Electromagnetic shielding with discontinuous adhesive

Abstract: A shield blocks passage of electromagnetic energy through a seam of indefinite length between conductive elements and is adhesively attached to at least one of the elements. A preferably non-conductive adhesive is disposed on a conductive surface of a seal in a discontinuous pattern along the length of the seam, defining areas of 5-40% adhesive for physically attaching the conductive surface, separated by 60-95% exposed areas for direct electrical contact. The seal can include a compressible body or can be defined by a conductive sheet, for example a fabric provided with a plating or other conductive coating. The areas of adhesive preferably define longitudinally non-overlapping inclined lines extending laterally across the seam. A release liner can removably attach to the adhesive. In a preferred embodiment the conductive surface is the surface of a compressible seal body, such as a conductive sheath on a compressible core. A conductive path extends without interruption along the length of the seam and across the lateral width of the seam at the adhesive lines, providing improved physical attachment and conductivity characteristics for the shielded seam as a whole.

Description: This invention relates to electromagnetic shielding, including conductive seals and gaskets for sealing between abutted parts of conductive bodies, and shielding material in sheets, applied as a barrier to electromagnetic radiation passing into or out of enclosures, cables, conduits and the like. The shielding material is affixed to a conductive surface by a discontinuous nonconductive adhesive. Shielding against electromagnetic interference (EMI) involves providing a conductive barrier in which currents induced by incident electromagnetic fields are grounded and/or dissipated in eddy currents. Conductive seals and gaskets typically render continuously conductive a junction between conductive parts of enclosures of electrical or electronic equipment. The conductive parts may be relatively movable such as a door of a cabinet, or stationary such as panels held together by fasteners. Shielding cables and the like typically involves surrounding the entire cable in a shield material. Similarly, conductive seals and gaskets may have a conductive sheet material wrapped on a compressible form. While it is possible to form a seamless tube of conductive sheet material for a cable shield, it is also possible to wrap the cable with conductive sheet material, defining a longitudinal or helical seam across which the conductive material must be joined so as to conduct. Seams are also defined in use between conductive material of a seal or gasket and the cabinet panels or the like to which the seal or gasket is affixed. The shielding may confine or exclude electromagnetic interference. In microwave ovens and the like, for example, the microwave field is confined to the enclosure. In connection with communications equipment and the like, the objective may be either or both of isolating circuitry from ambient electromagnetic interference (EMI) and protecting other equipment from interference generated by the equipment. Shielding can protect against potential damage as well as potential improper operation due to induction of currents. Shielding also may protect vulnerable circuitry from damage due to electromagnetic pulses, such as produced by nuclear detonations. The need to conductively fix the shield material at least at one seam is common to all shielding arrangements. The shield resides across the path of incident electromagnetic radiation, defining a substantially continuously conductive body (either alone or in conjunction with conductive panels or the like), to block propagation of electromagnetic fields. One known seal has a conductive sheath of woven or knitted wire, the sheath enclosing a compressible core. The seal can be mounted, for example, in a slot in a first conductive body to bear against a second body brought into abutment with the first. In movable panel arrangements, one of the panels normally carries the seal and the other of the panels simply abuts against the seal. If the sealed panels remain immovable, the seal can be attached to one or both. Typically, such a seal is attached to the conductive panel at least partly by an adhesive. For shielding efficiency there is a need to minimize electrical resistance between the seal and the surface against which it abuts, including resistance due to the adhesive between the panel and the conductive material of the seal. The typical technique is to place a continuous bead of conductive adhesive along the seal, on a side of the seal to be disposed against a conductive panel. Conductive adhesive has conductive particles in an adhesive binder, forming a conductive path by surface contact of the particles. The binder typically consists essentially of a nonconductive elastomer which otherwise would function as an insulator or as a dielectric. Conductive adhesives tend to break down over time. The conductive particles also can migrate in the binder, particularly with compression and decompression of the seal. As a result, the conductive particles can become spaced from the surface of the adhesive as the elastomer flows viscously. The conductivity of the seal as a whole gradually decreases with increase in resistance across the adhesive. The conductive particles also can deteriorate from mechanical and environmental effects. Breakdown is accelerated where the sheath is a wire mesh or the like, where movement of the wire portions of the sheath kneads the adhesive. U.S. Patent 4,857,668 - Buonanno discloses a seal with a conductive sheath on a resilient foamed core. The sheath can be ripstop nylon, a polymer fabric, plated with conductive material (e.g., metal), mounted by conductive clips, engaged in a groove or attached by conductive adhesive. Other forms of seals are also known, including sheet metal structures and resilient rubber or plastic forms enclosed in conductive sheathing. Each form has conductive surfaces contact the conductive panels and must be attached, typically by an adhesive. In a seal formed by wrapping a strip of conductive sheathing, the lateral edges of the sheathing must be conductively attached together along a seam so that the sheath is uniformly conductive, which often requires an adhesive. A conductive adhesive will provide a continuous conductive path when attaching the sheath to itself or to a panel. However, conductive adhesives tend to break down and become less conductive over time, and have inherent resistance. Conductive adhesive is more expensive than nonconductive adhesive, and is less sticky than a comparable quantity of nonconductive adhesive. For these reasons, it would be helpful to avoid or minimize reliance on conductive adhesive. According to the invention, we provide a seal for shielding against passage of electromagnetic energy through a seam with a conductive body, comprising an elongated conductive seal operative to conduct laterally of a longitudinal extension of the seal, and an adhesive disposed on a surface of the conductive seal directed against the conductive body, characterised in that the adhesive is nonconductive and is discontinuous along said length of the seal, the adhesive defining a plurality of areas of adhesive, for attaching the seal physically across the seam, separated by areas wherein the seal is exposed directly into contact with the conductive body, the seal being in contact with the conductive body at every point along a longitudinal extension of the seam for coupling the seal electrically across the seam. The discontinuously applied adhesive may be in the form of regularly spaced adhesive dots, or preferably along spaced lines at the conductive and physical junction of the sheath or seal. Contrary to expectations, the lack of an adhesive material in the area between the adhesive lines or dots does not reduce the effectiveness of the seal, particularly if the adhesive is arranged in the form of longitudinally non-overlapping lines inclined laterally across the connecting seam of the seal. In fact, the seal is more effective because greater direct contact between the panels and the conductive sheath of the seal provides an overall conductive EMI shielding barrier that intersperses only the contact resistance of the sheath and the panel between the abutting elements along the path of electromagnetic propagation. Also according to the present invention, we provide a method for mounting a seal against passage of electromagnetic energy through a seam between conductive bodies, comprising the steps of providing an elongated conductive seal operable to conduct laterally of a longitudinal extension of the seam, applying a nonconductive adhesive to a surface of the conductive seal to be directed against one of the conductive bodies, the adhesive being discontinuous along said length of the seal, thereby defining a plurality of areas of adhesive for attaching the seal physically across the seam, separated by areas wherein the seal is exposed directly into contact with the conductive body, the areas exposed for contact extending along every point of a longitudinal extension of the seam for coupling the seal electrically across the seam, and attaching the seal via said adhesive to a surface of at least one said conductive body. The proportion of adhesive area to exposed space is preferably about 40% adhesive to 60% exposed, and can be as little as 5% adhesive to 95% exposed. The conductive shield can have a release liner removably attached to seal via the adhesive, pulled off to expose the adhesive. A conductive path is provided, including surface contact at the exposed areas. Where inclined lines of adhesive are disposed along the junction, the lines can be laterally non-overlapping to provide a single thickness of adhesive line at all points along the junction. Such arrangements reduce the cost and complexity of mounting the seal, while retaining full effectiveness of the seal for electromagnetic shielding between conductive panels by minimizing electrical resistance along a path between such panels, improve the conductivity in the long term of shielding junctions, where the seal is used, and provide a seal or sheath that is easy to use, physically strong and of moderate cost. They also maximise surface contact between conductive elements forming a barrier to electromagnetic propagation. Several preferred embodiments of a seal according to the invention are now described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a perspective illustration of a seal or gasket, with discontinuous adhesive in inclined non-overlapping lines; Fig. 2 is an elevation view showing the seal interspersed between conductive panels; Fig. 3 is a diagrammatic plan view showing the proportion of exposed area to adhesive area, the adhesive applied as spaced dots; Fig. 4 is a partial section view through a junction of the seal and a conductive panel, along line 6-6 in Fig. 2, showing the seal compressed; Fig. 5 is a perspective view showing the seal with a release liner; Fig. 6 is a plan view of an alternative embodiment, with a different pattern; Fig. 7 is a perspective view showing sheet conductive material wrapped on an elongated body; Fig. 8 is a section view longitudinally along a seam with conductive material attached along edges to conductive panels; and, Figs. 9a-9c are illustrations of additional forms of shielding seals in perspective and in elevation. The invention concerns the conductive connection of parts to block electromagnetic radiation, and is applicable to a number of types of shielding apparatus to be bridged at a seam with a conductive material. The conductive material can be more or less flexible and can be thick or thin, depending on the particular application. A discontinuous adhesive material is applied to affix the conductive shield at one or both of the edges of the seam. Fig. 1 is a perspective illustration of a compressible seal or gasket, showing the cross section of the seal body at the end. The seal can be any length, preferably long enough to encompass the full length of the line or seam to be sealed. In this case the compressible seal resides along a junction between conductive panels, and by conductively bridging between the panels provides a continuous conductive barrier against the propagation of electromagnetic interference either into or out of the area encompassed by the panels and the shield. Shield 30 includes a conductive sheath 32 on a compressible core 34. The seal can have a foamed elastomer core and a sheath of metal plated or conductively coated ripstop nylon. An anti-abrasive coating can be applied on the surface of sheath 32, formed of conductive particles (e.g., conductive carbon black) suspended in an elastomeric binder in sufficient concentration to obtain with the conductive sheath an overall conductive body. It is also possible to employ the invention with other forms of shielding seal structures, for example seals with wire mesh woven or knitted around a resilient core, and wholly metallic shielding seals, for example with helically wound metal strips or formed spring-like metal tongues protruding from a strip attachable to one panel and resiliently bearing against the opposed panel. Other forms of shield seals, as well as shields having a sheet of conductive material, can be attached by an adhesive according to the invention. Fig. 2 shows a shielding seal of the type shown in Fig. 1, between conductive bodies or panels 24, for example a movable door panel and a stationary cabinet panel against which the door panel closes. The seal provides a conductive path bridging across any space which may be defined between the panels 24, 24 due to minor misalignment, surface irregularities and the like. The seal blocks propagation of electromagnetic interference through the opening between the panels. The shield strip or body 30 is fixed to at least one of the panels via an adhesive disposed on the shield, e.g., on a surface of a conductive sheath. The adhesive is discontinuous along the length of the seal body, defining a plurality of areas 50 of adhesive for physically attaching the shield to at least one of the panels, separated by areas 60 wherein the conductive material of the shield is exposed directly for conductive surface contact with the panels. The sheathing 32 in Figs. 1 and 2 is wrapped circumferentially around a compressible core 34. Accordingly, an elongated longitudinal seam 35 is defined where the sheathing overlaps itself, the edges of the sheathing at the seam being in conductive contact such that the shield as a whole is uniformly conductive across the panels. This seam 35 can be affixed via a discontinuous adhesive according to the invention. The invention is described primarily with respect to a compressible seal adhesively fixed to one panel and abutted against a relatively movable opposed panel. The seal can be attached to both panels in an embodiment wherein the panels are to remain fixed, in the manner of a fixed gasket. The adhesive attachment of the seal to a conductive body applies to attachment of edges of sheathing along longitudinal or helical conductive seams, and to attachment of other forms of conductive sheets and strips to conductive bodies. In exposed areas 60, the conductive shield material (e.g., the sheath) is urged by the compressible foam core 34 directly against the panel to which the seal body is attached. The surface of the sheath bearing against the panel is made discontinuous along the length of the seal by the areas 50 of adhesive. On the opposite panel the foam core holds the conductive sheath in continuous contact. In a similar manner the pressure of spring-like seals of sheet metal and the like urges a surface of the shield against the opposite panel. On the attachment side(s), areas 50 of adhesive and areas 60 where the conductive shield material is exposed define a regular repetitive pattern along the length of the seal. The pattern can be simple spots or dots of adhesive as shown in Fig. 3, but preferably is arranged as a pattern of longitudinally non-overlapping lines extending laterally across the seam, to maximize both the extent of conductive surface contact and the extent of physical attachment of the material across the seam. The dimensions of the adhesive areas are a function of the dimensions of the seal and a function of the required extent of physical attachment. For larger seals and/or seals which are to be more securely affixed, larger adhesive areas are preferable. The proportion of adhesive area to exposed area can be a minimum to barely hold the seal, for example 5% adhesive area to 95% exposed area, or can range from 5% to 40% adhesive to exposed area of 95% to 60%. For better adhesion the relatively larger proportions of adhesive are needed. Favorable conductivity characteristics together with good adhesive bonding are provided in a proportion of substantially 40% adhesive area to 60% exposed area. Figs. 1-6 illustrate a seal having a rectangular cross section, for example of about 3 by 5 mm. According to Fig. 3, which shows the higher proportion of adhesive, adhesive spots on the wider side of the rectangular form seal are arranged in a non-overlapping manner, about 2 or 3 mm in diameter and spaced on 1 cm centers. This places a relatively larger proportion of the surface in contact with the panel and provides good adhesive bonding. In a preferred embodiment shown in Fig. 4, the adhesive defines a regular repetitive pattern of lines along the seal. The lines are inclined laterally of the longitudinal axis of the seam, and can extend laterally across the full width of the seam. In Fig. 4 the seal is compressed between panels 24. As a result of compression, the seal becomes widened slightly such that the adhesive lines do not extend clear to the edges 39, although the adhesive initially extended to the at-rest edges 38 of the seal, shown in broken lines. The adhesive lines are non-overlapping as viewed laterally of the seam. The end of each of the lines is longitudinally coextensive and laterally spaced from an end of a next one of the lines. As a result, an equal lateral width of adhesive is provided at substantially every longitudinal point along the longitudinal extension of the seam, securely attaching to the conductive element. At the same time, at every longitudinal point along the seam there is a substantial exposed conductive area, considering the lateral extension of the seam (i.e., the adhesive occupies a relatively small part along any lateral line across the seam). This leads to good mechanical attachment and electrical shielding attributes. Preferably, the conductive seal surface is supplied with a release liner, for example a coated strip of paper, removably attached to the seal via the adhesive as shown in Fig. 5. The user peels the release liner from the adhesive prior to attaching the seal across the seam. Inasmuch as the adhesive lines are longitudinally coextensive and the release liner is peeled longitudinally from the seal, the release liner peels continuously upward from the adhesive, rather than by discontinuously popping free at the exposed areas. The longitudinally continuous and laterally discontinuous adhesive pattern provides secure mechanical connection. The seal is equally conductive with the conductive element all along the seam because the proportion of exposed area to adhesive area does not vary. Due to these aspects, it is possible to obtain as good or better conductivity (and therefore shielding effectiveness) with a nonconductive adhesive, as a seal having a continuous conductive adhesive bead extending all along the seal parallel to the longitudinal axis. A preferred nonconductive adhesive material is hot melt adhesive and can be applied in production for example by passing the seal by a dispensing head which operates intermittently at the required rate, or which is arranged to sense the linear passage of the seal and to dispense a quantity of adhesive at the required spacing. Alternatively, the adhesive can be "printed" on the sheath, using an applicator wheel to which lines of adhesive are applied at regular spaces around the circumference, and transferred to the seal. The pattern of adhesive can be varied while retaining the non-overlapping arrangement described. In Fig. 6, a different shape of adhesive area is provided, wherein parts of the adhesive lines run parallel to the seal. While the shape of the adhesive can be varied, for example to provide particular characteristics in the adhesive bonding of the seal body and the panel, in each case sufficient space 60 between the adhesive areas 50 is provided to ensure good conductive surface contact between a substantial proportion of the seal body and the panels. A conductive seam using discontinuous adhesive can be used for other forms of shielding, as shown for example in Figs. 7 and 8. In these embodiments the shield involves the conductive attachment of a sheet-like conductive material to articles to be shielded. In Fig. 7, the shield material 32 shields a cable 25. The shield material can be wrapped to form a longitudinal seam, or as shown the material can form a helical seam along cable 25. The shield material is conductive, for example being made of a metal foil, metal plated or conductively coated paper or fabric, or the like. The shield material 32 overlaps each previous wrap of shield material to form a seam 35 at which the conductive material is physically attached and in electrical contact with the previous wrap. This is accomplished using a discontinuous pattern of adhesive, preferably with a broken line of segments which extend laterally of the helical seam and are non-overlapping along the line of the seam. The helical wrapping of the shield material 32 provides the necessary inward force to urge the wraps into contact in the exposed areas between the adhesive areas, and this inward force can be increased by using a conductive shield material which stretches resiliently, for example a conductively coated fabric. In Fig. 8, a sheet form of shield is attached via discontinuous inclined adhesive lines to each of two conductive panels 24. Fig. 8 is a cross-section viewed along the axis of the two seams between shield material 32 and panels 24. Very little of the lateral extension of material 32 bridging across the conductive panels is occupied by the adhesive and most of the lateral extension of the shield material is in direct contact with the panels. Accordingly, a secure and quite conductive bridge is formed between the two panels for blocking electromagnetic interference. Figs. 9a-9c illustrate application of the discontinuous adhesive to additional forms of conductive seals. Each of the illustrated embodiments is compressible. In Fig. 9a, a wire knit or mesh 131 forms the conductive material on a cylindrical compressible core. The adhesive lines 50 define a section of a helix on the conductive mesh or knit. When the seal is compressed the core flattens the adhesive against a panel (or a groove in a panel, etc.) to obtain a conductive seam. Figs. 9b illustrates discontinuous adhesive on a seal having resilient conductive tongues 133 on a conductive base strip 134. The base strip 134 is conductively and physically joined to a panel using the discontinuous pattern as described, and the tongues bear against an opposed panel. The seal can be integral sheet metal or conductively coated resilient material such as plated or coated plastic. In Fig. 9c, the seal has helically wrapped conductive material. The inclined lines run along a side of the substantially cylindrical shape. The lines shown are further made discontinuous by the gaps between successive wraps of the helical conductive material, which can be sheet metal, conductively coated plastic, etc., and is compressed between conductive panels or the like. The invention can concerns a method for mounting a seal against passage of electromagnetic energy through an abutment of conductive panels, or a method for blocking electromagnetic energy. The method includes providing a conductive shield material 32 as described, in an indefinite length, for example on a compressible core 34. Adhesive 50 is applied to a surface 36 of the conductive material and is discontinuous along the seam to be formed, defining a plurality of areas 50 of adhesive, separated by areas 60 where the conductive material 32 is exposed. The seam is formed by attaching the shield material via adhesive 50 to at least one conductive surface such as the surface of conductive panels to be bridged by the shield. The panels can be brought into abutment with the seal interspersed, whereby conductivity between the panels includes a conductive path from the panels directly to the exposed areas of the conductive shield. Conduction between the panels occurs through the conductive shield material 32. The conductive material can be arranged as a sheet or as a sheath on a compressible core. The shield material or sheath can be conductive due to its composition (e.g., including a metallic form, foil or mesh, and/or can have a conductive coating). Further variations will occur to persons skilled in the art of EMI shielding. Reference should be made to the claims rather than the discussion of exemplary embodiments to determine the scope of the invention in which exclusive rights are claimed.

Claims: A seal for shielding against passage of electromagnetic energy through a seam with a conductive body, comprising an elongated conductive seal (30) operative to conduct laterally of a longitudinal extension of the seal, and an adhesive (50) disposed on a surface of the conductive seal (30) directed against the conductive body, characterized in that the adhesive (50) is nonconductive and is discontinuous along the length of the seal (30), the adhesive defining a plurality of areas of adhesive (50), for attaching the seal (30) physically across the seam, separated by areas (60) wherein the seal (30) is exposed directly into contact with the conductive body for coupling the seal electrically across the seam. A method for mounting a seal against passage of electromagnetic energy through a seam between conductive bodies, comprising the steps of providing an elongated conductive seal (30), operable to conduct laterally of a longitudinal extension of the seam, applying a nonconductive adhesive (50) to a surface of the conductive seal to be directed against one of the conductive bodies, the adhesive being discontinuous along the length of the seal (30), thereby defining a plurality of areas of adhesive (50) for attaching the seal physically across the seam, separated by areas (60) wherein the seal is exposed directly into contact with the conductive body for coupling the seal electrically across the seam, and attaching the seal via said adhesive to a surface of at least one said conductive body. The invention according to claims 1 or 2, wherein the areas of adhesive (50) and the exposed areas (60) define a regular repetitive pattern along the length of the seal. The invention as claimed in claim 1, 2 or 3, wherein the plurality of areas of adhesive define a broken line on the surface of the conductive seal at the seam. The invention according to claim 1, 2, 3 or 4 wherein the areas of adhesive (50) and the areas (60) exposed are provided in a proportion of about 5% to 40% adhesive area (50) to 60% to 95% exposed area (60). The invention according to any one of the preceding claims, wherein the areas of adhesive (50) define a succession of lines inclined laterally of the longitudinal extension of the seam, the lines being non-overlapping as viewed laterally of the seam such that an end of each of the lines is longitudinally coextensive and laterally spaced from an end of a next one of the lines. The invention according to any one of the preceding claims wherein the seam comprises a compressible seal body (34). The invention according to any one of the preceding claims wherein the seam is defined between overlapping portions of a conductive sheath. The invention according to any one of the preceding claims further comprising a release liner removably attached to the adhesive.

Inventors: FERRIS MERLE C, HOGE WILLIAM C JR, FERRIS, MERLE C., HOGE, WILLIAM C., JR.
Applicants: SCHLEGEL CORP, SCHLEGEL CORPORATION
Application year: 1991
Main classification: H05K9
IPC codes: H05K9