RF SHIELD FOR CIRCUIT CARD

BLINDAGE HF POUR CARTE IMPRIMEE

English Abstract

A metallized plastic RFI/EMI shield is provided against electromagnetic energy
for a circuit card (32) wherein the shield is formed of a box enclosure with
two casing halves to enclose the card (32), the first casing half having a
solid flange side walls surrounding the card (32) and the second casing half
(24) having a flange (26) formed by flat, plate-like closely-spaced fingers
(28), the fingers confronting, being in electrical contact with and mating in
full length with substantially the entire periphery of the side walls of the
first casing half. The fingers (28) are preferably spaced with a separation of
less than 1/4 wavelength of any measurable harmonic of the intended operating
frequency, typically up to the tenth harmonic. A slot (18) may be provided to
receive the fingers (28) on the outside of the side walls, and closely-spaced
grounding pins arranged around the periphery of the card (32) may extend
between the first and second casing halves through ground plated perforations
(34) along the edge of the side walls.

French Abstract

Cette invention concerne un blindage en plastique métallisé contre les parasites radioélectriques et électromagnétiques (RFI/EMI) destiné à une carte imprimée (32). Ce blindage est constitué d'une enceinte de type boîte présentant deux moitiés de boîtier conçues pour enfermer ladite carte (32), la première moitié de boîtier comportant des parois latérales à rebord solide entourant ladite carte (32) et la seconde moitié de boîtier (24) présentant un rebord (26) constitué de doigts (28) plats, de type plaque et très rapprochés, lesdits doigts faisant face à sensiblement tout le pourtour des parois latérales de la première moitié de boîtier, pourtour avec lequel ces doigts sont accouplés sur toute leur longueur et en contact électrique. Les doigts (28) sont, de préférence, espacés d'une distance inférieure au quart de la longueur d'onde de toute harmonique mesurable de la fréquence de service voulue, typiquement jusqu'à la dixième harmonique. On peut prévoir une fente (18) destinée à loger les doigts (28) sur le côté extérieur des parois latérales, ainsi que de broches de mise à la masse très rapprochées, réparties sur le pourtour de la carte (32) qui sont disposées d'une moitié de boîtier à l'autre au travers de perforations (34) ayant un revêtement de mise à la masse arrangées le long du bord des parois latérales.

Claims

WHAT IS CLAIMED IS:
1. A shield against electromagnetic energy for an
electromagnetic circuit card, said shield comprising:
a box enclosure for said electromagnetic circuit
card, said box enclosure having a first casing and a second
casing, said first casing having a solid first flange of a
first contour defining side walls surrounding said
electromagnetic circuit card and said second casing having a
second flange of a second contour defining side walls, wherein
said second contour is formed by closely-spaced fingers, said
fingers confronting, being in electrical contact with and
mating in full length with said first contour along
substantially the entire periphery of said side walls.
2. The shield according to claim 1 wherein said
fingers are spaced from one another at a distance
substantially less than 1/4 wavelength of a measurable
harmonic of a fundamental operating frequency of said
electromagnetic circuit.
3. The shield according to claim 1 wherein said
fingers are flat plates having a face for flexibly confronting
said first contour.
4. The shield according to claim 3 wherein said
second casing is formed of metallized plastic, and wherein
said fingers flexibly conform to said first contour.
5. The shield according to claim 1 wherein said
side walls are disposed within boundaries defined by said
fingers.
6. The shield according to claim 5 wherein said
first casing further includes a lip disposed in parallel to
said side walls to define a receiving slot for said fingers.

6
7. The shield according to claim 6 wherein said
electromagnetic circuit card has a plated periphery and
further including a plurality of closely-spaced pins mating to
said periphery and protruding from said first casing into said
second casing through said periphery.
8. The shield according to claim 6 wherein said
electromagnetic circuit card has a periphery of grounded
perforations and said first casing further includes a
plurality of closely-spaced pins mating to said perforations
and protruding from said first casing into said second casing
through said periphery.
9. A shield against electromagnetic energy for an
electromagnetic circuit card, said shield comprising:
a box enclosure for said electromagnetic circuit
card, said box enclosure having a first casing and a second
casing, said second casing being formed of metallized plastic,
said first casing having a solid first flange of a first
contour defining side walls surrounding said electromagnetic
circuit card and said second casing having a second flange of
a second contour defining side walls, wherein said second
contour is formed by closely-spaced fingers, said fingers
confronting, being in electrical contact with and mating in
full length with said first contour along substantially the
entire periphery of said side walls, wherein said fingers are
flat plates having a face for flexibly confronting said first
contour and are spaced from one another at a distance
substantially less than 1/4 wavelength of a measurable
harmonic of a fundamental operating frequency of said
electromagnetic circuit
10. The shield according to claim 9 wherein said
first casing further includes a lip disposed in parallel to
said side walls to define a receiving slot for said fingers.

7
11. The shield according to claim 10 wherein said
electromagnetic circuit card has a plated periphery and
further including a plurality of closely-spaced pins mating to
said periphery and protruding from said first casing into said
second casing through said periphery.

Description

CA 022311~0 1998-03-04
W O 97/11592 PCTrUS96/14711
RF SHIELD FOR CIRCUIT CARD
BACKGROUND OF THE INv~NllON
This invention relates to electromagnetic shielding
problems confronting radio frequency circuits. More
particularly, the invention relates to a manufacturable
shielding enclosure for circuit cards which emit unwanted
electromagnetic radiation.
Shielding against unwanted high frequency r.f.
emissions is a challenge, since any seemingly small aperture
in a shielding enclosure may inadvertently form a radiating
aperture in the nature of a slot antenna structure. In the
prior art, it has been necessary to provide several levels or
layers of shielding to prevent unwanted emissions and to meet
regulatory st~n~Ards for emission suppression. It has been
observed that a major source of leakage is perimeter apertures
in the shield. However, manufacturability is an important
consideration in mass production applications. It is costly
and impractical to test and customize each and every enclosure
during assembly to assure emissions st~n~rds compliance.
What is needed is an enclosure which is readily manufactured,
easily assembled with a circuit card and essentially fully
immune to unwanted r.f. emissions.
SUMWARY OF THE lNV~NllON
According to the invention, a metallized plastic
RFI/EMI shield is provided against ele~lu ~gnetic energy for
a circuit card wherein the shield is formed of a box enclosure
with two casing halves to enclose the card, the first casing
half having a solid flange side walls ~ullounding the card and
the second casing half having a flange formed by flat, plate-
like closely-spaced fingers, the fingers confronting, being in
electrical contact with and mating in full length with
substantially the entire periphery of the side walls of the
first casing half. The fingers are preferably spaced with a

CA 022311~0 1998-03-04
W O 97/11592 PCT~US96/14711
separation of less than 1/4 wavelength of any measurable
harmonic of the intended operating frequency, typically up to
the tenth harmonic. A slot may be provided to receive the
fingers on the outside of the side walls, and closely-spaced
grounding pins arranged around the periphery of the card may
extend between the first and second casing halves through
ground plated perforations along the edge of the side walls.
The invention will be better understood upon
reference to the following detailed description in connection
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of a first casing half
of an enclosure.
Fig. 2 is a perspective view of a second casing half
of an enclosure which mates with the first half casing to form
an effective r.f. shield according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Fig. 1 is a perspective view of a first casing half
10 an enclosure according to the invention. Designated
arbitrarily the upper half casing, this half 10 comprises a
solid base 12 and a side wall 14 of a solid flange ext~n~;ng
along the entire enclosure periphery. Optionally there may be
a lip 16 outside the side wall 14 to define a slot 18 between
the lip 16 and the side wall 14 all along the periphery of the
base 12. AS understood, each circuitous path for an r.f.
signal increases path loss and thus adds to the effectiveness
of ~u~lession. Within the interior, there are internal
isolation walls 20, some of which are disposed along the side
walls, and some of which traverse the base. As shown in this
embodiment, the internal isolation walls have along their
(inwardly-directed) edges regular, closely-spaced indentations
2 2 or holes. Alternatively, the indentations may be
protrusions or pins which mate to such indentations 22. As
hereinafter explained, these indentations and ~r oLL.lsions form
another level of shielding and grounding for spurious
emissions.

CA 022311~0 1998-03-04
W O 97/11592 PCTAJS96/14711
Referring now to Fig. 2, there is a perspective view
of a second casing half 24 of an r.f shield according to the
invention. Together the casing halves 10 and 24 form an
effective box enclosure r.f. shield according to the
invention. The second casing half 24 mates intimately with
the first casing half 10 to form the desired level of
shielding. The structure of the second casing half 24 is
instructive. According to the invention, there is provided a
second flange 26 all along the periphery which is formed of
closely-spaced fingers 28. The fingers 28 are generally flat,
flexible plates wherein the inwardly-disposed surface conforms
closely with the contour of the outer surface of the flange 16
of the mating first casing half 10. The fingers 28 and the
flange 16 slidably contact one another when the two casing
halves are mated. The fingers 28 extend into the optional
slot 18 to establish the circuitous path for r.f.
The fingers 28 are closely spaced to one another,
typically having narrow slots with side-to-side spacing of far
less than a quarter wavelength. Preferably the spacing is
less than 1/4 wavelength of the highest measurable harmonic of
the operating frequency of the fundamental of the r.f.
emission. A typical spacing is 1/4 wavelength at the tenth
harmonic (1/8 wavelength of the fifth harmonic). The narrow
spacing has been found to be adequate to provide an effective
r.f. seal to the edges which providing a good mech~n;cal slide
fit. The plate-like ~ingers 28 produce a flexible, self-
sustaining lateral pressure fit against the solid flange 14.
The material forming the shield may be metal or
metallized plastic, that is, plastic coated or plated with
copper and nickel. The copper provides electromagnetic
shielding and the nickel provides abrasion resistance.
Various processes are known in the art to achieve a
substantially uniform effective coating.
The enclosure is for enclosing a circuit card 32 (in
phantom in Fig. 2) wherein the circuit card 32 conforms with
the enclosure. In accordance with the invention, the second
casing half 24 is provided with isolation walls 120 which
align with the isolation walls 20 of the opposing half. In

CA 022311~0 1998-03-04
W O 97/11592 PCT~US96/14711
this embodiment, closely-spaced protrusions 30 or pins are
disposed along the inward faces of the walls 120 and which
mate with indentations in the first casing half 10. The
alternative is also equivalent. The protrusions 30, from
whichever casing half, either extend directly through
perforations 3 4 in the circuit card 32 or press along the
periphery of the circuit card 32. The perforations 34 must be
plated-through or the periphery must be edge plated to form a
grounded boundary at and along the edge of the circuit card
32. The protrusions 30 provide a conductive path between the
casing halves and the circuit card at the isolation walls 120
and 20 and along the inside of the flange 14. This structure
further ~nh~nc~ internal as well as external r.f. shielding.
The invention has now been explained with reference
to specific embodiments. Other embodiments will be apparent
to those of ordinary skill in the art. It is therefore not
in~n~e~ that this invention be limited except by the appended
claims.

Representative Drawing