FR2713139A1 - Metallised rigid organic foam support for hyperfrequency electronic circuits or antenna arrays - Google Patents

Abstract

Metallised support has a rigid expanded foam organic material, e.g. a polymethacrylate imide or vinyl polymer, support (1) covered partially with a thermoplastic organic polymer, pref. a copolymer of propylene and methylpentene, film (3) covered with a metal, e.g. copper, aluminium or alloys, layer (5). An electronically conductive convection (6) passes through the foam or films to connect the separate metallised surfaces. Also claimed is the prodn. by: activating the surface of the thermoplastic material; placing the activated surface against the support foam; placing a metal layer on the other activated surface of the thermoplastic; joining the assembly under heat and pressure.

Description

METALLIC SUPPORT BASED ON ORGANIC FOAM,
ASSEMBLY OF AT LEAST TWO OF SUCH SUPPORTS AND METHOD
OF MANUFACTURE OF THIS SUPPORT
The invention relates to a metallized support based on organic foam, an assembly of at least two of these supports and the method of manufacturing this support. It applies in particular to the production of microwave electronic circuits or radiating elements such as flat antennas (transmitters or receivers) for connections between mobiles (car, boat, for example), between fixed stations or the two associates.

The production of these microwave electronic circuits or of these planar antennas requires the use of dielectric materials meeting in particular the following conditions
- lowest possible dielectric constant,
- low dielectric losses,
- possibility and ease of metallization by
electrically conductive materials, of which
the adhesion to the dielectric material is
good,
- the lowest possible manufacturing cost, and
- lightness of this material.

 A certain number of organic materials capable of being used for the abovementioned applications exist on the market. They are, for example, in the form of polymers in films, sheets or laminates such as polyimides, epoxides, polyesters, polysulfones, polystyrene, polyethylene, polytetrafluoroethylene, polypropylene or polymethylpentene, these materials being able to be marketed under different trade names.

 However, if these materials have dielectric characteristics close to those desired for the radioelectric applications of the invention, they nevertheless have two drawbacks, namely, their price as a raw material, and sometimes, metallization difficulties.

Therefore, the adhesion between the polymer and the metal is sometimes achieved by means of an adhesive.

We know however from the patent
EP-A-0 149 394 metallized supports based on polypropylene using no adhesive and a method for manufacturing these supports.

Also known from the patent application
FR-A-2 651 953 metallized supports based on polymethylpentene and their manufacturing process.

 All of the above unexpanded materials can be metallized on one or both of their faces. The most widely used metal for this coating is generally copper which can be deposited by vacuum deposition techniques, chemical or electrochemical metallization or by hot plating under pressure, for example. These materials make it possible to produce radiating circuits or elements using etching or printing techniques for printed circuits, subject to certain specific precautions.

 Furthermore, materials are known which are in the form of rigid expanded foams such as the polymethacrylate imide, marketed for example under the registered brand name ROHACELL, vinyl products, etc. which exhibit due to their expansion. , a dielectric constant close to 1, that is to say close to that of air. These different materials could therefore be very interesting for the aforementioned applications.

However, there is no known in the prior art such expanded materials in metallized form. Indeed, the existing foams have a surface state and roughness often incomptabile with the definition of certain microwave circuits or certain radiating elements. However, the production of radiating elements on metallized expanded foams would be very interesting for the microwave domain, more particularly for frequencies lying around 109 Hertz.

 Currently, the achievements of metallized foam-based structures are obtained by using radiant circuitry or planes formed on non-expanded organic laminates and by interposing between them a non-metallized expanded foam. These stacks are then maintained by mechanical tightening means such as screws.

 The technical problem to be solved is to be able to coat expanded foams, with a metal several tens of micrometers thick, having a surface condition compatible with the definition of microwave circuitry. In addition, the foam material used must allow the use of conventional photogravure techniques of printed circuits.

 The present invention therefore relates to a metallized support comprising at least one support material of expanded, organic, rigid foam of which at least one of the faces is covered at least partially with a first film of organic thermoplastic material covered with a layer of conductive metal.

 This support overcomes the drawbacks of the aforementioned prior art, in particular by providing a metallized support having interesting electrical characteristics, namely a dielectric constant s and a low loss angle tg5 (tt; tg & 10 ~ 3 to 10-4), and a thickness of metal between a few micrometers and several centimeters. In addition, this support has a fairly low manufacturing cost due in particular to the use of conventional metallization and etching techniques. Finally, it can be used in a frequency range located around the GHz and can reach higher values for specific applications.

 According to a preferred embodiment of the invention, the conductive metal is chosen from copper or its alloys, aluminum or its alloys, the thermoplastic film is a polymer or copolymer of propylene and / or methylpentene and the rigid organic foam is made of vinyl polymer or polymethacrylate imide.

 In applications other than microwave, for example in decoration or construction, the choice of materials may be different.

 According to another alternative embodiment, the foam material comprises a film of thermoplastic material on at least part of each of its opposite faces, this film being covered with a layer of metal, and an electrically conductive connection passes right through. the foam support material and the thermoplastic films to connect the two metal layers together.

 The invention also relates to an assembly of at least two metallized supports in accordance with the invention, connected together by a second thermoplastic film having a softening temperature lower than that of the first thermoplastic film of the metallized support.

The invention also relates to a method of manufacturing a metallized support based on organic foam, comprising:
a) activate a first surface with a layer of
thermoplastic material so that it has
open channels,
b) placing said first surface opposite a
face of an expanded foam support material,
organic, rigid,
c) heat the assembly, under pressure, to the
softening temperature of the layer of
thermoplastic material,
d) cool the assembly to a temperature
slightly greater than or equal to the temperature
ambient, always under pressure,
e) place a metal layer over a second
surface of the thermoplastic material opposite to the
first, after this second surface has been
enabled to have open channels and
that the metal layer has undergone a treatment
of surface,
f) heat the assembly under pressure until the
material softening temperature
thermoplastic, and
g) cool the assembly to a temperature
slightly greater than or equal to the temperature
ambient, always under pressure.

 The production of these supports can be carried out, for example, by hot plating, that is to say by pressing or by hot calendering.

 The surface treatment of the metal layer is carried out by surface oxidation. In the case of copper, it can be subjected to a chemical oxidation treatment. In the case of aluminum, the initial treatment may be an anodic oxidation not followed by clogging.

 Other characteristics of the invention will emerge more clearly from the following description, given purely by way of nonlimiting illustration, this description being made with reference to the accompanying drawings in which - FIGS. 1 and 2 are schematic sectional views of a support according to the invention during two stages of its preparation, - Figures 3 and 4 are schematic sectional views of supports according to the invention in which the metallization is carried out respectively discontinuously or continuously, after or with foam shaping, - Figures 5 and 6 are schematic sectional views respectively representing two examples of assemblies of at least one support according to the invention with another type of support or of three supports according to invention.

 FIG. 1 illustrates an organic foam / thermoplastic film laminate corresponding to the first step of the process for manufacturing the support according to the invention. The support material 1 made of rigid organic foam is covered with a film of thermoplastic organic material 3, on at least one of these faces and at least partially, (see FIG. 3) or on its two faces as shown in Figure 1.

 When this laminate is intended for a radioelectric application, the choice of organic foam 1 is dictated by the search for advantageous electrical characteristics, namely a low dielectric constant preferably close to 1 (or between 1 and 3) and small angles losses, between 10-3 and 10O. Its resistance to chemical agents must allow it to undergo conventional or slightly modified cycles of the manufacture of printed circuits. Advantageously, this foam 1 can then be made of a polymethacrylate imide (for example for a density of 51; dielectric constant 1.07, losses 0.0002 at 2 GHz or dielectric constant 1.11, losses 0.0050 to 26 GHz ) or vinyl polymer (dielectric constant 1.07, losses 0.0016 between 8 and 12 GHz for a density of 50).

 On the other hand, if the use envisaged for this type of laminate is not a radioelectric application, but an application in the field of transport, building, or decoration, for example, the choice of foam can be widened. In fact, the foams used in microwave are generally more expensive because they are purer and the plates are sold with guarantees of greater dimensional accuracy. The chemical composition of the foam can be the same, the choice will then be made according to the mechanical or thermal qualities of the latter.

 Whatever the application envisaged, the foam 1 must be a rigid foam capable of withstanding temperatures equal to or greater than 1200C in order to be able to be used in the method described later. Preferably, this foam must withstand pressures of at least 104 Pa.

 To make this first laminate, the foam 1 of thickness at least equal to the thickness of the desired dielectric, is carefully cut to a format whose volume corresponds to that of the cavity of the pressing tool. This must be done with great care.

 The thermoplastic film 3 is also chosen according to the applications envisaged. Its dielectric constant must be as low as possible (E varying from 1 to 3) and its dielectric loss angle close to 10-4, if one wishes to carry out a radioelectric application. In this case, the film of thermoplastic material 3 is advantageously a polymer or a copolymer of propylene and / or methylpentene. Whatever the application envisaged, the softening temperature of this film 3 must be lower than the deformation temperature under load of the foam 1.

 The thickness of the film 3 can vary from a few micrometers to several millimeters.

 In the case of a non-radioelectric application, the choice of the material constituting the film can be widened.

 To improve the adhesion of the thermoplastic film 3 to the foam 1, the surface of this film must be activated by various techniques making it possible to form open chains on its surface. Among the surface treatment techniques that can be used, we will cite: the action of an oxidizing solution, flame treatment, treatment by corona discharge in a gaseous plasma, irradiation with ultraviolet radiation or electronic bombardment , for example.

 The assembly of the thermoplastic film 3 on the foam 1 can be carried out after the surface treatment of the film by a process using pressure heating. More specifically, this process can be implemented by hot calendering or using presses with heating and cooling plates, for example. The choice of temperature and pressure depends on the thermal and mechanical characteristics of the film 3 and of the foam 1.

The assembly cycle includes the following steps
1 ") temperature rise until the film 3 has softened, while maintaining a simple holding pressure between the film and the foam (approximately 105 Pa);
20) maintaining the bearing temperature for approximately 5 minutes and applying the selected manufacturing pressure;
30) cooling at a speed of approximately 100C per minute to a temperature of a few tens of degrees above ambient temperature (approximately 80 "C), with maintenance of the selected pressure;
4) gradual decrease in pressure;
50) opening of the press and demolding of the support covered with the film obtained.

 In the case of the production of a foam imide of polymethacrylate / polypropylene film, the temperature rise (step 1) takes place up to 1730C, for approximately 50 minutes and the manufacturing pressure applied is 106 Pa about.

 The same embodiment can be obtained by introducing the various constituents into a press already brought to a temperature close to the bearing temperature. In this case, the temperature rise is much faster, which shortens the total duration of the cycle.

 The laminate thus obtained is capable of receiving metallization.

 FIG. 2 illustrates the metallization step which is carried out by depositing on the film of organic thermoplastic material 3, a layer of metal 5, the latter completely or partially covering the film 3. The metal 5 deposited is preferably chosen from copper , aluminum, or one of their respective alloys.

 Before proceeding with the plating of the metal layer 5, it is necessary to subject the thermoplastic film 3 to a treatment intended to activate its surface and create open chains.

This surface treatment of the face of the film intended to receive the metal is carried out by one of the techniques previously described for treating its opposite face.

 The choice of metal depends on the intended applications. For using the metallized support as a circuitry support for microwave, the metal used is preferably copper with a thickness ranging from 10 to 35 micrometers approximately. For other applications, this copper strip will advantageously be replaced by thicker copper plates or by aluminum or aluminum alloy plates.

 In all cases, it is necessary to subject the metal to a surface treatment which consists of a surface oxidation of the latter allowing the creation of physical and molecular bonds with the thermoplastic film 3.

 Oxidation of copper can for example be obtained after degreasing in a solvent and optionally pickling by a mechanical or chemical method, by immersion, for ten minutes, in an aqueous solution at 850C comprising 30 g / l of chlorite of NaClO2 soda, 10 g / l of NaPO4H2O trisodium phosphate, 50 g / l of NaOH soda and demineralized water. The metal strips or plates 5 are then carefully dried, not without having been rinsed beforehand.

 In the case of aluminum or its alloys, the surface treatment promoting adhesion to the thermoplastic film 3 can be any technique making it possible to promote the adhesion of the metal to the film. However, a simple technique to implement consists in producing on the metal a layer of Al 2 O 3 alumina by anodic oxidation, but not followed by clogging so that the layer of alumina remains with open pores. The thickness of the alumina layer is generally between 5 and 10 micrometers, although this criterion is not imperative. This treatment is also followed by rinsing and drying before the strip or the metal plate 5 is applied to the thermoplastic film 3.

 The pressing of the metal 5 on the foam laminate 1 / film 3 is carried out by the same series of pressing and temperature raising operations as that described above for the manufacture of the laminate.

 It will also be noted that the operations of pressing the metal on the foam laminate 1 / film 3 and pressing the thermoplastic film 3 on the foam 1 can also be done simultaneously.

 It is also possible to make an electrically conductive connection 6 between two opposite metal layers 5 by a method described later.

 Finally, later, the metal layer 5 can be etched as shown in the upper part of FIG. 2.

 The foam 1 can be shaped under hot pressure. It is therefore interesting to be able to metallize foams of different shapes. Thus, it is possible to metallize discontinuously different levels of a foam as shown in FIG. 3 or on the contrary to metallize the surface of the foam support 1, continuously along different levels, as shown in figure 4.

 In the first case illustrated in FIG. 3, the organic foam 1 is first of all shaped by hot pressing in a mold corresponding to the desired shape. Then placed at the places intended to be metallized, a film of thermoplastic material 3, for example polypropylene, which is then covered with a layer of metal, for example a copper strip, this film and this strip having been prepared. as previously described. The assembly is then subjected to hot pressing either in the mold previously used to form the foam 1, or in another mold which takes account of the thicknesses of the film and of the metal to be fixed on the foam.

 In the second case illustrated in FIG. 4, the film 3 of thermoplastic material and the metal layer 5 are placed on a non-preformed foam 1.

The assembly is then subjected to a hot pressing cycle in a mold adapted to the desired result. In this case, it is necessary that the depth of the mold recesses be reduced so as to avoid any breakage of the metal strip. It is also advisable to use a very ductile metal (for example copper).

 It is also possible to produce multilayer supports as illustrated in FIGS. 5 and 6. A laminate of the type of that shown in FIG. 2, one or both of the metal faces of which are engraved or not, can thus be associated either with a another laminate of the same type, either to another engraved laminate or not of the conventional printed circuit type, or to a thermoplastic dielectric.

 In the embodiment illustrated in FIG. 5, a laminate comprising a support material 1 made of foam, covered on one of its faces by a first thermoplastic film 3 and a layer of metal 5 is bonded, via a second thermoplastic film 3 ', to an insulating support 7 covered with a metal layer 5', engraved or not. The 5 ′ metal is preferably chosen from copper, aluminum or one of their alloys. It is not necessarily of the same nature as the metal layer 5.

 The thermoplastic bonding film 3 ′ must have a softening temperature lower than that of the first film 3 to prevent the new pressing operation (during assembly) from deforming the already assembled layers and more particularly the etched layers. The 3 'bonding film is also chosen according to its application and in the case of radioelectric applications, it must have a low dielectric constant (e = 1 to 3). It may for example be a polypropylene-ethylene copolymer.

 The surface treatment operations of the various layers and of assembly are identical to what has been previously described, however the temperature plateau is around 1680C in the case where the film 3 'is made of polypropylene-ethylene copolymer.

 Furthermore, it is possible (and even desirable in the case of stacks) to produce electrically conductive connections 9 between the different metal layers 5, 5 '. These connections 9 are produced by conventional methods, such as the metallized hole method of printed circuits, the method of metal inserts or the method of recharging using a conductive ink.

 In an embodiment illustrated in FIG. 6, three metallized supports according to FIG. 2 have been assembled. In this case, the central metallized support has a thermoplastic bonding film 3 'which makes it possible both to fix the metallic layer 5 on the foam support material 1 and to assemble several metallized supports together. In addition, the metal layers 5 of each laminate are linked together by a conductive connection 9.

Claims (17)

 1. Metallized support comprising at least one support material (1) of expanded, organic, rigid foam of which at least one of the faces is covered at least partially with a first film of organic thermoplastic material (3) covered with a layer (5) of conductive metal.  2. Support according to claim 1, characterized in that the foam material (1) comprises a film of thermoplastic material (3) on at least part of each of its opposite faces, this film being covered with a layer of metal (5), and in that an electrically conductive connection (6, 9) passes right through the foam support material (1) and the thermoplastic films (3) to connect the two layers of metal (5).  3. Support according to claim 1 or 2, characterized in that the metal (5) is chosen from copper, aluminum or one of their respective alloys.  4. Support according to claim 1 or 2, characterized in that the foam material (1) must withstand temperatures equal to or greater than about 1200C.  5. Support according to claim 1, 2 or 4, characterized in that the thermoplastic organic material (3) has a softening temperature lower than the deformation temperature under load of the foam (1).  6. Support according to any one of the preceding claims, characterized in that it is obtained by hot plating of its various constituents (1, 3, 5).  7. Support for microwave circuits according to any one of claims 1 to 6, characterized in that the foam (1) has a dielectric constant ranging from 1 to 3 and a dielectric loss angle ranging from 10-3 to 10-4 .  8. Support according to claim 7, characterized in that the thermoplastic film (3) has a dielectric constant ranging from 1 to 3 and an angle of dielectric losses of approximately 10-4.  9. Support according to any one of the preceding claims, characterized in that the thermoplastic material (3) is a polymer or a copolymer of propylene and / or methylpentene.  10. Support according to any one of the preceding claims, characterized in that the foam (1) is made of polymethacrylate imide or of vinyl polymer.  11. Assembly of at least two supports according to any one of the preceding claims, characterized in that said supports are interconnected by a second thermoplastic film (3 ') having a softening temperature lower than that of the first thermoplastic film ( 3).  12. Method for manufacturing a metallized support based on organic foam, consisting of  a) activating a first surface of a layer of thermoplastic material (3) so that it comprises open chains,  b) placing said first surface opposite one face of a rigid, organic expanded foam support material (1),  c) heating the assembly, under pressure, to the softening temperature of the layer of thermoplastic material (3),  d) cooling the assembly to a temperature greater than or equal to ambient temperature, always under pressure,  e) placing a metal layer (5) on a second surface of the thermoplastic material (3) opposite the first, after this second surface has been activated to have open chains and the metal layer has undergone a surface treatment,  f) heating the assembly, under pressure, to the softening temperature of the thermoplastic material (3), and  g) cool together to a temperature greater than or equal to room temperature, always under pressure.  13. Method according to claim 12, characterized in that the steps of assembling the foam material (1), the thermoplastic material (3) and the metal layer (5) are carried out simultaneously.  14. Method according to claim 12 or 13, characterized in that it comprises a step of shaping the organic foam support (1).  15. Method according to claim 12 or 13, characterized in that the surface treatment of the metal layer (5) is carried out by surface oxidation.  16. The method of claim 12 or 13, characterized in that it comprises a step of etching the metal layer (5).  17. The method of claim 12 or 13, characterized in that it comprises a step of forming a layer of thermoplastic material (3) and a metal layer (5) on at least part of the two opposite faces of the foam support material (1) and the formation of an electrically conductive bond (9) between the two metal layers (5).