Abstract:
The invention relates to circuit boards and to screening circuits and components on such boards from stray rf interference when they are mounted as arrays or stacks of such circuit boards. The circuit boards ( 12, 14 ) are individually screened by conductive screening layers ( 16, 18 ) as known in the art and the individual screening layers are coupled together by layered interconnects ( 34 ) which connect corresponding screening layers ( 16, 18 ) of the individual circuit boards ( 12, 14 ) together, instead of by vias.

Description:
RELATED APPLICATION INFORMATION 
     This application is a United States National Phase Patent Application of, and claims the benefit of, International Patent Application No. PCT/GB2005/050260 which was filed on Dec. 22, 2005, and which claims priority to British Patent Application No. 0 428 591.2, which was filed in the British Patent Office on Dec. 31, 2004, the disclosures of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention is concerned with circuit boards, in particular with printed circuit boards (PCB) and with circuit boards comprising other types of conductive layer, and with units comprising same. While circuit boards according to the present invention may be used in particular with signal frequencies in the range 500 MHz to 80 GHz, the exemplary embodiments and/or exemplary methods of the present invention may be applied for use in high speed digital circuits and with millimetric wavelength signals of up to 300 GHz, with appropriate choice of materials. 
     BACKGROUND INFORMATION 
     Printed circuits, in particular, are susceptible to interference by radio frequency (rf) emissions and particularly so when they are coupled together either in side-by-side or stacked relationship. In order to screen the components of PCBs from for example radio interference, PCBs are typically mounted, or ‘sandwiched’ between screening layers formed of conductive material which is maintained at ground potential, thereby to form a multilayer or composite PCB unit. 
     Typical arrangements of two PCB structures, each comprising a plurality of such multilayer PCB units in a stacked arrangement, are shown in  FIGS. 1 and 2  of the accompanying drawings which illustrate isometric views of the arrangements. 
     In  FIGS. 1 and 2 , the arrangement in each figure comprises four PCB units  1 ,  2 ,  3 ,  4  mounted in a stack and coupled physically and electrically by vias  5  while maintained in spaced relationship by intervening layers of insulating material  6 . The vias  5  provide electrical connection between corresponding layers of the PCB units to ensure that electrical communication can be maintained between them and the components thereof, according to the circuit design, thus to ensure that a uniform potential can be maintained where necessary, i.e. ground potential. 
     However, it has been observed that the use of vias does permit noise and crosstalk between the interconnected layers due to the vias acting as antennae at radio frequencies. 
     In our co-pending UK patent application no. GB 0326229.2, there is disclosed an interconnect for electrically connecting striplines or multilayer PCBs, which interconnect is formed as a sandwich of dielectric material held between two ground planes with a strip conductor extending through the dielectric and having an input portion and an output portion. The sandwich is formed by two pieces of dielectric material, each piece being backed by a ground plane and the two pieces of dielectric material being brought together in overlapping relationship. 
     The arrangement disclosed in the aforesaid application addresses to some extent the band-limiting characteristics of plated-through apertures and over-the-edge connections of the then prior art, which affected the performance of the PCB circuits and gave rise to some signal radiation. 
     SUMMARY OF THE INVENTION 
     The present invention provides, in one aspect, a circuit board structure comprising: 
     a first multilayered circuit board unit; 
     a second multilayered circuit board unit, 
     each multilayered circuit board unit comprising a first outer screening layer and a layer comprising a circuit board; and 
     an interconnect providing an electrical connection between the first and second circuit board units, the interconnect comprising a laminate having a first electrically conductive layer and a further layer, the first electrically conductive layer electrically connecting the first outer screening layer of each of the first and second multilayered circuit board units. 
     From this first aspect the circuit board units within the structure may each be provided with only one outer screening layer. Preferably the outer screening layers of the first and second circuit board units are mounted on opposite sites of the respective circuit board layers so that when assembled in the circuit board structure the first and second layers of circuit board may be disposed between two outer screening layers. 
     In a circuit board structure according to the present invention, the circuit board units may themselves be spaced apart. 
     In a circuit board structure according to the present invention, the interconnect may comprise an electrical connection for electrically connecting components of the circuit boards of the first and second circuit board units, the electrical connection being provided either within the further layer or mounted thereon. The interconnect in this case is essentially a section of microstrip or “buried” microstrip. Alternatively the further layer may simply be an insulating layer serving primarily to maintain a spaced relationship between electrically conductive layers. 
     In a preferred embodiment, in the circuit board structure according to the first aspect of the present invention: each of said first and second circuit board units further comprise a second outer screening layer such that the layer comprising a circuit board is disposed between the first and second outer screening layers; the interconnect is a laminate further comprising a second electrically conductive layer such that the further layer forms an intermediate layer disposed between the first and second electrically conductive layers; and the second electrically conductive layer electrically connects the second outer screening layer of each of the first and second multilayered circuit board units. 
     Where there is a requirement for electrically coupling a circuit or circuits of one circuit board unit to circuits of the adjacent circuit board unit, or simply to maintain parts of the circuits of the adjacent circuit board units at the same potential, this can be achieved by providing a conductive layer or strip in the intermediate layer of the interconnect. To this end, the intermediate layer is then preferably formed as two strata of insulating material that either enclose a conductive strip or sandwich a conductive layer therebetween, while permitting electrical contact to be made therewith. 
     The screening layers of each circuit board unit and the first and second layers of the interconnect are provided primarily to screen circuits of the circuit boards from radio interference though may also serve to screen those same circuits from other external interference or influences, such as magnetic fields. 
     The first and, preferably, the second layers of the interconnect are of planar form such that, when connected to the first and, preferably, second rf screening layers of the circuit board units, they can form a continuous outer surface with those first and second outer rf screening layers. 
     The circuit board units themselves can be mounted in various positions relative to each other. In a first arrangement of a structure according to the invention, the first circuit board unit can be superimposed partially or wholly over the second circuit board unit with the interconnect bonded to both units. A preferred arrangement is one in which one unit is positioned vertically over the other with the two units in parallel relationship, which may be a spaced relationship. 
     The two units may be of the same or similar dimensions as to length and width so that when positioned one above the other, their respective edges are vertically aligned. For ease of construction, with the two units arranged one above the other, it is advantageous that respective portions of two edges are vertically aligned. With such an arrangement, the interconnect is then mountable alongside the vertically aligned edge portions and spaced therefrom. The circuit board units may each be provided with a tab portion so positioned that the interconnect can extend between them. In one preferred example, the tab portions are formed at opposite end regions of respective aligned edge portions with the interconnect extending between them. 
     The interconnect itself may be in the form of an inclined ramp extending between the tab portions. Alternatively it may be formed as a series of ramps with a section between successive ramps that is parallel to the planes of the circuit board units. It will be appreciated that a number of variants may be considered. It has been observed however, that, no matter the manner in which the interconnect is formed, optimum results are obtained where the interconnect has continuous outer surfaces, uninterrupted by fold lines, creasing and the like. This continuity applies also, as described below, to the connection of the electrically conductive layers of the interconnect with the respective screening layers of the circuit board units. To avoid fold lines and creases, the interconnect may be formed to provide curved or bent layers that can be aligned with the outer screening layers of the circuit board units so that the outer surfaces thereof can be continuous with the outer surfaces of the respective outer screening layers of the circuit board units. To permit provision of curved surfaces by the interconnect, the conductive layers of the interconnect and the strata providing the intermediate layer, and also the conductive strip or layer embedded therein may be provided by flexible materials, or by materials that can be rendered flexible by the application of heat. 
     For ease of convenience, an interconnect is advantageously mounted externally of the circuit board units that it electrically connects. However, space is frequently a consideration in locating and mounting circuit board units and it is therefore considered to be useful to provide the interconnect within the confines determined by the dimensions of the circuit board units themselves. Accordingly each circuit board unit of a structure according to the present invention may be formed with an aperture or recess therein, with the apertures or recesses aligned when circuit board units are stacked one upon the other or in side-by-side relationship; each aperture or recess may then have an edge portion opposite to that of an adjacent circuit board unit so that the interconnect can be electrically connected to the units and extend from one edge portion to another, and within the overall confines of the stack of circuit board units. 
     It will be readily appreciated that provision for attaching an interconnect between circuit board units can be made when forming the boards and circuit board units comprising those boards, where these are intended to be mounted in side-by-side or stacked relationship. 
     The interconnect itself can be formed as a single part or as a plurality of interconnect segments. In either form, end portions of the single part or of each segment are preferably stepped so that the layers of the interconnect can be abutted with complimentarily formed edge portions of the circuit board units with which they engage, prior to bonding, and/or with each other where the interconnect is itself formed of a plurality of segments. 
     The interconnect is ideally bonded to the first and second circuit board units by a suitable bonding agent so that the first and second layers of the interconnect are electrically connected to the respective first and second screening layers of the first and second circuit board units, the bonding agent being electrically conductive and sufficiently robust, when necessary, to withstand for example, impact or severe vibration. Suitable bonding agents include solders, solder-based adhesives and other electrically-conductive materials such as epoxy resins containing for example tin, silver, gold or carbon as filler material. In environments where robust treatment is not anticipated, other materials for example electrically-conductive adhesive tape may be used. 
     Of course, the same bonding techniques may be applied whether the circuit board units included within the circuit board structure have one or two outer screening layers and hence whether the interconnect provides electrically conductive layers for linking one or two outer screening layers of such circuit board units. 
     Where the layers comprising a circuit board of adjacent circuit board units are desired to be electrically connected, and the intermediate layer of the interconnect includes a conductive strip or layer, a similar adhesive material to that described in the immediately preceding paragraph may be used for connecting the conductive strip or layer to the circuit boards. 
     In any structure provided by the present invention, it is contemplated that the first and second screening layers of each circuit board unit are spaced from and electrically insulated from the circuit boards themselves and from electrical and electronic components thereon. This insulation may be achieved simply by the spatial separation itself but it is preferred that such insulating material comprises for example epoxy resin material between the circuit board and the screening layers. 
     The present invention provides in another aspect a circuit board structure comprising three or more multilayered circuit board units, each of which comprises first and second outer screening layers and a circuit board located between the first and second outer screening layers and insulated therefrom, and an interconnect connected between each two of adjacent ones of the multilayered circuit board units, each interconnect comprising a laminate having a first electrically conductive layer, a second electrically conductive layer and an intermediate layer between the first electrically conductive layer and the second electrically conductive layer, the first electrically conductive layer electrically connecting the first screening layers of said any two adjacent multilayered circuit board units, and the second electrically conductive layer linking the second rf screening layer of said any two adjacent ones of the multilayered circuit board units. 
     The present invention provides in yet another aspect a circuit board structure comprising three or more multilayered circuit board units, each of which comprises first and second outer screening layers and a circuit board located between the first and second outer screening layers and insulated therefrom, and 
     an interconnect comprising a laminate having a first electrically conductive layer, a second electrically conductive layer and an intermediate layer between the first electrically conductive layer and the second electrically conductive layer, the first electrically conductive layer electrically connecting the first screening layers of at least three of said adjacent multilayered circuit board units and the second electrically conductive layer linking the second screening layer of at least three of said multilayered circuit board units. 
     Where three or more circuit board units are provided, a single interconnect can electrically connect the screening layers of all of the circuit board units, or selected ones thereof. 
     The present invention further provides in another aspect a circuit board structure comprising a plurality of multilayer circuit board units arranged in a stack, each circuit board unit comprising one or more circuit boards and upper and lower outer screening layers above and below and insulated from said one or more circuit boards such that a lower screening layer of one circuit board unit is in electrical contact with an upper screening layer of an adjacent circuit board unit, with the circuit boards being spaced from one another, and an interconnect external to said stack electrically connecting at least one set of adjacent ones of said upper and lower screening layers to other sets of adjacent screening layers. 
     The present invention further provides a circuit board structure comprising a plurality of multilayer circuit board units arranged in a stack, each circuit board unit comprising one or more circuit boards and upper and lower outer screening layers above and below and insulated from each of said one or more circuit boards, with a lower screening layer of one circuit board being in electrical contact with an upper screening layer of a circuit board immediately beneath said one circuit board, the circuit boards being arranged such that they form a stepped and/or overhanging arrangement at one end or side of the stack, and an electrically coupling interconnect that extends between an uppermost screening layer of the stack and a lowermost screening layer of the stack in electrical contact with exposed regions of the intervening screening layers of adjacent circuit board units. 
     The present invention further provides an interconnect for providing screening continuity between screening of adjacent circuit board units, each of which comprises first and second outer screening layers and a circuit board located therebetween and spaced apart and insulated therefrom, the interconnect comprising first and second conductive layers spaced apart by insulating material, the first conductive layer being arranged to provide an electrical connection between the first screens of the adjacent circuit boards and the second conductive layer being arranged to provide an electrical connection between the second screens of the adjacent circuit boards. 
     The insulating material which maintains the first and second conductive layers of the interconnect in spaced apart relationship may be formed as two or more separate strata of insulating material and, when so formed, can accommodate an electrically conductive strip or layer therebetween which can connect circuits or components of the circuit boards of adjacent units. 
     The present invention also provides a method of screening adjacent circuit boards from rf interference, the method comprising enclosing the circuit boards in electrically insulating material between upper and lower rf screens in a laminated arrangement, and electrically connecting a first planar interconnect between the upper screens and a second planar interconnect between the lower screens, each of the interconnects forming a continuous external surface with its respective screens. 
     The present invention also provides a method of screening adjacent circuit board units from rf interference, wherein each circuit board unit comprises a circuit board encapsulated by electrical insulation material and first and second electrically conductive screening layers with the encapsulated circuit board positioned therebetween, the method comprising the steps of: 
     providing an interconnect between the two circuit board units, the interconnect comprising a laminate itself comprising a first electrically conductive layer, a second electrically conductive layer and an intermediate electrically insulating layer between the first and second electrically conductive layers, 
     electrically connecting the first electrically conductive layer of the interconnect to the first electrically conductive screening layer of each of the circuit board units, and 
     electrically connecting the second electrically conductive layer of the interconnect to the second electrically conductive screening layer of each of the circuit board units, 
     with no discontinuity between outer surfaces of the connected layers. 
     In each of the preferred arrangements and methods defined above in which circuit board units have both first and second outer screening layers, it is intended that such preferred embodiments of the present invention may be implemented using circuit board units at least one of which has only one outer screening layer, with corresponding adjustments being made to the interconnects linking the screening layers of such circuit board units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of an arrangement of two PCB structures, each comprising a plurality of multilayer PCB units in a stacked arrangement; 
         FIG. 2  is an isometric view of an arrangement of two PCB structures, each comprising a plurality of multilayer PCB units in a stacked arrangement; 
         FIG. 3  is an isometric view of a first embodiment of the present invention in which two multilayer PCB units are electrically connected together; 
         FIG. 3A  is a side view illustrating connection of a conductive layer of an interconnect with that of a PCB; 
         FIG. 4  is an isometric view of an interconnect for use in the first embodiment of the present invention shown in  FIG. 1 ; 
         FIG. 5  is an isometric view of a second embodiment of the present invention, illustrating the manner in which four multilayer PCB units are electrically connected together; 
         FIG. 6  is a side elevation of a third embodiment of the present invention, illustrating a further arrangement for electrically connecting four multilayer PCB units together; 
         FIG. 7  is a plan schematic view of a variation of the embodiments of the present invention that are illustrated in  FIGS. 3 and 5 ; 
         FIG. 8  is an isometric view of a fourth embodiment of the present invention, showing three stacked PCB units of different dimensions electrically connected; 
         FIG. 9  is an isometric view illustrating a variation of the embodiment shown in  FIG. 8 ; 
         FIG. 10  is an isometric view of a fifth embodiment of the present invention; 
         FIG. 11  is an isometric view of a sixth embodiment of the present invention; 
         FIG. 12  is an isometric view of a seventh embodiment of the present invention; and 
         FIG. 13  is an isometric view of an eighth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of preferred embodiments of the present invention makes reference to circuit boards made using printed circuit techniques, hence printed circuit boards (PCBs). However, all references to printed circuit boards (PCBs) are intended to include reference to circuits formed in conductive layers other than those manufactured specifically using printed circuit techniques. For example, a reference to a PCB in the following description and drawings is intended to include a reference to circuits fabricated using semiconductor materials and the like. 
     There now follows a detailed description which is to be read with reference to  FIGS. 3 to 9  of the accompanying drawings of a number of embodiments of the present invention which have been selected for description to illustrate the invention by way of example. 
     Referring to  FIG. 3 , there is illustrated a printed circuit board (PCB) structure, generally indicated at  10 , of two connected multilayered PCB units provided by a first such PCB unit  12  and a second such PCB unit  14 . The two units  12  and  14  are held in spaced parallel relationship as shown and any suitable spacing means for so doing may be used. (It will of course be understood that the two PCB units may alternatively simply be stacked one directly on top of the other.) 
     Each PCB unit  12  and  14  preferably comprises a first outer screening layer  16  and a second outer screening layer  18 , and, between the two screening layers, a printed circuit board  20  carrying electrical circuits (not shown). 
     The two screening layers are provided to screen the electrical circuits from stray radio frequency signals. Between the PCB  20  and each of the screening layers  16  and  18 , a layer of insulation  22  is provided to insulate the PCB  20  from the screening layers  16 ,  18  and to protect the circuit components and conductive connections on the PCB. The screening layers  16 ,  18  may take the form of continuous sheets of conductor or they may be formed as a periodic pattern of conducting elements, in particular in the vicinity of interconnections between PCBs, to help to suppress parallel plate modes that may otherwise exist between layers during operation. The insulating layers  22  may be formed using a solid dielectric material or they may be formed using a dielectric material having a foam or a honeycomb structure to help to reduce losses at higher frequencies that may otherwise arise with solid dielectrics. 
     As shown in  FIG. 3 , the two PCB units  12 ,  14  are formed with tab portions  26 ,  28  respectively at one side and adjacent one corner of the unit. The two tab portions  26 ,  28  are provided at opposite corners of their respective PCB units and on the same side so that they face one another. 
     Between facing end surfaces  30 ,  32  of the two tab portions  26 ,  28  extends an interconnect  34  which provides a bridge between the tab portions. This interconnect  34  is formed as a laminate and has end faces shaped so that, when mounted in an inclined position such as shown in  FIG. 3 , they can mate with the end faces  30 ,  32  of the tab portions  26 ,  28  of the two PCB units  12 ,  14 , as hereinafter described. 
     The interconnect  34  comprises first and second electrically-conductive layers  36 ,  38 , formed of conductive material such as sheet metal, with an intermediate layer  40  between the two layers  36 ,  38 . The intermediate layer  40  is provided by two layers of insulating spacing material  42 ,  43  and a further conductive layer  45  sandwiched therebetween, the purpose of which is described below. The spatial separation between the three layers  36  and  45 , and  45  and  38  is such that, allowing for the angle of inclination of the interconnect  34  relative to the two PCB units  12  and  14 , the end faces of the five layers  36 ,  42 ,  45 ,  43  and  38  are aligned with end faces of the five layers  16 ,  22 ,  20 ,  24  and  18  respectively of the tab portions. The alignment of the end faces  30 ,  32  of the tab portions with the mating end faces of the interconnect is such that the end faces of the pairs of facing layers  16 ,  36 ;  20 ,  45 ;  18 ,  38  are in abutting or overlapping contact to provide an electrical connection therebetween. The end faces of the three conductive layers can then be bonded in position by means of solder, a solder-based adhesive composition or other electrically-conductive adhesive composition. Suitable adhesive compositions include epoxy resins containing an electrically conductive filler such as tin, silver or carbon, and silicone-based conductive adhesive compositions. Other polymers such as acrylate polymers and copolymers may also be used in conjunction with Ag, Au or similar conductive filler material; these latter adhesives may be curable very quickly by use of UV curing techniques for example. Alternatively, anisotropically conductive adhesives may also be used. 
     When the interconnect is bonded in position, it is highly desirable that a smooth surface of each of the layers  36 ,  38  and the corresponding adjacent outer surfaces of the respective layers  16 ,  18  of the two units  12 ,  14  is developed to avoid discontinuities that may give rise to noise or crosstalk. 
     It is to be understood that whereas PCB units may have both first and second outer screening layers  16 ,  18 , preferred embodiments of the present invention may be implemented using PCB units having only one outer screening layer with corresponding adjustments being made to provide interconnects  34  having only one electrically conductive layer for interconnecting the outer screening layer of two or more such PCB units. It will also be understood that a mixture of one and two screen PCB units may be provided in a PCB structure and interconnected with appropriate interconnects defined according to preferred embodiments of the present invention described herein. 
     In  FIG. 4  is shown one example of the manner in which one end of an interconnect  34  according to the present invention can be connected electrically either to a tab portion of a PCB, such as tab portion  26 ,  28  of the structure shown in  FIG. 3  or, where the interconnect  34  is itself formed as a plurality of segments, the manner in which one such segment can be connected to another. As shown in  FIG. 4 , the interconnect  34  is of two-part construction in which one segment  34   a  is to be connected to a second segment  34   b . It will however be readily understood that the same approach is to be adopted where either part, or segment, represents an edge portion of a PCB unit to which the interconnect  34  is to be electrically connected. 
     An end portion  35  of the interconnect segment  34   a  is formed such that the intermediate layer  40   a  is undercut from the top screening layer  36   a  and the intermediate layer  40   a  is itself undercut at a level where a conductive strip (not shown) corresponding to the layer  45  of the  FIG. 3  embodiment extends through the segment  34   a  and is exposed at the undersurface of the extension  40   b  of the intermediate layer  40   a . Corresponding end portion  37  of the interconnect segment  34   b  is complimentarily formed in a stepped shape to expose a portion of the lower second outer conductive layer  38   a  and to expose a further conductive strip  45   a  which, when the two segments are brought together, can make electrical contact with the corresponding strip in the segment  34   a . The two strips can be bonded together for example by a solder or by an electrically conductive adhesive composition. Similarly, the outer conductive layers can also be brought into electrical contact by bonding the respective layers together. 
     The interconnect  34 , or one segment  34   a ,  34   b  of the interconnect  34 , may be formed as part of PCB unit  12 ,  14  during fabrication, lying initially within the plane of the PCB unit  12 ,  14 . A laser forming and annealing technique may then be used to cause the interconnect  34  or interconnect segment  34   a ,  34   b  to bend to the angle required. This technique avoids the need to bond the interconnect  34  to at least one of the PCB units  12 ,  14 . If, instead of using printed circuit techniques, the PCB units  12 ,  14  comprise circuit boards fabricated using semiconductor materials and techniques, then the interconnect  34  or segments  34   a ,  34   b  thereof may be fabricated using similar techniques, or micro-machined from a semiconductor substrate as required. 
     Whereas the arrangement shown in  FIG. 4  has been described for use in connecting the ends of the interconnect segment to tab portions of PCB units, it may be used equally well either to connect multiple segments within the interconnect itself or to make a connection to a further intermediate PCB unit at a point other than at the ends of the interconnect. 
     As previously mentioned, it has been found advantageous to avoid fold lines and creases at the juncture of the interconnect surfaces with those of the PCB units, and so the interconnect may be formed to provide curved layers that could be aligned with the outer screening layers of the PCB units so that the outer surfaces thereof can be continuous with the outer surfaces of the respective outer screening layers of the PCB units, as shown in  FIG. 3A . To permit provision of curved surfaces by the interconnect, the conductive layers of the interconnect and the strata providing the intermediate layer, and also the conductive strip or layer embedded therein may be provided by flexible materials, or by materials that can be rendered flexible by the application of heat. 
     Referring now to  FIG. 5 , there is shown therein a PCB structure which is similar to that shown in  FIG. 3  but which comprises a stack  50  of four spaced-apart PCB units  52 ,  54 ,  56 ,  58  adjacent ones of which are electrically connected together by interconnects  60 ,  62 ,  64  which are of similar construction to the interconnect  34  of  FIG. 3 . The structure of the PCB units themselves is also similar to the PCB units  12 ,  14  of  FIG. 3 . Neither the PCB units nor the interconnect of  FIG. 5  will therefore be further described. However, it will be appreciated from the disclosure of  FIG. 5  that a structure such as is shown therein may consist of more than four PCB units. 
       FIG. 6  illustrates a further alternative embodiment of the present invention, in which interconnects  70 ,  72 ,  74  are positioned between opposed pairs of tab portions  76 ,  78 ;  80 ,  82 ;  84 ,  86  on adjacent PCB units  88 ,  90 ,  92 ,  94 . The tab portions  78 ,  80  and  82 ,  84  may be formed as a single tab portion as indicated by the dotted line extending between tab portions  78 ,  80 . the structure of the PCB units and of the interconnects is similar to that of the embodiment shown in  FIGS. 3 and 5  and will therefore not be further described. 
     In  FIG. 7  is shown a variant of the embodiments of the invention that are illustrated in  FIGS. 3 and 5 . A PCB unit is shown generically at  100 , the PCB unit having an elongate aperture  102  formed therein. the PCB unit  100  is formed with a tab portion  104  extending into the aperture  102 , and a similar PCB unit (not shown but positioned below or behind the PCB unit  100 ) is similarly formed and provided with an opposed tab portion  106 . Extending between the upper tab portion  104  and the lower, opposed tab portion  106  is an interconnect  108 . 
     The structure of the PCB unit and of the interconnect  108  is similar to that described with reference to the preceding  FIGS. 3 to 6  and will not be further described. It will be appreciated that the construction shown in  FIG. 7  is especially suited for use in structures where it may be subject to rough handling in instrumentation which is designed for use in rugged conditions. 
     Referring now to  FIG. 8 , there is shown therein an embodiment of the invention that is particularly useful with PCB units that are of a different size one from the other. As shown in  FIG. 8 , three PCB units  110 ,  112  and  114  are stacked one upon the other, though of course they may be in spaced relationship to each other. It is also to be clearly understood that the arrangement of the PCB units may be inverted from that shown in  FIG. 8  or may be re-arranged such that the shortest PCB unit  110  is located between the other two. It is also to be understood that the construction of each PCB unit is substantially similar to that shown and described with reference to  FIGS. 3 to 7 . Thus, PCB unit  110  has an upper screening layer  116  exposed while only portions only of the upper screening layers  118 ,  120  of the PCB units are exposed. 
     The screening layers  116 ,  118 ,  120  can be electrically connected by positioning electrically conductive material  122  at the end face of each of PCB units  110 ,  112  and  114 . The electrically conductive material  122  is deposited, for example by plating, as a conductive adhesive or bonded to the edges of the adjacent screening layers  116 ,  118  and  118 ,  120  in the same manner as described above with reference to the embodiment disclosed in  FIG. 3 , i.e. by use of solder, an electrically-conductive epoxy resin or the like. 
     Alternatively, as shown in  FIG. 9  a single length  124  of screening material may extend along the length of the screening layer  116 , down the end face of the PCB unit  110 , along the exposed portion of screening layer  118 , down the front face of the PCB unit  112  and along the exposed portion of screening layer  120  of PCB unit  114 , being bonded to the screening layers in the manner described above. 
     Turning now to  FIGS. 10 ,  11 ,  12  and  13 , there is shown, in each Figure, a PCB structure which comprises a stack of six PCB units  132 . In both of these embodiments of the invention, a means of electrically connecting electrically conductive layers which are within the body of a stack of PCB units is shown. 
     In all four embodiments, the adjacent screening layers  132  of the uppermost pairs of PCB units and of the lowermost pairs of PCB units of the stack are electrically connected. 
     In the embodiment of  FIG. 10 , this is achieved by a shaped electrically-conductive interconnect  134  that is bonded to one or both of the screening layers of any adjacent or non-adjacent pair of PCB units using an adhesive composition as described above, care being taken to ensure that the bonding adhesive composition forms a smooth continuous surface with surfaces of the screening layer(s) and with immediately adjacent surfaces of the interconnect. 
     In the embodiment of  FIG. 11 , electrical connection of the layers of any adjacent or non-adjacent uppermost and lowermost pair of PCB units  132  is achieved by bonding separate interconnects  136 ,  138  to one or both of the layers of each pair and joining the two interconnects  136 ,  138  with a bridging interconnect member  140  which may be a continuous electrically conductive material or a patterned electrically conductive material supported by an electrically insulating material. The interconnect member  140  is provided with spaced-apart apertures  142 ,  144  arranged to receive the interconnect  136 ,  138  which can be bonded in situ to the member  140 . 
     It will be readily appreciated that there may be any number of separate interconnects on any wall of the stack and that the area of the interconnect member  140  can be of any convenient shape or size. 
     In the embodiment of  FIG. 12 , the electrical connections are shaped so as to connect any desired combination of conductors within the stack. It will be readily appreciated that this is another means to effect shaped interconnections similar to those shown in  FIGS. 3 ,  5  and  6 . 
     In the embodiment of  FIG. 13 , electrical connection of the layers is achieved by an interconnect member  140  which is provided with spaced-apart apertures  145  and  146  arranged to lie adjacent to the conductive material at the edges of the PCB units and, after bonding the electrically-insulating material  139 , applying conductive material to the apertures  145  and  146  so as to make electrical connection between the electrical conductors on the interconnect member  140  and those within the stack, as for  20  in  FIG. 3 . It will be readily appreciated that the electrical connection may be made using electrically conductive adhesive or by a plating technique as known in the art and as used for what are known as “plated-through-holes” or “vias”. 
     In any of the embodiments  10 ,  11 ,  12  and  13 , the interconnection member  140  can also include a separate electrically conductive layer so as to provide a connection with controlled characteristic impedance. The patterns may be on the same layer, so as to form what is known as a co-planar waveguide, or on one or more parallel surfaces so as to form either microstrip or tri-plate.