Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-08-24
2002-04-23
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S751000, C361S772000, C361S816000, C361S818000
Reexamination Certificate
active
06376779
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to printed circuit boards and more particularly to high-frequency EMC containment therefor.
BACKGROUND OF THE INVENTION
Electronic components, including semiconductor components, integrated circuits and chips, carried on printed circuit boards are a source of electromagnetic interference or radio frequency disturbances when in use. Manufacturers of electronic devices must provide means to contain such electromagnetic interference or radio frequency disturbances when the devices are in use (EMC). For ease, both electromagnetic interference and radio frequency disturbances will be referred to as electromagnetic emissions.
Coating or “plating” a printed circuit board with a film of highly conductive material such as copper over all surfaces enables the printed circuit board to effectively operate electrically as a Faraday cage, and thereby constitute one means to contain such electromagnetic emissions.
To manufacture a conductively shielded, or “plated”, printed circuit board, it is typical to provide the printed circuit board with extra material, or “scrap”, which provides a location at which to support the printed circuit board during the plating step. The scrap border is typically connected to the printed circuit board by support tabs. Following plating, the scrap border is removed leaving the final printed circuit board.
However, the removal of the scrap border results in a “break” in the conductive shielding where the support tabs were severed, leaving the final board susceptible to electromagnetic emissions entering or leaving the board at the site of these support tabs.
One solution previously employed was to drill through a printed circuit board alternating rows of shielding holes, or vias, inward of each support tab. The vias are interiorly coated or filled with the conductive material. The vias come into contact with all ground layers in the printed circuit board, thereby connecting the ground layers to the top and bottom conductive layers of the printed circuit board. These vias attenuate frequencies of electromagnetic waves along their interior lengths, thereby effectively reducing the break in the conductive shield at the support tabs, and thus further reduce the chance of electromagnetic waves travelling on the inside surface of the top or bottom conductive surface of the printed control board from reaching and travelling along the outside surface and contributing to radiated emissions.
However, the attenuation of high frequency electromagnetic waves is problematic. As frequencies increase, the diameter and spacing of the vias must be decreased to accomplish signal attenuation. It is difficult to produce vias fine enough, or produce a fine enough piton to produce the vias, to attenuate signal energies in excess of 622 megahertz having regard to such factors as the thickness of printed circuit boards and manufacturing limitations.
Accordingly it is desirable to have an alternative means to attenuate high frequency signal energies on plated printed circuit boards.
SUMMARY OF THE INVENTION
The present invention seeks to provide a printed circuit board which minimizes the above problems.
According to one aspect of the invention, there is provided a printed circuit board having a plurality of spaced apart scrap border support tabs along the perimeter of the board. The board surfaces are coated with a conductive shielding material, except that each tab presents an uncoated, unshielded surface at the point of severance created by detachment of the scrap border subsequent to coating application. The printed circuit board includes a plurality of spaced apart elongated apertures adjacent to the perimeter, with each aperture being inwardly coincident to a respective one of each of the support tabs, each aperture defining an inner surface adjacent to the corresponding support tab with a portion of the inner surface being substantially parallel to the perimeter of the circuit board, the inner surface of each aperture also being coated with the conductive shielding material with the latter being attached to the conductive shielding material of the board surface, such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
The invention defined above extends to all forms of multiple layer printed circuit boards including backplanes and panels used in electronic systems.
In another aspect of the invention there is a printed circuit assembly provided with a scrap border detachably connected to the perimeter of a printed circuit board by a plurality of spaced apart support tabs, all of which are coated with a conductive shielding material during manufacture. The scrap border is detached from the printed circuit board subsequent to coating by severance of the tabs whereby each tab presents an uncoated, unshielded surface at the point of severance. The printed circuit board includes a plurality of spaced apart elongated apertures adjacent to the perimeter of the board, with each aperture being inwardly coincident to a respective one of each of the support tabs. Each aperture defines an inner surface and an edge adjacent to the corresponding support tab, with a portion of the inner surface and the edge being substantially parallel to the perimeter of the circuit board, with the entire inner surface of each aperture also being coated with the conductive shielding material during manufacture, with the latter being attached to the conductive shielding material on the board surfaces such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
In yet another aspect of the invention there is provided a method of manufacturing a printed circuit assembly having multiple layers in a sandwich arrangement comprising: providing a circuit assembly including a scrap border extending around and detachably connected to the perimeter of a centrally disposed printed circuit board by a plurality of spaced apart support tabs, fully coating the assembly with a conductive shielding material, and detaching the scrap border from the printed circuit board subsequent to coating by severance of the tabs, whereby each tab presents an uncoated, unshielded surface at the point of severance; the method being characterized by the formation in the centrally located printed circuit board of a plurality of spaced apart elongated apertures adjacent the perimeter, with each aperture being inwardly of the perimeter coincident to a respective one of each of the support tabs, and formed such that each aperture defines an inner surface and an edge adjacent to the corresponding support tab, with a portion of the inner surface and the edge being substantially parallel to the perimeter of the circuit board, with the coating step being carried out such that the inner surface of each aperture becomes coated with the conductive shielding material and attached to the conductive shielding material on the board surface such that a waveguide configuration is created which serves to attenuate electromagnetic emissions entering into or leaving the printed control board at the corresponding uncoated unshielded surface portions of the support tab.
Such an invention allows a plated printed control board to be EMC compliant at any operating frequency with signal energies up to and in excess of 40 gigahertz.
A separate enclosure around a printed circuit board carrying electronic components that can attenuate electromagnetic emissions may be used as another means to contain electromagnetic emissions. Such enclosures are typically referred to as Faraday cages.
Previously, Faraday cage enclosures surrounding a printed circuit board carrying electronic components on both major surfaces comprised two units which came into contact with each other. An electrically condu
Edginton Kyle G.
Kasprowicz Denis
Shearman Simon E.
Skanes Geoffrey G.
Alcala José H.
Gaffin Jeffrey
Nortel Networks Limited
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