FR2987545A1 - Multi-layer printed circuit structure for antenna of airborne radar, has layer whose metalized face is provided with electronic component, where metalized faces of layer are assembled with metalized faces of another layer - Google Patents

Abstract

The structure has a layer (CM1a) including first and second machined metalized faces. Another layer (CM2) comprises first and second machined metalized faces, where the first metalized face of the latter layer is provided with an electronic component. The metalized faces of the former layer are assembled with the metalized faces of the latter layer. The latter layer includes a track made of copper. The layers are attached with each other. Subsets (SE3a, SE3b) are metalized on the first or second metalized faces of the former layer. An independent claim is also included for a method for realizing a multi-layer printed circuit structure.

Description

BACKGROUND OF THE INVENTION The invention relates to the field of printed circuit boards, and more specifically to a multi-layer printed circuit board comprising integrated low dielectric loss transmission lines and a method of the present invention. associated development. The antennas of some airborne radars electronically scan the beam in a single plane. In the plane where the beam is fixed, to reduce costs, sub-networks of N radiating elements are fed with a single active channel, via a 1: N distributor, a distributor consisting of a set dividers connected together by sections of transmission line. To improve the transmit and receive performance of the antenna, it is essential to minimize the frontal losses between the active channel and the N radiating elements, that is to say the losses of the distributor. A partial solution was provided by French patent FR 2 496 996. This document describes a microwave transmission line 20 comprising two parallel conductive plates, spaced apart from each other, and electrically connected to one another. The space separating the two plates being filled with air, serving as a dielectric material, and a conductive central ribbon interposed between the two plates. Several pieces of dielectric material, distributed along each side of the ribbon and forming a support, are integral with the two plates. Each support piece has a notch in each of which is positioned the ribbon so as to be held in place. A disadvantage of the embodiment proposed in document FR 2 496 996 is the difficulty of implementing this method for a printed circuit board with a multilayer structure of dielectric materials. Previously, to overcome these drawbacks, a dielectric triplate technology compatible with a multilayer structure printed circuit board containing active components and radiating elements was used. The dielectric used was a commercial substrate. It had the advantage of a low cost.

It had the disadvantage of high dielectric losses, of the order of 10 dB / m, which degraded the transmission (EIRP) and reception (G / T) performance of the antenna. Another solution is proposed by N. Ehsan et al. "Broadband 5 Micro-Coaxial Wilkinson Dividers", Transactions on Microwave Therapy and Techniques, Vol. 57, No.11, November 2009. This document describes a multilayer structure printed circuit board including a distributor. The multilayer printed circuit comprising an integrated transmission line is made with "polystrata" technology (registered trademark), the dielectric material is replaced by air. In principle, this involves adapting the triplate technology to a multilayer structure of organic material. The "polystrata" technology consists of sequentially depositing layers of copper and photoresist on a silicon wafer. The transmission line is supported by a beam of dielectric material every 700 μm. This type of multilayer structure comprising a conductive element is described in patent application US 2004/0263290 and in the publication of Anthony A. Immorlica Jr. et al. The aim of the invention is to develop a printed circuit comprising a transmission line with low dielectric losses integrated into a circuit board. multilayer structure, comprising several elementary circuits, at low cost. For this, the inventors have used techniques described in patent applications EP1350418 and EP2205053. The patent application EP1350418 describes an interconnection method in a multilayer printed circuit. The process can be summarized as follows. In a first step, metallized holes are made. In a second step, the holes that must be electrically connected are covered with a metal interface. In a third step, the holes and the elements to which they must be electrically connected are covered with a component of a metal alloy. A hole being covered with a first component and the metal interface of the element to be electrically connected being covered with a second component of the metal alloy. The two metal components being brought into contact during the pressure exerted on the stack to form the multilayer printed circuit. In a fourth step, the assembly is heated to form an alloy by thermodiffusion. The patent application EP 2205053 describes the development of a printed circuit board comprising at least two superimposed elementary circuits metallized on their two faces and at least one interlayer disposed between said circuits. The interlayer comprises a thermoplastic material which is insensitive to chemical elements. With this type of interlayer, the risk of creep of the thermoplastic material is zero. The method of producing the printed circuit board, according to EP 2 205 053, can be summarized as follows. A first step is to perforate said elementary circuits and the interlayer vis-à-vis to connect said circuits together. A second step is to line the interior of the holes through the elementary circuits and the interlayer, by a metal surface. A third step is to cover the light holes of the elementary circuits to be connected, by a first metal, and the light of the holes of the intermediate layer, by a second metal. The first and the second metal are the constituents of an alloy. After stacking, a fourth step is to apply a pressure to braze the metal surfaces that cover the holes of the elementary circuit holes with the metal surfaces that cover the holes of the holes of the interlayer. The solder is by diffusion of the metals which then form an alloy. According to one aspect of the invention, there is provided a printed circuit of multilayer structure comprising: - a first layer comprising a first metallized face and a second machined face, so as to form cavities, and metallized, and - a second layer comprising a first metallized face comprising an electronic component and a second metallized face, the machined face of the first layer is connected to one of the two faces of the second layer.

The introduction of an electronic component inside a cavity integrated in a multilayer structure makes it possible to limit the dielectric losses of this component. According to another embodiment, there is provided a printed circuit in which the second layer comprises a track comprising copper. This embodiment makes it possible to produce a transmission line with low dielectric losses integrated into a multilayer structure. According to another embodiment, there is provided a printed circuit in which a second first machined layer is assembled with the second layer. This embodiment allows the circulation of a fluid, in particular air, under the electronic component thus enabling the thermal regulation of said component. According to another embodiment, there is provided a printed circuit 15 further comprising at least a first subset metallized on at least one face and assembled to at least a first layer. According to another embodiment, there is provided a printed circuit further comprising a dielectric material layer over 5 to 20% of the total length of the second layer so as to create bridges. According to a variant, the printed circuit further comprises a layer (CM3) of dielectric material over 5 to 10% of the total length of the second layer (CM2) so as to create reinforcements. This embodiment allows the mechanical maintenance of the track 25 comprising copper. According to another embodiment, there is provided a printed circuit, wherein at least one of the assemblies is made by thermodiffusion. The thermodiffusion assembly, as described in the patent application EP1350418, makes it possible to avoid the risks of creep due to the adhesive film 30 and solves the problems of limitations of configurations of Z connections between the various elementary circuits.

According to another embodiment, there is provided a printed circuit, in which a shielding by covering the walls of the cavity by a metallization layer and / or metallized holes on the sides of the printed circuit.

According to another aspect of the invention, there is provided a method for producing a transmission line with low dielectric losses integrated into a printed circuit board with a multilayer structure and comprising the following steps in which: a metal plate is assembled with a first facing a first layer comprising a first dielectric material and metallized on the second face, to form a first subset, - the first layer is machined to create a cavity. The use of a metal plate allows mechanical maintenance during all the manufacturing steps of the printed circuit. According to another aspect of the invention, there is provided a method in which the first layer is machined to create a channel. The machining of the first layer to create a channel allows to include a particular track including copper.

According to another embodiment, there is provided a method, further comprising a step in which the second metallized face of the first layer of the first subset is assembled with a first face of a second layer, comprising a dielectric material supporting at least one electronic component, to form a second subset. This embodiment makes it possible to integrate electronic components inside a cavity so as to limit the dielectric losses of this component, and in particular a transmission line. In another embodiment, there is provided a method wherein the second layer comprises a central track comprising copper. According to another embodiment, there is provided a method in which a third layer comprising a dielectric material is assembled with the second layer over 5 to 20% of the length of the second layer at regular intervals to create bridges. According to a variant there is provided a method in which a third layer (CM3) comprising a dielectric material (M3) is assembled with the second layer (CM2) over at least 5 to 10% of the total length of the second layer (CM2). ) at regular intervals to create reinforcements. This embodiment makes it possible to create reinforcements so as to maintain in particular the copper track. According to another embodiment, there is provided a method which further comprises a step in which a second first subset is assembled with the metallized face of the second layer of the second subassembly. This embodiment allows the circulation of fluid and in particular air under the electronic component so as to ensure a thermal regulation. According to another embodiment, there is provided a method in which the metal plates of the first and second first subassemblies are machined. According to a variant, the metal plates of the first and second subassemblies (SE1a; SE1b) are machined partially or totally. According to another embodiment, there is provided a method which further comprises a step in which the free face of at least a first subset is assembled with a subset. The invention thus makes it possible to create, at lower cost, a multilayer printed circuit board comprising at least one cavity or channel enclosing an electronic component or a transmission line with low dielectric losses. It also makes it possible to create configurations of Z connections between the elementary circuits.

The invention will be better understood from the study of some embodiments described by way of non-limiting examples, and illustrated by the appended drawings in which: FIG. 1 shows an embodiment of a method for producing a a cavity integrated in a printed circuit of multilayer structure, according to one aspect of the invention, and - Figures 2a, 2b and 2c illustrate the various steps of producing a multilayer structure printed circuit comprising a cavity enclosing an electronic component according to one aspect of the invention, and FIGS. 3a, 3b, 3c and 3d illustrate the various steps of producing a multilayer structure printed circuit comprising a channel enclosing a transmission line, according to one aspect of the invention, and FIG. 4 shows a multilayer printed circuit board comprising a transmission line with low dielectric losses, according to one aspect of the invention. FIG. 1 shows a method for producing a Cav cavity integrated in a printed circuit board C1 with a multilayer structure, according to one aspect of the invention. In a step 10, a face of the first layer CM1 comprising a dielectric material M1 is assembled with a metal plate PM to form a subset SE1a. The material M1 may comprise "RO 4003" (registered trademark) of 0.5 mm thickness, for example. The assembly is made from a glue film, type "4350" (registered trademark), for example. In a second step 20, the first CM1 layer is machined to create a cavity Cav and thus form a first subset SE1a 30 machined. Machining can be achieved either mechanically using a z-axis controlled trimming machine or laser radiation. Laser radiation makes it possible to have a good control of the machining depth. It is possible to combine the two machining modes: a mechanical pre-machining to eliminate a majority of the insulation followed by a laser machining so as to better control the operation. The machining of the first CM1 layer to create a cavity or a channel can lead in some cases to the formation of areas of independent materials M1, difficult to manipulate. The metal plate PM, preferably a copper plate with a thickness of 500 .mu.m, serves as mechanical support during all the steps of producing the cavity integrated in the multilayer structure.

Optionally, the walls of the cavity Cav formed may be metallized to effect electrical shielding of said cavity Cav. In a third step 30, a second CM2 layer is assembled, comprising a dielectric M2 material and supporting at least one electronic component CE, placed vis-à-vis the cavity Cav, with the subset SE1a machined. The assembly can be made, for example, from a glue film or by thermodiffusion. If the assembly is carried out by thermodiffusion, the machined face of the subset SE1a is then covered with a layer of copper and a first metal A, and the face of the layer M2 in contact with the subset SE1a. is covered with a layer of copper and a second metal B. The metals A and B are constitutive of the first alloy AxBy. Typically, the alloy is made of gold and tin or silver and tin. The electronic component may be etched on the second CM2 layer or inserted within Cav Cav. Optionally, the PM metal plate may be thinned totally or partially to serve as a ground plane for the final device. FIGS. 2a, 2b and 2c illustrate the steps of the method of producing a cavity integrated in a multilayer structure containing an electronic component. FIG. 2a illustrates a subset SE1a comprising the assembly of a metal plate PM, preferably comprising copper and a first layer CM1 comprising a dielectric material M1. The assembly is made by gluing, for example, from a "4350" type adhesive film (registered trademark). FIG. 2b illustrates the subassembly SE1a after machining the first CM1a layer to create a cavity Cav. The machined face of the subset SE1a is covered with a layer of copper and a layer of the first metal A constituting the first alloy AxBy. Optionally, the walls of the cavity Cav are covered with a metallization layer in order to carry out the shielding thereof. Advantageously, the shielding layer comprises a layer of copper and a layer of the first metal A. FIG. 2c shows the assembly of the subset SE1a machined with a second layer CM2 comprising a dielectric material M2 and supporting at least one electronic component THIS. The assembly can be carried out by gluing or by thermodiffusion. If the assembly is carried out by thermodiffusion, the second layer CM2 is covered on at least one of its two faces with a layer of copper and a layer of the second metal B, constituting the binary alloy AxBy. This process for producing a cavity Cav integrated in a printed circuit board C1 of multilayer structure makes it possible to electrically connect electronic components CE, such as resistors, or active components located inside the printed circuit board Cl. The assembly thus produced can then be integrated in a more complex multi-layer circuit board C1, either by adding more elementary circuitry in the stack or by increasing the dimensions of the multilayer structure along the x and / or y axes. FIGS. 3a, 3b, 3c and 3d illustrate the steps of the method for producing a channel Can, comprising a transmission line L integrated into a multilayer structure. Figures 3a to 3c show the process described in Figures 2a to 2c.

However, the CM1 layer of the subset SE1a is machined to form a channel Can (Figure 3b) and the second layer CM2 supports a transmission line L vis-à-vis the Can channel. Typically, the thickness of the rectangular section copper track 5 is between 50 and 200 μm and the track width is optimized to obtain a characteristic impedance of 50 ohms. The second layer CM2 can be: either a commercially available organic substrate with low dielectric losses, such as a "RO 4003" (registered trademark) of 100 μm in thickness, liquid crystal polymers of 50 μm in thickness, or 25 "thick" kapton "(registered trademark) or of the same type as the cavity substrate but with a lower thickness, - or a substrate obtained from a Fcol adhesive film, of a thickness between 30 and 100 pm, resting on a thin organic substrate 15 machined in the vicinity of the track, to reduce losses. With the second CM2 layer consisting of an adhesive film Fcol on an organic substrate, lower dielectric losses are obtained than with the thin dielectric substrate. However, this type of layer has less rigidity. In a severe vibration environment, such as that of airborne applications, it is possible for the line L to vibrate relative to the ground planes of the printed circuit C1, which can lead to degradation of the performance of the transmission lines L. To stiffen the second layer CM2 supporting the transmission line L, it is necessary to add reinforcement bridges R serving as a support for the transmission line L. Thus, the organic substrate is not completely hollowed out so as to create R bridges at intervals regular. To preserve the interest of the reduced dielectric losses, these R bridges are made over a small fraction of the total length of the line, typically 5 to 20%. Figure 3d illustrates the assembly of the subassembly SE1b and a second subassembly SE1a machined. The assembly can be carried out by gluing or by thermodiffusion.

The printed circuit board C1 of multilayer structure comprising an integrated transmission line L, according to one aspect of the invention, allows the circulation of the air under the transmission line L so as to thermally regulate the transmission line L. The dielectric losses are linear in the order of 5 dB / m in the X band, from 8000 to 12000 MHz, a decrease of half in dB / m compared to solid dielectric triplate lines with commercial dielectric substrates. The example describes the realization of transmission lines with low dielectric losses but it is also possible in the same way to create integrated cavities for different applications such as suspended filters or active or passive components. FIG. 4 represents a printed circuit C1 of multilayer structure comprising a transmission line L with low dielectric losses according to the method described above.

The printed circuit Cl is composed of five subsets. A first subset SE3a comprises a stack of N elementary circuits. The elementary circuits are electrically connected to each other via metallized holes allowing configurations of Z connections. One of the two faces of the subassembly SE3a is covered with a first metal A constituting a binary alloy AxBy. A first layer CM1 comprising a material M1 comprises a channel on one of its faces. The two faces of the first layer CM1 are covered with a layer of copper and a layer of a second metal B, constituting the alloy AxBy. Typically the alloy comprises gold and tin or silver and tin. The metallized face of the first subassembly SE3a and the unmachined face of the first layer CM1a are assembled, to form a sub-assembly SE2, by thermodiffusion to form the alloy AxBy. A second layer CM2 comprises a thin organic substrate CM3 and an adhesive film Fcol, on which is engraved a transmission line L vis-à-vis the channel Can formed in the subset SE1a.

To stiffen the second layer CM2 supporting the transmission line L, the organic substrate CM3 is not completely hollowed out so as to create R bridges acting as reinforcements at regular intervals. To preserve the interest of the reduced dielectric losses, these bridges R are made over a small fraction of the total length of the line, typically 5 to 20%. The CM2 layer is covered with a copper layer comprising the first metal A on both sides. A second machined first second layer CM1b is covered on both sides by a copper layer and a layer of a second metal B. The second first layer CM1b is assembled with the other side of the CM2 layer by thermodiffusion. A second first subassembly SE3b coated on at least one of its faces with a copper layer and a layer of a first metal A is assembled with the unmachined surface of the second first layer CM1b by thermodiffusion. . Optionally, the assembly by thermodiffusion of the entire printed circuit Cl can be done at one time. The integrated low dielectric loss transmission line L is shielded at the sides by metallized holes. The metallized holes have a diameter of 0.3 mm advantageously. The usual implantation rules impose a minimum distance of 0.25 mm to 0.30 mm between the walls of the cavity and the walls of the plated holes, preferably. The metallized holes may be replaced by continuous metal walls if the printed circuit fabrication technology permits. The invention thus produced allows the production of multilayer printed circuit board comprising electronic components, transmission lines or suspended filters with low dielectric losses at lower cost.

Of course, the invention is not limited to this embodiment. It is possible to assemble several circuits type "air plate" according to the invention. Moreover, the assembly of the different layers by thermodiffusion makes it possible to manage the configurations of connections in Z.

Claims (1)

REVENDICATIONS1. Printed circuit board (CI) of multilayer structure characterized in that it comprises: a first layer (CM1a) comprising a first metallized face and a second machined and metallized face, and a second layer (CM2) comprising a first metallized face comprising an electronic component (CE) and a second metallized face, the machined face of the first layer (CM1a) being joined to one of the two faces of the second layer (CM2). The printed circuit board of claim 1 wherein the second layer (CM2) comprises a track comprising copper. Printed circuit board according to one of Claims 1 or 2, in which a machined second second layer (CM1b) is assembled with the second layer (CM2). Printed circuit according to one of claims 1 to 3 further comprising at least a first subassembly (SE3a; SE3b) metallized on at least one face and assembled to at least a first layer (CM1a; CM1b). The printed circuit according to one of claims 1 to 4 further comprising a layer (CM3) of dielectric material on 5 to 10% of the total length of the second layer (CM2) so as to create reinforcements. Circuit board according to one of claims 1 to 5 wherein at least one of the assemblies is made by thermodiffusion. Printed circuit board according to one of Claims 1 to 6, comprising a cavity-covering shield (Cav) by a metallization layer and / or metallised holes Tm on the sides of the printed circuit (Cl) .2. 3. 4. 5. 6. 7.308. Process for producing a printed circuit according to one of the preceding claims, characterized in that it comprises the following steps: - a metal plate (PM) is assembled with a first face of a first layer (CM1) comprising a first M1 dielectric material and metallized on the second side, to form a first subset (SE1a), - the first layer (CM1) is machined to create a cavity (Cav). 9. The method of claim 8, wherein the first layer (CM1) is machined to create a channel (Can). 10.Procédé according to one of claims 8 or 9, further comprising a step in which is assembled the second metallized face of the first layer (CM1) of the first subset (SE1a) with a first face of a second layer (CM2), comprising a dielectric material (M2) supporting at least one electronic component (CE), to form a second subset (SE2). 11.Procédé according to one of claims 8 to 10, wherein a third layer (CM3) comprising a material (M3) dielectric with the second layer (CM2) is assembled on at least 5 to 10% of the total length of the second layer (CM2) at regular intervals to create reinforcements. 12.Procédé according to one of claims 8 to 11, wherein the second layer (CM2) comprises a central strip comprising copper. 13.Procédé according to one of claims 8 to 12, further comprising a step in which a second first subset (SE1b) is assembled with the metallized face of the second layer (CM2) of the second subset ( SE2). 14.Procédé according to one of claims 8 to 13, wherein the metal plates of the first and second first subset (SE1a; SE1b) are machined partially or completely.15.Procédé according to one of claims 8 to 14, further comprising a step in which the free face of at least a first subset (SE1a; SE1b) is assembled with a subset (SE3). 16.Procédé according to one of claims 8 to 15 wherein at least one of the assemblies is made by thermodiffusion.