GB2300340A

GB2300340A - Heating electrical components

Info

Publication number: GB2300340A
Application number: GB9603041A
Authority: GB
Inventors: Michael Owen Mccann
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1995-04-28
Filing date: 1996-02-14
Publication date: 1996-10-30
Also published as: GB9508631D0; DE19608858A1; GB9603041D0

Description

1 ELECTRICAL CIRCUITS This invention relates to electrical circuits.

2300340 Electrical components are often required to operate satisfactorily over a range of temperatures. In vehicle applications, such as in aircraft, the components may have to perform reliably in temperatures ranging from -55' to +125'C. Because of this, it has been the practice to use military specification, or other high specification, components certified to operate over these temperature ranges. There are, however, various difficulties with using these components such as their high cost, the small range of suppliers and components, and the increasing shortage of the components. This has led manufacturers to search for ways in which lower specification components can be used safely in circuits and, in particular, ways in which components can be used in circuits required to operate at below the rated temperatures of the components.

It is an object of the present invention to provide an electrical circuit assembly capable of operating satisfactorily at temperatures below the rated operating temperatures of the components of the circuit.

According to one aspect of the present invention there is provided an electrical circuit assembly including a circuit board, an electrical component mounted on the board, a conductive track formed on the board, the track having a region thereof of a resistive nature in close thermal proximity to the electrical component, and means for applying a voltage across said'track sufficient to cause the resistive region to increase in temperature so as thereby to heat the electrical component.

The track preferably has a first layer of a first material having a high resistivity and a second layer on top of the first layer of a second material having a lower resistivity, the 2 resistive region being formed by removal of the second layer along the region. The first material may be nickel and the second material may be copper. The assembly may include a thermal transfer member of a thermally- conductive and electrically-insulative material in thermal contact with the resistive region of the track and the underside of the electrical component. The circuit preferably includes a thermostat device connected in series in the track and located where it is subjected to a similar temperature as the electrical component, the thermostat device being arranged to prevent current flow along the track when temperature rises above a predetermined value. The circuit preferably includes a safety cut- out connected in series in the track, the safety cut-out being arranged to prevent current flow along the track when current flow along the track exceeds a predetermined value.

An electrical circuit assembly according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which..

Figure 1 is a plan view showing the circuit schematically; and Figure 2 is a sectional side elevation view of a part of the circuit along the line II-II of Figure 1.

The circuit assembly comprises a printed circuit board 10 supporting various electrical components 11 to 14, including a power supply 15.

The printed circuit board 10 may be of the multi-layer type, although only one layer is shown in Figure 2, for simplicity. The board 10 has an electrically-insulative substrate 16 and, on its upper surface, a sandwich of two electrically-conductive layers 17 and 18 directly on top of one another. The lower conductive layer 17 is of nickel, whereas the upper layer 18 is of copper, the upper layer being exposed on the upper surface of the board. The two layers 17 and 18 are etched through, in part, to the underlying substrate 16 to form a pattern of various electrically-conductive tracks 20 to 26 on the upper surface of the board 10, by which the 3 electrical components 11 to 15 are interconnected. The power supply 15 is connected across opposite ends of one of the tracks 20 so that a voltage is applied across the track.

In one region 30 of the track 20, the upper copper layer 18 is etched away to expose the underlying nickel layer 17. Because the resistivity of nickel is about four times that Of copper, this region 30 has an increased resistance compared with other parts of the track 20. Other resistive materials, instead of nickel, could be used. If the thickness of the nickel layer 17 is less than that of the copper layer 18, the increase in resistance caused by removal of the copper layer is greater. The resistance can be further increased by reducing the width of the track in the resistive region 30 so that the cross-sectional area of the nickel track is reduced.

One of the electrical components 11 is a video RAM, or V-RAM, which is a conventional commercial device rated to operate between 0' and 7CC. The component 11 Is mounted on the upper surface of the board 10 directly above the region 3 0 of the track 20. The component 11 makes electrical connection to other tracks 21 to 26 on the upper surface, or to buried tracks (not shown) within the thickness of the board. A thermal transfer member or wafer 31 of an electrically-insulative but thermally-conductive material is sandwiched between the component 11 and the board 10 so that the lower surface of the wafer contacts the resistive region 30 of the track 20 and the upper surface of the wafer contacts the underside of the component.

The track 20 is broken at two locations 27 and 28 on opposite sides of the resistive region 30. One of these breaks 27 is bridged by one of the components 12, which is a surfacemounted thermostat. The thermostat 12 is connected to the track 20 on opposite sides of the break 27 and overlaps a part of the resistive region 30. The thermostat is closed, or on, at low temperatures to enable current flow along the track 20 and opens, or turns off, at temperatures above about 1 O'C (that is, within the operating range of the V-RAM component 11) to prevent current flow along the track. When the temperature drops below this value, the thermostat 12 closes again. The other location 28 is bridged by another one of the 4 components 13, being a thermal safety cut-out, which is a conventional re-settable temperature and current overload trip, which opens the circuit when current flow exceeds a predetermined safety value.

When the electrical circuit is turned on, the power supply 15 applies a voltage across opposite ends of the track 20. If the circuit is cold, that is below the cut-off limit of the thermostat 12, the thermostat will be closed and current will flow along the track 20. Because of the low resistance of the upper copper layer 18, there will only be a low voltage drop along most of the length of the track. However, where the copper layer 18 has been removed, that is, in the region 30, there will be an appreciable resistance and a significant heating effect will be produced. Heat produced in the region 30 is conducted through the wafer 31 to the component 11, so as to raise its temperature well within the operating range of the component. The thermostat 12 also will be heated by heat from the resistive region 30 and, when the temperature rises above the thermostat cut-off temperature, the thermostat will open and stop any further heating of the component I I until the temperature falls again. This ensures that the temperature of the component is not raised too high and reduces power dissipation. If the track 20 should be shorted, or some other fault occur, leading to an excessive current flow through the track, the thermal cut-out 13 would trip, preventing any further damage.

The present invention enables electrical components to be readily heated without the need for attaching separate heaters to the components. A heating facility can thereby be provided in an electrical circuit using conventional automated fabrication and assembly techniques and without substantially increasing the weight of the circuit.

It will be appreciated that resistive regions could be provided at several locations along a track or along several tracks, so that several components could be heated. Additional heating effect could be produced by bending the track in the resistive region into serpentine shapes, to increase the length of the resistive region. The resistive region need not be formed by removal of an overlying layer but could, for example, be formed by reducing the cross-sectional area of the track in the desired region, by substitution of that part of the track with a more resistive material, by treatment of the track material or by other techniques. The electrical component should be in close thermal proxinfity to the resistive region of the track but it may not be necessary to use a thermally-conductive wafer, instead, the component could be in direct contact with the resistive region or a potting compound or thermally-conductive grease could be used to improve conduction of heat from the resistive region to the component.

Alternatively, the resistive region could be formed as a part of an inner conductive layer of a multi-layer printed wiring board. A thermal interface to the outer surface of the board could be provided by a series of conductive plated holes, or thermal vias. These thermal vias would be terminated with a thermally-conductive member on the outer surface, to establish a conductive path to the component.

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Claims

An electrical circuit assembly including a circuit board, an electrical component mounted on the board, a conductive track formed on the board, the track having a region thereof of a resistive nature in close thermal proximity to the electrical component, and means for applying a voltage across said track sufficient to cause the resistive region to increase in temperature so as thereby to heat the electrical component.
An assembly according to Claim 1, wherein the track has a first layer of a first material having a high resistivity and a second layer on top of the first layer of a second material having a lower resistivity, and wherein said resistive region is formed by removal of the second layer along said region.
3. An assembly according to Claim 2, wherein said first material is nickel and said second material is copper.
4. An assembly according to any one of the preceding claims including a thermal transfer member of a thermally-conductive and electrically-insulative material in thermal contact with said resistive region of the track and the underside of the electrical component.
5., An assembly according to any one of the preceding claims, wherein the circuit includes a thermostat device connected in series in the track and located where it is subjected to a similar temperature as the electrical component, and wherein the thermostat device is arranged to prevent current flow along the track when temperature rises above a predetermined value.

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6. An assembly according to any one of the preceding claims, wherein the circuit includes a safety cut-out connected in series in the track, and wherein the safety cut-out is arranged to prevent current flow along the track when current flow along the track exceeds a predetermined value.
7. An electrical circuit assembly substantially as hereinbefore described with reference to the accompanying drawings.
8. Any novel feature or combination of features as hereinbefore described.