Microwave coaxial interposer

Abstract

The invention relates to a microwave coaxial interposer intended to be arranged between two heterogeneous elements, comprising: - a printed circuit (1); - at least one cylindrical diffuse contact (2), inserted into a hole (3) in a board (4) forming a shim and an insulating guide for the diffuse contact (2), one end (2a) of the diffuse contact (2) extending to a portion (1a) of the printed circuit, in contact with same, and the other end (2b) extending from the hole (3); - an RF track (5) in the printed circuit (1), one end of the RF track extending to the bottom of the hole (3) in the board (4) opposite the diffuse contact (2), and configured to be in contact with the diffuse contact (2), and the other end (5b) extending to the outer face of the printed circuit (1), located outside the interposer; and - a cylindrical ground via (6) arranged in the board (4) coaxially around the diffuse contact (2), extending to one face of the board (4), as well as extending to the other face of the board (4) and being configured to be electrically connectable by means of the outer face of the printed circuit (1), located outside the interposer.

Description

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 interconnect amplifier based on a modular principle, it is necessary to be able to connect the module grounds while being removable. Without this, performance cannot be achieved.

[0004] Several solutions were 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 GT 1000 from getelec

[0005] None of these solutions work due to the RF contact quality and associated losses. A clamped interconnect 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 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 Integrated 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 to address the above-mentioned problems are conventional solutions for which the inventions have fallen into the public domain. These solutions use either flexible materials (silver-filled epoxy glue or soft solders), or coaxial connector principles (SMP for "Sub Miniature Push-on" 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 preform or "preform glue" in English (ablefilm or ablestick 5025E or equivalent); but the 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 board, 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. Add to this the fatigue linked to differential thermal expansion constraints. The elements are not removable.

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

[0012] Coaxially connected solutions using SMP spring bullets are too bulky, too expensive and induce too many 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 ground copper via arranged in the plate coaxially around the diffuse contact, opening onto one face of the plate, and opening onto the other face of the plate while being configured to be electrically connectable by the external face of the printed circuit located outside the interposer.

[0016] In a printed circuit board, a via is a metallized hole that 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 board 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 low loss RF laminated substrate made of ceramic or glass / PTFE composite.

[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: - at least one radiating element; - a printed circuit equipped with at least one BFIC component; - an interposer according to one of the preceding claims arranged between the radiating element and the printed circuit; - a layer of electrically conductive epoxy glue between the radiating element and the interposer plate; and - solder balls between the BFIC component and the external face of the printed circuit (BGA package).

[0025] The invention will be better understood by studying a few embodiments described as non-limiting examples and illustrated by the appended drawings in which:

[0026] [Fig.1] schematically illustrates a coaxial microwave interposer, according to one aspect of the invention; and

[0027] [Fig.2] schematically illustrates another coaxial microwave interposer, according to one aspect of the invention.

[0028] [Throughout the figures, elements with identical references are similar.

[0029] 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.

[0030] This interwoven structure provides adequate elasticity and excellent contact quality when properly compressed (typically 30% compression ratio). Such contacts are custom-manufactured with diameters ranging from 0.25 mm to 7 mm and lengths greater than 1 mm up to 7 mm.

[0031] 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).

[0032] [Fig.1] schematically illustrates a coaxial microwave interposer intended to be arranged between two heterogeneous elements according to one aspect of the invention.

[0033] 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 with a portion 1a of the printed circuit at one end 2a, and opening from the hole 3 at its other end 2b.

[0034] 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.

[0035] The coaxial microwave interposer further comprises a cylindrical copper ground via 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.

[0036] This coaxial microwave interposer of [Fig.1] is represented between two elements, in this case a radiating element 7 of an active antenna and a card 1 equipped with a BFIC component 8 in BGA format for the acronym of "bail grid array" in English, i.e. equipped with solder balls 9.

[0037] In the example of [Fig.1], the printed circuit 1 is multi-layer, but could alternatively be single-layer.

[0038] 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.

[0039] The epoxy glue used is loaded with silver and preferably in preform form, which guarantees the thickness of the glue (unlike a viscous formulation).

[0040] 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 onto the external face of the printed circuit 1 located outside the interposer allowing the RF grounds to be connected.

[0041] Plate 4 can be made of metallized polytetrafluoroethylene or a glass / ceramic / polytetrafluoroethylene composite laminated RF substrate, which allows working 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 can be used.

[0042] 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.

[0043] 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 intended for operate on a standardized impedance of 50 ohms in a ball grid array (BGA) format. The diameter of the balls, their spacing and their interface with the integrated circuit (PCB) are taken into account in the design of the component. This routing constraint is therefore imposed by the component and its package.

[0044] 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 but 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.

[0045] 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 created all around the diffuse contact. This limits the number and spacing of 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.

[0046] Attention should be paid to the required compression ratio 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" and 0.15 mm thick. Insulation is required between the contact area and the ground area to ensure that there is no short circuit.

[0047] This new type of assembly is particularly suitable for active antenna applications given the density of signals to be processed.

[0048] 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.

[0049] The cylindrical ground via 6 opens onto the external face of the printed circuit 1 located outside the interposer.

[0050] [Fig.1] 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.

[0051] The interposer of [Fig.2] is similar to that of [Fig.1] 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 gluing 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 1 1 are identical.

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

[0053] 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 easy to implement; - Connection allowing the adaptation of RF constraints between 2 substrates of different relative permittivities 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, comprising interlaced metal wires, 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 (1 a) 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. Interposer according to claim 1, in which the printed circuit (1) is single-layer.

3. Interposer according to claim 1, in which 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 (11 a) opposite the cylindrical via (6), and opening at its other end (11 b) 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 plate (4) is made of metallized polytetrafluoroethylene or ceramic / glass / polytetrafluoroethylene composite RF laminated substrate.

7. Interposer according to claim 3, in which the plate (4) is composed of at least one layer of the printed circuit (1).

8. 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.

9. Active antenna comprising: - at least one radiating element (7); - a printed circuit (1) according to claim 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