BACKGROUND ART In Euro Patent 0 518 900 there is described, amongst other aspects of the invention, apparatus for measuring dynamic torque transmitted by a body having an axis of rotation wherein one or more SAW transducers are located relative to a body with the or each transducers located on the body; a first signal input and a signal transmitter coupled to the signal input by inductive, capacitative or radio wave means of low power; a second signal input and a signal receiver coupled to the signal input by way of inductive, capacitative or radio wave means of low power; and a signal output common to the outputs of both transducers and a signal receiver coupled to first signal output by way of inductive, capacitative or radio means of low power; each transducer being located at discrete locations on or near the outside of the shaft for rotation therewith; each transducer comprising a piezoelectric substrate having mounted on one side a pair of interdigitated electrodes; one electrode of the pair being connected to the signal inputs; a first generator for applying an input signal at a pre-determined frequency to the first signal input; a second generator for applying an input signal to the second signal input; and a mixer for receiving a signal from the first generator and from the second generator whereby changes in signal output from the strain applied to the substrate can be derived. Hereafter a SAW transducer of this type is referred to as being'of the type described'.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a transducer of the type described in which the substrate has mounted upon it an electrode structure of a thin film conductor such as aluminium; one or more conducting tracks extending from the electrode structure to the periphery of the substrate boundary; a closure for components mounted on the substrate comprising: a lid component; and a peripheral component of plastics material secured by one side to the lid component and by the other side to the one side to the substrate; the peripheral component serving to space the lid component from the substrate so that in combination the substrate, the lid component and the peripheral component serve to define a hermetically sealed enclosure for the electrode structure while leaving a region of the or each track outside the enclosure in the vicinity of the periphery of the substrate exposed outside the enclosure.

According to a first preferred version of the first aspect of the present invention the peripheral component is derived from a sheet of plastic adhesive film. Typically the adhesive film is ABLEFILM 550.

According to a second preferred version of the first aspect of the present invention or of the first preferred version thereof a region of the substrate on the opposite side to that bearing the peripheral component is adapted for attachment to a surface of a component which is subject to strain and which is to be examined by means of the transducer. Typically the region includes an adhesive layer whereby the transducer can be attached to the surface of the component. Alternatively the region includes a metallised layer whereby the transducer can be attached to the surface of the component.

According to a second aspect of the present invention there is provided a workpiece bearing a transducer according to the first aspect or any preferred version thereof to enable strain generated in the workpiece to be examined by way of the transducer.

A development program was undertaken on the basis of the present invention to develop a process for packaging of SAW devices in a manner that provides for a hermetic seal and permits the maximum strain transfer from the substrate into the SAW transducer. The process was required to be suitable for low-cost, high volume manufacture.

BRIEF DESCRIPTION OF DRAWING An exemplary embodiment of the invention will now be described with reference to the accompanying drawing of which the sole figure is a diagrammatic section of a packaged SAW transducer.

MODE FOR CARRYING OUT THE INVENTION SAW transducer device 11 comprises an electrode structure 12, manufactured in thin- film aluminium, on a quartz substrate 13. The substrates 13 measure approximately 9mm by 7mm by 250 em thick. An aluminium track 14 extends from the electrode structure 12 to edge 15 of substrate 13 to allow electrical connection of the device 11 to an external network. Lid component 16 is also made of 250 *m thick quartz but is slightly narrower in outside dimensions then the substrate 13 so that ends 14A of the aluminium track 14 is exposed. The packaged design was based on the assumption that sealing would be achieved by a peripheral component 17 in the form of a 1mm thick annular ring-of material linking substrate 15 to lid component 16 so as to provide a hermetically sealed chamber 18 for the structure 12. Underside R of the substrate 13 is adapted for attachment to a workpiece to be examined by means of the structure 12.

INITIAL STEPS Initially there was an intention to use a peripheral component 16 in the form of a metallic joint in order to provide a wall between the SAW device and its quartz lid component. This approach had the merit that it that a metallic joint would be clean (free from organic compounds), highly reliable and would provide for a fully hermetic enclosure for the electrode structure. Accordingly a program of work was initiated to establish a process for providing the necessary diffusion bonded joint between the peripheral component 16 and the substrate 13 and the lid component 15. Having made a number of functioning SAW devices it was realised that a simpler and lower cost approach would be to use a plastic peripheral component 16 instead. This involved the use of a peripheral component 17 forming a ring frame and made of a sheet of adhesive film to fix the 250 *m thick quartz plate lid 16 component over each SAW device 12. Initially it was felt a plastic component would be incapable of providing the same degree of hermetic sealing as could be derived from a metallic peripheral component. However it would serve to readily provide for electrical insulation, so avoiding the complication of bridging the SAW connections arising from the use of a metallic peripheral component 17. An adhesive component is physically compliant which would help maximise the strain transfer through the substrate 13 to the SAW devices 12. Furthermore the provision of a plastic peripheral component 17 would be relatively simple to implement in volume production. The materials and process costs would be low and the provision of plastic peripheral components in the form of ring- frames preforms of adhesive sheet could be purchased as ready-made items from adhesive film suppliers. For other applications different plastics materials might be required.

Test were made to find a suitable sheet plastic for the peripheral component and to establish appropriate process conditions for its application. Although the maximum continuous service temperature is 150°C, the plastic component needs to be able to survive intermittent exposure to 300°C. This later characteristic is required if a packaged SAW device is to be bonded onto a steel shaft using silver-tin solder which melts at 221°C. A proprietary adhesive known as ABLEFILM 550 in the form of a sheet material was eventually chosen which had be in use in the electronics industry for some twenty years. The film is available in a range of thickness, the thinnest of which is 0.125mm. This would provide an adequate stand-off distance between the SAW device and a package lid.

The adhesive material for the peripheral component was obtained in the form of a flexible, translucent sheet that was slightly tacky at room temperatures. Preforms were readily cut from the sheet using standard metal punches or by hand using scissors and a modelling scalpel. A spare lid was used as a template to cut out a rectangle of adhesive film of the correct shape and a scalpel blade used to prepare the peripheral component preform by cutting away the central portion, fairly reproducible preforms were prepared in this manner with negligible yield loss.

To activate and cure the plastic preform as a component was said to require simultaneous exposure to heating and compressive load for a minimum time. The manufacturers recommended conditions were a temperature of 150°C and a pressure of 5 PSI for 0.5 hours and these were used initially. Subsequent tests were based on progressively altering the parameters to achieve the required end product. It was found that the recommended conditions resulted in considerable flow of the adhesive and an almost total collapse of the joint gap. Accordingly the pressure and temperature were decreased in successive increments and the time increased proportionately in order to achieve a full cure. During the course of this work it was discovered that accurate regulation of temperature is critical to controlling the flow of the adhesive. A high performance fan oven was used to enable temperature to be controlled within +/-0.1°C of the set point. This was found to give a reasonably reproducible result.

The process conditions finally established were 100°C, with no applied load, maintained for at least 30 minutes. This process time requires some qualification namely that after 30 minutes at 100°C, no further flow of the adhesive resulted either on extended heating at the same temperature (up to 16 hours) or on increasing the temperature to 150°C for a final 30 minutes. Either the extended heat-treatment or the short higher temperature treatment is necessary to fully cure the adhesive. The decision as to which approach to take and whether the dual temperature process is undertaken in a programmable temperature oven or in a two separate ovens requires further consideration on cost grounds. Technically they are equally viable.

Tests were carried out on sample devices encapsulated in the suggested manner for hermeticity in accordance with Military Standard 883E, Method Al. In summary this involves placing a package in a chamber, evacuating to 5mb, pressurising to 60psi with helium for two hours and then measuring the rate of helium leak at atmospheric pressure. The military specification for a hermetically sealed package is for a helium leak rate not to exceed 5x10-8 atm/cc/sec. The package volume was estimated to be 0.5 3 mm.

Following the leak test the packaged SAW devices were tested for functionality following packaging. The packaging process did not appear to have effected the electrical performance of the SAW devices.

INDUSTRIAL APPLICABILITY The experimental results indicated that the proposed method used for packaging of SAW devices involving an plastic adhesive preform provided for a hermetic seal without adversely effecting the electrical performance of the component. The process involves the use of commercially available sheet film adhesive and a simple heating process. Particular attention needed to be given to control of the process temperature to achieve a satisfactory seal without producing excessive flow of the adhesive over the SAW device.

While the experimental program served to show the possibility of achieving a marketable product making use of a package with a top of quartz (corresponding to the substrate) and a peripheral component of a plastic preforms. The selection of an appropriate fabrication process involves consideration of the intended use and the eventual working location of the resulting transducer. The packaging materials and process of manufacture of the transducer could provide for one or more desired functions such as the ability to sustain temperature or pressure conditions, electromagnetic shielding, chemical resistance, shock and vibration resistance. In addition to the electrode structure the packaging could serve to enclose other components enabling the transducer to provide further information or to enhance or optimise the operation of the basic electrode structure.

In use it is envisaged that the packaged component will be attached by way of the underside R of the substrate 13 (for example using an adhesive) to a surface of a component subject to strain which is to be examined by means of the transducer. In this way non-contact processes can be used to transmit information into the transducer and to recover information from it representative of the torque, strain or pressure to which the component is being subjected. The transducer is small and light and so, in addition to the non-contact communication path provided by the invention, it has virtually no significant effect on the component to which it is attached. In its packaged form the invention of the proposed transducer provides a basis for measuring virtually any magnitude of torque, strain or pressure on any size of component rotating at virtually any speed.

The back R of such a device can be metallised to give a better distribution of strain and than is likely to be possible from an adhesive attachment which are thought likely to suffer from creep over long periods of use (such as could arise in an automotive context).