Abstract:
A shield device is provided in which a layer of deformable electrically conductive material is conformed to fit over the components [692] on the board. In one embodiment of the invention the deformnable material is conductive foam [600], such as metalized foam. One or both sides of the foam layer [6001 can be covered with dielectric material. Portions of the dielectric material and foam can be removed, such as from the bottom layer [615 ] to create insulating slants [615 a ] over the component. The board can be placed over the components, which are received in recesses in the shield which are either preformed or result from compression of the deformable material at the location of the components. In one embodiment of the invention, regions of a conductive layer are removed and the layer is placed over the components. A top layer [610] is placed thereover. The invention also relates to the method of foaming the board level shield.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates generally to electromagnetic force (EMI) shielding materials and more particularly, to a shield for various components mounted on a circuit board.  
           [0002]    Many electrical components generate EMI and other noise in operation and various electrical components are sensitive to EMI generated elsewhere. Accordingly, it is often advantageous to shield an electrical component to prevent it from emitting EMI or to shield an electrical component from EMI emitted elsewhere.  
           [0003]    Many electrical devices include electrical components arranged on a circuit board. One way that such components have been shielded is to mount five sided metal cans over each individual component. The cans can all be grounded. Often, this is done by soldering the can to a ground line. It is necessary to remove the cans in the event any of the components need to be serviced, which can be inconvenient.  
           [0004]    Conventional shielding products have proved to be not fully satisfactory, both in terms of cost and convenience of use. Accordingly, it is desirable to provide an improved board level shield which is less expensive and easier to assemble and remove than conventional shielding devices.  
         SUMMARY OF THE INVENTION  
         [0005]    Generally speaking, in accordance with the invention, a shielding device is provided in which a layer of deformable electrically conductive material is conformed to fit over the components on the board. In one embodiment of the invention the deformable material is conductive foam, such as metalized foam. One or both sides of the foam layer can be covered with dielectric material. Portions of the dielectric material and foam can be removed, such as from the bottom layer to create insulating islands over the components. Cuts in the deformable material lead to compression only over the components. The board can be placed over the components, which are received in recesses in the shield, which are either preformed or result from compression of the deformable material at the location of the components. In one embodiment of the invention, a conductive sheet is taller than the electrical components and openings are formed through the sheet with dimensions slightly greater than those of the components. This sheet is placed over the board and the components are received in the openings. A second conductive sheet is then placed thereover. The invention also relates to the method of forming the board level shield.  
           [0006]    Accordingly, an object of the invention is to provide an improved shield for electrical components.  
           [0007]    Another object of the invention is to provide an improved method of forming an EMI shield for electrical components.  
           [0008]    Still other objects and advantages of the invention will in part be obvious and more in part be apparent in the specification and drawings.  
           [0009]    The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article embodying features of construction and arrangements of parts, which are exemplified in the following detailed disclosure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    For a fuller understanding of the invention, reference is had to the following description, taken in connection with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1A is a cross sectional view of a compressible conductive sheet for a shielding member in accordance with a preferred embodiment of the invention;  
         [0012]    [0012]FIG. 1B is a cross sectional view of a the conductive sheet of FIG. 1A, with a dielectric layer on the top and bottom surfaces thereof;  
         [0013]    [0013]FIG. 1C is a cross sectional view of the composite of FIG. 1B, with partial die cuts in the lower dielectric layer and the lower portion of the conductive sheet;  
         [0014]    [0014]FIG. 1D is an enlarged partial cross sectional view of the composite of FIG. 1C, with portions of the lower dielectric layer removed to form insulating islands on the bottom of the conductive sheet;  
         [0015]    [0015]FIG. 1E is a partial cross sectional view of the shield of FIG. 1D installed over electrical components mounted on a circuit board with the islands resting on the top of electrical components and the foam over the islands in a compressed state;  
         [0016]    [0016]FIG. 2 is a perspective view of a board level shield in accordance with the invention, positioned above a circuit board having an electrical components mounted thereon;  
         [0017]    [0017]FIG. 3A is a cross sectional view of a board level shield in accordance with preferred embodiments of the invention positioned over a circuit board having electrical components mounted thereon in the unassembled condition;  
         [0018]    [0018]FIG. 3B is cross sectional view of the shield and board of FIG. 3A, in the assembled condition;  
         [0019]    [0019]FIG. 3C is an enlarged partial cross sectional view of the assembly of FIG. 3B; and  
         [0020]    [0020]FIG. 4 is a perspective view of a conductive sheet for a board level which in accordance with the invention;  
         [0021]    [0021]FIG. 5 is a cross sectional view along line  5 - 5  of the sheet of FIG. 4, with a second sheet placed thereover;  
         [0022]    [0022]FIG. 6 is a perspective view of a circuit board having electrical components mounted thereon and a shield for fitting over the board;  
         [0023]    [0023]FIG. 7A is a partial cross sectional view of the board and shield of FIG. 6, in the unassembled condition; and  
         [0024]    [0024]FIG. 7B is a partial cross sectional view of the board and shield of FIG. 6, in the assembled condition. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Preferred embodiments of the invention employ a multi-layer laminate material as a board level shield. A sheet of conductive foam, such as metalized foam or other conductive compressible material can be covered on its top and bottom, or merely its bottom or top, or not at all, with a layer of insulating (dielectric) material. In a preferred embodiment of the invention, a layer of insulating material is at least on the bottom surface of the conductive sheet.  
         [0026]    In one embodiment of the invention, the bottom of the shield is selectively die cut to remove portions of the lower dielectric layer and expose portions of the conductive foam around islands of insulating material. The shield is then pressed over components on a circuit board and portions of the metalized foam contact conductive lines on the board at the edges of the components to serve as a connection to ground. Portions of the insulating layer on the bottom surface of the foam layer can be selectively removed to facilitate such contact. Partial die cuts can also be provided in the foam to facilitate localized compression of the foam in the areas above the electrical components only.  
         [0027]    As one exemplary non-limiting embodiment of the invention, reference is had to a conductive (e.g. metalized) foam layer  100  in FIG. 1A. In FIG. 1B, an upper dielectric layer  110  and a lower dielectric layer  115  are shown disposed on the top and bottom surfaces of conductive foam layer  100 . The actual order of assembly is immaterial and the conductive foam can be disposed over a first dielectric layer and the second dielectric layer formed thereon or the dielectric layers can be disposed on opposite sides of the conductive foam layer  100 . Use of one layer is also acceptable.  
         [0028]    Referring to FIG. 1C, it can be seen that a series of die cuts  130   a - 130   d  are made through insulating layer  115  and that an additional series of die cuts  120  are made into the body of conductive foam layer  100 . The die cuts in the foam layer are advantageously made in between pairs of die cuts in the insulating layer, such as between cuts  130   a  and  130   b.    
         [0029]    Referring to FIG. 1D, it is seen that portions of the insulating layer between die cuts  130   a  and  130   b  are removed, to form an open area  131  as defined by a pair of walls  132   a  and  132   b  to form a modified dielectric layer  115 ′, which is comprised of a series of dielectric segments  115   a , bounded by open spaces. In preferred embodiments of the invention, segments  115   a  are rectangular islands bounded by a continuous rectangular open area  131 . The result is a board level shield  160  which can be disposed over electrical components  192  arranged on a circuit board  195 , as shown in FIG. 1E.  
         [0030]    Referring to FIG. 1E, it can be seen that when dielectric island portion  115   a  contacts the top of electrical component  192 , this causes a portion of conductive foam layer  100   b , bounded by die cut  120  to be compressed above electrical component  192 . Depending on the relative dimensions of dielectric portion  115   a  to electrical component  192 , shield  160  can be formed so that portion of conductive foam layer  100  contacts either a top or side of electrical component  192  or is prevented from contacting electric component  192 . Because conductive foam layer  100  includes cuts  120  a portion of form layer  100   a  will not be compressed as a result of layer  100  contacting electrical component  192  and will contact the upper surface of circuit board  195 . Because portions of insulating layer  115  were removed in void areas  131  a trace portion  125  can be urged past the height of insulating layer  115   b  and contact circuit board  195 . It should be noted that the relative heights, widths and thicknesses of portions of shield  160  and the electrical components are not drawn to scale, for purposes of illustration.  
         [0031]    In general, conductive foam layer  100  can be about 0.03-0.5 inches thick and insulating layer  115  can be about 1 to 5 mils thick. Insulating layer  110  and  115  can be formed of various materials, such as polycarbonate, polypropylene, PC/ABS, polyethylene and mylar. Cuts  120   a  and  120   b  should be about 0.005 to 0.125 inches from the edge of electrical component  192  and traces  125   a  and  125   b  should be about 0.01 to 0.125 inches wide.  
         [0032]    Referring to FIG. 2, a shield  260  is shown in accordance with an embodiment of the invention. Shield  260  includes a lower insulated layer  215 , a middle conductive metalized foam sheet  200  thereover and a top insulated layer  210  on foam sheet  200 . Shield  260  has portions of bottom insulated layer  215  removed at cuts  230   a  and  230   b , for the removal of a rectangular void  231 , where there is no lower dielectric layer  215 .  
         [0033]    Referring to FIG. 3A, it can be seen that shield  260  is positioned over circuit board  295  and then assembled. As shown in FIGS. 3A and 3B, insulating portion  215   a  will contact the top of component  292 . This will cause a portion  200   b  of foam layer  200  to be compressed over component  292 . In addition, because foam layer  200  includes die cuts  220  that are spaced apart from island  215   a , trace portions  225  of foam layer  200  will come in contact with circuit board  295 .  
         [0034]    In other embodiments of the invention, die cuts  120  can be formed in the same operation and line up with die cuts  130   b . Likewise, die cuts  220  can be formed at the same time as and line up with die cuts  230   b.    
         [0035]    In still another embodiment of the invention, as shown in FIGS. 4 and 5, a shield  500  can be formed with two insulating foam layers, a first lower foam layer  400  having openings  470  cut therein to fit over electrical components on a circuit board and a second foam layer  401  which is disposed over foam layer  400 . The bottom of lower foam layer  400  can include an insulating surface over its entirety or regions of the dielectric surface can exposed portions of foam layer  400  for contacting the circuit board. The top surface of upper foam layer  401  can include an insulating surface. In certain embodiments of the invention, upper layer  401  can be bonded and selected portions to foam layer  400  or otherwise fastened thereto with electrically conductive or non-conductive joining mechanisms.  
         [0036]    In each of these embodiments of the invention, it can be advantageous to mount a structure, such as a plastic cover, metal cover or circuit board over the shield to hold and compress it in place.  
         [0037]    [0037]FIG. 6 shows a perspective view of a circuit board  601  having a plurality of electrical component  692  mounted thereon. A ground trace  693  is disposed around each component  692 . FIG. 6 also shows a board shield  660  having a layer of metalized foam  600  with a lower dielectric layer  615  and an upper dielectric layer  610  on its respective lower and upper sides.  
         [0038]    Referring to FIGS. 7A and 7B, it can be seen that open areas  631  were formed where portions of dielectric layer  615  surrounding a plurality of insulating layer islands  615   a  were removed from lower dielectric layer  615 . Islands  615   a  align with the tops of components  692 . Referring to FIG. 7B, which shows board  601  and shield  660  in the assembled condition, regions of compressed foam  600 b exist above each component  692 . Such localized compression is facilitated as a result of partial die cuts  620  formed around each of the areas where the electrical components  692  are positioned. It can also be seen that portions of exposed conductive foam  600  will contact ground traces  693  when shield  660  is compressed towards board  601 .  
         [0039]    As is evident from the exemplary embodiments discussed and shown in the Figures, one aspect of the invention is a board level EMI shield, in which a conductive sheet includes die kiss cuts in the general shape of the electrical components arranged in a circuit board, so that when the sheet is placed over the components, the components will compress the foam bounded by the die cuts and the remainder of the foam will extend to the surface of the board, to effectively shield each of the components on the board. In preferred embodiments of the invention, shield includes insulating material, such as various dielectric materials, within the boundaries defined by the die cuts, to rest on the top surface of the electrical component and space the conductive foam from the component. In other embodiments of the invention, the shield comprises two layers of conductive material, such as conductive foam material. Voids are formed in the lower of the two layers where the electrical components can be housed, either in contact with or spaced apart the conductive material. A second layer is then disposed over the first to provide an effective shield.  
         [0040]    A preferred method of making a shield in accordance with the preferred embodiments of the invention is to provide a conductive foam sheet and coat, such as by spraying or dipping insulating material on the bottom, top or both the bottom and top thereof or by laminating a dielectric layer to the foam with flame laminating techniques or adhesive bonding. The insulating material is removed to form islands of insulating material bordered by voids, with the reconductive material exposed. Cuts are made in the conductive sheet, in the void region and then when the board is placed over the components on the circuit board, the islands separate the foam from the components and the die cuts permit the foam to be compressed over the components and uncompressed in areas between the components, so that the foam can reach the surface of the circuit board. Alternatively, one or both of the cuts into the insulating layer can extend into the conductive sheet to promote selective compression of the conductive material over the electrical components.  
         [0041]    Thus the several aforementioned objects and advantages are most effectively attained. Although preferred embodiments of the invention have been disclosed and described in detail herein, it should be understood that this invention is in no sense limited thereby and its scope is to be determined by that of the appended claims.