Earthed intermediate frame for circuit boards
The present invention relates to an earthed (grounded) intermediate frame to be fitted between circuit boards. 
     It is known in the art to join circuit boards together with earthed metallic intermediate frames fitted therebetween for example so that two circuit boards are joined with a metal frame disposed therebetween into a box-like structure, in which the earth (ground) folios each circuit board constitute the opposite outer surfaces of said box-like structure, whereas the metallic intermediate frame forms its sides. In this manner, the signal conductors of the circuit boards remaining inside said box-like structure and the connectors connecting the circuit-boards are shielded, not only against external interferences but also against causing interference to the environment. The signal conductors and connectors remaining thereinside are not, however, prevented by this kind of structure from causing mutual interference unless said connectors and conductors are separately shielded by coating said connectors and conductors with an earthed sheath. Such separate shielding will however become expensive and particularly the signal conductors on the surface of the circuit boards are difficult to protect in this manner. 
     The object of the present invention is therefore to provide an earthed intermediate frame to be fitted between said circuit boards, which not only shields the signal conductors on the surface of the circuit boards and the connectors connecting the circuit boards against external interferences and against causing interference to the environment but also prevents the signal conductors on the surfaces of the circuit boards and the connectors connecting said circuit boards joined by the intermediate frame from causing mutual interference so that no separate shielded connectors or signal conductors be needed for said purpose. 
     The intermediate frame according to the present invention is provided with through apertures or substantially transversal sleeves for connectors connecting circuit boards, and/or grooves in register with the signal runs located on the surface of the circuit boards on the surface of the intermediate frame placed against said surface of the circuit board. By means of the intermediate frame according to the invention the signal conductors on the surface of the circuit boards joining it and the connectors connecting said circuit boards can be shielded, not only against the external interferences of the structure constituting the intermediate frame for joining them but also against the mutual interfering influences caused by said signal conductors and connectors, not having to resort to separate shielding of the signal conductors or connectors. 
     The intermediate frame according to the invention may be made entirely of a metal or e.g. of plastic, whereby the inner faces of the grooves at the signal runs thereof and of the apertures around the connectors, or of the substantially transversal sleeves have been coated by earthed metal folio. Said metal folios form preferably a uniform earth folio. 
    
    
     The invention is described below more closely, reference being made to the accompanying drawing, in which: 
     FIG. 1 presents a cross-sectional perspective partial view of the frame structure according to the invention, fitted between two circuit boards, and 
     FIG. 2 presents a longitudinal section of an alternative intermediate frame according to the invention, likewise fitted between two circuit boards. 
    
    
     In FIG. 1 the circuit boards are generally indicated by reference numeral 1 and the intermediate frame fitted therebetween generally by reference numeral 2. As is seen in FIG. 1, the intermediate frame 2 is provided with a sleeve 3 extending on opposite sides substantially rectangularly, positioned at the connectors 4 connecting the circuit boards 1 so that each connector 4 is surrounded by a sleeve 3 of the intermediate frame 2 which at the same time keeps the circuit boards 1 spaced from one another. The intermediate frame 2 or at least its sleeves 3 may be made of a metal, so that the intermediate frame, or at least its sleeves 3, are earthed. Alternatively, the inside face of the sleeve 3 may be coated with an earth folio 7, which may be combined to the earth folio of one or both of the circuit boards 1, said folio being located on the outer face of the structure. In this case the intermediate frame may be e.g. of plastic. 
     In the embodiment presented in FIG. 1, the circuit boards 1 are fitted with the earth folios thereof placed outwards on the opposite sides of the planar intermediate frame 2 to form a sandwich structure. Both of the opposite surfaces of the intermediate frame 2 placed against the circuit boards 1 are provided with grooves 6 in register with the signal conductors 5 on the opposite surfaces of the circuit board 1. In addition, the plate-like intermediate frame 2 is provided with through apertures 8 in register with the connectors 4 connecting the circuit boards 1. The plate-like intermediate frame 2 can be made entirely of a metal and be earthed for isolating the connectors 4 and signal conductors 4, not only from the environment but also from each other. Alternatively, the plate-like intermediate frame 2 can be made of a non-conducting material, such as plastic, whereby the surfaces of the apertures 8 and the grooves 6 are coated with a metal folio to be earthed. The metal folios coating the inner surfaces of the grooves 6 and apertures 4 may form a uniform earth folio, this being preferably connected to the earth folio at least of the other circuit board 1. 
     The embodiments presented in the drawing represent only a few of the examples of the numerous embodiments of the intermediate frame of the present invention. It is also obvious that more than two circuit boards are connectable with intermediate frames fitted therebetween into a multiple-layered structure in which the earth folios of the outer circuit boards are located on the outer surface of said structure.