WO2003013207A1

WO2003013207A1 - Shields for components on circuit boards

Info

Publication number: WO2003013207A1
Application number: PCT/GB2002/003419
Authority: WO
Inventors: Michael Hennessy Sullivan; Johannes Waltherus Kuijpers
Original assignee: Rosti, A/S
Priority date: 2001-07-26
Filing date: 2002-07-26
Publication date: 2003-02-13
Also published as: EP1410707A1

Abstract

A shield for electronic components on circuit boards forms a housing (10,22) comprising conduction material defining at least one recess with an open face. The periphery of the open face has a flange (6, 30, 32) for engaging the surface of a circuit board. In use the housing (10,22) is placed over one or more components on the circuit board and held in place with the flange (6, 30, 32) against the circuit board surface. Various mechanisms (14,36) are disclosed for holding the housing in place.

Description

SHIELDS FOR COMPONENTS ON CIRCUIT BOARDS

This invention relates to circuitry in electronic equipment, and particularly to such equipment for personal use. The invention has especial application in the telecommunications field, and in mobile telephones which are of necessity operated in close proximity to a user's head.

Much concern has been expressed at the potentially harmful effect of electromagnetic radiation from electrical tools and instruments. There has been particular concern about the emissions of such radiation from mobile telephones. Shields have been designed and applied to circuit boards in mobile telephones, and while they have been successful in containing the electromagnetic radiation, they have created some problems in the manufacturing process. The primary problem arises from their permanent securement to the respective circuit boards. If after a subsequent final inspection there is a fault, removal of the shield is a cumbersome process, and re-assembly can sometimes be problematic.

Shields for electronic components on circuit boards must of course be conductive. Typically the shields are made as thermoformed plastics housings, with an internal coating of conductive paint or ink. The housing is then fixed on the circuit board by means of an adhesive, and typically the adhesive used is itself also conductive. The result is an effective shield, but one which is very difficult to remove. Recognising that there was a need to have subsequent access to components enclosed by the shield, some shields have been designed with a closable aperture, so that access can be had to the respective components after the shield has been secured. However, this has been found to be an unsatisfactory solution, as the access provided through the aperture is too small for many corrective operations.

The present invention is directed at a shield for electronic components on a circuit board, which can be mounted on the circuit board without a need for adhesive securement, and which can also be removed and replaced without losing its effectiveness. According to the invention the shield takes the form of a housing with a housing body having an open face, the housing body itself being electrically conductive. The housing body edge defining the open face is adapted to engage the surface of the circuit board around the electronic components. The edge itself comprises a flange resiliently deformable relative to the housing body whereby, upon engagement with the surrounding surface the flange bends relative to the housing body to conform with the surrounding surface. With the housing held in place, the shield can effectively contain electromagnetic radiation emitted by the respective enclosed electronic components, but the shield itself can be removed without altering the surface of the circuit board to any substantial extent. This means that if upon subsequent testing a fault is found, the shield can be removed complete and intact, and then replaced after the fault has been corrected.

Various mechanisms can be used to secure the housing of a shield according to the invention to a circuit board, and it is of course preferred that any securing means is reusable. A preferred securing mechanism for shields according to the invention comprises pins attached to the housing and extending beyond the edge defining the open face, for engagement in holes formed in the circuit board. The pins will normally be a friction fit in the holes, but if desired the pins can be locked in place by means on the opposite side of the circuit board, after testing and the completion of any corrective work.

A number of shields according to the invention can be used on a single circuit board, and the shields can of course be designed and shaped for particular applications. Normally the housing body will have a wall which, when the housing is mounted on the circuit board, extends perpendicular to the board surface. Typically, the wall will extend from a base, to define the open face opposite the base, with the wall consisting of a plurality of straight sections to facilitate bending of the flange relative thereto. Thus, in a particular embodiment the housing can be a box with four orthogonal sides forming a wall, and the flange comprising four sections, one extending along the respective edge of each of the four sides.

While the flange extending from the housing body can be a simple flap, it is preferred that its distal edge be discontinuous to facilitate it conforming with the surface of the circuit board. A serrated edge can be effective in ensuring good compliance, other more substantial discontinuities are preferred. A saw-tooth or stepped edge is most preferred, but some forms of crimping can also be very effective. In some embodiments, the flange or sections can comprise discrete elements separately identifiable and attached to the housing body.

Normally, in shields of the invention the housing is formed in a plastics material, to which a conductive coating is applied. Preferred plastics materials are polycarbonate and ABS, and a conductive surface can be in the form of conductive paint or ink, or applied by means of simple metallizing or sputtering. The shaped housing body can be formed by folding from a pre- marked and cut blank. Alternatively, the housing can be formed or thermoformed from a plain sheet of plastics material. The latter process is preferred, as it avoids the risk of leakage between abutting edges, and is also quicker and more reliable. In this context it is relevant that the housing material is normally a film of thickness no greater than 200μm, preferably around 100μm (before metallization).

In order to improve handleability, and provide a convenient means by which the housing body can be held in place on a circuit board, in preferred embodiments of the invention the housing includes a cage extending around the housing body. Typically, such a cage will comprise a wall or series of wall sections, of thickness no more than 0.5mm, preferably around 0.3mm. It will normally be moulded in a plastics material, and can be moulded directly around the housing body. If mutually compatible materials are used for the cage and the housing body, moulding in this way creates a bond between the two with the result that the assembled shield is a fully integral unit. The same plastics materials; polycarbonate and ABS, are preferred for the cage as for the housing body. The preferred mechanism described above for securing the housing body to the circuit board can be embodied in this integral shield. Pins can be moulded as part of the cage.

In a variant of the invention, a securing mechanism is used which is different from that described above. In order to enhance and secure the flange at the edge of the shield housing against the circuit board, a resilient element is disposed over the flange. A closure member is engaged over the element to compress it against the flange and thereby the flange against the circuit board, and itself secured relative to the circuit board to form a complete assembly in which a respective component is fully enclosed under the shield. According to this variant, a shield unit for electronic components for a circuit board has a shield in the form of a housing comprising conductive material and defining a recess with an open face, the housing wall at the periphery of the recess extending to create a flange for engaging the surface of a circuit board; and a resilient element for engaging the flange along the length thereof for transmitting a closing pressure to the flange to press the flange against the surface of a circuit board.

This embodiment of the invention has a particular advantage in that it facilitates the formation of a plurality of recesses in a single shaped body for enclosing a corresponding plurality of components on a circuit board. Flanges round the recesses will be defined at the edge of the body and in grooves formed in the body to define the separate shields.

The resilient element will not normally be formed separately from the shield itself or a unitary body in which a plurality of shields are defined. In order to ensure an accurate correlation between the shape of the element and the respective flange sections, the resilient element can be cast using the shield or unitary body, and the flange or flange sections as the or part of the mould. It is preferred that the material of the resilient element is compatible with that of the shield or unitary body, so that it can bond with the respective flange sections, thereby enhancing comprehensive engagement.

The resilient element will preferably have a height at least equalling the height of the or each shield housing, so that the closure member can easily engage its upper surface to urge it downwards, and the flange into engagement with the circuit board surface. Additionally or alternatively, the resilient element can be formed with a ridge provided particularly for engagement by the closure member and facilitate the application of pressure.

The closure member securing the assembly will normally be a closure or cover for the device in which the circuit board is being installed. For example, it can be the cover of a mobile telephone, PDA or handheld computer. Conveniently, and in the preferred variants referred to above, it will have a substantially flat inner surface that engages the top surface or ridge of the resilient element, providing uniform pressure along the full extent of the element, urging it into engagement with the flange, and thereby the flange into engagement with the circuit board surface.

Where a resilient element is used, as described above, to hold the flange against the circuit board surface, the application of continuous pressure on the flange assists in preserving a good seal between the flange and the circuit board surface in both high and low temperatures. This has particular value in devices that are to be used in extreme climatic conditions.

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings wherein: Figure 1 shows in simplified form the various stages in the manufacture of a shield according to a first embodiment of the invention;

Figure 2 is a perspective view of a circuit board with shields of the kind -shown in Figure 1 fitted thereto;

Figure 3 is a perspective view of the unitary body in a second embodiment of the invention, and in which three separate shield housings are formed;

Figure 4 is a perspective view of a resilient element for use with the body of Figure 3;

Figure 5 is a perspective view showing the resilient element installed on the unitary body of Figure 3;

Figure 6 is a section taken on line 6-6 of Figure 5;

Figure 7 is an enlarged section taken from Figure 5; and

Figure 8 is a reduced plan view showing how the illustrated embodiment can be fitted into the housing of a handheld device.

Figure 1 shows a sheet blank 2 formed with wall sections 4, each of which has at its distal edge a flange 6. The central section 8 of the blank forms a square. Thus, when the wall sections are folded as illustrated at 10, a housing body is formed as a square block with four mutually orthogonal sides 4, with a flange extending from the lower edge of each wall section 4. However, and as noted above, the housing 10 may be formed or thermoformed on a mould tool from a plain sheet not reshaped. However, the detailed shape of the various flange sections can of course be finalised only after the housing body has been formed. After the housing 10 has been formed, it may itself be used as the mould around which a cage 12 is formed or cast. The cage 12 includes a pin 14 depending from each wall section 16 thereof, and these will align with complementary holes in the circuit board 18 with which the shield is to be used. In order to enhance a friction fit of the pins in such holes, the pins are preferably moulded with a square cross-section, and the holes in the circuit board made round. We have found this mismatch facilitates the re-use of shields according to the invention, the friction fit being equally effective upon installation a second time.

If the material of the housing body is compatible with the material of the cage, then upon moulding the cage around the housing body a bond is formed between the two. The result is a very handleable unit, that may be installed on a circuit board, removed and reinstalled without detracting from its performance as a radiation shield.

The drawing shows the cage 12 with the wall sections 16 merging at the corners to form a continuous barrier. However, the wall sections 16 may remain separated at the corners, and linked by cross bars 20 (Figure 2) traversing the base (central section 8) of the housing 10. This is particularly convenient when the housing is formed from the sheet blank shown at 2. The wall sections 4 can be extended at either end to form flaps which project through slots formed between the cage wall sections 16 at the corners and separately bonded to avoid leakage.

A number of shields according to one aspect of the invention embodied in Figure 1 will normally be fitted to a circuit board. In a mobile telephone for example, a generally square unit of the kind illustrated in the drawing will have a side length of around 10mm, a depth of around 3mm with the width of the flange being around 0.5mm. The depth of the unit would be around 3mm, with the pins extending 1mm below the side walls. The internal surface of the housing body would be metallized, preferably by sputtering with a suitable metal such as nickel, and metallization would of course extend to the adjacent under surfaces of the flanges. The flanges are inclined relative to the walls of the housing body, typically at an angle of around 15°, such that when the shield is installed on a circuit board, the flanges are forced outward with their conductive surfaces brought into contact with the respective circuit board surface. These surfaces are held in contact by the resilience of the flanges and of course the pressure between the base of the cage and the circuit board.

Shields of the kind shown in Figure 1 can be manufactured individually, in batches, or in strips. Individual manufacture is quite satisfactory, but relatively slow and therefore potentially labour intensive. In batches, a plurality of blanks as described above can be simultaneously cut from a single sheet, and then folded or formed into the housing body. As in the preferred embodiment the housing body is the mould around which the cage is formed, the subsequent casting of the cage can also be conducted simultaneously around some of or the same plurality of housing bodies. To facilitate the subsequent use of the shields and their application to circuit boards, it is also preferred that they are linked for ease of handling. Thus, they can be provided in strip form, with successive shields being connected by strands of material extending between juxtaposed wall sections of successive cages. Conveniently the strands extend between the corners, and these strands can be moulded simultaneously with the cages around the housing bodies. It will be appreciated then, that multiple strips of shields according to the invention can be manufactured in a single batch, with the housing bodies formed from a common sheet. Alternatively, longer strips of shields can be manufactured in a continuous process from a strip of material from which housing bodies are formed seriatim. The unitary body 22 shown in Figure 3 defines three discrete recesses 24, 26 and 28 projecting upwards from a flange section that extends around and between them. The section 30 extends around the periphery of the body. The sections 32 extend across the body forming grooves between the various recesses. The body is formed with an opening 34 for components mounted on the circuit board which do not require shielding.

Figure 4 shows a resilient element shaped to correspond with the flange sections 30 and 32 in the body 22. At the upper surface of the various limbs of the element a continuous ridge 36 is formed which, when the element is coupled to the body, projects above the upper surface of the shield recesses. This is apparent from Figure 5, which shows the combination of element and body, in which the element substantially fills the spaces between the shield recesses, thereby enhancing the ability of an additional component to apply uniform pressure to the element and the flange section beneath it.

It is preferred that the resilient element is cast or moulded directly in and around the flange sections of the body 22. The benefits of this are apparent from Figure 6 which shows how the element closely follows the contours of the unitary body and will create a bond therewith if compatible materials are used. A separate mould element will of course be required around the periphery of the body, and to create the central opening 34. However, and as shown in Figure 7, the resilient element and the ridge 36 can assist in creating a sturdy unit.

Figure 8 shows how the unit comprising the body 22 and the resilient element can be fitted as a single unit into a handheld device such as a mobile telephone. When the telephone cover is fitted, it will engage the ridge 36 of the resilient element, and apply a compressive force thereto urging the element and the flange beneath it into engagement with a circuit board therebeneath. While the telephone cover provides a convenient closure member for applying the requisite compressive force, it will be appreciated that other mechanisms might be used. With various electronic components being enclosed within the recesses 24, 26 and 28, they will be effectively shielded, and leakage of electromagnetic radiation beneath the flange sections will be minimised.

The material of the body 22 shown in Figure 3 will typically be a moulded plastics material such as polycarbonate or ABS, to which a conductive surface is applied. The resilient element is preferably a compatible pliable plastics material that will form a chemical or thermal bond with the body when moulded therein.

Claims

1. A shield for electronic components on a circuit board, the shield taking the form of a housing with a housing body for enclosing said components and having an open face, and an edge defining the open face and for engaging the surrounding surface of said circuit board, the housing body being electrically conductive, and the edge comprising a flange resiliently deformable relative to the housing body whereby, upon engagement with said surrounding surface the flange bends relative to the housing body to conform with said surrounding surface.

2. A shield according to Claim 1 including means for securing the housing to a circuit board.

3. A shield according to Claim 2 wherein the securing means are reusable.

4. A shield according to Claim 2 or Claim 3 wherein the securing means comprises pins attached to the housing and extending beyond the edge for engagement in holes formed in a said circuit board.

5. A shield according to any preceding Claim wherein the housing has a base with a wall extending therefrom to define the open face opposite the base, the wall being oriented to be substantially perpendicular to said surrounding circuit board surface upon engagement therewith.

6. A shield according to Claim 5 wherein the housing is a base with four orthogonal sides forming the wall, and wherein the flange comprises four discrete sections, one extending along the respective edge of each of the four sides.

7. A shield according to any preceding Claim wherein the distal edge of the flange is discontinuous.

8. A shield according to Claim 7 wherein the distal edge of the flange is serrated.

9. A shield according to Claim 7 wherein the distal edge of the flange has a saw-tooth profile.

10. A shield according to any preceding Claim wherein the distal edge of the flange is crimped.

11. A shield according to any preceding Claim wherein the housing is formed in a plastics material.

12. A shield according to Claim 11 wherein the plastics material is one of polycarbonate and ABS.

13. A shield according to any preceding Claim wherein the housing is formed by folding a sheet of material; the sheet being shaped to define respective fold lines.

14. A shield according to any of Claims 1 to 12 wherein the housing is formed directly from a plain sheet of material.

15. A shield according to any preceding Claim wherein the thickness of the housing material is no more than 200um.

16. A shield according to any preceding Claim wherein the housing includes a cage extending around the housing body.

17. A shield according to Claim 2 and Claim 16 wherein the securing means are mounted on the cage.

18. A shield according to Claim 16 or Claim 17 wherein the cage comprises a continuous wall extending around the housing body.

19. A shield according to Claim 16 or 17 wherein the cage comprises a series of separate wall sections extending around the housing body.

20. A shield according to any of Claims 18 or 19 wherein the wall thickness is no more than 0.5mm.

21. A shield according to any of Claims 16 to 20 wherein the cage is moulded in a plastics material.

22. A shield according to Claim 21 wherein the cage is moulded around the housing body.

23. A shield according to Claim 11 and Claim 22 wherein the material of the cage is compatible with that of the housing body and forms a bond therewith upon moulding.

24. A circuit board with electronic components thereon enclosed by a shield according to any preceding Claim.

25. A circuit board according to Claim 24 including a shield according to Claim 4, wherein the pins are a friction fit in the holes in the circuit board.

26. A circuit board according to Claim 25 wherein the pins have a square cross-section and the holes are circular.

27. A method of manufacturing a shield for electronic components on a circuit board comprising creating a housing body according to any of Claims 1 to 23; and moulding a cage around the housing body.

28. A shield unit for electronic components on a circuit board, having a shield in the form of a housing comprising conductive material and defining a recess with an open face, the housing wall at the periphery of the recess extending to create a flange for engaging the surface of a circuit board; and a resilient element for engaging the flange along the length thereof for transmitting a closing pressure to the flange to press the flange against the surface of a circuit board.

29. A shield unit according to Claim 28 wherein the resilient element is shaped to complement the flange and the housing wall adjacent thereto.

30. A shield unit according to Claim 29 wherein the resilient element is a cast body moulded using the housing wall and flange as part of the mould.

31. A shield unit according to Claim 30 wherein the resilient element is attached to the housing.

32. A shield unit according to Claim 31 wherein the resilient element is bonded to the housing.

33. A shield unit according to any of Claims 28 to 32 wherein the height of the resilient element exceeds that of the housing.

34. A shield unit according to any preceding Claims 28 to 33 wherein the upper surface of the resilient element is formed with a ridge or projections for receiving a closure member to apply said closing pressure.

35. A shield unit according to any of Claims 28 to 34 wherein the housing defines a plurality of recesses with the flanges of adjacent recesses forming grooves in which the resilient element when fitted, is received.

36. An assembly comprising a circuit board with at least one electronic component thereon; a shield unit according to any of Claims 28 to 35 enclosing the component or components; and a closure member fixed relative to the circuit board, and applying a closing pressure to the resilient element which is transmitted to the flange or flanges to compress the flange against the circuit board surface.

37. An assembly according to Claim 36 in a hand held device wherein the closure member is the cover of the device.