Abstract:
In a shielding scheme for circuit boards, conductive shields have contoured mating surfaces that become flat, or co-planar, at the circuit board-shield interface when the shields are fastened to opposite sides of the circuit board. The contoured mating surfaces compensate for deformation of the shields resulting from the fastening so that uniform mechanical pressure is applied at all designated points along the circuit board-shield interface even though fastening points are intermittently spaced throughout the shields. When compressible conductive gaskets are optionally interposed between the shields and the circuit board, stops are included to accommodate for the thickness of the gasket. High signal isolation is achieved without correspondingly high contact area on the circuit board and without closely-spaced fastening points. Low assembly time and manufacturing cost results for shielded circuit board assemblies incorporating the shielding scheme.

Description:
BACKGROUND OF THE INVENTION 
     Conductive shields provide signal isolation in electrical circuits and systems. Typically, conductive shields are positioned on opposite sides of a printed circuit (PC) board and screwed together, sandwiching the PC board between the shields. Compressible conductive gaskets placed between the PC board and each of the shields lowers electrical resistance at each of the board-shield interfaces. Lower electrical resistance generally produces higher signal isolation. Since performance specifications in many circuits and systems are only attainable if signal isolation is sufficiently high, providing a low electrical resistance at the board-shield interfaces is critical. 
     The electrical resistance at each board-shield interface decreases as pressure on the gasket increases, causing signal isolation to be limited by the point along the interface at which the mechanical pressure on the gasket is lowest. Therefore, sufficiently high mechanical pressure must be applied to the compressible conductive gasket uniformly, at all points along the board-shield interface, to achieve sufficiently high signal isolation. Providing uniform mechanical pressure to the compressible conductive gaskets is a difficult task. Typically, attachment screws are used to fasten the conductive shields to the PC board, and when the screws are tightened, the shields deform. This deformation results in uneven pressure on the gasket. The pressure on the gasket in the vicinity of the attachment screw is higher than the pressure on the gasket at other points along the interface. 
     In presently used shielding schemes, uniform mechanical pressure is provided to the gasket by minimizing deformation of the conductive shields. Deformation is minimized by making the shields rigid and by positioning the attachment screws at closely-spaced intervals. However, making the shields rigid results in the walls of the shields being thick which causes a large contact area between the compressible gasket and the PC board. This large contact area occupies PC board area that would otherwise be useable for mounting electrical components. In addition, closely spacing the attachment screws increases the total number of attachment screws in the shielded PC board assembly, causing the assembly time and manufacturing costs of shielded PC board assemblies to correspondingly increase. Accordingly, there is a need for a PC board shielding scheme that achieves sufficiently high signal isolation while minimizing contact area and minimizing the number of attachment screws. 
     SUMMARY OF THE INVENTION 
     In a shielding scheme for circuit boards constructed according to the preferred embodiment of the present invention, conductive shields have contoured mating surfaces that become flat, or co-planar, at the circuit board-shield interface when the shields are fastened to opposite sides of the circuit board. The contoured mating surfaces compensate for deformation of the shields resulting from the fastening so that uniform mechanical pressure is applied at all designated points along the circuit board-shield interface even though fastening points are intermittently spaced throughout the shields. When compressible conductive gaskets are optionally interposed between the shields and the circuit board, stops are included to accommodate for the thickness of the gasket. High signal isolation is achieved without correspondingly high contact area on the circuit board and without closely-spaced fastening points. Low assembly time and manufacturing cost results for shielded circuit board assemblies incorporating the shielding scheme. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B show a prior art shielding scheme. 
     FIGS. 2A and 2B show a shielding scheme constructed according to the preferred embodiment of the present invention. 
     FIG. 3 shows a detailed view of a predistorted conductive shield for use in the shielding scheme shown in FIGS.  2 A and  2 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1A and 1B show a prior art shielding scheme. A conductive shield  1   a ,  1   b  is fastened to each side of a circuit board  2 , such as an epoxy-glass printed circuit (PC) board. Typically, the surface of at least one side of the PC board  2  has exposed electrical components and conductive traces (not shown). The conductive shields  1   a ,  1   b  provide signal isolation between electrical components and traces within different sections of the board, and signal isolation between circuits or systems external to the PC board  2 . 
     In the side view shown in FIG. 1A the conductive shields  1   a ,  1   b  are fastened together with the circuit board  2  sandwiched between the shields to form a shield circuit board assembly  6 . Typically, one of the conductive shields  1   a ,  1   b  has threaded holes for receiving screws  4  that fasten the shields  1   a ,  1   b  on either side the board  2 . Screws  4  are closely-spaced to ensure that intimate electrical contact is made at all points along the board-shield interfaces  5   a ,  5   b . To lower electrical resistance at the board-shield interfaces  5   a ,  5   b , compressible conductive gaskets  7   a ,  7   b  are positioned between each of the shields  1   a ,  1   b  and the corresponding surfaces of the PC board  2 . Were the screws  4  not spaced sufficiently close to each other, deformation of the shields (as indicated by the dashed deformation profiles in FIG. 1A) would result, thereby producing nonuniform mechanical pressure on the compressible conductive gaskets  7   a ,  7   b . The mechanical pressure on the gaskets  7   a ,  7   b  would be higher in the vicinity of the screws  4  than at other points along the interfaces  5   a ,  5   b . While the closely spaced screws  4  provide some degree of uniformity in the mechanical pressure imposed on the gaskets  7   a ,  7   b  by limiting deformation or bowing of the shields  1   a ,  1   b , to further reduce deformation and further increase the uniformity of the mechanical pressure, the shields  1   a ,  1   b  are constructed to be rigid. 
     To achieve rigidity, the walls  8  of the shield as shown in FIG. 1B are made thick. However, thicker walls  8  result in correspondingly larger contact area on the circuit board  2 . The contact area consumes valuable circuit board area, that would otherwise be useable for mounting electrical components. When interior walls  9  are included in the shield  1  a for isolating electrical components and traces within different sections of the circuit board  2 , the contact area could consume a significant portion of the over-all area of the circuit board  2 . 
     The prior art shielding scheme provides sufficient signal isolation, but the required wall thickness consumes valuable circuit board area and requires a large number of attachment screws  4  to fasten the shields to the circuit board  2 . 
     FIGS. 2A and 2B show predistorted conductive shields  11   a ,  11   b  for use in a shielding scheme constructed according to the preferred embodiment of the present invention. Predistortion, or contouring, of the mating surfaces  12   a ,  12   b  of the conductive shields  11   a ,  11   b  is exaggerated in the side view shown in FIG.  2 A. In a neutral position shown in FIG. 2A, the conductive shields  11   a ,  11   b  are not fastened to a circuit board  16  and the mating surfaces  12   a ,  12   b  are predistorted to have a convex profile  13 . This profile  13  is designated so that the mating surfaces  12   a ,  12   b  become flat, or coplanar, in a loaded position that results when the conductive shields  11   a ,  11   b  are fastened to the circuit board  16  at fastening points  14  to form a shielded circuit board assembly  20  as shown in FIG.  2 B. The profile  13 , in this example, accommodates compressible conductive gaskets  15   a ,  15   b  positioned between each of the shields  11   a ,  11   b  and the corresponding surfaces of the circuit board  16  to lower electrical resistance at the circuit board-shield interfaces  17   a ,  17   b.    
     A threaded hole is provided at each fastening point  14  to receive an attachment screw  18 . A series of attachment screws  18  are used to fasten the shields  11   a ,  11   b  together while sandwiching the circuit board  16  between the shields  11   a ,  11   b . Alternatively, a series of clamps, rivets or other fasteners are used at the fastening points  14  to apply pressure on the shields  11   a ,  11   b , so that the circuit board  16  is sandwiched between conductive shields  11   a ,  11   b . When the shielding scheme includes compressible conductive gaskets  15   a ,  15   b  interposed between the predistorted conductive shields  11   a ,  11   b  and each of the contact surfaces of the circuit board  16 , the fastening points  14  include stops  19 , or stand-offs that contact the surface of the circuit board  16  and accommodate for the thickness of the gaskets  15   a ,  15   b , preventing the gasket from being overly compressed when the shields  11   a ,  11   b  are fastened to the circuit board  16 . The compressible conductive gaskets  15   a ,  15   b  in this example is a Cho-Form material, available from Chomerics, Division of Parker Hannifin Corp. that is dispensed onto the contoured mating surfaces  12   a ,  12   b  of the shield. Alternatively, the gaskets are tubular conductive elastomers or conductive paste that is screened or otherwise deposited onto the contoured mating surfaces. When Cho-Form material is used for the compressible conductive gaskets  15   a ,  15   b , the stops  19  are formed by pedestals that are raised from the mating surfaces by a height of approximately one-half of the uncompressed thickness of the gaskets. 
     FIG. 3 shows a detailed view of a predistorted conductive shield  11  for use in the shielding scheme of FIGS. 2A and 2B. The predistorted conductive shield  11  is formed using milling, casting or other fabrication processes to produce contoured mating surfaces  12 . The profile  13  of each contoured mating surface  12  is determined empirically, by computer modeling or by other techniques. Empirical determination involves constructing pilot shields (not shown) that are similar to the predistorted conductive shields  11  in all respects, with the exception that the pilot shields have mating surfaces  12  that are flat, as opposed to contoured, when in the unattached, neutral position. The pilot shields are then put in the loaded position by fastening them to the circuit board  16  using the same type of fasteners as chosen for the shielding scheme in the shielded circuit board assembly  20 . The fastening deforms the pilot shields, bowing each pilot shield away from the contact surface of the circuit board, similar to the dashed deformation profiles shown in FIG.  1 A. The pilot shields contact the circuit board at the fastening points, but the deformation of the pilot shields produces a gap of variable thickness between each pilot shield and the corresponding surface of the circuit board at points along the interface that are between the fastening points. These gaps are equivalently filled by the contoured mating surfaces  12  of predistorted conductive shield  11  shown in FIG.  3 . The thickness of the each gap at various points along the pilot shield-board interfaces is measured optically, or mechanically. These measured gap thicknesses determine how much higher the walls of the pilot shield must be at each corresponding location on the mating surfaces, relative to the flat mating surfaces of the neutrally positioned pilot shields, to displace or fill each of the gaps. The added heights define the contour of the profile  13  of the predistorted conductive shield  11 , completing the empirical determination. 
     The contours of the mating surfaces  12  of the predistorted conductive shields  11  are alternatively determined using computer modeling. In this determination, finite element analysis (FEA) software computes the profile  13  of the contoured mating surfaces  12 . Physical dimensions of a pilot shield is entered into a three-dimensional (3D) modeling program, such as the HP Solid Designer, available from Hewlett-Packard Company. The pilot shields entered into the 3D modeling program are similarly dimensioned to the predistorted conductive shields  11  in all respects, with the exception that the pilot shields have mating surfaces that are flat, as opposed to contoured, when in the unattached, neutral position. Mechanical forces on the pilot shield are then simulated by the FEA software. The simulated forces are applied along the mating surfaces and are equal in magnitude, but opposite in direction, to the forces on the mating surfaces of pilot shields were the pilot shields in the loaded position, fastened to a circuit board at the fastening points. The magnitude of the forces is determined by the force on the mating surfaces exerted by the compressible conductive gaskets. For example, Cho-Form material, exerts forces of approximately 6 pounds per square inch when the gasket material is compressed by approximately 50%. While this force is in the direction that points away from the circuit board when the shields are in the loaded position, that is fastened to the circuit board, the force entered into the FEA software is in a direction that points toward the circuit board. Based on the entered forces, the FEA software indicates the profile  13  of the contoured mating surfaces  12  for the predistorted conductive shields  11 . 
     Once the contours of the profiles  13  are defined using computer modeling, empirical determination or other techniques, the profiles  13  are established in the wall heights to form the contoured mating surfaces  12  of the predistorted conductive shields  11 . The profiles  13  are implemented in a manner consistent with the manufacturing technique. For example, when the predistorted conductive shields are milled, the contours of the profiles  13  are approximated in a segmented fashion (as shown in FIG. 3) that includes discrete steps in the height of the walls. The length L and height H of each step is chosen to best fit to the profile  13 . When the predistorted conductive shields  11  are cast, the profiles  13  are more readily formed by continuously varying the wall heights of the predistorted conductive shields  11  (as shown in FIG.  2 A). 
     While the preferred embodiment of the present invention has been illustrated in detail, it should be apparent that modifications and adaptations to this embodiment may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.