FI90935C - Method for mounting a coaxial transmission line on a printed circuit board - Google Patents

Description

i 90935

Method of installing a coaxial transmission line on a printed circuit board - Forfarande for montering av en coaxial transmissionsledning på en tryckt kretsskiva 5

The invention relates to a method by which a fixed-sheathed coaxial cable or the like can be mounted on a printed circuit board.

10 In the electronics industry, there is often a need to provide a transmission line with good electrical properties on a printed circuit board. If the transmission line is to be implemented by circuit board technology, it requires expensive circuit board material. It is often more advantageous to use a cheaper circuit board material-15 material (FR-4) and add loose ends of the transmission line to the desired locations. A fixed-sheathed, semi-rigid coaxial cable can be used as such a transmission line with good electrical properties. It is known from the prior art to install such a coaxial cable on the surface of the circuit board, whereby the contact of the middle conductor to the desired point is obtained by bending it appropriately on the surface of the circuit board. In this case, there must be a ground plane under the outer sheath, from which the signal ground contact is formed between the ground plane of the circuit board and the ground conductor of the cable. In this case, however, the problem is often the irregular shape of the center line of the cable as it passes from the cable to the surface of the circuit board at the connection point, whereby a point of discontinuity in the signal path is formed. This can be a disadvantage, especially at high frequencies.

The present invention provides a method by which this coaxial cable installation can be performed so as to avoid the above problem. According to the invention, a recess or opening is formed in the circuit board, for example by stamping or milling, into which the coaxial cable sinks so deep that the center conductor of the cable comes level with the surface of the circuit board without bending (Figures 1 and 2). In this way, the characteristic impedance of the strip made on the circuit board and the characteristic impedance of the coaxial cable are made equal, whereby the signal is transferred from the strip to the cable and vice versa without discontinuity points and 2 mismatches (Figure 3). The signal ground contact is provided between the ground plane of the cable and the ground plane of the circuit board by bringing this ground plane to the edge of the recess or opening formed in the circuit board, from which the contact is made by tin soldering. An even better contact is created if the recess or opening is metallized (Fig. 4), whereby solder contact is made between the walls of the recess or opening and the outer sheath of the cable. Figures 4 and 5 also show the opening of the through-metallization at the center conductor to avoid a short circuit during soldering.

If milling or the like is used as the method, and the thickness of the circuit board and the diameter of the coaxial cable correspond to each other appropriately, it may make sense to form only a recess in the circuit board. The edges and bottom of this recess may be metallized.

The essential features of the invention are set out in the appended claims.

20

It is also essential for the invention that the structure becomes very compact when the diameter of the coaxial cable and the thickness of the circuit board are suitably selected so that about half of the cable sinks inside the circuit board (Figure 1).

25

The installation method according to the invention also meets the RF protection requirements, if the lower surface of the circuit board is an intact ground plane, i.e. the lower surface of the circuit board is metallized (acts as part of the RF shield); an opening formed in the circuit board is covered by a cable sheath (aperture walls soldering is made around the entire opening. At the opening of the through-through metallization, a solder contact is made between the sheath and the circuit board surface when the ratio of the circuit board thickness to the cable diameter is selected so that the outer surface of the kaa-35 game is lateral to the circuit board surface at this point (Figure 6). In the case of a recess, soldering can be performed along the sides of the cable sheath.

II

90935 3

The invention will be described in more detail with reference to the figures.

Figures 1-3 show a coaxial cable mounted on a circuit board from the top, from the side and from above, respectively; Figure 4 shows a part of the circuit board in a perspective view; Figure 5 shows the geometry of a coaxial cable and a circuit board; and Figure 6 is a bottom perspective view of the coaxial cable and circuit board.

Figure 1 shows a coaxial cable 1 mounted on a circuit board 2 as seen from the end of the cable. An opening 3 is formed in the circuit board 2, in which the coaxial cable 1 is mounted 15 so that the center conductor 4 of the cable remains at the level of the surface of the board. The center conductor is surrounded by a dielectric medium 5 and further by a copper sheath 6 of the cable. Figure 2 shows a side view of a fixed-sheathed coaxial cable 1 mounted in an opening formed in the circuit board 2.

20

Figure 3 shows a top view of how the strip 7 of the printed circuit board and the center conductor of the coaxial cable are connected to each other without points of discontinuity. When the characteristic impulses of the strip made on the circuit board and the coaxial cable are the same, a signal is obtained to move over the cable without mismatch.

Figure 4 shows a through-metallized opening 3 formed in the circuit board, in which a through-metallization opening 8 is made at the center conductor of the coaxial-30 dirt cable. The figure also shows a strip 7 on the surface of the printed circuit board. Figure 5 shows the opening 8 of the opening through-metallization seen from the front. The figure also shows the dimensions of the cable and the circuit board.

Figure 6 shows the opening of the circuit board from below, showing that the metallized lower surface 9 of the circuit board, i.e. the ground plane, is lateral to the outer surface of the cable at the through-metallization opening, making solder contact between the sheath and the circuit board surface.

Claims (7)

A method for mounting a fixed-sheathed coaxial cable or the like on a printed circuit board, characterized in that a recess or opening 5 (3) is formed in the circuit board (2) into which the coaxial cable (1) is embedded so deep that the cable center conductor le without bending, whereby the middle conductor (4) is connected to the strip conductors (7) on the surface of the circuit board. Method according to Claim 1, characterized in that the recess or opening (3) is metallized. Method according to Claim 2, characterized in that an opening (8) is left in the metallization at the center conductor (4) of the coaxial cable (1). Method according to one of Claims 1 to 3, characterized in that the lower surface (9) of the circuit board is metallized. 20 Method according to one of Claims 1 to 4, characterized in that the formed recess or opening (3) is dimensioned so as to be covered by the cable sheath (6). 25 A method according to claim 5, characterized in that the thicknesses of the coaxial cable and the circuit board are selected so that the sheath of the cable is lateral to the surface (9) forming the ground plane of the circuit board. 30 Method according to Claim 5 or 6, characterized in that the cable sheath (6) is soldered to the circuit board (2). 90935