GB2379089A

GB2379089A - RF screening for printed circuit boards

Info

Publication number: GB2379089A
Application number: GB0120292A
Authority: GB
Inventors: Richard Townsend
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2001-08-21
Filing date: 2001-08-21
Publication date: 2003-02-26
Also published as: GB0120292D0

Abstract

A multi-layer printed circuit board (2) has an upper ground plane (4), a lower ground plane (6) and an intermediate signal plane (10) each separated by an electrically insulative substrate layer. The board has a pair of slots (16, 18) formed either side of a signal path (14) formed on the signal plane (10), the slots extending between the ground planes and being through plated (22, 24) to form a generally continuous conductive tube around the signal path (14). The conductive tube prevents RF signal emissions from interfering with adjacent circuitry on the board and prevents noise being received on the signal path.

Description

RF SCREENING FOR PRINTED CIRCUIT BOARDS This invention relates to electrical screening for printed circuit boards and in particular to the screening of signal paths intended to carry RF signals.

The transmission of radio frequency signals over signal paths in a printed circuit board is problematic. If RF signals are permitted to pass through a bare signal path on the circuit board then the signal path typically will act as an antenna which (if the signal being carried is of high power) will cause RF signal emissions which may interfere with adjacent circuitry or (if the signal levels being carried are of low power) may allow the signals being carried to be affected by noise received from external sources of RF interference.

Thus, RF signals are conventionally carried using suitably matched transmission lines such as a co-axial transmission line having inner and outer conductors located coaxially with dielectric material between the conductors.

Although the use of c-axial conductors (typically with the outer conductor connected to the or one of the earth planes in the circuit board) overcomes the problems associated with transmitting RF signals on a PCB, it will be appreciated that the formation of suitable lengths transmission line, the bending of it into suitable shapes and the attaching of it to the PCB (with soldered connections being made to the inner and outer conductors), makes automated assembly of such PCBs difficult and therefore expensive. However, the removal of screening for RF signals is likely to make the electrical circuit formed by the PCB and its associated components inoperable either through interference from external sources or through unacceptably high RF emissions from the circuit during operation.

In accordance with a first aspect of the invention there is provided a multi-layer printed circuit board having an upper ground plane, a lower ground plane and an intermediate signal plane each separated by an electrically insulative substrate layer, the board being arranged to have a pair of slots placed either side of a signal path formed in the signal plane, the slots extending between the ground planes and being through-plated to form a generally continuous conductive tube around the signal path.

In a second aspect, the invention provides a method of electrically screening a signal path in a multi-layer printed circuit board having a plurality of ground planes and signal plane, comprises forming a pair of slots either aside of a signal path and which extending between a pair of the ground planes which are located in respective board layers which are above and below the signal plane carrying the signal path, and through plating the slots to form an electrically continuous tube around the signal path formed by the innermost walls of the slots and the portions of the pair of ground planes which is located between the slots.

Embodiments of the invention will now be described by way of example with reference to the drawings in which: Figure 1 is an enlarged cross-section through a portion of a three layer multi-layer PCB in accordance with the invention; and Figure 2 is a plan view of a PCB in accordance with the invention.

With reference to Figure 1, a printed circuit board 2 has an upper ground plane 4 and a lower ground plane 6. The PCB also has a signal layer 8 which is printed on an intermediate substrate 10.

In this example, a screened signal path is shown which travels to a surface mount RF connector 12. It will be appreciated that the techniques described below are applicable to connections between any electrical components and also to multi-layer PCBs having more than two ground planes and more than one signal plane.

A signal path 14 is formed by a track printed on the intermediate substrate 10 taking the desired path which terminates at the surface mount connector 12. It will be noted that the ground planes 4 and 6 form earthed surfaces above and below the track 14. However, in order to provide an adequate screen for the signal path, it is necessary to complete the screening on either side of the signal path so that lateral radiation (generally along the direction of the plane of the board) are also prevented.

In circuit boards in accordance with the invention the lateral radiation is prevented by cutting (typically using the type of routing or milling operation used to separate boards

after the plating process) the board to produce generally parallel slots on either side of the signal path 14. This can be seen more clearly in Figure 2 where slots 16 and 18 have been cut through the board on either side of the signal path 14.

Once the slots have been cut, conventional through-plating techniques allow the side walls of the slots to be plated and to be electrically conductively connected with the upper and lower ground planes 4 and 6.

As a consequence, it will be noted that the innermost sides of the slots 22 and 24 then form generally vertical sides to a hollow tube which surrounds the signal path 14 (the upper and lower surfaces of the tube are formed by the ground planes 4 and 6 as noted above).

Thus, with a simple routing and through-plating operation, an electrically conductive screen has been produced around a signal path without needing to use additional transmission line components.

Thus in the arrangement of Figure 2, a component 26 which operates at RF frequencies is (as is conventionally known) enclosed in a screening can 28. The signal path 14 with surrounding slots 16 and 18 exits the can and passes to the surface mount RF connector generally at the edge of the board 2. The RF component and its can may be mounted in an entirely conventional manner (with the can being electrically coupled to the upper ground plane 4 in the conventional manner) and once this is done and the RF component is coupled to the distal end of the track 14, the connection between the RF component and the surface mount RF connector 12 is complete.

Thus it will be appreciated that the techniques described above provide an equivalent level of screening to that of a c-axial cable but using only standard PCB manufacturing techniques used in a through-plated multi-layer printed circuit board. Furthermore, applying strip line techniques, it is possible to set the impedance of the signal track to a desired impedance (50 or 75 ohms for example). The signal path 14 may exit the screened tube using a blind via hole, for example.

Claims

Claims 1. A multi-layer printed circuit board having an upper ground plane, a lower ground plane and an intermediate signal plane each separated by an electrically insulative substrate layer, the board being arranged to have a pair of slots placed either side of a signal path formed in the signal plane, the slots extending between the ground planes and being through-plated to form a generally continuous conductive tube around the signal path.
2. A printed circuit board according to claim 1, wherein the slots are generally parallel.
3. A printed circuit board according to claim 1 or claim 2, wherein the slots extend generally vertically between the ground planes.
4. A printed circuit board according to any preceding claim, wherein the tube has a generally rectangular cross-section and wherein the ground planes form one pair of opposing sides of the tube and the respective innermost walls of the slots form the other pair of opposing sides of the tube.
5. A printed circuit board according to any preceding claim, wherein the slots extend through the whole thickness of the circuit board.
6. A printed circuit board according to any preceding claim, wherein the distance between the respective slots and the signal path, the distance between the ground planes and the signal path, and the dielectric constant of the insulative substrate layers are each selected such that the tube forms a transmission line of predetermined impedance.
7. A method of electrically screening a signal path in a multi-layer printed circuit board having a plurality of ground planes and signal plane, comprises forming a pair of slots either aside of a signal path and which extending between a pair of the ground planes which are located in respective board layers which are above

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and below the signal plane carrying the signal path, and through plating the slots to form an electrically continuous tube around the signal path formed by the innermost walls of the slots and the portions of the pair of ground planes which is located between the slots.
8. A printed circuit board constructed and arranged as described herein with reference to the drawings.
9. A method of electrically screening a signal path in a multi-layer printed circuit board as described herein with reference to the drawings.