PRINTED CIRCUIT BOARDS

Abstract: A printed circuit board (1) comprises a multi-layered structure. One of the layers (3) provides a heating layer such that heat applied to that layer (3) is transferred to electronic components (2) mounted on the board (1). The components (2) can thus be maintained at a suitable operating temperature.

Description: PRINTED CIRCUIT BOARDS TECHNICAL FIELD OF THE INVENTION The present invention relates to printed circuit boards and in particular to a method and apparatus for heating selected areas of a printed circuit board. DESCRIPTION OF THE RELATED ART In order to keep electronic components working reliably, they must be maintained within their desired operating temperature range. This is a particular problem in remote cellular or satellite telephony base stations, which may be required to operate in extreme climatic conditions. In such conditions, components having wide operating temperature ranges have conventionally been used. However, such components are expensive. US Patent No. 5,103,071 discloses a surface mount technology heater which is used to provide heat to a circuit board in order to solder surface mount connector terminals to contact pads . The connector terminals are configured as spaced integrally formed projections of a selectively actuable heater. The heater develops thermal energy which is conducted along the projections to the solder tails to melt fusable conductive material. However, such a heater is only used to solder terminals to a PCB, and is not suitable for use in heating electronic components mounted on the PCB during operation. SUMMARY OF THE PRESENT INVENTION In accordance with the invention, means are- provided for heating a temperature-sensitive component on a PCB, to maintain it within a desired operating temperature range. According to a first aspect of the present invention, there is provided a printed circuit board comprising a multi-layered structure. One of the layers of the structure provides a heating layer such that heat applied to that layer is transferred to electronic components mounted on the printed circuit board. According to a second aspect of the present invention there is provided a method of • heating electronic components mounted on a printed circuit board. The method comprises supplying thermal energy to a layer of the printed circuit board. Preferably, heat is supplied by ohmic heating of a resistive element within the printed circuit board. Advantageously, the temperature of the components is detected, and the sensed temperature is used to control the supply of heat thereto. BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows a schematic view of a first printed circuit board embodying the present invention; and Figure 2 shows a second printed circuit board embodying the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, a printed circuit board (PCB) l has a number of electronic components 2 mounted thereon. The PCB 1 is generally of conventional construction, i.e. having a substrate layer, a conduction layer, a ground layer and insulating layers. The conduction and ground layers are usually copper and are configured to provide appropriate connections between the electronic components 2- mounted on the PCB 1. In particular, these components, which may for example be integrated circuits or may be discrete devices, may be such that they only operate satisfactorily within a specific temperature range (e.g. 0"C-70 o C) . The printed circuit board forms part of a device, for example a cellular telephony base station, which needs to be operable in harsh climatic conditions, for example in temperatures well below 0 β C A number of surface mount resistors, for example 0.25 watts, are mounted around the PCB 1, in contact with a heating layer 3 which is shown as a upper layer of the PCB J.. A temperature sensor 7 is -also mounted on the PCB l and provides a temperature signal to a controlled voltage source 9. The controlled voltage source 9 is connected to the surface mount resistors 5 in order to provide a controlled voltage to those resistors 5. In order to heat the electronic components 2 mounted on the PCE 1, and thus maintain them wi-hin their operating temperature range, a voltage is supplied to the surface mount resistors 5 by the controlled voltage source 9. This causes the surface mount resistors 5 to heat up by ohmic heating, thereby transferring heat to the heating layer 3 of the PCB 1. Heat is then transferred to the electronic components 2 mounted en the PCB 1. The temperature sensor 7 provides a temperature signal to the controlled voltage source 9 in order to regulate the voltage supplied to the surface mount resistors 5. The temperature cf the PCB l and the electronic components 2 can therefore be controlled. The surface mount resistors 5 can be arranged in any convenient configuration to provide a desired heating pattern on the PCB. For particular heating patterns, different individually controlled groups of resistors can be arranged across the PCB. Heat can then be supplied in different amounts to different regions of the PCB. Similarly, more than one temperature sensor may be provided. The heating layer 3 can be provided by the normal conduction laver of the PCB, or by the normal earthing layer of the board and so can be within the board layers . Alternatively, an additional copper heating layer can be provided which is exclusively used for heating the electronic component mounted on the board. Figure 2 shows a view of a second PCB 10 embodying the present invention. The PCB 10 has an insulating upper layer 12 on which are arranged a number of surface mount resistors 13. Within the board there is provided a copper layer 1 . The resistors 13 are in thermal contact with the copper layer 14 by way o-f a plurality of via holes 15. When a voltage is applied, by way of connectors 16 and 17, to the resistors 13, they heat up so that heat is transferred to the copper layer 14. This in turn raises the temperature of the whole PCB 10. The voltage supplied to the resistors can be controlled in dependence upon an output from a temperature sensor (not shown) , as in the Figure 1 circuit board.

Claims: CLAIMS 1. A printed circuit board comprising a multi- layered structure, one of the layers of the structure providing a heating layer such that heat applied to that layer is transferred to electronic components mounted on the printed circuit board; a plurality of surface mount resistors arranged in groups, wherein the resistors are mounted in contact with the said heating layer; a controlled voltage source for providing a control voltage to the resistors; and a temperature sensor mounted on the board for providing a signal representing the temperature of the board to the controlled voltage source, the controlled voltage source being operable to provide control voltages to the surface mount resistors in dependence upon the temperature signal, each group- being individually controlled by the control voltage source. 2. A printed circuit board as claimed in claim 1, comprising a substrate layer, a ground layer and a conducting layer, the ground and conducting layers being isolated from one another by an insulating layer, one of the conducting and ground layers providing the said heating layer. 3. A printed circuit board substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings. 4. A method of heating electronic components mounted on a printed circuit board; substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: GUIVER ANDREW PETER JONES, MORTON EDWARD PETER, GUIVER, ANDREW, PETER, JONES, MORTON, EDWARD, PETER
Applicants: ERICSSON MOBILE COMMUNICATIONS, GUIVER ANDREW PETER JONES, MORTON EDWARD PETER, ERICSSON MOBILE COMMUNICATIONS (UK) LIMITED, GUIVER, ANDREW, PETER, JONES, MORTON, EDWARD, PETER
Application year: 1998
Main classification: H05K1
IPC codes: H05B3, H05K1