Coaxial microwave interposer.

Abstract

The invention relates to a coaxial microwave interposer intended to be disposed between two heterogeneous elements, comprising: a printed circuit board (1); at least one cylindrical diffuse contact (2), inserted into a hole (3) in a plate (4) forming a wedge and insulating guide for the diffuse contact (2), the diffuse contact (2) opening into contact with a portion (1a) of the printed circuit board at one end (2a), and opening out of the hole (3) at its other end (2b); an RF trace (5) in the printed circuit board (1) opening at one end (5a) into the bottom of the hole (3) of the plate (4) opposite the diffuse contact (2), and configured to be in contact with the diffuse contact (2), and opening at its other end (5b) into the external face of the printed circuit board (1) located outside the interposer;and a cylindrical ground via (6) disposed in the plate (4) coaxially around the diffuse contact (2), opening onto one face of the plate (4), and opening onto the other face of the plate (4) being configured to be electrically connectable via the external face of the printed circuit board (1) located outside the interposer. Figures for the abstract: [Fig.1], [Fig.2];

Description

Title of the invention: Microwave coaxial interposer.

[0001] The invention relates to a coaxial microwave interposer.

[0002] The invention is intended for electrical interconnections in a frequency range from direct current (DC) to microwave frequencies (40 GHz and above). It applies to all types of board-to-board or board-to-metal element electrical connections (such as amplifiers, filters, combiners, radiating elements, circulators, isolators, etc.). It is particularly useful for amplification, transposition or active antenna functions for which signal density and integration constraints are high and when the associations between functions require the management of different expansion coefficients.

[0003] To develop a clamped interconnection amplifier based on a modular principle, it is necessary to be able to connect the masses of the modules while being removable. Without this, performance cannot be achieved.

[0004] Several solutions have been tested: - Use of a braided O-ring; - Use of gluing (epoxy glue loaded with silver); - Use of a pyrolytic graphite sheet shim (PGS); and - Use of a conductive seal type GT1000 from getelec

[0005] None of these solutions work because of the RF contact quality and associated losses. A clamped interconnection is not feasible in this case.

[0006] It is also necessary to be able to pass RF signals from one board to another (boards stacked on top of each other in the driver module). Spacing constraints are imposed by the DC board-to-board connector and it is difficult to find an off-the-shelf solution that meets exactly the same spacing, and this, in a constrained footprint.

[0007] Finally, in the context of an active antenna in Ka band (31GHz), it is necessary to associate a card or printed circuit integrating one or more components of beam-forming integrated circuits or BFIC for acronym of "Beam-Forming In-tegrated Circuit" in English, with the metallic radiating elements, the differential expansion of these elements not allowing the use of conventional soldering. The assembly is called a BFIC card.

[0008] The main solutions for addressing the problems mentioned above are conventional solutions for which the inventions have fallen into the public domain. These solutions implement either flexible materials (silver-filled epoxy glue or soft solders), or connector principles coaxial (SMP for acronym of "Sub Miniature Push-on" in English with "spring bullets" or spring beads in English.

[0009] Solutions based on soft conductive epoxy glue loaded with silver cannot be used because the losses are too high, leading to heating and then vaporization of the glue when the RF power is higher than one watt. Furthermore, in the case of an active antenna, it is impossible to correctly dose the quantity of glue required (in the case of a viscous solution). This process would not be industrial. It would then be necessary to use a conductive epoxy glue loaded with silver with a preform or "preform glue" in English (ablefilm or ablestick 5025E or equivalent); however, RF contacts, which have a diameter of around 0.5 mm in Ka band, do not allow this. The glue deteriorates on very small surfaces and tends to crack over time.

[0010] Soldered solutions also do not work in an industrial production flow because on the BFIC card, the components are wired on one side only, which causes buckling of the circuit: the contact quality would then not be controlled with the radiating elements. Added to this is the fatigue linked to differential thermal expansion constraints. The elements cannot be dismantled.

[0011] Sintered solutions cannot be used because the re-melting profile of components mounted on a BFIC card or on an RF module is incompatible with temperatures used in the sintering process.

[0012] Solutions connected coaxially using SMP spring bullets are too bulky, too expensive and induce too high losses.

[0013] Contact pressure solutions, or "clamp" in English, are impossible to implement in certain cases (especially on an active antenna interposer or an RF ground contact of an amplifier module).

[0014] One aim of the invention is to overcome the problems mentioned above.

[0015] According to one aspect of the invention, there is provided a coaxial microwave interposer intended to be arranged between two heterogeneous elements, comprising: - a printed circuit; - at least one diffuse contact of cylindrical shape, inserted into a hole in a plate forming a wedge and insulating guide for the diffuse contact, the diffuse contact opening into contact with a portion of the printed circuit at one end, and opening out of the hole at its other end; - an RF track in the printed circuit opening at one end at the bottom of the hole in the plate opposite the diffuse contact, and configured to be in contact with the diffuse contact, and opening at its other end on the external face of the printed circuit located outside the interposer; and - at least one cylindrical copper ground via placed in the plate of coaxially around the diffuse contact, opening onto one side of the plate, and opening onto the other side of the plate while being configured to be electrically connectable by the external side of the printed circuit located outside the interposer.

[0016] In a printed circuit, a via is a metallized hole which allows an electrical connection to be established between two or more layers.

[0017] In one embodiment, the printed circuit is single-layer.

[0018] Alternatively, the printed circuit is multi-layer.

[0019] According to one embodiment, the interposer comprises a layer of electrically conductive epoxy glue disposed between the plate and the printed circuit.

[0020] In one embodiment, the interposer comprises an RF track in the printed circuit opening at one end opposite the cylindrical via, and opening at its other end onto the external face of the printed circuit located outside the interposer.

[0021] For example, the plate is made of PTFE for metallized polytetrafluoroethylene or a low-loss ceramic or glass / PTFE composite RF laminated substrate.

[0022] According to one embodiment, the plate is composed of at least one layer of the printed circuit.

[0023] In one embodiment, the cylindrical ground via opens onto the external face of the printed circuit located outside the interposer.

[0024] According to another aspect of the invention, there is also provided an active antenna comprising:

[0025] - at least one radiating element;

[0026] - a printed circuit provided with at least one BFIC component;

[0027] - an interposer according to one of the preceding claims arranged between the element radiant and the printed circuit;

[0028] - a layer of electrically conductive epoxy glue between the radiating element and the interposer plate; and

[0029] - solder balls between the BFIC component and the external face of the circuit printed (B GA case).

[0030] The invention will be better understood by studying some embodiments described as non-limiting examples and illustrated by the appended drawings in which:

[0031] [Fig.l] schematically illustrates a coaxial microwave interposer, according to one aspect of the invention; and

[0032] [Fig.2] schematically illustrates another coaxial microwave interposer, according to one aspect of the invention.

[0033] [Throughout the figures, elements having identical references are similar.

[0034] The present invention is based on the use of diffuse contacts originally intended to produce flexible card-to-card contacts. Such a diffuse contact is composed of interlaced metal wires composed, for example, of a gold-plated copper and beryllium alloy.

[0035] This interwoven structure provides adequate elasticity and excellent contact quality when properly compressed (typically 30% compression ratio). Such contacts are custom-manufacturable with diameters ranging from 0.25 mm to 7 mm and lengths greater than 1 mm up to 7 mm.

[0036] The invention is therefore an assembly which implements this type of contact either in an assembly of RF modules or active antennas (or any other type of cards or heterogeneous assemblies intended to be stacked with an RF contact which can go up to 40GHz).

[0037] [Fig.l] schematically illustrates a coaxial microwave interposer intended to be arranged between two heterogeneous elements according to one aspect of the invention.

[0038] The coaxial microwave interposer intended to be arranged between two heterogeneous elements comprises a printed circuit 1 and at least one diffuse contact 2 of cylindrical shape, inserted in a hole 3 in a plate 4 forming a wedge and insulating guide for the diffuse contact 2, the diffuse contact 2 opening into contact on a portion 1a of the printed circuit at one end 2a, and opening out of the hole 3 at its other end 2b.

[0039] The coaxial microwave interposer also comprises an RF track 5 in the printed circuit 1 opening at one end 5a at the bottom of the hole 3 of the plate 4 opposite the diffuse contact 2, and configured to be in contact with the diffuse contact 2, and opening at its other end 5b on the external face of the printed circuit 1 located outside the interposer.

[0040] . The microwave coaxial interposer further comprises a copper via cy ground contact 6 arranged in the plate 4 coaxially around the diffuse contact 2, opening onto one face of the plate 4, and opening onto the other face of the plate 4 while being configured to be electrically connectable by the external face of the printed circuit 1 located outside the interposer.

[0041] This coaxial microwave interposer of [Fig.l] is represented between two elements, in this case an active antenna radiating element 7 and a card 1 provided with a BFIC component 8 in BGA format for the acronym "bail grid array" in English, i.e. provided with solder balls 9.

[0042] In the example of [Fig.l], the printed circuit 1 is multi-layer, but could alternatively be single-layer.

[0043] For example, the microwave coaxial interposer comprises a layer of electrically conductive epoxy glue 10 disposed between the plate 4 and the printed circuit 1.

[0044] The epoxy glue used is loaded with silver and preferably in the form of preform, which guarantees the thickness of the glue (unlike a viscous formulation).

[0045] In the example of [Fig. 1], the interposer according to one of the preceding claims, comprises at least one RF track 11 in the printed circuit 1 opening at one end 11a opposite the cylindrical via 6, and opening at its other end 11b on the external face of the printed circuit 1 located outside the interposer allowing the RF grounds to be connected.

[0046] The plate 4 may be made of metallized polytetrafluoroethylene or a glass / ceramic / polytetrafluoroethylene composite laminated RF substrate, which makes it possible to work with a low relative permittivity, typically less than 3, for which the geometries are compatible with the manufacturing tolerances of diffuse contacts for high frequencies of the order of 40 GHz and above. For frequencies below the Ku band (12 GHz), other substrates with higher relative permittivity may be used.

[0047] This embodiment makes it possible to adapt completely different materials to each other without thermomechanical stress because the silver-filled epoxy glue is flexible and thermally conductive. This makes it possible to avoid, for the printed circuit 1, any recourse to a stack of layers of different thicknesses leading to an asymmetry which could cause delamination of the printed circuit 1.

[0048] In the case of using beam-forming integrated circuits or BFICs, the substrate used is generally a ceramic / glass / polytetrafluoroethylene PTFE composite RF laminate substrate with a relative permittivity close to 3.48 on a substrate thickness of 10 mils (0.254 mm). This is due to the fact that the footprints of the BFICs are components designed to operate on a standardized impedance of 50 ohms in a ball grid array format or BGA. The diameter of the balls, their spacing and their interface with the integrated circuit or PCB, for the acronym of "Printed Circuit Board", are taken into account in the design of the component. This routing constraint is therefore imposed by the component and its package.

[0049] On the other side of the interposer, it is the radiating elements that impose the dimensional constraints. When these differ from those imposed by the BFIC component, it is impossible to obtain a symmetrical multilayer printed circuit. However, symmetry is essential to avoid buckling and delamination effects. There is then no other choice than to use a bonded interposer. A prepreg could have been used, but bonding must take place after the BFIC circuits have been transferred, which makes the use of a prepreg impossible.

[0050] The use of low-temperature bonding with a preform is therefore imperative. Furthermore, the use of a ceramic / glass / polytetrafluoroethylene composite RF laminate substrate allows ground vias to be made all around the diffuse contact. This limits the number and spacing of the diffuse contacts (which would then also have been used for the grounds). However, it is then necessary to connect the grounds vertically as illustrated above. The silver-filled epoxy glue is used to connect the grounds together. Without this layer of glue, the ground paths could have been broken from time to time depending on the natural buckling of the BFIC board (all components being soldered on a single side) or the interposer.

[0051] Attention should be paid to the compression ratio required to ensure good contact quality. As a general rule, it is advisable to apply a compression ratio of 30% to the diffuse button 2. To absorb the excess compressed part, a bore is made at the ends of the through-hole areas with an angle of 30* over 0.15 mm thickness. Insulation is necessary between the contact area and the ground area to ensure that there is no short circuit.

[0052] This new type of assembly is particularly suitable for applications with active antennas given the density of signals to be processed.

[0053] In the example of [Fig.2], the plate 4 is composed of at least one layer of the printed circuit 1. In other words, the printed circuit also acts as a plate 4 forming a wedge and insulating guide for the diffuse contact 2.

[0054] The cylindrical ground via 6 opens onto the external face of the printed circuit 1 located outside the interposer.

[0055] [Fig.l] and [Fig.2] represent the interposer of the invention arranged in an active antenna, i.e. between at least one radiating element 7 and a BFIC card 8 provided with solder balls 9 between the BFIC card 8 and the external face of the printed circuit 1, with a layer of electrically conductive epoxy glue 12 between the radiating element 7 and the plate 4 of the interposer

[0056] The interposer of [Fig.2] is similar to that of [Fig.l] except that it is simplified. Indeed, in this case, the plate 4 is composed of at least one layer of the printed circuit 1, which therefore directly acts as a wedge and integrated guide for a diffuse contact 2. A diffuse contact 2 rests inside the hole 3 on the face of one of the internal layers. The advantage of this solution is to avoid the bonding 10, the cost and implementation of an additional plate 4 and a shortened RF path length (less losses). On the other hand, it is imperative that the routing constraints applied to the printed circuit 11 are identical.

[0057] This type of interposer assembly is new and is particularly suitable for active antenna applications given the density of signals to be processed.

[0058] The present invention has the following advantages dedicated to active antennas or to the assembly of heterogeneous cards or elements by gluing: - Non-hyperstatic (flexible) RF connection capable of managing high differential expansion coefficients (notion of heterogeneity). This solution makes it possible in particular to associate a printed circuit with a mechanical part (additive manufacturing or 3D metal printing) while respecting RF and thermomechanical requirements; - Low-loss and reproducible (industrial) connection provided that bonding is used using a conductive glue preform; - Extremely dense connection, inexpensive and simple to implement; - Connection allowing to adapt the RF constraints between 2 substrates of per different relative mittivities without generating asymmetry in the substrate or delamination; and - Ultra short connection if the contacts are integrated into the chimneys (depends on the constraints on either side of the printed circuit.

Claims

Claims

1. Microwave coaxial interposer intended to be arranged between two heterogeneous elements, comprising: - a printed circuit (1); - at least one diffuse contact (2) of cylindrical shape, inserted into a hole (3) in a plate (4) forming a wedge and insulating guide for the diffuse contact (2), the diffuse contact (2) opening into contact on a portion (la) of the printed circuit at one end (2a), and opening out of the hole (3) at its other end (2b); - an RF track (5) in the printed circuit (1) opening out at one end (5a) at the bottom of the hole (3) of the plate (4) opposite the diffuse contact (2), and configured to be in contact with the diffuse contact (2), and opening out at its other end (5b) on the external face of the printed circuit (1) located outside the interposer;and - at least one cylindrical copper via (6) for grounding arranged in the plate (4) coaxially around the diffuse contact (2), opening onto one face of the plate (4), and opening onto the other face of the plate (4) while being configured to be electrically connectable by the external face of the printed circuit (1) located outside the interposer.;

2. An interposer according to claim 1, wherein the printed circuit (1) is single-layer.

3. Interposer according to claim 1, wherein the printed circuit (1) is multi-layer.

4. Interposer according to one of the preceding claims, comprising a layer of electrically conductive epoxy glue (10) arranged between the plate (4) and the printed circuit (1).

5. Interposer according to one of the preceding claims, comprising at least one RF track (11) in the printed circuit (1) opening at one end (11a) opposite the cylindrical via (6), and opening at its other end (11b) onto the external face of the printed circuit (1) located outside the interposer.

6. Interposer according to one of the preceding claims, in which the

7.

8.

9. plate (4) is made of metallized polytetrafluoroethylene or ceramic / glass / polytetrafluoroethylene composite RF laminated substrate. Interposer according to claim 3, wherein the plate (4) is composed of at least one layer of the printed circuit (1). Interposer according to claim 7, in which the cylindrical ground via (6) opens onto the external face of the printed circuit (1) located outside the interposer. Active antenna including: - at least one radiating element (7); - a printed circuit (1) provided with at least one BFIC component (8); - an interposer according to one of the preceding claims arranged between the radiating element (7) and the printed circuit (1); - a layer of electrically conductive epoxy glue (12) between the radiating element (7) and the plate (4) of the interposer; and - solder balls (9) between the BFIC component (8) and the external face of the printed circuit (1).

Images