FR2771889A1 - High frequency chip mounting construction - Google Patents

Abstract

The chip can be held in position in the upper surround earthed sides and precision mounted onto the printed circuit below. The side walls of the carrier also form a chip cap section and screening.

Description

 The present invention relates to a method of mounting a microwave box on the surface, comprising an electronic chip, on a printed circuit and a box and a printed circuit designed for the implementation of this method.

 It is known to mount a microchip directly, chip in the Anglo-Saxon literature, on a host printed circuit it is also known to mount the chip on a mini printed circuit to form a microwave package which is then surface mounted on the home circuit board. In both cases, it is known to make the connections with the host printed circuit by wired wiring and to cover the chip or the microwave box with a protective cover. However, the implementation of such mounting techniques, in quantity, is expensive because, for high operating frequencies, it requires, for the realization of the wiring, when equipping the host printed circuit, the use of microelectronics equipment; without such equipment it is practically impossible, using these mounting techniques, to work at frequencies above 12 GHz.

 The object of the present invention is to avoid this drawback.

 This is obtained by mounting a mini printed circuit carrying the chip on a receiving printed circuit, where the active face of the mini printed circuit carrying the chip is soldered on the receiving printed circuit and where ground planes distributed on the two printed circuits form a sort of cage constituting a shield around the chip.

 According to the invention there is provided a method of mounting a microwave box on a printed circuit, characterized in that it consists of - using a microwave box having two parallel faces, one face of which with an electronic chip, connected conductors to the chip and ground planes interposed between the conductors and a second face constituted by a ground plane connected to the ground planes of the first face make a printed circuit having a first face with, in an area dedicated to the housing, conductors and ground planes and a second face constituted by a ground plane connected to the ground planes of the first face of the printed circuit, the first two faces having on the one hand their ground planes and on the other hand their conductors which, in a predetermined position of the housing on the printed circuit, are in one-to-one contact, - to place the housing in the predetermined position - and to con Connect the box to the printed circuit by soldering the conductors and ground planes in one-to-one contact.

 According to the invention, a microwave box is proposed, for implementing a method of mounting a box on a printed circuit, this microwave box comprising a mini printed circuit with a first and a second face, the first face. comprising an electronic chip and conductors connected to the chip and the second face being constituted by a ground plane, characterized in that the first face comprises ground planes, in that these ground planes are interposed between the conductors and in that the mini printed circuit includes connections between the ground planes of the first face and the ground plane of the second face.

 According to the invention there is proposed a printed circuit for receiving a microwave housing, for implementing a method of mounting a housing on a printed circuit, this printed reception circuit comprising a first and a second face , the first face comprising, in a reception area, conductors and the second face being constituted by a ground plane, characterized in that the first face comprises ground planes in the reception zone, in that these ground planes are interposed between the conductors and in that the host printed circuit includes connections between the ground planes of the first face and the ground plane of the second face.

The present invention will be better understood and other characteristics will appear from the following description and the figures relating thereto which represent
FIGS. 1 and 2, the views of that of the faces of two printed circuits which carries, among other elements, conductors, the view according to FIG. 2 being a partial view,
- Figure 3, a sectional view of the printed circuits of Figures 1 and 2, after they have been joined by soldering.

 In the various figures, the corresponding elements are designated by the same references.

 In what follows and in the claims when it is a question of a printed circuit of which one of the two sides is entirely metallized and that this side is soldered on a metallic support, it will be considered that the metallic support is part of the printed circuit and in constitutes the ground plan.

 Figure i shows a microwave package of an electronic chip, P; it consists of a mini printed circuit whose figure 1 shows a square face, ABCD, with conductive tracks L1 to L4, ground planes M1 to M4, inserted between the tracks, and the chip P associated with four decoupling elements such as E. The chip P is located substantially in the middle of the face ABCD; wires welded at their ends provide the connections between the various elements of this housing.

 Tracks L2, L4 are connected respectively to two microwave accesses of the chip while tracks L1, L3 bring bias voltages to the chip. Each of the ground planes M1 to M4 is pierced with holes such as Ha, Hb; in total there are fourteen holes distributed in the vicinity of the periphery of the square ABCD.

 The housing is shown in section, in a rectangle located at the upper part of Figure 3; the section plane, of track XX, is perpendicular to the plane of FIG. 1 and is parallel to the tracks L2, L4 in their middle. Figure 3 comprises, in addition, a lower rectangle, attached to the rectangle of the housing; this rectangle corresponds to a sectional view of a printed circuit on which the ABCD face of the housing rests; the housing is said to be surface mounted on the printed circuit.

 The mini printed circuit of the housing comprises an insulating substrate 1 with, on one side, the face shown in FIG. I and, on the other side, a ground plane in three successive layers: a metallic deposit and solder, 2, and a mechanical brass support, 3; for ease of drawing, the metal deposit on the substrate 1 and the weld between this deposit and the mechanical support 3 have been combined in the drawing in FIG. 3.

 The housing has the particularity, compared to a conventional housing where the support, 3, is a simple brass plate with parallel faces, to have a boss or tab 3b in its support, on the side of the substrate, at the level of a hole used in the substrate to solder the P chip on the ground plane. Usually, that is to say without the boss 3b, the chip P is soldered on the ground plane, directly on the support 3; its face opposite to the ground plane is at a given distance from the face of the mini printed circuit which carries the tracks L1 to L4.

When, to protect it, resin is deposited on the chip, it turns out to be impossible to have only the thin layer of resin strictly necessary for protection; the resin, while strongly modifying the propagation medium on the side of the links of the chip, disturbs the operation beyond frequencies above 10 GHz. The presence of the boss 3b greatly reduces said given distance; that of the faces of the mini printed circuit which carries the tracks L1 to L4 is then used as a reference level for the quantity of resin to be deposited, which makes it possible to control the thickness of resin on the chip.

 FIG. 3 shows, in dotted lines, the holes Ha, Hb which have been mentioned during the description of FIG. 1; as for the other twelve holes drilled in the ground planes M1 to M4, these are holes with metallized walls which constitute parallel conductive columns, between the ground plane 2, 3 and the ground planes M1 to M4.

Figure 2 is a partial view of a host printed circuit, in its part dedicated to the housing which has just been described above with the aid of Figure 1 and the rectangle of the upper part of Figure 3 The rectangle in the lower part of figure 3 corresponds to a sectional view of the printed circuit of figure 2, in a trace section plane
XX; the cutting plane is perpendicular to the plane of Figure 2. It should be noted that it is intentionally that the cutting traces bear the same reference in Figures 1 and 2; it is to mark the fact that, when the box is mounted in the predetermined position which it must occupy on the printed circuit, the two cutting planes are merged.

 The printed circuit, according to FIG. 2 and according to the lower part of FIG. 3, comprises an insulating substrate 6 with, on one side of the conductive tracks L1 'to L4', and ground planes, MI 'to M4', shown in Figure 2 and, on the other side, a ground plane in three successive layers: a metal deposit and solder, 7, and a mechanical support in brass, 8; here again, for ease of drawing, the metallic deposit on the substrate 6 and the weld between this deposit and the mechanical support 8, have been combined in the drawing of FIG. 3.

 The tracks L2 ', L4' are microwave tracks which the section plane crosses longitudinally in their middle and the tracks L1 ', L3' are tracks intended to bring bias voltages.

 Each of the ground planes Mi 'to M4' is pierced with holes such as Ha ', Hb'. These holes, the wall of which is metallized, provide a conductive connection between the ground planes M1 'to M4' located on one side of the printed circuit and the ground plane 7, 8 located on the other side of the printed circuit.

In Figure 2 four points A ', B', C ', D' forming the vertices of a square, have been identified by arrows; they correspond to the exact positions which the vertices must respectively occupy
ABCD of the face of the mini printed circuit shown in FIG. 1, during assembly of the box on the receiving printed circuit; it is in this position of the housing with respect to its receiving printed circuit that the cut is made according to FIG. 3. It should be noted, by comparing FIGS. metallized holes located in the square A'B'C'D 'correspond point for point, in a symmetry with respect to a plane, to the ground planes and to the orifices of the metallized holes located in the square ABCD; for the conductive tracks this symmetry also exists but vis-à-vis only a part of the tracks of the square ABCD because the tracks extend less inside the square A'B'C'D 'than at the inside the square ABCD. Thus, when the vertices ABCD of the mini printed circuit are respectively above the points A'B'C'D ', the conductive tracks Li' a L4 'and the ground planes Ml' to M4 'are entirely opposite the tracks and corresponding ground planes of the housing; in addition, the fourteen metallized holes in the housing are an extension of the fourteen metallized holes in the receiving printed circuit.

The connection of the box to its receiving circuit consists, in the example described, of first applying a thin layer of solder on the tracks and the ground planes which are opposite when the square
ABCD is above the square A'B'C'D '; then the box is positioned on the reception circuit in such a way that the square ABCD is exactly above the square A'B'C'D 'and that, by heat sealing, the facing tracks and the facing ground planes can be welded. In Figure 3 the intake of calories ensuring heat sealing has been symbolized by an arrows Th; as for the solder layer which connects tracks L4 and L4 'after heat sealing, it has been denoted S in FIG. 3.

Thus mounted on the receiving printed circuit, the box has its chip which is enclosed in a sort of cage, forming a shield, limited upwards by the ground plane 2, 3, downwards by the ground plane 7, 8 and laterally by the fourteen pairs of metallized holes in the housing - by pair of metallized holes is meant the holes such as
Ha and Ha 'which are in the extension of one another during the mounting of the box on its receiving printed circuit, as well as the thickness of solder between the holes such as Ha and Ha'.

 Figures 2 and 3 show that the receiving printed circuit is pierced with a cavity, K, of oval section which opens under the chip P when the housing is mounted on the receiving printed circuit. The dimensions of this cavity are such that, relative to the same assembly but without cavity, the distance from the chip P to the walls closest to the ground plane 7, 8 is of the order of five times greater; the role of this cavity is to avoid disturbances in the operation of the chip, due to radiation from too close a metallic plane, in this case the ground plane 7, 8.

 The present invention is not limited to the example described, thus, in particular that the chip P can be mounted in a conventional manner, that is to say on a ground plane 2, 3 without the boss 3b; the chip may also not be protected by a deposit of resin, since, after assembly of the housing on its receiving printed circuit, the chip is in an almost closed space whose sealing can moreover be improved by a seal made of an insulating material, for example an adhesive, placed all around the housing, at the junction of the housing printed circuit housing; taking into account the performance to be achieved, it is even possible, in certain cases, not to provide a cavity in the receiving printed circuit, directly above the chip.

 In addition, the box can be produced with the chip placed on the mini printed circuit according to a so-called humpback or flip-chip assembly in English literature; but this arrangement where the studs of the chip are directly in contact with the mini printed circuit does not allow good heat dissipation because the chip is no longer plated on a ground plane which serves as its radiator.

 Another variant of the example which has been described for, as has been said, enclosing the chip in a sort of Faraday cage, consists in replacing, in the mini circuit, the metallized holes by metallizations carried out on the edge of the mini printed circuit, between the outer edges of the ground planes M1 to M4 and the ground plane 2, 3.

Claims (6)

 1. Method for mounting a microwave box on a printed circuit, characterized in that it consists in using a microwave box (L1-L4, Ml -M4, 1, 2, 3) having two parallel faces including a first side with an electronic chip (P), conductors (L1-L4) connected to the chip and ground planes (M1M4) interposed between the conductors and a second face constituted by a ground plane (2, 3) connected to the planes mass of the first face make a printed circuit (L1'-L4 ', M1'-M4', 6, 7, 8) having a first face with, in an area dedicated to the housing, conductors (L1'-L4 ') and ground planes (MI'M4 ') and a second face constituted by a ground plane (7, 8) connected to the ground planes of the first face of the printed circuit, the first two faces having on the one hand their planes earth and on the other hand their conductors which, in a predetermined position (ABCD / A'B'C'D ') of the box on the printed circuit, are in cont one-to-one act, placing the box in the predetermined position and connecting the box to the printed circuit by soldering (S) of the conductors and ground planes in one-to-one contact.  2. Microwave box, for implementing a method of mounting a box on a printed circuit, this microwave box comprising a mini printed circuit with a first and a second face, the first face comprising an electronic chip (P ) and conductors (L1-L4) connected to the chip and the second face consisting of a ground plane (263), characterized in that the first face comprises ground planes (M1-M4), in that these ground planes are interposed between the conductors (L1-L4) and in that the mini printed circuit comprises connections (Ha, Hb) between the ground planes of the first face and the ground plane of the second face.  3. Microwave housing according to claim 2, characterized in that the connections are made by holes with metallized walls (Ha, Hb).  4. Microwave housing reception printed circuit, for implementing a method of mounting a housing on a printed circuit, this reception printed circuit comprising a first and a second face, the first face comprising, in a reception area, conductors (L1 ' L4 ') and the second face being constituted by a ground plane (7, 8), characterized in that the first face comprises ground planes in the reception area, in that these ground planes are interposed between the conductors (Ll'-L4 ') and in that the receiving printed circuit includes connections (Ha', Hb ') between the ground planes of the first face and the ground plane of the second face.  5. Reception printed circuit according to claim 4, characterized in that it comprises a cavity (K) which opens into the reception area by an opening which is surrounded by the conductors and the ground planes of the first side.  6. Home printed circuit according to claim 4, characterized the connections are made by holes with metallized walls (Ha ', Hb').