GB2336432A

GB2336432A - Temperature indicating means for electronic components

Info

Publication number: GB2336432A
Application number: GB9808228A
Authority: GB
Inventors: Stuart Edwin Stanton Wilkie
Original assignee: Warth International Ltd
Current assignee: Laird Technologies Ltd
Priority date: 1998-04-17
Filing date: 1998-04-17
Publication date: 1999-10-20
Anticipated expiration: 2018-04-17
Also published as: GB9808228D0; GB2336432B

Abstract

A thermal interface pad for electrically insulating an electronic component whilst allowing heat dissipation includes means to indicate the temperature of the electronic component. The temperature indicating means comprises a thermochromatic material distributed either on or in the surface of the pad or substantially throughout the pad. The thermochromatic material may give a visual indication that the temperature of the electronic component falls within two or more discrete temperature ranges or may provide a continuous visual change over a single temperature range for more accurate temperature determination. The visual change may be permanent or reversible. Such thermal interface pads may be used to monitor the temperature of electronic components, particularly inaccessible electronic components, during operation in electronic assemblies. More particularly they may provide an indication that a maximum desired temperature has been exceeded.

Description

2336432 1 TENPERATURE INDICATING MEANS FOR ELECTRONIC CONPON-ENTS This

invention relates to a method and material for indicating the temperature of electroffic components, particularly suitable for monitoring the temperature of inaccessible electronic components in electronic assemblies.

For electronic assemblies, in order to be able to predict the reliability of a panicular electronic component such as a semiconductor when used within a particular system, and consequently the reliability of the system as a whole, it is generally necessary to determine the maximum temperature which the component reaches in situ when the system is operated. To this end, the maximum temperature reached by such electronic components is generally determined by attaching a temperature indicating meter. probe or gauge to the surface of the component and thus monitoring its temperature in situ. However, it is not always possible or practical to gain access to the component surface with such temperature indicating means, particularly where the component is located in a densely packed electronic assembly.

There is therefore a need for a temperature indicating means which allows the temperature of relatively inaccessible components to be monitored more easily.

It would furthermore be useful in certain cases to be able to record whether a particular electronic component had exceeded a specified temperature at any time during its operation in a system, for example in order to enable service or design enaineers to inspect the system periodically to identify electronic components which exceeded their temperature specification during use. However, temperature probes or gauges currently used to monitor the component surface temperature will generally indicate the instant temperature at any given moment only, without recording the maximum temperature attained by the component over a period of time.

2 Therefore, a finther need exists for a means of recording whether a specified temperature has been exceeded by a component over a given time period.

Thermal interface pads (TIPs) are thermally conductive, electrically insulating structures used as interface media to allow heat conduction whilst preventing electrical conduction between separated components. Conventionally, they are used in electronic assemblies to conduct heat generated by electronic components to heat dissipaters such as heatsinks whilst electrically isolating the electronic components &om the heat dissipaters. Thermal interface pads have been used for this specific purpose in electronic assemblies throughout the electronics industry.

We have now found that it is possible to achieve the desired aim of indicating the temperature of components in electronic assemblies by using thermal interface pads which have been specially modified to be able to indicate temperature.

Accordingly, the present invention provides a thermal interface pad comprising thermochromic material.

In a first embodiment of the invention, the thermochromic material is distributed substantially uniformly throughout the thermal interface pad. Alternatively. in a second embodiment, the thermochromic material is only present substantially on or in a surface of the thermal interface pad.

The TIP according to the invention indicates the temperature of a component with which it is placed in contact by adopting a colour, hue or shade which corresponds to a specific temperature or temperature range. By visually observing or otherwise measuring the colour, hue or shade assumed by the TIP during contact with an 7W electronic component, the maximum temperature attained or whether a specified threshold temperature has been exceeded by the electronic component can readily be determined.

Preferably, the temperature indicating TIP is bi-stable, meaning that below a specified threshold temperature it adopts a first colour, hue or shade, and at or above the threshold temperature it adopts a second colour, hue or shade which is discretely different from the first. The thermochromic material is selected accordingly to undergo a transition at the specified threshold temperature, so as to be capable of indicating two discrete temperature ranges.

For certain applications it will be useful to indicate whether the temperature reached by a component falls within one of three or more temperature ranges. In these circumstances, by using a suitable combination of thermochromic compounds a multistable TIP may be prepared which undergoes a discrete change in colour, hue or shade at two or more specified threshold temperatures.

In certain cases it will furthermore be useful to indicate the temperature reached by the component more specifically. In these circumstances an appropriate combination of thermochromic compounds may be used to prepare a TIP which undergoes a gradual change across a continuous spectrum of colours, hues or shades according to the temperature of the component in contact with the TIP.

Thus, a temperature or teme range which an electronic component has exceeded or reached may be indicated by the colour, hue or shade adopted by the TIP corresponding to that temperature ortemperature range.

The colour, hue or shade of the TIP during or following contact with the C1 component is preferably observed visually, although other suitable reading means may be contemplated to interpret the colour, hue or shade indicated.

The temperature indicating -TIPs according to the invention may convem ently be prepared from thermochromic matedal and the components of a conventional TIP.

4 Suitable thermochromic materials are commercially available, and can be selected as appropriate by those skilled in the ail according to the specified temperatures or temperature ranges which it is desired to indicate. The thermochromic material may comprise one or more thermochromic compounds. Thermochromic compounds suitable for use in accordance with the invention include for example 1,3,3)-trimethylindolinobenzopyrylospiran, 1,3,3-trimethylindolino-6bromobenzopyrylospiran, 1,3),,"j-trimethylindolino-8methoxybenzopyrylospiran, 1,3,3)-trim ethylindolino-betanaphthopyrylospiran, 1,3,-')-trimethylindolino-6-m'trobenzopyrylospiran and 1, 3,3 -trim ethylin dolin onaphthosiroo xazine.

It will be appreciated that any suitable thermochromic material which can indicate the temperatures of interest may be used to produce TIPs in accordance with the invention. The amount of thermochromic material used is not important provided it is sufficient to permit changes in colour, hue or shade to be detected by observing the TIP.

Preferably, the thermochromic material used undergoes a permanent or residual change in colour, hue or shade when the specified threshold temperature is exceeded. Thus, the TIP can record whether an electronic component has exceeded a specified temperature during its operation so as to permit an observer to discover the same by subsequent inspection of the TIP. Alternatively, the thermochromic material is selected from materials which revert to their origm'al colour, shade or hue when the temperature of the electronic component drops below the transition temperature or the TIP is removed from the component, so that the temperature attained by the electronic component during operation can only be discovered by observing the TIP in contact with the electronic component in situ during its operation.

Suitable components of conventional TIPs which may be used together with the thermochromic material to prepare the temperature indicating TIPs of the invention include electrically non-conductive materials such as silicone polyester compounds, in particular room temperature vulcanising silicones, mixed with thermally conductive materials such as alumina and boron nitride. These materials may be mixed together with the thermochromic material, in a suitable solvent such as toluene, to produce a curable mixture. The relative amounts of constituents of the mixture can be varied according to the grade of TIP being produced, as is known to those skilled in the art. Typically, the ratio of non-conductive silicone to thermally conductive boron nitride is from 90: 10 to 65:35 by weight. The mixture may then be cured and extruded or calandered to the desired shape and size in conventional manner.

Where appropriate, the temperature indicating TIPs according to the invention may be reinforced with suitable reinforcing materials known in the art for the preparation of conventional reinforced TIPs. For example, reinforcing materials such as fibreglass or polyimide, suitably in the form of a fabric sheet, may be coated with a curable mixture as described above, and cured to produce a reinforced temperature indicating TIP material. The cured material can be formed or cut to the required shape and size in conventional manner.

In a preferred process for manufacturing temperature indicating TIPs according to the invention, two or more coa of curable mixture are applied and cured successively onto a suitable carrier in the form of a sheet. Thus, TIPs in accordance with the first embodiment may be "ared by application and curing of successive coatings from one curable mixture comprising the thermochromic material, to obtain a TIP comprising thermochromic material distributed substantially throughout its crosssection. Alternatively, two differeatcurable mixtures may be used, with only one of the mixtures comprising the thermochromic material, so as to apply the thermochromic material only in the final coating. Thus, a TIP according to the second embodiment having the thermochromic materi.ent only in the surface layer may be prepared.

A 6 Although the temperature indicating TIPs according to the invention are particularly suitable for monitoring the temperature of electronic components in electronic assemblies, it will be appreciated that they may be used to indicate the temperature of any appropriate structure or surface with which they can be placed in contact. Accordingly, the TIPs according to the invention are not limited in their application to electronic components, but may be used in other applications where the temperature of a structure or surface which must be electrically isolated from a heat dissipater is of interest.

The invention may be finther illustrated by the examples which follow:

Example 1

Temperature indicating TIPs in accordance with the invention were manufactured from the silicone, boron nitride and glass fibre materials typically used to prepare conventional TIPs. and commercially available thermochromic particles, as follows:

The silicone and boron nitride particles 'm a weight ratio of 80:20 are mixed in toluene as solvent using a 'Z' blade mixer. The thermochromic particles are added in an amount sufficient to provide between 1 and 1 Og per square meter of finished product and mixed to form a curable coating mixture. The coating mixture is poured into two header tanks, each feeding a continuously circulating dip tank. A continuous fabric sheet of glass fibre dispensed from a roll is drawn through the first dip tank and thus coated with a thin layer of the coating mixture. After exiting the first dip tank the coated sheet is fed up and down a curing tower. Next, the cured sheet enters the second dip tank where a second coating is applied over the cured first coating. On exiting the second dip tank the coated sheet is passed through thickness control rollers before travelling, up and down a second curing tower. The cured sheet is wound on a roll for storage and transport, and cut to size to form finished TIPs containing a uniform distri'bution of thermochromic material.

7 Example 2

The process described in Example 1 was followed, except that only the second dip tank contained a mix comprising thermochromic particles. The finished TIPs obtained by the process contained thermochromic material only in the surface layer.

8

Claims

CLAIMS:

1. A thermal interface pad comprising thermochromic material.

7 A pad according to claim 1 wherein thermochromic material is present on or in a surface of the pad.

A pad according to claim 1 or claim 2 wherein thermochromic material is C> distributed substantially throughout the pad.

4. A pad according to any of claims 1 to 3 wherein the thermochromic material comprises a mixture of thei-mochromic compounds.

5. A pad according to any of claims 1 to 4 wherein the thermochromic material can indicate two discrete temperature ranges.

6. A pad according to any of claims 1 to 4 wherein the thermochromic material can indicate at least three discrete temperature ranges.

7. A pad according to any of claims 1 to 4 wherein the thermochromic material c dicate any specific temperature within a range.

an in Z A method of indicating the temperature of an electronic component, which comprises contacting the electronic component with a thermal interface pad as defined in any of claims 1 to 7.

8.

9. A method of monitoring the temperature of an electronic component i. n si. w. which comprises disposing a thermal Miterface pad as defined in any of claims 1 to 7 in contact with the electronic component and observMig the pad during or after operation of the component.