Patent Number: 048333344
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With a view to simplifying the description, the latter refers to a protective box formed from a monoblock cover fixed to base and formed by a single part, in which the element liable to be irradiated by X-rays is the box. The possible directions of the X-ray flux are represented by arrows F in FIGS. 1 to 3. As stated hereinbefore, the invention clearly has a much wider application. FIG. 1 diagrammatically shows in longitudinal section a box for protecting electronic circuits having a base 2, on which is fixed a cover 4, e.g. using screws 6. This box receives electronic circuits 8. In particular, these circuits can be fixed by any known means to bosses 10 provided for this purpose on the inner face of the box cover 4. In very diagrammatic manner as it does not form part of the object of the invention, the electronic circuits 8 are connected to electrical circuits outside the box by conductive tracks indicated at 12 and which e.g. pass through the box base 2. The specific shape of the box, as well as the fixing and connection of the electronic circuits located in the X-ray protection box are linked with the type of components used. These components can be encapsulated in boxes TO 5, TO 66, etc. According to the invention, cover 4 has a rigid structure 14 ensuring the mechanical protection of the electronic circuits. Structure 14 is formed from a thermosetting plastics material, such as bakelite, polyimide resin or silicones, reinforced by organic or non-organic fibres. Structure 14 is e.g. made from KINEL 5504 marketed by Rhone Poulenc, said material being a polyimide resin reinforced by long glass fibres arranged in a random manner. This rigid structure 14 is formed by moulding, either by injection, or by compression, said procedures being well known in the art. It can have a thickness of 2 mm. The protection against X-rays of the electronic circuits 8 located in the box is ensured with the aid of a layer material 16 in contact with the inner surface of the rigid structure 14 or, as shown in the drawing, in contact with the outer surface of the rigid structure. The X-ray protection material 16 covers all the outer surface of structure 14 liable to be irradiated by X-rays. In the case shown in FIG. 1, the material layer 16 covers the entire outer surface of structure 14, namely upper face 15 and side faces 17 of said structure. In order to ensure a good adhesion of the protective material 16 to rigid structure 14, the latter can have lots 18 formed during the molding of the rigid structure 14. These slots 18 are filled with X-ray protection material during the potting of the latter. The X-ray protection material 16 is e.g. formed from a tungsten powder representing 30% by volume of the finished product and which is regularly dispersed in a PA11 resin produced by ATOCHEM. This resin is a thermoplastic polyamide resin. The tungsten powder has an average grain size of 4 .mu.m, a dispersion of 2.5 and a purity of 99.9%. This protective material can be obtained by melting granules of PA11 resin, to which the tungsten powder is added. The premix obtained is introduced into a Werner ZSK30 extruder-granulator, in order to obtain mixture granules which can then be introduced into a mold already containing the rigid structure 14 to be covered by the X-ray protection material. The introduction of the mixture granules into the mold takes place by injection. The potting of the X-ray protection material is assured on rigid structure 14 heated in order to facilitate the adhesion of the X-ray material. The protective material obtained can have a thickness of 1.5 mm. The molding and potting respectively of rigid structure 14 and X-ray protection material 16 make it possible to directly obtain protective box covers with the requisite dimensions and accuracies without further machining being required. Moreover, these molding procedures are particularly advantageous from the financial and manufacturing time standpoints, because the runs of box covers to be produced make it possible to reduce the costs of specific tools for each box model. The X-ray protection material can also be constituted by a powder containing 6% by volume of tungsten and 24% by volume of uranium dioxide (UO.sub.2) embedded in PA11 resin. This material can have a thickness of 2 mm, in order to ensure effective filtering. In the same way, the PA11 polyamide resin can be replaced by a polyether block amide resin, such as that marketed under the trade name DINYL by Rhone-Poulenc. The resin of protective material 16 can also be bakelite or silicone. To avoid raising the electromagnetic level due to electron emission by the outer walls of the protective box during X-ray irradiation and in particular by the metal contained in the X-ray protection material 16, an anti-SGEMP material 20 forming the outer surface of cover 4 can be provided. In the case shown in FIG. 1, material 20 covers the entire X-ray protection material 16. It is made from beryllium so that, apart from the anti-SGEMP function, it provides the necessary protection of electronic circuits 8 against electromagnetic waves. The closing of the Faraday cage is obtained for the case described here by the presence of base 2 made from or covered by a good electricity conducting metal, such as nickel, silver, aluminium, beryllium or copper. Material 20 must have a thickness exceeding the free mean travel of the electrons emitted by the walls of the cover and also constitutes a covering for the final layer. In particular, in the case of an X-ray protection material covering the inner surface of the mechanical structure, the anti-SGEMP material advantageously constitutes the internal surface of the cover. It is possible to envisage the simultaneous use of an anti-SGEMP layer constituting the inner surface of the box and an anti-SGEMP layer constituting the outer surface of the box. FIG. 2 shows a second embodiment of the protective means according to the invention, in which the material used for preventing the emissivity effects of the walls of the case exposed to X-rays and in particular material 16 is made from a poor electricity conductor, such as carbon or boron. Under these conditions, the protection of integrated circuits 8 against electromagnetic waves is not assured. In order to assure this protection, the inner surface of the rigid structure 14 is completely covered with a good electricity conducting layer 22. In particular layer 22 is made from a metal, such as nickel or silver. Layer 22 has a thickness of approximately 0.1 mm. The other parts of the protective box and in particular cover 4 are unchanged compared with FIG. 1. FIG. 3 shows a third embodiment of the protective box according to the invention. This embodiment differs from that of FIG. 2 only in that the layer serving as the Faraday cage is positioned between the anti-SGEMP material layer 20a and the X-ray protection material layer 16. Layer 24 has a thickness of 0.1 mm and is in particular made from silver or nickel. As in the case of FIG. 1, the Faraday cage is closed by the presence of base 2 made from a metal or covered with a metal which is a good electricity conductor. In order to simplify the manufacture of the protective box according to the invention and ensure a good insulation of the electronic circuits 8 against electromagnetic waves, it is possible to provide a faradization layer 22-24 covering both the inner face of mechanical structure 14 (FIG. 2) and the outer surface of the X-ray protection material 16 (FIG. 3). This can be carried out by simply immersing the mechanical structure 14 coated with protective material 16 in a bath containing the metals to be deposited to serve as a Faraday cage (chemical deposition). It should be noted that the metal faradization layer cannot be applied to a silicone surface due to the poor adhesion of such a metal to such a resin. The description given hereinbefore has obviously been given in a non-limitative, illustrative manner and modifications are possible without passing beyond the scope of the invention. In particular, the X-ray protection material 16 may only cover the upper face 15 of the mechanical structure or only the side faces 17 thereof. In this case, the anti-SGEMP material 20 or 20a forming the outer surface of the box cover 4, covers all the X-ray material and that part of the rigid structure 14 not covered by X-ray protection material 16. Moreover, when the anti-SGEMP material is a poor electricity conductor, a faradization layer can be inserted between the anti-SGEMP layer 20a and those parts of the outer surface of structure 14 not covered by the X-ray protection material 16. The protective means according to the invention can be used wherever electronic circuits have to be protected against X-rays. This protection makes it possible to withstand severe surrounding climatic and mechanical conditions. In particular, the invention applies when minimum weight conditions are required. Thus, the protective box according to the invention makes it possible, in the case of an equivalent filtering efficiency to that of a solid material sheet covering a metal mechanical structure, permit gains as regards weight and overall dimensions, as well as a reduction in manufacturing costs. Thus, the protective box according to the invention can be advantageously used for producing very high performance electronic means on-board aircraft.