Abstract:
For sealing and shielding purposes, a laminate is used, including a layer of ionomer plastics bonded to a metal sheet or a metal plate. The bonding is made without any addition of adhesive agents between the plastics layer and the metal surface by pressing, in a heated state, the plastics layer against a well cleaned surface. The metal should be such a kind, that its surface is oxidized when stored in air. A multi-layer laminate is used, where one metal layer provides a good electrical shielding and an outermost metal layer provides good mechanical strength. The plastics layer at the inner side of a laminate is in the same way bonded directly to margins of electrical connector terminals made of metal.

Description:
TECHNICAL FIELD 
     The present invention relates to protection of delicate electric equipment, in particular to a laminate suitable for capsules for such equipment, and also to a method for manufacturing such laminates. 
     BACKGROUND OF THE INVENTION 
     Mechanical and electrical equipment, which is to be used in a difficult environment must often be protected thereagainst in order not to be damaged or interrupted in the its function. 
     In many cases enclosures are used which are type plastics. For more demanding applications laminates are used, for instance composed by several plastic materials, or by plastics and other materials such as metal sheets, in the established art. Laminates of plastics and metal have the advantage of being both non-permeable to diffusion of water vapour and of being electrically shielding. Depending on the metal layer and in particular the thickness thereof both flexible and stiff laminates can be obtained. The latter ones can also for certain material compositions be deep drawn, that provides a simple manufacturing method of boxes and lids for sealing and encapsulation. 
     The manufacture of plastic/metal laminates are usually performed in one of the two following manners: 
     1. One of the surfaces, which are to be joined, is coated with a polyurethane glue having a sticky consistency, whereafter the surfaces are pressed together in heat. The glue contains solvent, which must be removed by drying the glued materials in large, space-consuming drying machines. Moreover, an afterhardening of the glue must be made by storing the laminate for a couple of months in order for the water to diffuse into the glue for hardening it. The glue joints have a bad humidity resistance. 
     2. The plastics layer is extruded in the shape of a thin layer on a metal sheet path. High requirements are put on the wide slit nozzle used. Furthermore, commercially used extruders and their screws are large, which results in that a readjustment for extrusion of different materials is difficult, circumstantial and very time-consuming to perform. This is unsuitable for manufacturing laminated materials to be used in small series, when a flexible manufacturing method is required, such as is the case in making different types of capsules for example for delicate electronic circuits. 
     Also, plastic/metal laminates can be manufactured where the bonding to the metal layer is provided by a layer of ionomer plastics, where the bonding layer also can be the entire plastic component of the laminate, see e.g. the European patent application EP-A2 0 057 994, U.S. Pat. Nos. 5,376,446, 4,439,810, 3,725,169 and the German published patent application DE 2 233 958. 
     SUMMARY 
     The object of the present invention is to solve the above mentioned problems, which are associated with the established art. 
     In particular, an object of the invention is to provide a laminate suitable for protecting mechanically and shielding electrically electric and electronic components. 
     Another object is to provide laminates for protecting electrical components which can be manufactured at a lower cost in small series. 
     Another object is to provide a method for manufacturing such laminates, that can be used at a reasonable cost for electrical components used in small series. 
     Another object is to provide a method of protecting in a secure way delicate electrical components. 
     For sealing and shielding a laminate is used comprising a layer of ionomer plastics bonded to metal sheets and/or plates. The bonding is thus accomplished without addition of any adhesive agents between the plastics layer and the metal surface, and is obtained by pressing the plastics layer against a well cleaned metal surface in heat. The metals used shall be such that their surfaces are oxidized when stored in air. Thus a multi-layer laminate is provided, where one metal layer provides a good electrical shielding and an outermost metal layer provides a good mechanical strength and/or chemical resistance to corrosion. The plastics layer at the inside of a laminate bonds in the same manner directly at margins of electrical connectors made of metal. 
     Generally, a laminate for diffusion tight storing of objects comprises a metal layer and a polymer layer, in particular comprising a first layer of a metal, which is oxidized in air on its surface, and a second layer of an ionomer plastics material. These layers are directly bonded to each other, in particular by means of heating during compression. The temperature during the compression shall be such, that the material of the ionomer plastics layer is only half-melted or very viscous. After the compression in heat, a heat treatment at a higher temperature is advantageously conducted, at which the ionomer plastics layer is significantly more mobile than during the compression, which considerably increases the adherence between the layers of metal and of polymer. The second layer can be a surface material layer on a thermoplastics layer, in particular polythene or polypropene, which surface layer has been obtained by means of a suitable treatment of the layer of thermoplastics. 
     The laminate further has a third layer of a metal, which is also oxidized on its surface in air. This third layer is directly bonded to a layer of an ionomer plastics material, which can be the second layer or a surface material layer on the other side of the thermoplastics layer. 
     Furthermore, a fourth layer of an ionomer plastics material can be present, which is joined to the third layer, in particular so that this fourth layer is an inner layer of the laminate or so that it is a surface material layer on a base layer of thermoplastics. 
     The laminate is intended for sealing and encapsulation of electronic components, in particular for an electronic circuit board having electronic components mounted thereon, and then the first, outer metal layer is a mechanically strong and/or corrosion resistant metal material, such as a layer of a foil or a sheet of Ni or of steel plate. The third layer is then a metal layer having a good electrical conductivity, in particular Al or Cu. Further, the metal material layers are advantageously the kind, that are suitable for deep-drawing, so that for instance stiff capsules can be formed. 
     The different layers can have substantially the same thickness and for instance the different layers can have a thickness of about 50 μm. 
     When an extra good electrical shielding of a joint between such laminates is required, the joint can be short-circuited, for instance by means of a method that is described in the International patent application PCT/SE94/00255, which is incorporated herein by reference. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail by way of non-limiting embodiments with reference to the accompanying drawings, in which: 
     FIG. 1 is an exploded perspective view of an encapsulation for a printed circuit board, where a laminate has been used as a bottom plate, 
     FIGS. 2 a  and  2   b  are sectional views of marginal areas of two joint laminates used for encapsulation and/or for shielding, 
     FIGS. 3 a  and  3   b  illustrate manufacturing procedures for the laminate of FIG. 2 a.   
    
    
     DETAILED DESCRIPTION 
     A material, which when heated has a good adherence to many metal surfaces, is thermoplastic ionomers, such as the types of plastic materials that are commercially available under the name “Surlyn” from the company E. I. Du Pont de Nemours and Company, for example type 1652, or “Iotek” from the company Exxon Chemical Americas. These thermoplastic materials contain carbon chains having carboxylic side groups —COO — and these carboxylic groups are interconnected by means of zinc ions Zn 2+ and/or other metal ions such as sodium ions Na + . The bonding to metal surfaces can in principle be conceived to occur by the metallic ion being attracted to negative charges associated with oxygen atoms at a metal surface that is at least partly oxidized. That this mechanism is operative is supported by the fact that no bonding or a bad bonding is obtained to non-oxidized metal surfaces, such as surfaces of gold Au, and that also ions having less charge such as Na + provides a poorer bonding than Zn 2+ . Therefore, ionomer materials based on zinc ions are preferred herein, but of course thermoplastic ionomers based on other metal ions can be used, provided that they give the above mentioned good adherence to metal surfaces. 
     Hence, for a good adherence of the ionomer layer, the metal surfaces used need not to be extremely well cleaned but it is only needed to rid them of dirt, oil and possible particles. An oxide layer must remain on the metal surface. Typical materials to be used in metal layers or surfaces are nickel, aluminium, iron in the form of steel, for instance deep drawn steel, steel covered with a protective layer such as zincified steel, aluzinc, copper, etc. The metal can be present in the form of rigid metal plates, flexible metal sheets or metal foils, to which the ionomer layer is bonded. 
     In FIG. 1 an exploded perspective view of an encapsulation suitable for an electronic circuit board  11  is shown. The circuit board  11  is attached to a bottom part  13 , which consists of a lower metal plate or metal sheet  15 , for instance made of nickel, and a layer  17  of ionomer plastics attached thereon. The ionomer layer  17  can have been attached to the metal sheet  15  by means of pressing in heat, as will be described below. Exterior electrical connector terminals, not shown, can be provided in the lower plate  13 . Further, there is a top part or a lid  19 , which comprises a bowl-shaped, centrally located portion  20  for enclosing electronic components mounted on the circuit board  11  and a flange or marginal portion  21 , which extends along the whole circumference of the upper portion  19  and has a flat lower surface. When sealing and encapsulating the circuit board  11  the upper part  19  is put in place onto the lower part  13 , so that the lower surfaces of its flanges  21  come in contact with the marginal portions of the ionomer plastics layer  17 . Then the compressive force and heat are applied to the whole marginal region, whereby a tight sealing and a mechanical fixing or cementation of the lid  19  to the lower part  13  is obtained. 
     In encapsulating electronic components to be used at high frequencies it can be desirable to obtain a complete electrical shielding of the components inside the encapsulation. With the methods above, no completely tight encapsulation is obtained but a small area is obtained at the plastics material  17 , where radio waves can leak in and out to the otherwise well shielded electronic circuits on the circuit board  11 . A shielding of the marginal region can then be obtained in the manner described in the above mentioned International patent application. Then a contact element having protruding parts, which are inserted in the margin areas, where the sealing is obtained between the lower part and upper part  13 ,  19 , is used. It can in this case be advantageous to arrange two metal layers in the encapsulating material. Laminates  22 ,  22 ′ suitable for such a device are shown in FIG. 2 a  illustrating a sectional view of a marginal joint of two identical laminates. Outermost in such a laminate  22  there is an outer layer  23  of a corrosion resistant metal, such as nickel. Inside this layer there is an ionomer plastics layer  25  and inside it another metal layer  27 . The metal layer  27  shall be a good electrical conductor and here aluminium can be chosen. Innermost there is yet another layer  29  of ionomer plastics. The different layers in the laminate can have approximately the same thickness and for instance thicknesses of 50 μm can be used. 
     The laminates illustrated in FIG. 2 a  are well suited, at least for the dimensions indicated above, for deep drawing for manufacturing for instance a lid  19  according to FIG.  1 . In an electrical shielding of the marginal region two laminates  22 ,  22 ′ of for instance the same kind are placed having the free ionomer plastics layers  29 ,  29 ′ in contact with each other. Between the marginal areas of the laminates the elongated electrically conducting material having protruding parts, for instance in the shape of a thin metal strip turned into a screw shape, is placed as shown at  31  in FIG. 2 a . When compressing and heating the marginal areas of the laminates parts of the metal strip  31  will then pass into the laminates  22  and  22 ′ and for suitable material dimensions and material choices be pressed into and penetrate through the inner ionomer plastics layers  29 ,  29 ′ and the inner, metal layers  27 ,  27 ′, for instance made of aluminium. An electric contact is then obtained to the shielding, inner metal layers  27 ,  27 ′ in the laminates  22 ,  22 ′. The contact material  31  then does not penetrate through the outer protecting material layers  23 ,  23 ′, in particular if these are made of a material having a high mechanical strength such as nickel foil. 
     When using the laminates  22 ,  22 ′ for manufacturing a shielding enclosure according to FIG. 1 the inner ionomer plastics layer  29  or  29 ′ can possibly be excluded, however not within those regions, in which electrical connector terminals shall be arranged, since they are suitably sealed along their edges by means of compression in heat to an ionomer plastics layer. A sectional view of a marginal region of an encapsulation made of two different laminates  22 ″ and  22 ′ is shown in FIG. 2 b.    
     The laminates according to FIGS. 1,  2   a  or  2   b  can easily be manufactured by means of compression in heat, as for instance is shown schematically in FIG. 3 a . For manufacturing the laminate  22  according to the embodiment in FIG. 2 a  the different sheets  23 - 29 , cut into a suitable shape, are placed between heated compression surfaces of press plates  33  in a press. As an alternative the sheets or foils can be laminated together between heated cylinders  35 , see FIG. 4 b . After the compression, a heat treatment is conducted at a higher temperature in order to further increase the adherence between plastics and metal. 
     In an experiment for testing the adherence of an ionomer plastics layer to a metal sheet, a laminate comprising a surface layer of an aluminium sheet, an intermediate sheet of ionomer plastics of type 1652 Surlyn, an additional aluminium sheet and at the bottom an ionomer plastics layer were manufactured. All the layers present had a thickness of 50 μm and they were pressed together during a time period of 1 minute at a compression force of 1·10 6  N/m 2  at a temperature of 100° between pressing plates of the kind shown in FIG.  4 . Thereupon a heat treatment was conducted without compression by maintaining the laminate in an oven having the temperature of 140° during 1 hour. After cooling, the laminate thus obtained was tested again by means of boiling under water for 15 hours. No traces of delamination were noticeable and in particular there were no traces of water penetration at the boundary between ionomer plastics and metal. In the corresponding treatment of a polythene sheet bonded to a metal sheet in a conventional manner by adhesive bonding using a polyurethane glue, a pronounced hydrolysis of the boundary between plastics and metal was clearly noticeable after boiling in water for 5 minutes. 
     Experiments have also been conducted for the laminate by maintaining it in a humid atmosphere—85% relative humidity and at a high temperature—85°. For the ionomer plastics laminate no change was noticeable after storage for 1500 hours while a change occurred in the bonding area between plastics and metal sheets after 200 hours for the corresponding conventionally manufactured laminate. 
     The pressing of the laminate should be performed at a temperature as high as possible, that is when the ionomer plastics layer or layers still are half molten and not too liquid. The good adherence between metal and ionomer plastics layer are considerably improved by the final heat treatment that should be made at a temperature where the plastics material has a low viscosity and flows easily. If the final heat treatment is performed at a lower temperature, for instance 120°, a maximal adherence is not obtained, while if higher temperatures, such as 160°, is used, the plastics layer or layers become milky white, which indicates a thermal decomposition of the plastics material.