WO1995011433A1

WO1995011433A1 - Manufacturing of strain gauged sensors

Info

Publication number: WO1995011433A1
Application number: PCT/GB1994/002299
Authority: WO
Inventors: John David Barnett
Original assignee: John David Barnett
Priority date: 1993-10-21
Filing date: 1994-10-20
Publication date: 1995-04-27
Also published as: GB9321699D0

Abstract

A force sensor comprising a printed circuit strain gauge has the strain gauge pattern (42) formed on a dielectric layer (38) in situ on the surface of the structural support (16), the strain of which is to be measured. The strain gauge may be formed from a conductive layer by etching and further areas of the same or another conductive layer on the dielectric layer may be simultaneously treated to provide processing circuit elements for the strain gauge. The method of construction can simplify the manufacture of force sensors and also improve the quality of the sensors.

Description

Manufacturing of Strain Gauged Sensors

This invention relates to sensors which incorporate strain gauges and to methods for the manufacture of such sensors.

Strain gauges are usually produced by etching the electrical elements of the gauges from a thin conductive layer in the form of either a rolled foil or a sputtered layer on a dielectric substrate. The sputtering method has many disadvantages, because of the difficulty of controlling it precisely and the limited range of materials which can be used. The high plant costs of sputtering method also count against it. The use of foil to form the strain gauge elements is not without its problems, however. The production of force sensors incorporating bonded foil strain gauges is usually carried out in two distinct stages. In the first stage a batch production process is employed in which a sheet of thin (eg. 5 microns) metal foil is adhesively bonded to a dielectric backing sheet some 50 microns thick and resistance elements (eg. individual resistors or pairs of matched resistors) for the strain gauge circuits are etched in the metal foil using multiple image photo tools. In the second stage the elements making up each sensor are bonded through their dielectric backing onto the support substrate or base for the sensor and are then electrically connected by conductor wires to each other and to a signal processing circuit.

Although the resistance elements are produced to closely controlled tolerances, when making up each sensor on its support, circuit trimming and calibration measures are normally required. For example, it is usually necessary to adjust the resistance of the strain gauge, which typically comprises a Wheatstone bridge circuit which must be balanced, because the characteristics of the circuit are seldom unaffected by the bonding process. These adjustments add significantly to the costs of production, and furthermore they can affect the performance of the force sensor.

According to one aspect of the present invention, there is provided a method of manufacturing a sensor having a strain gauge, comprising the steps of applying a dielectric coating and a conductive foil layer to a surface of a structural support or base with said coating disposed between said layer and said surface, and thereafter etching an electrical strain gauge pattern in said conductive foil layer.

By producing the strain gauge pattern only after the foil has been secured to the base that is to form its final structural support, the manufacture of the sensor is simplified. Furthermore, if the strain gauge pattern is etched individually, as compared with the batch production used in practice to produce the gauge resistors, it is possible to avoid the need for adjustment and calibration of the circuit formed by the pattern if the tolerances of the etching process can be appropriately controlled.

It may be arranged that the conductive foil layer is adhered to the dielectric coating after it has been applied said surface. However, a simpler procedure can be achieved by using a composite or laminated material that has respective dielectric and conductive foil layers, so that they are secured to said support ' surface in a single operation. When the etched resistance elements of a strain gauge are assembled on the support in the conventional method of manufacturing force sensors referred to above, further elements of the sensor are also put in place. For example, the strain gauge circuit usually requires trimming resistances. These are similarly manufactured by etching and are then bonded onto the structural support through their dielectric backing.

These further elements may be of the same material as the strain gauge pattern, in which case when employing the method of the present invention they can conveniently be etched in the foil simultaneously with the pattern. If they are of a different material, as is usually the case with trimming resistances, a layer of that material can be adhered to the dielectric coating and, like the strain gauge pattern, can be etched while in place on the structural support or base. It is similarly possible to form on the dielectric coating the electrical circuitry that is coupled to the strain gauge circuit for processing its signals and which has hitherto been required to be produced separately on its own printed circuit board.

Thus, according to another aspect of the invention there is provided a sensor comprising a dielectric layer secured to a surface of a structural support or base and having an electrical strain gauge pattern and electrical processing circuitry for the strain gauge signals etched from conductive foil material adhered to said dielectric layer.

Following the method according to the invention it is possible to bond a suitable dielectric material directly onto the base providing the final substrate or support without an intervening adhesive layer such as is required at present.

According to a further aspect of the invention, therefore, there is provided a sensor comprising a dielectric layer applied directly onto a surface of a structural support or base and an electrical strain gauge pattern etched from a conductive foil layer adhered to said dielectric layer.

Preferably, the thickness of the dielectric layer is not substantially more than 20 microns. Because the dielectric coating is never required to be self supporting, it can be made thinner than is conventional. Consequently, the strain gauge circuit can be located closer to the support surface than in known force sensors. It is then possible for the circuit to respond more closely to strains in the support and the accuracy of compensation for temperature variations can be improved, particularly when these are of a transient nature. The invention will be further described by way of example with reference to the accompanying drawings, wherein:

Figs. 1 and 2 are axial sectional views of, respectively, a known form of pressure sensor and a pressure sensor according to the present invention.

Fig. 3 is a cross-sectional view of a portion of the pressure sensor in Fig. 2,

Fig. 4 is a process diagram for the manufacture of the sensor in Fig. 2. Fig. 1 illustrates a known form of pressure sensor comprising strain gauges. The sensor has a stainless steel casing comprising a body part 2 provided with a threaded union 4 for attachment to a pressure vessel (not shown) with the interior bore 6 of the body part exposed to the pressure in the vessel. A hollow cover 8 of the casing is secured over the outer end face of the body part and carries a sealing gland 10 through which an electrical cable 12 having input/output leads 14 extends. The relatively thin end wall 16 closing the bore in the body part forms a diaphragm subject to flexure in dependence upon the pressure within the vessel. A strain gauge 18 is mounted on the end face to respond to the flexure of the diaphragm and comprises a self-supporting substrate layer of a dielectric material carrying metal foil elements connected in a strain gauge pattern by wire leads (not shown) . The foil is bonded to its substrate before being etched to form the individual resistors or pairs of resistors of the strain gauge pattern and the completed gauge is then in turn placed on the diaphragm outer face and bonded thereto through its substrate.

A carrier 22 fitting within the cover 8 supports a printed circuit board spaced, from the diaphragm. The printed circuit on the board and the associated surface-mounted components form a processing circuit 28 for the strain gauge signals. The processing circuit is connected to the strain gauge by fine wire leads 30 and the leads 14 of the input/output cable are soldered to terminals 32 of the processing circuit. Also mounted on the carrier 22 are further electrical elements (not shown) such as trimmer resistances for adjustment of the strain gauge values. Figs. 2 and 3 illustrate a pressure sensor that is constructed in accordance with the present invention. Parts already described are indicated by the same reference numbers. The gauge and all the associated electrical circuitry are now arranged as a single unit on the end face of the casing body part 2, which again provides a final structural support or base formed as a diaphragm 16.

As Fig. 3 shows in more detail, directly bonded onto the face of the diaphragm 16 by adhesive layer 36 is a dielectric coating 38. A coating of adhesive 40 over the dielectric coating secures an etched foil circuit 42 that forms the strain gauge of the sensor, eg. being in the form of a Wheatstone bridge. Fig. 3 also illustrates leads 44 that are attached by solder 46 to terminals of the strain gauge to connect it to a processing circuit (visible only in its surface mounted components) performing such functions as amplification and voltage regulation and possibly to a trimming resistance (not shown) , although it will be understood from the following description of the manufacture of the sensor, these may not be needed. The reference 42' in Fig. 2 indicates a common printed circuit that combines the strain gauge and elements of its electrical processing circuit. The circuitry on the dielectric layer is protected by a final encapsulating layer 48.

The main steps of a process for the manufacture of the sensor illustrated in Figs. 2 and 3 are indicated in Fig. 4, beginning with cleaning of the flexure, that is to say the body part of the sensor that has the diaphragm 16. This first cleaning step renders the top surface of the diaphragm chemically inert. It is then abraded lightly, for example by sand blasting, and cleaned again to facilitate bonding of the adhesive 36, eg. phenol epoxy resin, that is next applied by spinning. A preformed disc of dielectric material, preferably of polyimide 10 to 15 microns thick, is laid on the adhesive coat, care being taken to ensure no bubbles or voids are trapped under the disc. A further coating 40 of the same adhesive is next spun onto the dielectric coating 38, to a thickness preferably not more than about 5 microns, and is used to secure in place a disc of rolled metal foil, being preferably about 5 microns thick, substantially centrally on the flexure or diaphragm.

The foil -disc may be made of any appropriate electrical conducting material . A typical material for strain gauges, because of its low temperature coefficient of resistance (TCR) and the ease of attaching connector leads, is constantan, a solderable nickel-copper alloy, but a Ni-Cr alloy such as Evanohm or Stabiloy may be preferred for some applications because of its higher resistivity.

The disc is pressed firmly in place to ensure secure bonding without the creation of voids as the adhesive is cured. The pressure must be applied in a way that avoids introducing an unacceptable degree of a pre- strain in the diaphragm. For example, if the sensor is intended to be used to sense relatively high pressures (above 10 bar) a pressure of about 3 bar can be applied through a silicon rubber pad, preferably about 2.5mm thick, which is isolated from the adhesive by a PTFE disc, eg. some 50 microns thick.

When the adhesive is cured, a layer of photoresist is spun over the disc surface to a thickness of about 1 micron and this layer is set by baking. Conventional photo tools may then be employed to expose the desired strain gauge pattern on the photoresist layer. After the photoresist has been developed, the foil is etched to leave the required circuit pattern in place.

To achieve higher uniformity of product, it is preferred to employ the same photo tool for each sensor, and to locate the tool by a servo positioning system. A pre-production stage may be introduced in which the photo tool is modified after the trial manufacture and testing of prototype samples to obtain a more finely tuned circuit. In this way it is possible to take advantage of the uniformity of product that can be achieved by the process to avoid or reduce any need to adjust the final product. Preferably, the photo tool is adjusted while it is in its negative form and the negative is converted to a thin film coated glass ready for production use. By adopting this method and controlling the quality of the materials employed, strain gauge circuits can be produced within a tolerance with regard to the balance of the bridge that is satisfactory for many purposes, without resort to subsequent adjustments in the product, eg. by secondary etching, or laser or abrasive trimming. After etching the circuit pattern, the photoresist is chemically stripped from the flexure, which is then cleaned thoroughly before testing the circuit electrically. The encapsulation layer is spun onto the circuit pattern and is cured by baking. Lastly, the external leads 14 to the terminals 32 are soldered on to complete the sensor.

The above process can be simplified by using a composite sheet of dielectric and foil layers, to eliminate the step of securing the two layers in place separately. Alternatively, the dielectric coating can be spun on in a fluid condition and, after curing and spinning on an adhesive layer, the conductive foil is applied. In the process described above, the use of a foil as thick as 5 microns enables electrical elements ancillary to the strain gauge itself, in particular the printed circuit for processing the gauge signals, to be etched simultaneously from the same piece of foil. It is then possible to achieve a standard satisfactory for a general purpose sensor. For greater sensitivity, the gauge elements themselves are preferably of a thinner foil to increase their resistance while the processing circuit elements are of a thicker foil to reduce their resistance. This result can be obtained starting with a thicker, eg. 10 microns, foil by using a two-stage etching process. In a preferred contact masking process, as a first stage the printed circuit area is masked with the photoresist in the circuit pattern, as in the process first described, to produce the required etched pattern. After cleaning the surface again, further photoresist is laid on it to leave only the strain gauge area is exposed so that the foil there can be thinned by etching eg. to about 2.5 microns. Alternatively, using a projected masking process the two stages are performed in reverse order.

To improve the operating characteristics of strain gauge sensors, it is known to provide metal foil trimming resistors with a high TCR, such as nickel, on the diaphragm or flexure close to a strain gauge of constantan for temperature compensation of the gauge factor and diaphragm modulus. In the process according to the invention, an electrical resistance layer can be placed on the adhesive layer alongside the strain gauge layer before the adhesive is cured and the photo tool can incorporate the resistor pattern as well as the strain gauge pattern. Where there are reasons why a single piece of foil cannot provide both the strain gauge and processing circuit elements, the same procedure can be used to form the processing circuit for the strain gauge bridge from a separate foil layer on the diaphragm. It may be arranged that interconnects with the gauge are etched from the foil simultaneously with the processing circuit. It is also possible to apply a conductive mechanism, such as a silver-loaded epoxy layer, to the surface, eg. by screen printing, in known manner, to form the required interconnects after the etching operations, in particular if two or more pieces of foil are etched to provide the circuitry. Both methods avoid the need to solder delicate connecting wires in place. If layers of different materials are put in place side by side prior to etching, a common etching fluid suitable to all the materials may be used or the different materials may be masked successively. Because the processes described avoid the need for a self-supporting and therefore relatively thick dielectric backing, creep can be reduced and output performance improved. The processes also alleviate the problems of handling delicate foils that are present in conventional manufacturing techniques, so that it is possible to use thinner foils than usual and thereby to produce higher resistance strain gauges. The processes are particularly suitable for the production of relatively small force sensors. By bonding the foil in place before etching the strain gauge pattern and automatically controlling the tooling position, a more consistent product can be obtained and plant and overall production costs reduced. Not at least of the advantages is the ability to avoid many of the subsequent adjustment operations after the formation of the strain gauge pattern. This is also an aid to consistency in the product.

Simplification and improved consistency can also result from the ability to locate the strain gauge and electrical circuitry elements in the same plane and on a common substrate in a manner which makes it unnecessary to form delicate wire interconnections in the usual way. The only connections that need to be made by hand are those to the input/output cable.

The invention also lends itself to mechanical production techniques for surface mounted assemblies. This factor together with the reduced number of components necessary and the avoidance of the need to make delicate electrical connections can increase the reliability of the final product.

Although.the invention has been exemplified in the preceding description by a pressure sensor in which the deformation of the diaphragm in response to the fluid pressure upon it generates the sensor input signals, it will be understood that force sensors have a wide variety of uses and they may therefore take many different configurations within the scope of the present invention to measure different types of strain in different structures.

Claims

1. A method of manufacturing a sensor having a strain gauge, comprising the steps applying a dielectric coating and a conductive foil layer to a surface of a structural support or base with said coating disposed between said layer and said surface, and thereafter etching an electrical strain gauge pattern in said conductive foil layer.

2. A method according to claim 1 wherein the dielectric coating is first applied to said surface and the conductive foil layer is adhered to the said coating.

3. A method according to claim 1 or claim 2 wherein at least one further electrical element is etched from said conductive foil layer simultaneously with the strain gauge pattern.

4. A method according to claim 3 wherein the conductive foil layer is etched to different thicknesses in respective areas provide said strain gauge elements and said electrical circuit elements respectively.

5. A method according to any one of the preceding claims wherein a further conductive layer is adhered to said dielectric coating and is etched to form at least one further electrical element for cooperation with said strain gauge pattern.

6. A method according to claim 5 wherein said strain gauge pattern and said further element are connected by a conductive medium deposited upon the dielectric layer.

7. A method .according to any one of claims 3 to 6 wherein electrical processing circuitry for the strain gauge signals is etched from the or at least one said conductive foil layer on the dielectric coating.

8. A sensor comprising a dielectric layer applied directly onto a surface of a structural support or base and an electrical strain gauge pattern etched from a conductive foil layer adhered to said dielectric layer.

9. A sensor comprising a dielectric layer secured to a surface of a structural support or base and having an electrical strain gauge pattern and electrical processing circuitry for the strain gauge signals etched from conductive foil material adhered to said dielectric layer.

10. A sensor according to claim 9 wherein said strain gauge pattern and said electrical processing circuitry are etched from a single conductive layer adhered to said dielectric layer.

11. A sensor according to claim 10 wherein the conductive layer for said strain gauge pattern and said processing circuitry are formed from a foil layer having a uniform thickness of approximately 5 microns.

12. A sensor according to claim 10 wherein said strain gauge pattern and said processing circuitry is formed from a one-piece conductive foil layer which has different thicknesses in the respective regions of said strain gauge and further elements.

13. A sensor according to claim 9 wherein said strain gauge pattern and said processing circuitry are etched from separate conductive layers.

14. A sensor according to claim 12 wherein said strain gauge pattern and said processing circuitry are connected by a conductive medium deposited on the dielectric layer.

15. A sensor according to any one of claims 10 to 13 wherein said strain gauge pattern and said processing circuitry lie in substantially the same plane.

16. A sensor according to any one of claims 8 to 15 wherein the dielectric layer is not substantially more than 20 microns thick.