GB2314699A

GB2314699A - Detecting overheating of printed circuit boards

Info

Publication number: GB2314699A
Application number: GB9613528A
Authority: GB
Inventors: Andrew Pinnell-Hutchison; Thomas Paul Groves; Jason Robert Reuben Jackson
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1996-06-27
Filing date: 1996-06-27
Publication date: 1998-01-07
Also published as: GB9613528D0

Abstract

A printed circuit board has first and second detection planes 23,24 across which a test voltage is applied so as to detect the resistance or capacitance between them. The second plane 24 includes a lattice of interconnected pads 25. If the value of the detected parameter varies outside a preset range, an alarm signal is generated to shut down or reduce the power supplied to the board.

Description

Method and Apparatus for Detecting Overheating of Printed Circuit Boards Field of the Invention The present invention relates to a method and apparatus for detecting damage through overheating of one or more printed circuit boards which constitute an electrical load receiving power from an electrical power supply.

Background of the Invention Conventional over-current devices used to protect electrical printed circuit boards from a current overload are ineffective to prevent damage because the energy required to damage or destroy a printed circuit board may be as low as a few tens of watts. The trip characteristics of the overcurrent device are such as to be insensitive to this low level of overload. Such conventional devices are also ineffective in protecting current carrying conductors in electrical distribution systems in the presence of a loose connection. In these cases localised overheating occurs due to arcing at the point of the poor connection, but the overcurrent device cannot protect against this condition because the mean current being drawn may not exceed the rated current under normal conditions.

A heat detector may not be effective in equipment that has forced air cooling or a large number of components because the detector has to be in close thermal contact with the source of the heat. This would require the use of a large number of detector devices distributed throughout the equipment to guarantee satisfactory detection of overheating at a large number of sites. Thermal detector devices are also difficult to specify if they are expected to detect temperatures over a wide temperature range of operation, e.g.

-300C to +550C. A device that gives reliable indications at high ambient temperatures may not detect a problem in time at a low ambient temperature. A heat detecting device sized for the early detection of overheating at low ambient temperatures may operate falsely at high ambient temperatures. Furthermore, a heat detector would not be practical for detecting loose connections because a separate device would be required at everv interconnection point.

Smoke detectors can be used to detect the smoke caused by overheating at a loose connection, but they have many disadvantages associated with them. They require routine maintenance (annual cleaning), they are of large size relative to the circuit boards to be protected, they have a limited operating temperature range, they can provide false alarms through detecting dust particles, and it is difficult to position them effectively in a forced air environment.

Finally it may be necessary to install more than one detector.

Summary of the Invention The present invention seeks to provide an improved method and apparatus to detect the presence of overheating in one or more printed circuit boards constituting an electrical load receiving power from an electrical power supply.

According to the present invention there is provided a method of detecting damage through overheating of an electrical load comprising one or more printed circuit boards receiving power from an electrical power supply, the method comprising; monitoring a parametric characteristic of the board or boards to detect a variation in the characteristic which would indicate thermal damage to the load, and generating an alarm signal representing the onset of thermal damage to the load when the monitored characteristic varies outside a preset range.

Further according to the present invention there is provided apparatus to detect damage through overheating of an electrical load comprising one or more printed circuit boards connected to receive power from an electrical power supply, the apparatus comprising; monitoring means to monitor a parametric characteristic of the board or boards to detect a variation in the characteristic which would indicate thermal damage to the load, and alarm signalling means, included in the monitoring means, to generate an alarm signal representing the onset of thermal damage to the electrical load when the characteristic being monitored varies outside a preset range.

Brief Description of the Drawings The invention will now be described, by way of example, with reference to the accompanying drawings in which; FIG. 1 is an illustration of an apparatus embodying the invention to detect the breakdown of a printed circuit board load.

FIG. 2 is an illustration of an apparatus embodying the invention to detect the breakdown of a printed circuit board load.

FIG. 3 is an illustration in flow diagram form of an operation of the apparatus illustrated in FIG.1 and FIG. 2.

Description of a Preferred Embodiment In FIG. 1, a source of electrical power 20 is connected to supply an electrical load 21. The load 21 is constituted by a number of printed circuit boards although it is to be understood that the invention is applicable to a load which consists of only one such board. The boards are arranged in a series of layers as shown more particularly in FIG. 2.

Monitoring means 22 monitor one or more parametric characteristics of the circuit board material by measuring one or more electrical properties between two or more layers or tracks in the electrical load.

During the early stages of overheating of a printed circuit board, the parametric properties of the printed circuit board substrate will change. In the case where the parametric characteristic being monitored is indicated by the resistance of the substrate material, the monitoring means will register a decrease in resistance as the substrate material oxidises and carbon deposits form. In the case where the parametric characteristic being monitored is indicated by the capacitance presented to the monitoring means, the monitoring means may register an increase or a decrease in capacitance.

Variations in capacitance may be caused by a change in the dielectric constant of the substrate material as it heats and oxidises. Additional variations in capacitance may result from catastrophic conditions where layers or tracks on the printed circuit boards physically move relative to one another.

As shown more particularly in FIG. 2, the circuit boards include pairs of detection planes on adjacent layers. Each pair of adjacent detection planes includes a first detection plane, which may be a 0 volt detection plane, associated with a second detection plane such as is illustrated by the reference numeral 24 in FIG. 2. The second detection plane includes a lattice of interconnected pads 25.

The monitoring means 22 includes leads connected to apply a test voltage between adjacent detection planes so as to test the resistance between them. A fault is registered by a change in the detected resistance which occurs outside a preset range. Alternatively or additionally, the monitoring means applies a switching voltage between adjacent detection planes so as to test the voltage rise time and hence the capacitance between them. A fault is registered by a change in the detected capacitance which occurs outside a preset range.

The monitoring means 22 generates an alarm signal when a fault is detected. The alarm signal may be used as a shut down signal to switch off the supply current or may be used to reduce it to a safe level.

Referring now to FIG. 3, the power supply is powered up as represented by the block 26 and the monitoring means continuously monitors to determine whether the resistance of the substrate material is within specification as represented by the block 27 and whether the capacitance is within specification as represented by the block 28. If either the block 27 or the block 28 detects a fault because the parameter being monitored in either block 27 or 28 is outside the preset range of values, an alarm signal is triggered in the block 29 followed by shut down of the power supply.

Claims

1. A method of detecting damage through overheating of an electrical load comprising one or more printed circuit boards receiving power from an electrical power supply, the method comprising; monitoring a parametric characteristic of the board or boards to detect a variation in the characteristic which would indicate thermal damage to the load, and generating an alarm signal representing the onset of thermal damage to the load when the monitored characteristic varies outside a preset range.

2. A method according to claim 1, wherein the step of monitoring comprises monitoring the resistance through the substrate material of the one or more circuit boards in the electrical load.

3. A method as claimed in claim 1 or 2, wherein the step of monitoring comprises monitoring the capacitance through the substrate material of the one or more circuit boards in the electrical load.

4. A method as claimed in any one of the preceding claims, wherein the alarm signal is used to switch off the power supply.

5. A method as claimed in any one of claims 1 to 3, wherein the alarm signal is used to reduce the supply of current from the power supply.

6. Apparatus to detect damage through overheating of an electrical load comprising one or more printed circuit boards connected to receive power from an electrical power supply, the apparatus comprising; monitoring means to monitor a parametric characteristic of the board or boards to detect a variation in the characteristic which would indicate thermal damage to the load and, alarm signalling means, included in the monitoring means, to generate an alarm signal representing the onset of thermal damage to the electrical load when the characteristic being monitored varies outside a preset range.

7. Apparatus as claimed in claim 6, wherein the monitoring means is adapted to monitor the resistance through the substrate material of the one or more circuit boards of the electrical load.

8. Apparatus as claimed in claim 6 or 7, wherein the monitoring means is adapted to monitor the capacitance through the substrate material of the one or more circuit boards of the electrical load.

9. Apparatus as claimed in claim 6, 7 or 8, wherein the power supply has means responsive to the alarm signal to switch off the power supply.

10. Apparatus as claimed in claim 6, 7, or 8, wherein the power supply has means responsive to the alarm signal to reduce the current supplied by the power supply.