Abstract:
A secure electronic circuit on a printed circuit board is disclosed. The electronic circuit is enclosed in a housing comprising two shells located in abutment with opposite faces of the printed circuit board. One shell is provided with a first element that extends through an aperture in the printed circuit board and the other shell is provided with a second element that irreversibly engages with the first element and thereby secures both shells to the circuit board and inhibits removal of either shell. The construction is such that removal of either shell results in irreversible damage to the housing.

Description:
This application is a continuation of U.S. application Ser. No. 09/493,191 filed Jan. 28, 2000, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a construction of secure housing and in particular to a construction of secure housing for preventing access to circuits on a printed circuit board. 
     It is known in the construction of postage meters to enclose electronic circuits and mechanical components that are critical to the proper functioning of the postage meter in a secure housing to prevent unauthorised access to said circuits and mechanical components. The housing may comprise two parts assembled together and then a seal is applied so that unauthorised entry to the housing results in breaking of the seal thereby providing evidence that unauthorised entry to the housing has been attempted or has occurred. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a secure electronic circuit on a printed circuit board includes a secure housing, said secure housing including a first shell secured in abutting relationship with a first face of the printed circuit board; at least one first element projecting from said first shell and extending through an aperture in said printed circuit board beyond a second face of the printed circuit board opposite to said first face; a second shell located adjacent to said second face of the printed circuit board; at least one second element integral with said second shell and located within said second shell; said second shell being secured in abutting relationship with the second face of the printed circuit board by irreversible engagement of said at least one second element and said at least one first element so that removal of or attempt to remove the second shell from the printed circuit board results in fracture of at least one of said first and second elements to provide evidence of tampering. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       An embodiment of the invention will now be described by way of example by reference to the drawings in which: 
         FIG. 1  is transverse section through a secure housing preventing access to electronic circuits, and 
         FIG. 2  is a longitudinal section through the secure housing. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, a printed circuit board  10  comprises a substrate  11  carrying discrete electronic circuit components  12  that are interconnected by electrically conductive tracks (not shown) extending on one or both faces of the substrate  11 . If the printed circuit board is a multi-layer device further electrically conductive tracks may extend in internal planes within the thickness of the substrate  11 . In order to protect electronic circuits comprising the components  12  and conductive tracks from unauthorised access, a secure housing consisting of a first shell  13  and a second shell  14  is provided. 
     The first shell  13  comprises a first wall  15  from which pairs of side walls  16 ,  17  extend. Free edges of the side walls  16  abut against a first face  18  of the substrate  11 . The side walls  17  extend further than the side walls  16  so as to extend beyond the first face  18  of the substrate  11  across edges  19  of the substrate  11 . Studs  20  extend, in the same direction as the side walls from the first wall  15 . The ends of the studs are substantially co-planar with free edges of the side walls  16  and hence the studs and the side walls  16  abut the first face of the substrate  11 . The first shell is secured to the substrate  11  by means of screws  21  passing through apertures in the substrate  11  and threadedly engaging in the studs  20 . It will be appreciated that with the first shell secured to the substrate  11 , the side walls  16  abut against the first face and the side walls  17  extend across the edges of the substrate  11 . Accordingly access to the first face of the substrate  11  and to any electrical tracks or connections thereon is prevented. If desired the studs may be slightly shorter than the side walls  16  so as to ensure that when the first shell is secured by the screws  21  to the substrate  11 , the free edges of the side walls  16  are held tight against the first face of the substrate  11 . 
     The first shell is provided also with first elements  22  in the form of pins that extend from the first wall  13  through apertures  23  in the substrate  11  and beyond the substrate  11 . 
     The second shell  14  comprises a second wall  24  from which pairs of side walls  25 ,  26  extend. Free edges of the side walls  25  abut against a second face  27  of the substrate  11  and the side walls  26  extend to a greater extent than the side walls  25  so as to extend beyond the second face of the substrate  11  and overlap the side walls  17  of the first shell. The second shell is provided with second elements  28  in the form of studs extending from the second wall  24  toward the substrate  11 . The second elements  28  are located in opposition to the first elements  22  and have a bore  29  to receive the first elements  22 . The bores  29  and the first elements  22  may be slightly tapered to enable entry of the first elements into the bores  29 . However an essential relationship of the first elements and the bores is that the second shell can be pressed onto the first elements to a sufficient extent as to bring free edges of the side walls  25  into tight abutment against the second face of the substrate and that when the second shell is located with the side walls  25  abutting the substrate the first elements are held in an interference fit in the bores. The interference fit of the first elements in the bores is sufficiently strong as to provide irreversible engagement between the first and second elements so that in any attempt to remove the second shell the first elements remain engaged in the bores and force applied to attempt to remove the second shell results in fracture of the first or second elements. 
     Preferably, the second shell includes weakened regions  30  in locations adjacent bases of the studs  28 . The weakened regions may each comprise a groove in the second wall extending around the stud  28 . Accordingly if a force is applied to attempt to remove the second shell, removal of the shell will be resisted by the engagement of the first elements in the bores of the second elements and the force will result in fracture of the second wall at one or more of the weakened regions  30 . It will be appreciated that the fracture of the second wall provides evidence that tampering has occurred or has been attempted. It is to be understood that the weakened region may be located adjacent the second elements of the second shell as indicated in the drawings or adjacent the first elements of the first shell. Furthermore, locations of the first and second elements may be interchanged, second elements having a bore therein being formed as part of the first shell and the elements  22  being formed as part of the second shell. 
     While it is convenient to secure the first shell to the printed circuit board substrate by screws, if desired this securing may be omitted. The first shell may then be located relative to the printed circuit board by the first elements extending through apertures in the printed circuit board substrate and be secured to the substrate by the substrate being sandwiched between the first and second shells. 
     As described hereinbefore, the walls  17  and  26  of the first and second shells extend across the edges  19  of the printed circuit board. However if desired the side walls of one of the shells, for example side walls  17  may abut against the face of the circuit board and only the side walls of the other shell, for example side walls  26  extend across the edges of the printed circuit board so as to overlap the side walls  17 . 
     Generally the first and second shells can be manufactured to be sufficiently rigid and to tolerances such that invasive tampering via access between the side walls  16  or  23  and the substrate to the electronic circuits is prevented. However in order to provide greater protection from such invasive tampering, elements  31  may be provided on one or both faces of the substrate and secured to the substrate. These elements  31  extend internally of the shell adjacent the side wall thereof. The elements  31  extend from the face of the substrate to an extent sufficient to prevent any implement inserted between the side wall  23  and the second face of the substrate from reaching the electronic circuits housing with the secure housing. The elements may be of any convenient form suitable to impede insertion of an implement and for example may be of ‘L’ section as shown in FIG.  2 . 
     The first and second shells are each manufactured as integral mouldings so that the side walls and first and second elements are integral with the first and second walls respectively. The first and second shells may be formed of synthetic plastics material or of metal. 
     It will be appreciated that, as shown in  FIG. 1 , access at the edges  19  of the printed circuit board to the electronic circuits is prevented by the overlap of the side walls  17  and  26  of the first and second shells. Where the printed circuit board extends beyond the side walls  16 ,  25  of the first and second shells, access to the electronic circuits enclosed within the secure housing is prevented by close abutting engagement of the side walls with the surface of the printed circuit board and additionally is prevented by the elements  31 .