Protective shield for electrical components

The present invention relates to a protective shield (1) of EMC-type, for e.g. electrical components (40) on a circuit board (30). The protective shield (1) comprises a frame (10) with associated lid (20), where said frame (10) at the end facing towards the circuit board (30) comprises a flange (12). The frame is arranged to be fastened to the circuit board (30) by means of gluing, welding or soldering. The frame (10) is intended to surround at least one component (40) so that electrical contact is formed between said circuit board (30) and said frame (10) and between said frame (10) and said lid (20).

TECHNICAL FIELD 
The present invention relates to a surface-mounted protective shield for 
components on e.g. a circuit board. 
STATE OF THE ART 
The screening of components on circuit boards is becoming more important, 
especially in the case when the signal speed approaches the Gbit region. 
It is well known that radiation from certain components at high 
frequencies can cause interference with communications e.g. 
telecommunication. This phenomenon is called RFI, radio frequency 
interference. The interfering circuits can either be shielded with closed 
boxes or the sensitive components can be shielded with closed boxes in 
order to prevent interference. 
In GB 2 226 187 there is described a miniature protective shield for 
electronic components arranged on a circuit board. The protective shield 
comprises a number of contact springs and a lid. The contact springs are 
arranged around the component and are in electrical contact with the 
circuit board. The contact springs are in general U-shaped. The lid 
comprises a flat surface with side walls along all the edges so that the 
lid is similar to an open box. The size of this box is adapted to the 
contact springs on the circuit board. The sides are intended to be 
inserted into the U-shaped contact springs so that a protective shield is 
formed. 
In WO 95/28074 a contact spring is described which can be used to connect a 
protective shield to a circuit board. This contact spring has a different 
appearance compared to the above mentioned contact springs, which means 
that the contact between lid and contact spring is ensured and that the 
shielding increases. 
The problem of the above contact springs is that they are quite complicated 
to arrange around the component on the circuit board. This solution is, 
furthermore, not completely optimal, both with regard to space 
requirements and the degree of protection. 
DISCLOSURE OF THE INVENTION 
On circuit boards where certain components produce much electromagnetic 
radiation, the placement of these components must be chosen with great 
care so that the radiation shall not cause operational problems. On e.g. a 
single-sided circuit board, where both a transmitter and a receiver are 
placed, the minimum size of the circuit board is limited to how near each 
other the transmitter and receiver can be placed without causing 
operational disturbances. 
A problem which occurs when different products tend towards smaller and 
smaller dimensions is that the screening between sensitive components must 
be increased. 
Another problem is that known protective shields take up too much room and 
are complicated and time-consuming to place on the circuit board, either 
because they should be mounted in holes in the circuit board or because 
they should be held fixed during the soldering operation. The above 
mentioned problems often require hand assembly. 
The present invention intends to solve above said problems through a 
surface-mounted protective shield, a so-called EMC-shield, being arranged 
over sensitive or disturbing components on a circuit board. The EMC-shield 
is in its most simple embodiment a frame with an associated lid arranged 
around at least one component on the circuit board. The frame is attached 
so that it is in electrical contact with the circuit board. The lid is 
adapted to the shape and size of the frame in order to obtain an 
EMC-protection which is as effective as possible. 
The frame, which can have any possible shape, e.g. circular, triangular and 
rectangular, is optimized in size to precisely surround the component or 
components which are to be shielded. The frame, which can be made of some 
electrically conductive material or a material which is coated with an 
electrically conductive material, can have a flange at its end facing 
towards the circuit board. This flange can align the frame when it is 
fastened to the circuit board with the help of e.g. soldering, welding or 
gluing. A joint material, which is placed at the intended position on the 
circuit board next to the component which is to be shielded and with a 
shape corresponding to the contour of the frame, causes a frame which is 
not too greatly mispositioned to be displaced to the right position with 
the help of the surface tension of the joint material. The lid, which 
comprises a flat surface with walls arranged around all its sides, has its 
shape and size adapted to the frame so that the lid is either slipped onto 
the outside of the frame or placed inside it. 
An advantage of the present invention is that the protective shield takes 
up minimal space and that it does not leak radiation through the frame. 
Another advantage of the present invention is that the fastening is 
simplified in that the frame is formed of a unit which cannot tip over. 
Yet another advantage of the present invention is that with a flange on the 
frame it can adjust itself to the right position. 
Yet another advantage of the present invention is that the mounting on an 
assembly line is simplified and that simpler "pick and place" robots can 
be used.

PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of a protective shield of the EMC-type 1 
according to the invention. The protective shield 1 in this Figure is 
mounted on a substrate 30. The protective shield comprises a frame 10 and 
a lid 20. The frame 10 can be fastened to the substrate 30 with the help 
of gluing, welding or soldering. On the inside of the walls of the frame 
10 there are a number of contact elements 11. These contact elements 11 in 
this embodiment are shaped as tension prongs. The outer dimensions of the 
lid 20 are adapted to the inner dimensions of the frame 10 according to 
FIG. 2. When the lid 20 is forced down into the frame 10, the side walls 
21 of the lid 20 are gripped between the walls of the frame 10 and the 
tension prongs. The component 40, which one desires to shield, is placed 
inside the frame 10. Said tension prongs can also be arranged on the 
outside of the frame 10. The inner dimensions of the lid 20 are in this 
case adapted to the shape and size of the outer dimensions of the frame 10 
so that the side walls 21 of the lid 20 are clamped between the tension 
prongs and the outside of the frame 10. 
A variant for contacting the lid 20 to the frame 10 is that the outer 
dimension of the frame 10 is precisely adapted to the inner dimension of 
the lid 20 so that these details are mechanically held together and are in 
electrical contact with each other by the lid 20 being slipped over the 
frame 10 and contacting it by press-fitting. It is naturally also 
conceivable that the outer dimension of the lid 20 is adapted according to 
the inner dimension of the frame 10 so that the lid 20 fits inside the 
frame 10 so that said electrical contact and mechanical holding between 
these parts is again formed by press-fitting, see FIG. 3. 
Instead of using tension prongs or press-fitting for forming electrical 
contact between the frame 10 and the lid 20, a number of other embodiments 
can be conceivably used. One variant could be that the frame 10 just like 
earlier is fastened to the substrate 30 in a suitable way. The lid 20 is 
dimensioned to either fit inside the frame 10 or be threaded over the 
frame 10. If the lid 20 is intended to fit into the frame 10, then on the 
outside of the side walls 21 of the lid 20 there could be arranged a 
number of contact means in the shape of ribs and on the inside of the 
frame 10 there could be arranged corresponding ribs. The ribs of the lid 
20 and the ribs of the frame 10 snap into each other in order in this way 
to form the mechanical and electrical contact between the lid 20 and the 
frame 10, see FIG. 5. 
Yet another variant can be that the frame 10 has a number of bulges 15 in 
the form of e.g. hemispheres which are adapted to corresponding spheres on 
the side walls 21 of the lid. If there are a number of bulges 25 arranged 
on the side walls 21 of the lid 20, then there are corresponding 
depressions 15 arranged on the frame 10. If on the side walls 21 of the 
lid 20 there are arranged a number of depressions 25, then there are 
corresponding bulges 15 arranged on the frame 10. The lid 20 can either be 
adapted to be slipped over the frame 10 or to fit inside it, the 
depressions or bulges 25 on the side walls 21 fitting into the 
corresponding bulges or depressions 15 on the frame 10, see FIG. 5. This 
embodiment plus the one with ribs are especially suitable in the case of 
small dimensions of the protective shield. 
Yet another variant for contacting the lid 20 to the frame 10 is that the 
lid 20 is arranged with a number of tongues 24 along the side wall 21. 
These tongues 24 are bent in towards the center of the lid 20 when the lid 
20 is to be slipped over the frame 10 and bent out from the center of the 
lid 20 when the lid 20 is to fit into the frame 10, see FIG. 4. The 
tongues can also at their free ends be bent in towards the side wall 21. 
On the frame 10 there can be arranged a flange 12 at the end which is to be 
connected to the circuit board. The flange 12 can be directed in towards 
the center of the frame 10 as in FIG. 3, or out from the center of the 
frame 10 as in FIG. 4. The flange 12 means that the frame 10 can 
self-adjust to the right position with the help of the surface-tensioning 
of the fastening material 50. If the frame 10 is incorrectly placed in 
relation to a soldering material arranged on the circuit board 30, the 
frame 10 can move itself to the center of said soldering material when 
this begins to melt. 
In order to achieve as good a shielding bandwidth as possible, the material 
in the EMC-protection can be manufactured in some electrically conductive 
material with magnetic qualities. 
The walls of the frame 10 can be somewhat bent 12 at the bottom close to 
the substrate 30 to which the frame 10 is fastened. The bending of the 
walls close to the substrate can either be in towards the frame 10 or out 
from the same. 
The invention is naturally not limited to the embodiments described above 
and shown in the drawings, but can be modified within the scope of the 
accompanying claims.