Patent Abstract:
The present invention provides a security assembly for protecting a device includes first and second security wraps fitted to the device. The first security wrap covers a first area of the device. The second security wrap partially overlaps the first security wrap and covers a second area of the device. Each of the first and second security wraps has a security screen having first and second screen terminals and a conductive track extending between the first and second screen terminals. A conductive structure is disposed in an overlapping area between the first and second security wraps and coupled to the second screen terminal of the first security screen and to the first screen terminal of the second security screen.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of co-pending application Ser. No. 13/971,094, filed on 20 Aug. 2013, for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 1214823.5 filed in United Kingdom on Aug. 20, 2012 under 35 U.S.C. §119, the entire contents of all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a tamper indication device and in particular to a stackable security wrap for an electronic circuit to protect against tampering. 
     Although this invention will be described in relation to security wraps for a printed circuit board as an example of the invention, the invention can be used with any printed electronics (PE) flex having a need for protection against or detection of tampering. 
     BACKGROUND OF THE INVENTION 
     Traditional security wraps form a solid security screen masking an area of the electronics to be protected. Removal of the security wrap is physically difficult due to the manner in which the security wrap is attached to the device, usually by gluing, soldering or encapsulation by a resin material. Modern security wraps have a security screen electrically connecting a pair of terminals of an alarm circuit. The security screen may be damaged or broken during attempts to tamper with the device to thereby set off an alarm condition. The alarm circuit may disable the device or simply give a visual indication that the security wrap has been tampered with. 
     In a co-pending commonly assigned patent application, there is disclosed a security wrap of the breakable conductor type, having a security screen with a conductor that is relatively thin and densely packed over the area to be protected to prevent tampering and arranged or designed to easily fracture should an attempt be made to tamper with the security wrap once fitted. However, security wraps are often required to cover complex shapes which may be best protected by using two or more security wraps. With only a limited number of alarm terminals available, connection of the security screens of multiple security wraps may be problematic. 
     Hence, there is a desire for a security wrap which can be stacked with another security wrap with the security screens of the two security wraps being electrically interconnected. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect thereof, the present invention provides a security assembly for protecting a device, comprising first and second security wraps fitted to the device. The first security wrap covers a first area of the device, and has a first security screen comprising a pair of first screen terminals and a conductive track extending between said first screen terminals. The second security wrap partially overlaps said first security wrap, covers a second area of the device, and has a second security screen comprising a pair of second screen terminals and a conductive track extending from said second screen terminals. A conductive structure is disposed in an overlapping area between said first security wrap and said second security wrap and coupled to said screen terminals of said first security screen and to said screen terminals of said second security screen. 
     Preferably, said first screen terminals of said first security screen and said second screen terminals of said second security screen are coupled to two corresponding terminals of an alarm circuit of the device. 
     Preferably, said first security wrap further includes a first substrate; said first security screen includes said first screen terminals and said conductive track formed on said first substrate; and said conductive structure includes a conductive plug formed in said first substrate and coupled to said first screen terminals of said first security screen and to said second screen terminals of said second security screen. 
     Optionally, said conductive structure includes a conductive resilient disc disposed between said first security wrap and said second security wrap, said conductive resilient disc being in contact with said first screen terminals of said first security screen and with said second screen terminals of said second security screen in response to a compression in the overlapping area between said first security wrap and said second security wrap. 
     Optionally, an adhesive layer in the overlapping area between said first security wrap and said second security wrap, said adhesive layer compressing said conductive resilient disc in contact with said first screen terminals of said first security screen and with said second screen terminals of said second security screen. 
     Optionally, further comprising a spigot over the overlapping area between said first security wrap and said second security wrap, said spigot compressing said conductive resilient disc in contact with said first screen terminals of said first security screen and with said second screen terminals of said second security screen. 
     Optionally, said conductive structure includes a carbon pad disposed in the overlapping area between said first security wrap and said second security wrap and in contact with said second screen terminals of said second security screen; said carbon pad and said first screen terminals of said first security screen define a gap there between; and said carbon pad is in contact with said first screen terminals of said first security screen in response to a compression of the overlapping area between said first security wrap and said second security wrap. 
     Preferably, said conductive structure includes a printed conductive through hole electrically connected to said first screen terminals of said first security screen and said second screen terminals of said second security screen. 
     Preferably, said second security wrap includes a folded wrap. 
     Preferably, said first and second security screens include first and second breakable conductive tracks formed on said first and second security wraps, respectively. 
     According to a second aspect thereof, the present invention provides a security assembly for protecting a device includes first and second wrap. The first wrap comprises a substrate having first side and second sides opposite to each other, a first conductive track bonded to the first side of said substrate and having two ends forming first screen terminals coupled to the device and second screen terminals, and a first adhesive layer covering said first conductive track over the first side of said substrate and bonding said substrate to the device. The second wrap has an overlapping area with said first wrap and comprises a substrate having first and second sides opposite to each other, a second conductive track bonded to the first side of said substrate and having two ends forming second screen terminals coupled to the first screen terminals of said first conductive track and second screen terminals coupled to the device, and a second adhesive layer covering said second conductive track over the first side of said substrate and bonding the substrate to the device. A conductive structure is disposed in the overlapping area between said first wrap and said second wrap and in said substrate of said first wrap, said conductive structure being coupled to said first screen terminals of said first conductive track and to said second screen terminals of said second conductive track. 
     Preferably, said first and second conductive tracks includes first and second breakable conductive tracks. 
     Preferably, further comprising an intermittent pattern of release ink disposed between the first side of said substrate of said first wrap and said first conductive track to selectively modify a bonding strength between said first conductive track and said substrate of said first wrap. 
     Preferably, said first screen terminals of said first conductive track are coupled to third terminals of the alarm circuit of the device; said second screen terminals of said second conductive track coupled to third terminals of the alarm circuit of the device; said first conductive track and said second conductive track form a series conductive path between the first and second terminals of the device. 
     Preferably, said first wrap is fitted to the device and covers a first area of the device; and said second wrap is fitted to the device and covers a second area of the device. 
     By overlaying and interconnecting security wraps, complex areas can be covered or protected by the security wraps without having to form large complex security wraps which have high material wastage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
         FIG. 1  illustrates an exemplary PCB, protected by two interconnected security wraps; 
         FIG. 2  is an exploded view of the device of  FIG. 1 ; 
         FIG. 3  illustrates a folded security wrap, being one of the security wraps shown in  FIG. 1 ; 
         FIG. 4  illustrates the security wrap of  FIG. 3  before it is folded into its final form; 
         FIG. 5  is a sectional schematic of the electrical connection between the two security wraps; 
         FIG. 6  is an enlarged view of a connection shown in  FIG. 5 ; and 
         FIGS. 7 &amp; 8  illustrate alternative connection methods. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  illustrate an electronic device  10 , by way of example only, in the form of a printed circuit board (PCB)  12  having electronic components such as a CPU  14  that is desired to be protected against tampering. The PCB  12  also has a key pad having a number of keys represented as circles  16  on the lower section of the PCB  12 , for entering data and instructions for the CPU  14 . Two security wraps  20 ,  30  are fitted to the PCB  12 . A first security wrap  20  covers the key pad portion of the PCB  12 . It may also cover smaller electronic components. A second security wrap  30  covers the CPU  14  and larger electronic components. To accommodate the CPU  14 , which is too large to be securely covered by a flat security wrap, the second security wrap  30  is of the folded type wherein the wrap  30  is folded to form a chamber for the CPU  14 . Alternatively, the second security wrap  30  may be an embossed security wrap having a preformed chamber to accommodate the CPU  14 . Preferably the second security wrap  30  is mounted to the first security wrap  20  as this allows a more simple assembly and construction of the first security wrap  20 . Each security wrap  20 ,  30  has a conductive circuit  22 ,  32  between a pair of screen terminals  24 ,  34  forming a security screen. The screens, in use, form a conductive path between terminals  18  of an alarm circuit. When the electrical connection between the alarm terminals  18  is interrupted, such as when the conductor of a screen is broken or the security wrap is removed, an alarm condition is triggered. The response to the alarm condition depends on the device but may include shutting down of the device, resetting the memory of the CPU, disabling or total destruction of the device. 
     The PCB  12  and the two security wraps  20 ,  30  are shown in exploded view in  FIG. 2 . As can be seen, two conductive spring discs  56 , known as domes, are used to electrically connect the security screens of the security wraps  20 ,  30  together. It is preferable for the security screens to be connected in series so that to the alarm circuit, the two security screens appear and function as a single security screen. A pair of third terminals  36  is shown formed on the second wrap  30 . The third terminals  36  are arranged to connect to the alarm terminals  18  and form the ends of the conductive path formed by the screens. As will be realized, for the third terminals  36  to be the ends of the conductive path, the second wrap  30  must have two security screens, one screen extending between one of the third terminals  36  to one of the second terminals  34  and the other screen extending between the other third terminal  36  and the other second terminal  34 . Alternatively the third terminals  36  could be formed on the first wrap  20 . This depends on the layout and location of the terminals of the alarm circuit and the layout of the wraps. It is also possible for each wrap to have one third terminal and one first or second terminal such that each wrap has only one conductive screen. This has the advantage of requiring only one through connection to be formed in the first substrate. 
     As mentioned, the second security wrap  30  is preferably formed as a folded wrap. This means that the wrap  30  is formed from a flat sheet of flexible substrate material. Once the flat wrap has been formed and cut to shape, it is folded into the desired shape. The unfolded or developed state is shown in  FIG. 4 . Fold lines are indicated by dashed lines  41 . The security screen  32  is also partially indicated by a dashed line. In practice, the security screen  32  covers substantially the entire under surface of the substrate which is then covered with an adhesion to fix the wrap  30  to the device. As can be seen, the pre-folded wrap has a central portion  40 , four side portions  42  extending from the central portion  40  across fold lines. Panels  46  extend from each side portion  42  across a fold line. The two longer side portions  42  also have two flaps  44  extending from the short edges. When folded, the flaps  44  lay over the adjacent shorter side portion  42  and are fixed thereto by adhesive. Panels  46  are turned out to form a rim that is arranged to be fixed to the PCB  12 , with one panel slightly raised to sit on the first security wrap  20  and thus is fixed to the PCB  12  via the first security wrap  20  whereas the other panels, in the embodiment shown, are fixed directly to the PCB  12 . 
     The general principles of construction of a security wrap will now be described with reference to the first security wrap  20  and  FIG. 6 . The security wrap has a substrate  50 , a conductive circuit  22 , and a layer of adhesive  54  to bond the security wrap to the parent device, i.e., to PCB  12 . An insulating layer, such as a dielectric layer, may be used to provide insulation between the conductive circuit  22  and the adhesive or device. The adhesive, if non-conductive, may function as the insulating layer. 
     For a security wrap  20  with breakable conductors, an intermittent layer of adhesive modifier or release layer  52  is applied between the substrate  50  and the first conductive circuit  22 . It should be noted that the use of breakable conductors is optional and that breakable conductors can be formed using a different method not involving a release layer. 
     The substrate  50  is preferably a polymer film, typically a polyethylene terephthalate (PET or commonly referred to as polyester) film, that provides a base for a security wrap circuit. Optionally the substrate is flexible, being a film of thickness between 25 μm and 175 μm but can be greater depending on functional requirements and may include other variants of polymer film including, but not limited to, polycarbonate, PEN, polyimide and PVC. The substrate  50  may be clear but preferably is opaque and pigmented, for example black or white, to hide the configuration of the conductive screen  22  and the underlying circuitry on PCB  12 . 
     The release layer  52  is preferably, a ultra-violet (UV), infra-red (IR) or thermally cured ink system used to provide a different adhesion level between the substrate  50  and the security screen  22 . The ink is thus an adhesion modifier. The release layer  52  is intermittent and applied to the substrate  50  in a predetermined pattern by a printing process and is not a complete layer such that there are areas of substrate which are not covered by the adhesion modifier ink. Optionally, the pattern of the release layer  52  is simple stripes or dots. 
     The conductive circuit  22  is composed of a pattern formed by a conductive trace or conductor preferably formed by thermoset or thermoplastic conductive ink printed over the substrate  50  in variable trace widths and serpentine mesh patterns forming an electrically conductive path between a pair of screen terminals  24 . Preferably, the screen terminals  24  are simply the ends of the conductors. 
     The conductive inks can be silver, silver-coated copper or gold containing conductive or resistive ink, each with specific properties that suit the necessary requirement for the operation and functionality of the security wrap flexible circuit. The conductive ink can also be carbon, graphite, clear conductive polymer or other conductive or resistive ink, each with specific properties that suit the necessary requirement for the operation and functionality of the security wrap. 
     A dielectric layer such as a UV curable ink system with electrically insulative properties may be used to electrically insulate the security screen  22  to avoid short circuits, if needed. 
     The adhesive layer  54  is preferably a pressure-sensitive adhesive (PSA), typically an acrylic adhesive that forms a bond between surfaces when pressure is applied. The adhesive may be applied as an adhesive ink or as a laminate. The adhesive layer is used to bond the security wrap to the parent device. Alternatively, the adhesive maybe a liquid adhesive such as an epoxy, or moisture-cure urethane etc. which is dispensed or printed between the security wrap and the PCB, which is then cured by moisture, thermal or UV energy and forms a permanent bond between wrap and PCB. This type of adhesive is not pressure sensitive, but could work under the same disclosed principle. 
     Depending on the material of the parent device  10  to which the security wrap  20  is adhered a variant PSA with specific adhesion properties could be used. Specifically the adhesion to the parent device  10  must be stronger than the adhesion to the substrate  50 , so that on removal of the security wrap  20  from the parent device  10 , the adhesive layer  54  will remain adhered to the parent device  10  in order to break the conductor of the conductive circuit  22 . 
     First wrap  20  differs from the second wrap  30  in that normally the screen terminals are on the same side of the substrate  50  as the screen and the adhesive so that the security wrap protects the connections to the alarm terminals. However, as the screen of the first wrap  20  needs to be connected to the screen of the second wrap  30 , the first screen terminals  24  extend through the first substrate  50  as shown in  FIG. 6 . This is preferably achieved by a printed or filled through hole, although a plated through hole could be used. A printed through hole is formed by making a suitable hole in the substrate  50  and filling it with conductive ink, typically at the time of forming the conductors of the screen, thus providing a conductive path through the substrate  50 . Such holes are known as a via. The connection is protected by the second wrap  30 . 
     The screen terminal  34  of the second wrap  30  is formed as a projection of the end of the conductor  32  forming the second screen and preferably forms a projection extending into the adhesive layer  54 . The actual connection between the first and second screens  20 ,  30  may take different forms. 
       FIG. 6  illustrates the preferred method using conductive domes  56 . The dome  56  is inserted into a recess  58  in the adhesive layer  54  aligned with the corresponding second screen terminal  34  such that the dome  56  is in electrical contact with the second screen terminal  34  of the second wrap  30  and is aligned with the corresponding first screen terminal  24  of the first wrap  20 . The dome  56  can be arranged in two ways. Firstly as a resilient contact whereby the adhesive provides a mild compression force on the dome  56  so that the resilience of the dome  56  allows the connection to tolerate slight movement as may occur for example by rough handling or thermal expansion of the various components. Alternatively, the dome  56  acts a spring loaded lift off contact requiring an external force to be applied to the connection in order to compress the dome to establish an electrical connection between the screen terminals. The dome  56  is shown in the relaxed state in  FIG. 6 . 
       FIG. 5  is an exploded schematic illustrating a preferred method of applying force to the connections. The dome  56 , in the relaxed state, lifts of the screen terminal  24  breaking electrical contact there with. Spigots  19  formed on a part of the housing for the PCB  12  are arranged to bear down on the second security wrap  30  so as to press the second screen terminals  34  against the domes  56  and the domes  54  against the first screen terminals  24  to establish an electrical connection between the screens as the housing is closed. When the housing is opened, the spigots  19  separate from the security wrap  30  and the dome  56  relaxes breaking the connection and raising an alarm condition. In this way, an alarm condition is triggered by merely opening the housing before any direct attempt to remove the security wrap  30  is made. 
       FIG. 7  is an enlarged sectional view, similar to  FIG. 6 , of a different method of connecting the screens. The dome is replaced by a carbon pad  60  which is slightly thinner than the adhesive layer  54  so that the carbon pad  60  faces the first screen terminal  24  across a small air gap  62 . The carbon pad  60  makes direct contact with the second screen terminal  34  and may actually form the second screen terminal  34 . When an external force is applied to the second wrap  30  in the region of the carbon pad  60 , the second substrate  50  is resiliently deformed and the carbon pad  60  is pressed against the corresponding first screen terminal  24 , establishing an electrical connection between the screens. On removal of the external force, the substrate  50  relaxes and the carbon pad  60  separates from the first screen terminal  24  to break the connection thus raising an alarm condition. 
       FIG. 8  illustrates another connection method wherein a plug  64  of conductive material replaces the carbon pad  60  shown in  FIG. 7 . The plug  64  completely extends across the thickness of the adhesive layer  54  to make direct contact between the first and second screen terminals  24 ,  34 . The plug  64  may be of any suitable material such as conductive foam but preferably it is formed of conductive ink, formed at the time of printing the conductors of the second screen. The plug  64  maybe directly connected to the first screen terminal  24  by pressure from the adhesive layer, or by conductive adhesive, conductive paste, etc. 
     While the preferred embodiment uses security wraps with breakable conductors the present invention is also applicable to more traditional security wraps where the conductors of the security screen as not specifically designed to be broken if the security wrap is removed from the device. Depending on the complexity of the alarm circuit, a security screen may have any number of conductors. 
     While the drawings have been enlarged for better clarity of observation and description, in the preferred embodiments, the width of the conductive traces and the spaces there between are in the range of 1 to 1,000 microns. The preferred embodiment uses a trace width between 200 and 300 microns. This produces a good compromise between costs and security level as the finer the widths the higher the security level but the printing process is more expensive. 
     In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
     Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.

Technology Classification (CPC): 7