GB2306790A

GB2306790A - Gasket for electric field isolation

Info

Publication number: GB2306790A
Application number: GB9522103A
Authority: GB
Inventors: Malcolm Perry
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-10-28
Filing date: 1995-10-28
Publication date: 1997-05-07
Anticipated expiration: 2015-10-28
Also published as: GB2306790B; GB9522103D0

Abstract

A gasket (50), made from compressive material, comprises a curved upper contact portion (54) and a lower body portion (56) having a slot (58) for receiving a protruding member (64) of a "can" (or mounting) assembly (52). When the gasket (50) is placed in contact with a board and upon application of pressure to the "can" (52), compression of the upper contact portion (54) occurs over an area substantially corresponding to a lateral dimension of the protruding member 64 (and hence a base 60 of the slot), thereby forming a seal, for producing an EMC enclosure.

Description

GASKET AND MOUNTING THEREFOR Field of the Invention This invention relates, in general, to gaskets and is particularly, but not exclusively, applicable to gaskets for Electro-Magnetic Compatibility (EMC) enclosures for use on printed circuit boards (PCBs) for electric (E-) field isolation of components and circuits in cavities on the PCB.

Summarv of the Prior Art When energised, certain components or electrical circuits generate substantial electro-magnetic fields that adversely effect the operation of adjacent components or circuits. As such, the layout of components on PCBs is carefully considered to mitigate against the effect of such electro-magnetic fields. Additionally, component screening may be adopted, where necessary, to isolate (attenuate) further these electro-magnetic fields. By way of illustration, screening is prevalent in communication devices that operate, generally, in the radio frequency (RF) spectrum.

With respect to screening techniques, cast or metal-plated U-shaped "cans" may be placed directly on a PCB to produce an EMC enclosure.

However, abutting faces of the PCB or "can" (or both) must be extremely flat to produce a good sealing surface and hence relatively good attenuation. Unfortunately, the fine manufacturing tolerances required to produce these extremely flat surfaces substantially increases manufacturing costs, and reduces the production efficiency for such structures and PCBs. Alternatively, conductive gaskets are more often used in association with the cast or metal-plated "cans" to produce an EMC enclosure. More particularly, the conductive gasket abuts against an end of the "can" and then produces a seal upon pressured contact with the PCB.Typically, attenuation of-80dB can be achieved with such low-cost, fitted gaskets, with the amount of attenuation predominantly determined by the contact surface area (resulting from compression of the gasket) between the gasket and the PCB. Unfortunately, constraints imposed on both the size of the PCB and overall packaging (compartment) dimensions for the equipment restrict the amount of attenuation that can be provided by conventional gaskets, such as O-rings. Moreover, conventional gaskets typically provide a contact surface having a diameter of less than 0.5 millimetres (mm), while requiring a substantially larger "can" assembly to abut and house the gasket. Indeed, the abutting surface of the "can" will typically have a width of 5mm or more.

Therefore, it would be desirable to have a low-cost gasket and "can" assembly that together provide improved isolation with reduced physical size.

Summarv of the Invention According to a first aspect of the present invention there is provided a gasket made of compressible material comprising an upper contact portion having a curved surface and a lower body portion containing a slot with a base, adjacent the upper contact portion, for receiving an abutting mounting member, the curved surface arranged to flatten under pressure exerted on the base of the slot by the mounting member to produce a sealing surface having a profile substantially corresponding to the base of the slot.

In a second aspect of the present invention there is provided a mounting assembly for mounting the gasket of any preceding claim to produce a "can", the mounting assembly comprising a protruding member for abutting engagement with the slot.

Typically, the "can" is used on a printed circuit board to isolate a component radiating an electro-magnetic field.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings FIG. 1 is a cross-section of a prior art O-ring gasket.

FIG. 2 shows a prior art "can" assembly and abutting O-ring gasket in situ on a PCB.

FIG. 3 is an alternate prior art gasket.

FIG. 4 shows a prior art "can" assembly and abutting gasket (of FIG. 3) in situ on a PCB.

FIG. 5 represents a "can" assembly and abutting gasket according to a preferred embodiment of the present invention.

FIG. 6 represents an alternate gasket and "can" assembly according to an alternative embodiment of the present invention.

FIG. 7 illustrates the "can" assembly and abutting gasket of FIG. 6 in situ on a PCB.

Detailed Description of a Preferred Embodiment Referring initially to FIG. 1, a cross-sectional view of a prior art O-ring gasket 10 is shown. As the name suggests, the cross-section of the O-ring gasket 10 is substantially circular, with a diameter (d) of typically 2mm.

The O-ring gasket 10 is typically engaged by a metal or metal-plated "can" assembly 12, as shown in FIG. 2. Particularly, the "can" assembly comprises a square-shaped cavity 14 into which the O-ring gasket 10 is compressively seated. As can be seen, compression of the O-ring gasket 10 in the cavity 14 causes a flattening of the circumference of the O-ring gasket, and hence produces contact (c) surfaces 16 that abut against sides of the cavity 14. As will be appreciated, the O-ring gasket 10 protrudes beyond the cavity 14, such that when the combined "can" assembly 12 and gasket 10 are placed adjacent a PCB 17 compression of the O-ring gasket 10 further occurs and produces a sealing (contact) surface 18 against the PCB 17.By applying pressure (P) 20 across the "can", the size of the sealing surface 28 is varied, as will be understood, thereby producing an EMC enclosure. It will also be noted from FIG. 2 that edges of the "can" assembly abutting against the PCB 17 are non-regular and that only certain points along these edges actually engage against the PCB 17. Also, although the PCB 17 has been shown to have a planar abutting face 21, in reality this face 21 may have a non-regular profile. As previously described, the "can" will typically have a width (w) of 5mm (or greater), with the sealing surface 18 (under pressure) of 0.5mm (or less). Also, shielding is provided by the metal-to-metal contact of the "can" 12 and portions of the PCB 17.

FIGs. 3 and 4 show an alternate prior art gasket 26 having an undulating contact profile 28. More particularly, the undulating contact profile 28 comprises a plurality of substantially semi-circular lugs 30-33 evenly distributed across the width of the gasket 26 and formed as an integral part of the gasket 28. In operation, a planar face 34 (opposite the undulating contact profile 28) of the gasket 26 abuts (or is perhaps glued) against an edge of a "can" 36, with pressure (P) 20 being applied to produce an EMC enclosure through the compression of the semi-circular lugs 30-33 against a face 21 of a PCB 17. As previously described, the "can" will typically have a width (w) of 5mm (or greater), with multiple sealing surfaces 18 providing a seal (under pressure) having a total linear dimension of 0.5mm (or less).In operation, substantial pressure 20 must be provided over a wide area of the "can", with the contact area, in general, relatively poor.

Having regard to FIG. 5, an improved gasket 50 and associated "can" assembly 52 (according to a preferred embodiment of the present invention) are shown. Typically, the gasket 50 will be of one-piece moulded construction and manufactured from silver on glass (or the like) in an elastomer (such as CH0.SEALTM 1390, for example, manufactured by Chomerics) having a Shore hardness of between 60 and 70, although any suitable combination of materials (and properties) may be used. Generally, the gasket 50 can be considered as U-shaped, with a curved (convex, arcuate or dome-like) upper contact portion 54. It is the upper contact portion 54 that is predominantly flattened during application of the gasket 50, as will be subsequently described in relation to FIG. 7. A lower body portion 56 of the gasket contains a slot (or cavity) 58, which preferably has a substantially flat base 60 that is substantially parallel to an imaginary tangent drawn horizontally across the top of the curved upper contact portion 54. Typically, the slot has a square or rectangular cross-section. A bed of the lower body portion 56 is arranged to mate against shoulders 62 of the "can" 52. Usually, the shoulders 62 of the bed will be substantially parallel to the base 60 of slot 58.

The "can" of the preferred embodiment is relatively narrow, having a diameter or width of approximately 2mm. A protruding member 64, corresponding to the shape of the cavity 58, extends beyond the shoulders 62 of the "can" 52. As such, the protruding member 64 engages exactly into the slot 58 of the gasket 50. Typically, the protruding member 64 (and hence the base 60 of the slot) will have a lateral dimension of approximately 1.5 mm, which protruding member will typically be integrally formed with the "can". Furthermore, with general regard to the lateral dimension of the protruding member 64 (and hence the base 60 of the slot), it will be noted that this lateral dimension is substantially larger than a point of contact (or apex) made by the upper contact portion 54 upon mere contact with a surface, i.e. when the gasket is not under compressive forces.

Also, although sides 66 of the "can" 52 are shown to be parallel, tapering of the sides 66 towards the gasket 50 is considered to be a possible design variant, thereby increasing the pressure applied through the protruding member 64.

FIG. 6 represents an alternate gasket 80 and "can" (or mounting) assembly 82 according to an alternative embodiment of the present invention. In this alternative form, the gasket 80 retains the curved upper contact portion o4, but the gasket contains side lobes 85 that therefore distort the U-shaped appearance of the first described embodiment. Also, slot 88 (and hence protruding member 94 of"can" 82) are wedge-shaped, although a base 90 of the wedge 88 is still substantially parallel to an imaginary tangent drawn horizontally across the top of the curved upper contact portion 84.

Furthermore, shoulders 96 of the "can" 82 are slopped, which results in a mating surface of the gasket 80 being correspondingly inclined. Side lobes 85 are included in the gasket 80 to accommodate for the wedge-shaped mating arrangement of the gasket 80 with the "can" 82. Specifically, under compression, the upper contact portion 84 of the gasket 82 is flattened whereby the side lobes 85 ensure that a minimum distance 98 is maintained between the protruding member 94 and the gasket's exterior.

Referring to FIG. 7, the present invention of the "can" assembly and abutting gasket (of FIG. 6) is shown in situ on a PCB 17. Pressure (P) 20 applied (essentially perpendicular to a plane of the PCB 17) through the protruding member 64 results in compression of the curved upper contact portion 54 of the gasket 50, producing a sealing surface 100 having an area (and profile) substantially corresponding to an area of the base 60 (and hence the protruding member 64). As such, the sealing surface 100 will typically have a lateral contact dimension of approximately 1.5mm, with an EMC enclosure 102 formed thereby.The relative orientation of the base 60 of slot 58, the protruding member 64 and the applied downward pressure 20 ensures that a relatively high pressure is applied across the entire sealing surface 100, causing substantially uniform compression (and distortion) of the gasket 50 (or 80) at the sealing surface 100, thereby resulting in the upper contact portion 54 (or 84) lying in parallel abutment with a surface 21 of the PCB 17 over a relatively large area.

Furthermore, when under applied pressure, side walls of the slot 58 (or 88) of the gasket 50 (or 80) clench the protruding member 64 (or 94), i.e. apply additional pressure (P') 104 is applied to side walls of the protruding member 64, thereby generally improving the seal.

In a preferred embodiment, compression of the complete gasket is regulated to approximately 75% through the use of integrally mounted compression stops (not shown) located at fixing points on the "can", as will be appreciated by the skilled addressee.

As such, construction of a "can" and gasket according to the present invention advantageously provides an improved gasket-seal that yields a substantial increase in isolation, and a reduction in the physical size of the "can" assembly. More particularly, as a consequence of being able to apply accurately a relatively high pressure across the entire sealing surface (defined by the internal presence (coupling) of the "can" to the gasket), use of the gasket of the present invention increases the sealing (contact) area between mating parts of the gasket and the PCB, for example. Indeed, results have indicated that an isolation in the range of -135dB can be achieved using the gasket and "can" of the present invention.

Furthermore, the gasket of the present invention is ideal for use in enclosures having closely specified tolerances, such as those prevalent in electronic equipment, e.g. radios.

It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the invention, for example the gasket may be applied against a PCB for E-filed isolation or may be applied against other substrates and circuits around which electro-magnetic isolation is desirable.

Claims

1. A gasket made of compressible material comprising an upper contact portion having a curved surface and a lower body portion containing a slot with a base, adjacent the upper contact portion, for receiving an abutting mounting member, the curved surface arranged to flatten under pressure exerted on the base of the slot by the mounting member to produce a sealing surface having a profile substantially corresponding to the base of the slot.

2. The gasket according to claim 1, wherein the base of the slot is flat and the profile is substantially parallel thereto.

3. The gasket according to claim 2, wherein the sealing surface has a lateral dimension of approximately 1.5mm.

4. The gasket according to claim 1, 2 or 3, wherein the lower body portion contains shoulders adjacent the slot, which shoulders are substantially parallel to sealing surface and are arranged to abut further against the mounting member.

5. The gasket according to any preceding claim, wherein the slot has a substantially rectangular cross-section.

6. The gasket according to any preceding claim, wherein the gasket is substantially U-shaped.

7. The gasket according to any one of claims 1 to 6, wherein the slot has a substantially wedge-shaped cross-section.

8. The gasket according to claim 7, wherein the upper contact portion contains side lobes.

9. The gasket according to any preceding claim, wherein the compressible material has a Shore hardness in an approximate range of 60to70.

10. A mounting assembly for mounting the gasket of any preceding claim to produce a "can", the mounting assembly comprising a protruding member for abutting engagement with the slot.

11. The mounting assembly according to claim 10, wherein the protruding member is integrally formed in the mounting assembly.

12. A gasket substantially as hereinbefore described with reference to FIGs. 5 to 7 of the accompanying drawings.

13. A mounting and gasket combination substantially as hereinbefore described with reference to FIGs. 5 to 7 of the accompanying drawings.

14. An electrical device comprising: a) a printed circuit board containing a component radiating an electro-magnetic field; and b) a mounting assembly and gasket combination substantially as hereinbefore described with reference to FIGs. 5 to 7 of the accompanying drawings.