Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TITLE
Method o$ mounting a printed circuit board a~d securing
the earthing ~o a casing
BACXGROUND OF THE INVENTION
Field of the invention
This invention relates to a method of mounting a printed
circuit board in a casing, and more specifically to a method
of mounting a printed circuit board into a radio frequency
shielding casing, the casing having an electrically con-
ducting surface, and securing the earthing between theprinted circuit board and the casing.
Description of the prior art
Electronic equipment operating in the radio frequency range
is conventionally installed in`metal casings, in order to
ensure a shielded environment free of interference from
the outside and also to prevent the equipment itself from
radiating disturbances to the environment. There may be
several units in the same casing, which then is divided
into compartments by walls. Usually, in order to ensure a
good radio frequency shielding, the casing and the com-
partments are closed with a metal lid, a sealing strip of
electrically conducting material running between the edges
of the casing and the lid.
The earthing of the printed circuit boards is convention-
ally provided by a contact between the earthing strlp on
the printed circuit board and the casing. This is in most
cases feasible at those points where the board is fastened
to the casing, e.g. by screws. Alternative earthing con-
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tacts have been made with other mechanical arrangements,such as springs, clips and the like.
The conventional mounting methods then need space on the
printed circuit boards for screws and the like, which re-
duces the available space for components, or necessitates
correspondingly larger printed circuit boards. On khe other
hand, the fastening by screws traditionally requires
special studs to be molded in the casing and provided with
threading for the fastening screws.
The conventional mounting methods require many separate
components many manufacturing phases, including precision
machining and manual work.
SUMM~RY OF THE INVENTION
It is an object of the present invention to provide an im-
proved method of mounting a printed circuit board and se-
curing the earthing to a casing, overcoming the drawbacksof the prior art mentioned above.
According to the invention there is provided a method of
mounting a printed circuit board into a radio frequency
shielding casing, the casing having an electrically con-
ducting surface, and securing the earthing between the
printed circuit board and the casing, the method compris-
ing the steps of:
providing the printed circuit board with projecting parts
on at least two sides of the printed circuit board, and
arranging earthing means on the printed circuit board to
extend on at least one face of said projecting parts;
providing the edges of the casing with a corresponding num-
ber of recesses dimensioned and positioned so as to tightly
receive said projecting parts therein;
mounting the printed circuit board into the casing so that
the projecting parts fit into the recesses to support the
printed circuit board in the casing, the outer face of the
printed circuit board situated at least as low as the plane
defined by the edges of the casing;
attaching a flexible sealing strip of electrically con-
ducting material along the edges of the casing continuingover the projecting parts; and
placing a lid on said sealing strip, tightening the lid
against the sealing strip and securing the lid to the casing
Preferably, said projecting parts are provided as integral
parts of the printed circuit board. Suitably, the method
further includes the step of providing the earthing means
on both faces of the projecting parts of the printed cir-
cuit board.
Suitably the recesses of the casing are provided to a depth
exceeding the thickness of the printed circuit board by a
predetermined distance less than the thickness of the flex-
ible sealing strip. The lid may also be provided with swel-
ling portions at.the locations of the recesses and at theside facing the sealing strip.`
The casing and the lid may constitute homogeneous metal
parts or they may be of some other material covered with
an electrically conducting surface layer.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described by way of example with
reference to the accompanying drawing, in which:
Figure 1 illustrates schematically in a perspective view
the arrangement for carrying out the method according to
the invention; and
Figure 2 shows in an end view one projecting part of a
printed circuit board mounted in a casing and fastened be-
tween the casing and the lid, according to the inventivemethod.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The accompanying drawing shows in figure 1 a printed cir-
cuit board 10, which according to the invention is pro-
vided with projecting parts 11, 12, 13, 14. The earthing
means 15 (printed circuit conductors) are provided on both
faces of the printed circuit board and of the projecting
parts (16 and 17 in figure 2). The number and the loca-
tion of the projecting parts may vary in accordance with
individual re~uirements.
15 The casing 20 is provided with recesses 21, 22, 23, 24 on
the edges 25 at locations corresponding to the projecting
parts 11, 12, 13, 14, the recesses provided with a depth
equalling the thickness of the printed circuit board plus
a distance L (cf. figure 2), and a width that allows them
to receive the respective projecting parts with a small
clearance.
The printed circuit board 10 is mounted in the casing 20
by placing the projecting parts in the corresponding
recesses. Then the outer face 16 of the board will be a
distance L away from the plane defined by the edges 25 to
allow a space between the printed circuits and the lid 40.
The components on the printed circuit board will "hang"
downwards in the casing. The board 10 is then firmly held
in place against movement in the board's plane.
A commercially available flexible sealing strip 30 of elec-
trically conducting material (plastic or synthetic rubber,
with a metallic surface or including metal particles) is
placed along the edges 25 so that the s-trip continues over
the recesses and over the projecting parts 11, 12, 13, 14.
The sealing strip 30 may be applied in several parts, the
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ends of which are placed so that they contact adjacent
parts, thereby creating a continuous ring of electrically
conducting sealing strip around the edges of the casing
compartment.
The fastening of the printed circuit board is provided by
placing a metal lid ~0 over the casing 20 so that the seal-
ing strip will be compressed between them when the lid 40
is fastened to the casing. The fastening is completed by
tightening screws (not shown) through bores 41 in the lid
to the studs 26 in the casing.
} The situation for one projecting part after fastening of
the lid is shown schematically in figure 2.
The projecting part (11, 12, 13 or 14) is firmly held in
the recess (21, 22, 23 or 24) by the pressure applied
through the sealing strip 30. The electrically conducting
earthing means 15 (printed circuit conductors) on both
faces 16 and 17 of the printed circuit board 10 are then
contacting the bottom of the recess 27 and the sealing
strip 30 respectively, thereby securing the earthing of
the printed circuit board 10 to the casing 20.
To enhance the pressure on the printed circuit board, the
lid is provided with a swelling 42 somewhat narrower than
the recess. Although the clearances between the sides of
the projecting part and the recess are small, there may be
vibrating installation environments, where the best fric-
tion (fastening) is achieved with a radio frequency seal-
ing strip of synthetic, rubber-like composition blended
with electrically conducting metal particles.
The inventive method described above dispenses with screws
and other laborious fastening of the printed circuit board.
This leads to easy handling e.g. by service personnel; the
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~'13
printed circuit board may be removed at once when the lid
is taken off the casing.
Applications of the principles of the invention in this
S and similar manners are all intended to fall within the
scope of the invention.
