Abstract:
A method ( 10 ) for coating a substrate including at least an elastomer substrate portion with a conductive coating is described. The method includes steps of (i) providing a substrate comprising elastomer material and (ii) coating ( 13 ) the substrate with an adhesion promoting layer. The layer comprises a thin layer of B stage heat curable epoxy. The method also includes steps of (iii) pre-curing ( 15 ) the adhesion-promoting layer, (iv) disposing ( 17 ) a conductive coating atop the layer and (v) post-curing ( 19 ) the adhesion-promoting layer.

Description:
FIELD OF THE INVENTION 
     This invention relates to deposition of metal conductive layers on substrate materials, in general, and to metallization of flexible materials as well as metallization of substrates including both flexible and rigid portions, in particular. 
     BACKGROUND OF THE INVENTION 
     Conductive elastomer materials are used to form gaskets for sealing electronic modules which are typically enclosed in a rigid enclosure. The gaskets provide physical integrity by preventing unwanted materials from entering the enclosure. The gaskets are also provided to further radio frequency (rf) signal shielding to prevent unwanted emission of signals from within the module and to prevent unwanted reception of radio frequency signals from entering the module. Where rf shielding is of concern, it is highly desirable that the gasket be electrically conductive. To provide a good physical seal, the gasket is desirably of a flexible material such as an elastomer. 
     Conductivity has been imparted to elastomer materials by mixing finely divided conductive particles, usually metallic, with the materials that are cast to provide the conductive elastomer. The mixture is typically cast as a sheet from which gasket shapes are then cut. There are disadvantages associated with such conductive elastomer gaskets, including poor conductivity and poor thermal cycling behavior. The poor conductivity occurs as a result of several factors, including the fact that inter-particle contact is required for the material as a whole to be conductive and achieving such inter-particle contact over the continuum of the gasket is difficult to achieve. Additionally the conductive particle loading of the elastomer material may be inadequate either overall or in localized areas. Also, such gaskets may likewise have relatively poor thermal cycling behavior of the conductivity because the particles may not stay in continuous conductive contact as the material is heated and cooled. 
     Poor or variable (e.g., with thermal cycling) conductivity results in sporadic and/or inadequate electronic shielding of the equipment using such conductive elastomer material for rf gaskets. This may lead to failure to meet specifications and/or inappropriate electrical performance. 
     Various methods have been attempted in the past to deposit metal layers on non-metallic and insulating substrates, however, none of the prior approaches have produced conductive layers on flexible materials which retain adherence to the flexible material and continuity after flexing of the material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawing figures in which like reference designators are used to designate like elements, and in which: 
     FIG. 1 is a flow chart of a process for metallization of a substrate comprising an elastomer material in accordance with the present invention; 
     FIG. 2 is an elastomer gasket in accordance with the principles of the invention; 
     FIG. 3 illustrates an assembly in accordance with the principles of the invention; and 
     FIG. 4 illustrates a portion of the assembly of FIG. 3 in greater detail. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIG. 1, a process  10  for metallization of a substrate comprising an elastomer material is described in accordance with the present invention. In the illustrative embodiment, the elastomer material is a vulcanized ethylene propylene diene monomer (EPDM/PP). The process  10  may be utilized to form a conductive metal coating on a substrate such as the elastomer gasket shown in FIG. 2 or on a composite substrate such as the one shown in FIG.  3 . Process  10  begins with the selection  11  of a substrate to be processed in accordance with the invention. An example of a preferred substrate material is a thermoplastic elastomer (TPE) identified as Santoprene® and which is available from Advanced Elastomers of Akron, Ohio. The substrate is coated in step  13  with an adhesion agent. In a preferred embodiment, the selected substrate is coated with B-stage epoxy material. The B-stage epoxy material is 3M&#39;s 2290, diluted 1:4 with a dilutent comprising four parts methyl ethyl ketone and one part tetrahydrofuran. With this material, coating in a range of about one to ten microns thickness is possible by either spray or dip coating. Films having a thickness of less than about a micron tend to lack the strength for subsequent processing steps. 
     After coating the substrate with an adhesion agent, the substrate is subjected to a first curing or pre-curing step  15 . Pre-curing is usually carried out by heating the coated substrate to a temperature of 65° C. to 140° C. and could go to as high as 170° C., provided that the substrate can accommodate this temperature, for a period of time ranging from two to three minutes (at the upper end of the temperature range) to half of an hour (at the lower end of the temperature range) until the adhesion coating elasticity matches that of the substrate material. One useful test for determining adequacy of the pre-curing treatment is to fold the elastomer material double over itself so that there is no radius; if the adhesion coating splits or delaminates, further baking is needed. Pre-cure baking may be carried out by transporting the coated articles on a conveyor under an infrared heater. This method lends itself to higher temperatures, shorter cure times and high throughout. 
     The pre-curing stage has several purposes: (i) to reduce liquefaction and especially flowing of the epoxy film during metallization deposition and (ii) to reduce crazing of the film during post-deposition curing. 
     Metallization  17  follows the pre-curing step  15 . Metallization may be carried out via several metallization processes, i.e., electroless plating, vacuum evaporation, sputtering and the like, however, vacuum deposition is preferred. Vacuum deposition has the advantage of not requiring disposal of environmentally hazardous materials encountered with some electroless and/or electroplating processes. Metallization desirably includes a depositing a nickel flash layer  16  (i.e., a thin coating having a thickness of less than one micron and preferably less than one-tenth micron) followed by the full metallization step  17  of depositing a coating of copper up to approximately five microns in thickness, but preferably 0.5 to 1.0 micron thick. The metallization step  17  is carried out in any readily available rf sputter coating apparatus. Typically, the substrate and epoxy film will be heated to at least 70° C. and possibly higher by the metal. It is desirable to pre-cure the epoxy film at the highest temperature likely to be encountered in the deposition process. 
     Vacuum evaporation of aluminum is preferred for coating of the substrate for several reasons: (i) aluminum thin films have surprisingly high conductivity, compared to other materials; (ii) aluminum has a relatively high thermal coefficient of expansion, allowing matching of thermal properties to a wide range of substrate materials and (iii) aluminum films are surprisingly strong compared to what would be expected from the properties of bulk aluminum. The films so deposited are less than 10 microns thick and are typically in the range of 0.5 to 1.0 microns. 
     Following the metallization step  17 , a curing step  19  of the composite assembly is carried out. The curing step  19  is typically carried out at a temperature of 65° C. to 140° C. for a time of between several and twelve hours, with times of ten to eleven hours at about 75° C. being common. The curing temperature is usually close to the pre-curing temperature although this is not necessary. The objective of the curing stage is to fully react the epoxy without incurring crazing or delamination of the deposited metal film. Following the curing step  19 , the process of metallization of the substrate is complete as indicated at end step  21 . 
     Applicants have successfully deposited films of stainless steel, copper, nickel copper, aluminum and even silver on B-stage epoxy films. The substrate material is selected to withstand pre-curing of the adhesion agent  15 , metallization  17  of the coated substrate and curing  19  of the composite assembly, following which process  10  ends. Additionally, the substrate must be able to meet the requirements of the end use to which the article is to be put (i.e., temperature range of operation, elasticity etc.). 
     Substrates on which this process has been successfully employed include polycarbonates, different blends of ABS, Santoprene and urethanes. In addition, the method has been successfully applied to metallization of crystalline substrates such as ceramic substrates. Substrates on which this process has not shown success include fluorocarbons, materials having mold release agents therein and inhomogeneous quick molds, which later tend to ooze unreacted materials when heated, floating off the epoxy films. 
     Turning now to FIG. 2 a gasket  201  shown in cross-section and processed in accordance with the invention is shown. The gasket  201  includes a substrate  203  of Santoprene elastomeric material. Substrate  203  includes a peripheral portion  209  which surrounds an opening  211 . The substrate includes a layer  205  of adhesion agent in the range of five to ten microns thick. A metallization layer  207  is provided upon layer  205 . The gasket  201  may have any number of shapes and may be used for to provide a radio frequency shielding gasket. Although not shown in the drawing, a thin metallization flash coating of nickel may be deposited intermediate the adhesion layer  205  and the metallization layer  207 . The shape of gasket  201  is intended to demonstrate a general gasket or flexible seal and is not intended to in any way limit the invention. The gasket or seal may take any shape which is desirable to provide a gasket or seal for specific applications. The seal or gasket is illustratively formed of a flexible material and the adhesion agent is a B-stage epoxy film as described above. The gasket or seal  201  is processed in accordance with the process described above with reference to FIG.  1 . The gasket or seal  201  may also be metallized on additional surfaces using the process of FIG.  1 . Portions of the gasket may be masked off during the metallization process if is desired to metallize a pattern on gasket  201 . Gasket  201  may be used in various shapes to provide a seal for an electronic assembly. 
     Turning now to FIGS. 3 and 4, an assembly is shown to which the invention is advantageously applied. The assembly includes a housing portion  301  for electronic equipment which mates to a second housing portion  303 . The assembly shown in the FIGS. 3 and 4 is a cellular telephone housing shown in cross-section. The housing includes a front or top portion  300  and a bottom or back portion  303 . The housing portion  301  carries an elastomeric gasket  305  comprising an elastomeric substrate  304 , an adhesive agent  307  and a metallization layer  309 . The gasket  305  is carried on housing portion  301  in a gasket or seal receiving portion  311  which is in the form of a receiving channel into which the gasket  305  is carried prior to assembly of the housing. Housing portion  301  and gasket  305  are treated as a two part or composite substrate which is processed in accordance with the principles of the invention. Housing  301  and substrate  304  are both coated with an adhesion agent layer  307 . The adhesion layer  307  is disposed principally on the interior of housing  301  and on the substrate  304 . In practice, the substrate  304  is disposed into the seal receiving portion  311  prior to coating the housing portion  301  with the adhesion layer  307 . Advantageously, assembly is simplified since the substrate  304  may be processed concurrent with the processing of the housing  301  to provide a metallization layer. A metallization layer  309  five to ten microns thick is deposited upon the adhesion layer  307 . Similarly, an adhesion layer  307   a  is deposited on the interior surface of housing portion  303 . A metallization layer  309   a  is deposited on adhesion layer  307   a . Advantageously, by processing the composite substrate formed by the housing portion  301  and gasket  305  in accordance with the process of the invention, a structure is provided in which the metallization layer  309  bridges the gap between the solid portion  303  of the substrate and the elastomeric substrate  304  at the interior of the housing  301 . The resulting gasket  305  is flexible so that it will conform to the shape of the edge of housing portion  303 . This permits a continuous contact to be established and maintained between the metallization layer  309   a  of housing portion  303  and metallization layer  309  disposed on housing portion  301  and on gasket  305 . A continuous seal is thereby obtained between metallization layer  309  and metallization layer  309   a  This structure has the advantage that improved high frequency radio frequency shielding is provided. By providing the mating housing portion  303  with a metallization layer  309 , a substantially continuous metallization layer is provided for the two piece housing thereby providing an electronic unit having a continuous rf shield. One significant advantage to a structure formed in accordance with the invention is that galvanic action between the gasket and the housing is eliminated because both the gasket and the housing have the same metallization layer rather than dissimilar metals. As will be appreciated by those skilled in the art, the housing could also include more than two pieces and each piece could carry or mate to a gasket carried on another piece. Thus a multiple piece housing can be provided in which a continuous metallization layer is provided to thereby provide a continuous rf shield. By utilizing thin metallization layers, the quality of the unit is increased because the thin layer in combination with the adhesion layer permits the gasket to be flexible. 
     Thus, elastomeric substrates and composite substrates are provided having a continuous conductive coating. The conductive coating materials has been described which overcomes specific problems and provides certain advantages relative to prior art conductive substrates. The improvements over known technology are significant. Problems with poor particle loading and poor thermal stability noted with prior art conductive gaskets are obviated by employing a gasket in accordance with the invention. Additionally, the ability to use various top-coat metallization systems allows galvanic matching of the gasket to the housing. Reliable and robust rf gasket performance is realized by the method of the present invention. Further, a continuous and flexible metallic film is firmly bonded to the elastomeric material. Finally, a myriad of off-the-shelf molded gasket materials may be coated, rather than attempting to cut gaskets to shape from sheets of elastomeric materials loaded with conductive particles, which has been the practice prior to the present invention. 
     It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims. 
     Although the invention has been described in terms of the illustrative embodiment, it will be appreciated by those skilled in the art that various changes and modifications may be made to the illustrative embodiment without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by the claims appended hereto.