Patent Publication Number: US-2011067918-A1

Title: Emi shielding materials

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending International Application No. PCT/US2009/34242, which designated the United States, and which claims priority to U.S. provisional application Ser. Nos. 601/074,748, filed Jun. 23, 2008, and 61/088,162, filed Aug. 12, 2008, the disclosure of each of which is expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates broadly to electromagnetic interference (EMI) shielding materials such as in the form of gaskets and gap fillers, backplanes, grounding pads and the like, and particularly to such materials which are corrosion resistant. 
     The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed “electromagnetic interference” or “EMI” as being known to interfere with the operation of other proximate electronic devices. 
     To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other “target” devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect. 
     For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. Requirements also may dictate a low impedance, low profile gasket which is deflectable under normal closure force loads. Other requirements include the ability to achieve a certain EMI shielding effectiveness for both the proper operation of the device and compliance, in the United States, with commercial Federal Communication Commission (FCC) EMC regulations. 
     EMI shielding gaskets typically are constructed as a resilient element, or a combination of one or more resilient elements having gap-filling capabilities. One or more of the elements may be provided as a tubular or solid, foamed or unfoamed core or strip which is filled to be electrically-conductive. Such core or strip may be extruded, molded, or otherwise formed of an elastomeric polymeric material. Conductive materials for the filler include metal or metal-plated particles. 
     Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, die-cutting, and form-in-place (FIP). 
     In view of the foregoing, it may be appreciated that many different types of materials employed in the production of EMI shielding material such as gaskets. As electronic devices continue to proliferate, it is believed that additional EMI shielding alternatives and options would be well-received by the electronics industry. 
     SUMMARY OF THE INVENTION 
     The present invention relates broadly to electromagnetic interference (EMI) materials such as in the form of gaskets and gap fillers, backplanes, grounding pads and the like. More particularly the invention relates to a corrosion-resistant electromagnetic interference (“EMI”) shielding material provided as an admixture of an elastomeric polymeric component, and a filler component. The filler component itself is provided as glass or aluminum particles which are electroless-plated with a plating of a nickel-phosphorous alloy. 
     As compared to conventional silver-based, nickel, or nickel-graphite filled materials, at comparable volume loading levels, the nickel-glass and nickel-aluminum fillers material of the present invention exhibit comparable or improved EMI shielding effectiveness and corrosion resistance such as when in contact with an aluminum or nickel-plated housing or other device part. 
     The present invention, accordingly, comprises the materials, material forms, and/or methods possessing the construction, combination of elements, and/or arrangement of parts and steps which are exemplified in the detailed disclosure to follow. Advantages of the present invention include an EMI shielding material which is both cost-effective and corrosion-resistant, and which exhibits superior EMI shielding effectiveness. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein: 
         FIG. 1  is a perspective end view, partially in cross-section, of a handheld electronic communication device representative of a typical application for the corrosion-resistant EMI shielding material of the present invention; 
         FIG. 2  is a magnified view of a portion of the enclosure of  FIG. 1  showing in enhanced detail the corrosion-resistant EMI shielding material of the present invention as interposed between a first and a second housing part of the device of  FIG. 1 ; and 
         FIG. 3  is a plot comparing the EMI shielding effectiveness of conventional EMI those in accordance with the present invention. 
     
    
    
     The drawings will be described further in connection with the following Detailed Description of the Invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology may be employed in the following description for convenience rather than for any limiting purpose. For example, the terms “forward” and “rearward,” “front” and “rear,” “right” and “left,” “upper” and “lower,” “top” and “bottom,” and “right” and “left” designate directions in the drawings to which reference is made, with the terms “inward,” “inner,” “interior,” or “inboard” and “outward,” “outer,” “exterior,” or “outboard” referring, respectively, to directions toward and away from the center of the referenced element, the terms “radial” or “vertical” and “axial” or “horizontal” referring, respectively, to directions or planes perpendicular and parallel to the longitudinal central axis of the referenced element. Terminology of similar import other than the words specifically mentioned above likewise is to be considered as being used for purposes of convenience rather than in any limiting sense. Further, the term “EMI shielding” should be understood to include, and to be used interchangeably with, electromagnetic compatibility (EMC), electrical conduction and/or grounding, corona shielding, radio frequency interference (RFI) shielding, and anti-static, i.e., electro-static discharge (ESD) protection, and the terms “magnetic,” “dielectric,” “ferritic,” or “lossy” to be used interchangeably with EMI absorbing, absorptive, dissipating, dissipative, or attenuating, or as otherwise having a capability to attenuate electromagnetic energy by absorption or another dissipation mechanism. 
     In the figures, elements having an alphanumeric designation may be referenced herein collectively or in the alternative, as will be apparent from context, by the numeric portion of the designation only. Further, the constituent parts of various elements in the figures may be designated with separate reference numerals which shall be understood to refer to that constituent part of the element and not the element as a whole. General references, along with references to spaces, surfaces, dimensions, and extents, may be designated with arrows or underscores. 
     For the illustrative purposes of the discourse to follow, the corrosion-resistant EMI-shielding material of the invention herein involved is principally described in connection with its use an extruded, form-in-place, molded or other gasket which is interposed between two components, each of which may be a housing, case, circuit board, door, cover, can, shield, integrated circuit chip, or other part of an electronic device such as a mobile, i.e., cellular, telephone handset, or other electronic device such as a personal communications services (PCS) handset, PCMCIA card, global positioning system (GPS), radio receiver, personal digital assistant (PDA), notebook or desktop personal computer (PC), cordless telephone handset, network router or server, medical electronics device, modem, wireless communication base station, telemetry device, telematic component or system, or the like. It will be appreciated, however, that aspects of the present invention may find use in other EMI shielding applications, such as a gap filler or conformal coating which otherwise is interposed and then cured between two surface of one or more of the component parts of the devices. Such uses and applications therefore should be considered to be expressly within the scope of the present invention. 
     In accordance with the precepts of the present invention, a corrosion-resistant electromagnetic interference (“EMI”) shielding material is provided as an admixture of an elastomeric polymeric component, and a filler component. Such material may be provided as a cured or otherwise form-stable composition, such as in the form of a molded or extruded gasket profile, or as curable composition, such as for the spraying, dispensing, or other production of conformal coatings or form-in-place gaskets or gap fillers. By “cured” it is meant that the composition may be polymerized, cross-linked, further cross-linked or polymerized, vulcanized, cooled, hardened, or otherwise chemically or physically changed from a liquid or other fluent form into a sold or more solid or semi-solid elastomeric or polymeric phase or form. 
     The elastomeric polymeric component may be a thermoplastic or thermoset, and specifically may be selected as depending upon one or more of operating temperature, hardness, chemical compatibility, resiliency, compliancy, compression-deflection, compression set, flexibility, ability to recover after deformation, modulus, tensile strength, elongation, force defection, flammability, or other chemical or physical property. Depending upon the application, suitable materials, including blends, co-polymers, and other mixtures thereof, may include natural rubbers such as Hevea and thermoplastic, i.e., melt-processible, or thermosetting, i.e., vulcanizable, synthetic rubbers such as fluoropolymer, chlorosulfonate, polybutadiene, butyl, neoprene, nitrile, polyisoprene, buna-N, copolymer rubbers such as ethylene-propylene (EPR), ethylene-propylene-diene monomer (EPDM), nitrile-butadiene (NBR) and styrene-butadiene (SBR), or blends such as ethylene or propylene-EPDM, EPR, or NBR. The term “synthetic rubbers” also should be understood to encompass materials which alternatively may be classified broadly as thermoplastic or thermosetting elastomers such as polyurethanes, silicones, fluorosilicones, styrene-isoprene-styrene (SIS), and styrene-butadiene-styrene (SBS), as well as other polymers which exhibit rubber-like properties such as plasticized nylons, polyolefins, flexible epoxies, polyesters, ethylene vinyl acetates, fluoropolymers, and polyvinyl chloride. As used herein, the term “elastomeric” is ascribed its conventional meaning of exhibiting rubber-like properties of compliancy, resiliency or compression deflection, low compression set, flexibility, and an ability to recover after deformation, i.e., stress relaxation. Any of the forgoing materials may be used unfoamed or, if required by the application, blown or otherwise chemically or physically processed into an open or closed cell foam. 
     The elastomeric polymeric component generally may form a binder or other continuous or matrix phase into which the particulate filler component may be dispersed as a discrete phase. The filler generally may be included within the binder in a proportion sufficient to provide the level of EMI shielding effectiveness which is desired for the intended application. For most applications, an EMI shielding effectiveness of at least 10 dB, and usually at least 20 dB, and preferably at least about 40 dB or higher, over a frequency range of from about 20 MHz to 18 GHz may be considered acceptable. Such effectiveness translates to a filler proportion which generally is between about 10-50% by volume or 20-80% by weight, based on the total volume or weight, as the case may be, of the compound, and a bulk or volume resistivity of not greater than about 10 Ω-cm. However, the ultimate shielding effectiveness of the material can vary based on the amount of the EMI-absorptive filler, and of other fillers, such as other electrically-conductive fillers, and on the thickness of the material. 
     In accordance with the precepts of the present invention, the electrically-conductive filler component which is admixed with the elastomeric polymeric component is provided as either glass or aluminum particles which are electrolessly-plated with a plating of a nickel-phosphorous alloy. The plated particles may be of any shape, or combination of shapes, and may be referred broadly as being “particulate,” which should be understood to include solid or hollow spheres and microspheres or microballoons, flakes, platelets, fibers, rods, irregularly-shaped particles, nodules, fibers, which may be chopped or milled or whiskers, and powders. For many applications the aluminum particles may take the form of spheres, flakes, or nodules, while the glass particles, which may be silica, borosilica, non-silica, or a mixture of blend thereof, may be irregular or fibers which may allow a lower volume fill for a given conductivity. The mean average particle size or distribution of the filler, which may be a diameter, imputed diameter, length, or other dimension of the particulate typically will range from about 0.01 mil (0.25 μm) to about 20 mils (500 μm), but preferably for fibers a length of less than about 16 mil (400 μm) with a length to width aspect ratio of between about 5-20. The nickel-phosphorous alloy is electrolessly plated on the underlying glass or aluminum particles to form a plating thereon which plating may comprise less than about 40% of the weight of the filler. 
     Additional fillers and additives may be included in the formulation of the material depending upon the requirements of the particular application envisioned. Such fillers and additives, which may be functional or inert, may include wetting agents or surfactants, pigments, dispersants, dyes, and other colorants, opacifying agents, foaming or anti-foaming agents, anti-static agents, coupling agents such as titanates, chain extending oils, tackifiers, flow modifiers, pigments, lubricants such as molybdenum disulfide (MoS 2 ), silanes, peroxides, film-reinforcing polymers and other agents, stabilizers, emulsifiers, antioxidants, thickeners, and/or flame retardants and other fillers such as aluminum trihydrate, antimony trioxide, metal oxides and salts, intercalated graphite particles, phosphate esters, decabromodiphenyl oxide, borates, phosphates, halogenated compounds, glass, silica, which may be fumed or crystalline, silicates, mica, ceramics, and glass or polymeric microspheres. Typically, these fillers and additives are blended or otherwise admixed with the formulation, and may comprise between about 0.05-80% or more by total volume thereof. The formulation of the material may be compounded in a conventional mixing apparatus as an admixture of the polymeric and filler components, and any additional fillers or additives. 
     Referring now to the figures wherein corresponding reference characters are used to designate corresponding elements throughout the several views with equivalent elements being referenced with prime or sequential alphanumeric designations, an illustrative electronic device is shown generally at  10  in the perspective view of  FIG. 1  as including a case, housing, or enclosure, reference generally at  12 , which is modified in accordance with the precepts of the present invention as having a gasket, gap filler, coating or other layer,  14 , of the EMI shielding material of the present invention. For purposes of illustration, device  10  is shown to be a mobile telephone handset, but alternatively may be another handheld, portable, or other electronics device such as a personal communications services (PCS) handset, PCMCIA card, global positioning system (GPS), radio receiver, personal digital assistant (PDA), notebook or desktop personal computer (PC), cordless telephone handset, network router or server, medical electronics device, or the like. Enclosure  12  is shown to be of a 2-part construction including a upper half or cover,  16   a , and a lower half or base,  16   b , each of the parts  16   a - b  having a corresponding interior surface,  18   a - b , and an exterior surface,  20   a - b , which extend coterminously to form adjoining top and bottom walls,  21   a - b , side walls,  22   a - b  and  24   a - b , and end walls (not shown). The side and end walls together define the perimeters of each of the enclosure parts  16   a - b , which is demarked by a peripheral edge surface,  26   a - b . The edge surfaces  26  are mating and define a joint, parting line, or other interface,  30 . As is shown, enclosure  12  may house one or more printed circuit boards (PCBs),  32   a - b , or other circuitry of the device  10 . 
     With additional reference to  FIG. 2 , the detail of  FIG. 1  referenced at  40  is depicted in the magnified view of  FIG. 2 . As may be seen, a compressible gasket, gap filler, coating, or other form,  32 , of the material of the present invention is provided in the interface  30  as interposed between the surfaces  26  to provide electrical continuity and/or environmental sealing between the parts  16 . For the purposes of the present invention, one or both of the enclosure parts  16   a - b  and/or the surfaces  26   a - b , which may be constructed of the same or different materials, may be molded, cast, machined, or otherwise formed of a plastic or a metal such as an aluminum or aluminum alloy, or a plastic or metal which is nickel-plated or otherwise plated. 
     Example 
     Samples of representative EMI shielding materials according to the present invention were prepared for characterization. Irregular glass particles (average size 150˜200 μm) and nodular-shaped aluminum particles (average size 100˜120 μm) each were electrolessly plated with 20% by weight of a nickel-phosphorous alloy (&lt;6% phosphorous) to form fillers in accordance with the present invention. The mean average particle size of the nickel-glass filler was between about 150˜200 μm, and that of the nickel-aluminum fillers was between about 100˜120 μm. 
     These nickel-glass and nickel-aluminum fillers were admixed at, respectively, 39.2% and 38.6% by volume with a methyl and phenyl, peroxide-cured silicone to form EMI shielding compositions representative of the present invention. The galvanic corrosion resistance on hexavalent chromated aluminum (per CHO-TM100, Chomerics Test Procedure, Parker Chomerics Division, Woburn, Mass.), initial volume resistivity (per ASTM D991), and initial EMI shielding effectiveness (per Chomerics Test Procedure CHO-TM-TP08) of these materials was determined and compared to conventional materials. The results are summarized in the table below and in the EMI shielding effectiveness plot of  FIG. 3   
     
       
         
           
               
               
               
             
               
                   
                 TABLE 
               
             
            
               
                   
                   
               
               
                   
                 Corrosion 
                   
               
               
                   
                 Resistance 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 168 hrs 
                   
                   
               
               
                   
                   
                 Wt. 
                 504 hrs 
                 Volume 
               
               
                   
                 Conductive Filler 
                 Loss 
                 Wt. Loss 
                 Resistivity 
               
               
                 Elastomer 
                 (% by volume) 
                 (mg) 
                 (mg) 
                 (mOhm-cm) 
               
               
                   
               
               
                 Silicone 1   
                 40.3% Ag—Al 
                  7-11 
                 26 
                 3.5-5.1 
               
               
                 Fluorosilicone 2   
                 41.8% Ag—Al 
                 4-7 
                 45 
                 5.5 
               
               
                 Fluorosilicone 3   
                 41.4% Ag—Al 
                 3-5 
                 23 
                 4-6 
               
               
                 Silicone 4   
                 47.7% Ag-glass 
                 132 
                   
                 5 
               
               
                 Silicone 5   
                 32.6% Ni-graphite 
                 28-63 
                 138  
                 36 
               
               
                 Silicone 6   
                 37.4% Ni 
                 21-26 
                 52 
                 41-48 
               
               
                 Silicone 7   
                 24.5% Ni-graphite 
                  18 
                 46 
                 128-260 
               
               
                 Silicone 
                 39.2% Ni-glass 
                 3-6 
                 7-13 
                 356-832 
               
               
                 Silicone 
                 38.6% Ni—Al 
                 2-7 
                 6-17 
                 57-99 
               
               
                   
               
               
                   1 Cho-Seal 1285, Parker Chomerics Division, Woburn, MA 
               
               
                   2 Cho-Seal 1287, Parker Chomerics Division, Woburn, MA 
               
               
                   3 Cho-Seal 1298, Parker Chomerics Division, Woburn, MA 
               
               
                   4 Cho-Seal 1310, Parker Chomerics Division, Woburn, MA 
               
               
                   5 Cho-Seal S6305, Parker Chomerics Division, Woburn, MA 
               
               
                   6 Cho-Seal 6313, Parker Chomerics Division, Woburn, MA 
               
               
                   7 Cho-Seal 6330, Parker Chomerics Division, Woburn, MA 
               
            
           
         
       
     
     These data show that as compared to conventional silver-based, nickel, or nickel-graphite filled materials, at comparable volume loading levels, the nickel-glass and nickel-aluminum fillers material of the present invention exhibit comparable or improved EMI shielding effectiveness and corrosion resistance such as when in contact with an aluminum substrate. 
     Advantageously, the use of the EMI shielding materials of the present invention instead of silver-based materials can significantly reduce costs while providing comparable shielding effectiveness. Such materials, moreover, in offering improved corrosion resistance, such as when used in contact with aluminum or nickel-plated substrates, can extend the service life of seals, gaskets, gap fillers, coatings, and other products in maintaining their shielding and moisture sealing performance in uncontrolled environments. 
     As it is anticipated that certain changes may be made in the present invention without departing from the precepts herein involved, it is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense. All references including any priority documents cited herein are expressly incorporated by reference.