably a protruding rib from a cover, that exerts a compressive force on the top surface for urging the lower surface into an electrical coupling arrangement with an underlying planar conductor carrying the predetermined electrical potential (e.g., ground). The lower surface can include or be placed upon a layer of electrically conductive compliant material, such as a conductive adhesive, especially for high attenuation applications (e.g., RF or any high frequency applications, such as those equal to or greater than 1 GHz.)
In a preferred embodiment at least one of the sidewalls includes a projecting fluted structure having a size and location that is predetermined to engage a side of the structure that exerts the compressive force.
In a further aspect of this invention a cover is disclosed for use with an electronic device that contains a circuit board carrying components. The cover has, on an inner surface thereof, at least one rib structure protruding from the inner surface. The rib structure is located so as to at least partially surround a portion of the components to be shielded. The rib structure has a length selected for engaging the flange that extends outward from the sloping sidewalls of the electrically conductive shield structure forming the shielded cavity over the portion of the components. The length is preselected for exerting and maintaining the compressive force on the flange when the cover is placed into an intended mechanical engagement with a remainder of the electronic device. The rib structure may further include at least one crush compression feature on a terminal end portion thereof for engaging the flange.
Also disclosed is a method for assembling an electrical shield structure over circuitry to be shielded. The method includes steps of (A) providing a circuit board containing components; (B) placing over a component to be shielded a shield comprised of a metalized polymeric material having a shape that is predetermined to form within at least one cavity, the shield having sidewalls and a flange as discussed above; and (C) installing a cover over the circuit board, the cover including the at least one rib structure that protrudes from an inner surface. The rib structure is located so as to at least partially surround the shield, and has a length selected for engaging the flange so as to exert and maintain the compressive force on the flange.
REFERENCES:
patent: 5162980 (1992-11-01), Morgan et al.
patent: 5239125 (1993-08-01), Savage et al.
patent: 5252782 (1993-10-01), Cantrell et al.
patent: 5497292 (1996-03-01), Gandre
patent: 5519585 (1996-05-01), Jones et al.
patent: 5566055 (1996-10-01), Salvi, Jr.
patent: 5594199 (1997-01-01), Ciaccio
patent: 5638259 (1997-06-01), McCarthy et al.
patent: 5696669 (1997-12-01), Bassler et al.
patent: 5704117 (1998-01-01), Mok et al.
patent: 5763824 (1998-06-01
Berhane Adolf Deneke
Nokia Mobile Phones Limited
Ohlandt Greeley Ruggiero & Perle LLP
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