SPECIFICATIONPrinted circuit boardsThis invention relates to printed circuit boards, and particularly to such boards having a metal core. A printed circuit board in its simplest form consists of an insulating substrate carrying on at least one surface a pattern of metallic conductors.
If both sides of the board are used, then platedthrough holes are used to interconnect parts of the pattern on opposite sides of the board. Multi-layer printed circuit boards consists of several layers of interconnecting conductors insulated from one another. In all cases, the necessary circuit components are carried on the external surfaces of the board, usually attached to the conduction pattern by soldered connections.
One of the problems associated with printed circuits is that of removing heat generated by the circuit components. When discrete components were used it was usually possible to remove heat by natural convection, or if necessary by blowing air over or between the boards. However, the advent of the integrated circuit increased the component packing density and hence the number of possible heat sources on a single board. It is known for a printed circuit board to have a metal conducting pattern attached to one surface which was arranged to be in physical and thermal contact with the integrated circuit packages to remove heat from the packages. However, since the integrated circuit packages themselves could be formed from a plastics material, the heat had first to be conducted through the package.
Additionally, the presence of the metal pattern on one surface of the board meant that conductor patterns either had to be removed from that surface or well insulated.
A more recent development has been to form the printed circuit on a metal core, with single or multi-layer conductor patterns formed in the usual way on one or both surfaces of the core. Platedthrough holes between conductor patterns on opposite sides of the core pass through holes in the core. The metal core usually extends beyond the printed circuits along at least one edge to enable the core to be attached to a heat sink.
Whilst the presence of the metal core within the printed circuit board aids the removal of heat from circuit components, it is still necessary for this heat to pass through at least one layer of insulation to reach the metal core. It is an object of the invention to provide a metal-core printed circuit board in which the problem is avoided.
According to the present invention there is provided a printed circuit board which includes a metal core having two opposite parallel surfaces at least one of which carries a pattern of electric conductors spaced from the core by a layer of electrically-insulated material, one or more apertures formed through the layer of insulating material in areas where a heat-generating component is expected to be mounted on the surface of the board for connection to some at least of said conductors, and a heat-conducting member located in the or each aperture so as to ensure good thermal contact between the heatgenerating component and the metal core.
The invention will now be described with reference to the accompanying drawings, in which~ Figure 1 is a sectional view of part of a printed circuit board according to a first embodiment of the invention;Fig. 2 is a plan view of the embodiment ofFigure 1; andFigure 3 is a plan view of an alternative embodiment of the invention.
Referring now to Figure 1, this shows part of a printed circuit board comprising a metal core 10, in the form of a sheet of copper, or a copper/nickel-iron/copper sandwich. The surface of the core 10 carries a layer of electrically insulating material 11, on which is carried a pattern of electric conductors 12. These conductors may be formed using any of the wellknown printed circuit techniques. Figure 1 shows a component in the form of a ceramic chip carrier 13. This ship carrier may conveniently be in the form of a rectangular ceramic package containing one or more integrated circuit chips and having connection pads 14 formed on its outside edges.
The chip carrier is positioned on the printed circuit board so that the connection pads 14 are positioned over appropriate conductor 12 so that soldered connections 15 may be made between the pads 14 and the conductors 12.
The circuitry contained within the chip carrier 13 is likely to generate heat in one or more regions, which can be identified. The insulating layer 1 1 is removed under these regions to form an aperture 16 so that the metal core 10 is exposed. The aperture 1 6 is preferably filled with a thermally-conducting medium 18, and the chip carrier 13 is then placed in position and the soldered connections 15 are formed. The thermally-conducting medium 18 forms a good thermal connection between the heat-generating areas of the chip carrier 13 and the metal core 10, ensuring that heat is removed from the chip carrier.
Figure 2 is a plan view of an area of a typical printed circuit board to which the invention may be applied. The chip carrier is not shown, but its position is indicated by the broken outline 23. In the example illustrated the conductor terminations 22 are connected by way of plated-through holes 24 to other conductive patterns within a multilayer board, but they could equally well form part of a pattern of conductors on the exposed surface of the board. The aperture 16 is shown in the drawing as occupying the area covered by heatgenerating areas of the chip carrier.
It will be appreciated that the aperture may be of any required shape, and that several separate apertures may be used under a single chip carrier.
It will be equally clear that the chip carrier described in the above embodiment is only one example of a heat-generating component. Other types of integrated circuit package may be cooled in the same way, as may other components such as resistors. Figure 3 is a plan view of an arrangement which may be used, for example, with a dual-in-line package. In Figure 3 the conductor terminations 32 are arranged in two rows to conform with the pin arrangement of the package.
Between the two rows of terminations 32 are arranged one or more apertures 16, 30 in the board 1 1. If the apertures are small, sufficient heat conduction may be achieved without the use of a thermally-conducting medium as described above.
Clearly a single cup could be used if preferred.
In the case of a component such as a resistor the arrangement of Figure 4 could be used, in which a single aperture 16 is located between a pair of conductor terminations 41.
In all of the above embodiments the heatconducting member or medium may take one of a number of alternative forms. A thermallyconducting paste may be used, or an insert of a solid thermally-conducting medium. The latter may be metallic. Additionally, if a paste is used, it is possible to insert a metal cup into the aperture, secured to the metal core by sweating or some similar process. This may then be filled with a thermally-conducting paste if required.
The printed circuit board may be provided with a regular pattern of apertures or inserts, and, as already stated, the conductor pattern formed over the metal core may one of a number of layers of conductor patterns forming a multi-layer printed circuit board. In such cases the other layers will usually be on the opposite side of the core, interconnected by plated-through holes. Figure 5 shows a side view of such a construction. This shows three further layers of conductors 51, 52 and 53, separated from one another and from the core 50 by insulating layers 54. Plated-through holes 55 interconnect conductors in the various layers of the construction. Heat-generating components are only mounted on the upper side of the core where only a single insulating layer 11 exists. Heat may be removed from the core 10 by attaching it to a cold wall at the exposed righthand side in Figure 5.