Title of Invention

A PROCESS FOR THE PREPARATION OF THICK FILM RESISTOR USEFUL FOR MAKING STRAIN GAUGE

Abstract

This invention particularly relates to a process for the preparation of thick film resistor useful for making strain gauge. The thick film resistor of the present invention is obtained by inducing the strain sensitivity in thick film resistor after its fabrication on substrate by coating it with a formulation capable of gauge factor enhancement and subjecting it to high temperature in furnace. The usage of thick film resistor have industrial applications such as hybrid circuit and thermistros etc. The said process comprises: (a) cleaning an insulating substrate; (b) coating the said cleaned substrate with a known non—corrosive conductor thick film having a thickness at least 10 microns, (c) drying the conductor coated substrate at a temperature in the range of 60-120°C for atleast 15 minutes; (d) coating by known methods the prepared substrate with a known resistor thick film having a thickness of atleast 15 microns; (e) drying the resistor coated substrate at a temperature in the range of 60-120°C for atleast 15 mins; (f) firing the said coated substrate at temperature in the range of 8GO-900°C for a period of 10-20 mins to obtain a thick film resistor; (g) coating the prepared thick film resistor with a cyanometallic compound having a thickness in the range of 10-25 microns;(h) drying the compound coated substrate at a temperature in the range of 60-120°C for atleast 10 mins; and (i) firing the prepared substrate to a temperature in the range of 800-900°C for a period of 10 - 20 mins to obtain an improved thick film resistor useful for making strain gauge with enhanced gauge factor from 0 to 1000.

Full Text

This invention relates to a process for the preparation of thick film resistor useful for making strain gauge. The thick film resistor of the present invention is obtained by inducing the strain sensitivity in thick film resistor after its fabrication on substrate by coating it with a formulation capable of gauge factor enhancement and subjecting it to high temperature in furnace. The usage of thick film resistor have industrial applications such as hybrid circuit and thermistros etc. The hitherto known strain gauges are of two types. The first one. makes use of metal foil ( Micro-Measurements. Measurements Group, Vishay Inlet-technology. Inc.. P.O.Box 30(>, 38905 Chase Road, Romulus, Michigan 48174) pressed between two insulating laminates and the second one. uses the silicon material ( Kulite Semiconductor International Ltd.. One Willow Tree Road. Leonia, New Jersey 07605 ) having diffused layer formed by impurity drive-in at high temperature in an inert gas. The gauges thus formed exhibit 2 - 5 as the gauge factor for the former type and 45-175 for the latter version. Such gauges arc fragile and arc temperature sensitive too. The substrate is commonly used to mount the gauge onto it. There is a definite need for the enhancement of the gauge factor for improving the signal conditioners and for developing the sensors for medical applications. Our studies have shown that such strain gauge of high gauge lactor is not available and is not possible with existing material technology.Our studies have also provided us the experimental fact that it is possible to increase the gauge factor by enhancing the conductive phase size of thick film resistive material. As per our process Ihe surface morphology of the thick film resistor material gets converted to long conductive phases and their tunnel distance get enhanced. This in turn decreases the effective sheet resistance and enhances the gauge factor through geometrical and pic/.orcsistive effects. To the best of our knowledge till dale such gauges based on thick film technology having enhanced gauge factor are not available internationally . The main object of the present invention is to provide a process for the preparation o! an improved thick film resistor useful for making strain gauge with enhanced gauge fuclor upto 1000 overcoming the drawbacks of the hitherto known strain gauges. An embodiment of the process steps of the present invention is illustrated in Fig. 1 of the drawings accompanying this specification. Fig. 1 represent s the sectional view of the substrate (1) having conductor thick film (2) resistor thick film (3) and cyanometallic compound formulation (4) formed by silk screen printing and firing process respectively. Accordingly, the present invention provides a process for the preparation of film resistor useful for making strain gauge comprising: (a) cleaning an insulating substrate; (b) coating the said cleaned substrate with a known non—corrosive conductor thick film having a thickness at least 10 microns, (c) drying the conductor coated substrate at a temperature in the range of 60-120°C for atleast 15 minutes; (d) coating by known methods the prepared substrate with a known resistor thick film having a thickness of atleast 15 microns: (e) drying the resistor coated substrate at a temperature in the range of 60-120°C for atleast 15 mins; (f) firing the said coated substrate at temperature in the range of 800-900°C for a period of 10-20 mins to obtain a thick film resistor; (g) coating the prepared thick film resistor with a cyanometallic compound having a thickness in the range of 10-25 microns;(h) drying the compound coated substrate at a temperature in the range of 60-120°C for atleast 10 mins; and (i) firing the prepared substrate to a temperature in the range of 800-900°C for a period of 10 - 20 mins to obtain an improved thick film resistor useful for making strain gauge with enhanced gauge factor from 0 to 1000. The substrate (1) used may be such as a ceramic substrate an alumina, enameled steel, oxidized silicon wafer and any other material which is an insulator and is capable of withstanding high temperature. The substrate (1) may be cleaned by known methods such as chemical method, ultrasonic cleaning in acetone, boiling in trylene and followed by rinse in methanol. The coating of the substrate (1) with thick film (2,3,4) may be effected using standard silk screen printing technique commonly used for thick films. The non corrosive conductor thick film used may be such as Pt/Au, Pd/Ag, Pt and Au. The resistor thick film used may be such as Birox formulation etc. The cyanometallic compound used may be such as gold platinum solution, potassium ferrocyanide etc. The drying and firing of the coated substrate may be effected in an air atmosphere. The coated substrate after every heating at high temperature may be cleaned to remove dirt and impurities from it prior to next coating and heating. In the process of the present invention we have prepared and improved thick film resistor capable of being used as a strain gauge with enhanced gauge factor. This has been possible through modifying the fine conductive phase to long conductive phase with enhanced tunnel distance. The following examples are given by way of illustrations and should not be construed to limit the scope of the present invention. Example-l The sample A 1(4) was prepared by (a) cleaning the coors alumina substrate (2" x 2" x 0.6 mm), ultrasonically in methanol (b) coating it with Pd/Ag thick film paste of thickness 20 micron by screen printing; drying the coated substrate at 60 C for 15 mins (c) coating the thus prepared substrate with Birox resistor thick film paste of thickness 20 micron by screen printing; drying the coated substrate at 60 C for 15 mins; firing the thus prepared substrate at 850 C for 10 mins to obtain a thick film resistor (d) coating the thus prepared thick film resistor body with gold plating solution (acidic) 20 micron thick; drying the coated resistor at 60 C for 10 mins.; (c) firing the thus prepared substrate at 850 C for 10 mins. to obtained an improved thick film resistor useful for making strain gauge with enhanced gauge factor upto 1000. Fig. 2. shows the change in resistance as a function of strain for the sample AI (4) prepared by the above process. It follows that the gauge factor is about 900 in the linear range. Example-2 The sample W29(4) was prepared by the method exactly as outlined in an example-1 except the gold plating solution (acidic) was replaced by potassium ferrocyanide based solution of density 0.02 gm.cm-3. Fig. 3. shows the change in resistance with tensile and comprcssive strains for the sample W29(4) prepared by the above process. For the cantilever configuration used, 50 and 100 gms. of force produces 50 and 100 ppm of strain respectively. The gauge factor for the linear range, i.e., upto 50 ppm is 400. The main advantages of the process of the present invention are : 1. The process is very simple as this requires only the cyanometallic compound to be coated on resistor thick film of the substrate . 2. The process is very easy to monitor as it necessitates the resistance measurement after heating only. 3. The process is highly cost effective as the cyanometal lie compound coating requires no extra screen for printing. 4. Since the cyanometallic compound induces mixing therefore it is free from the problem of film peeling and thus increases the yield and the reliability. 5. The substrate is the integral part of the strain gauge. 6. The strain gauge can also be welded /bolted to the object of which the strain behaviour needs to be monitored by metallising the back surface of the strain gauge. We Claim: 1. A process for the preparation of thick film resistor useful for making strain gauge comprising: (a) cleaning an insulating substrate; (b) coating the said cleaned substrate with a known non—corrosive conductor thick film having a thickness at least 10 microns, (c) drying the conductor coated substrate at a temperature in the range of 60- 120 C for atleast 15 minutes; (d) coating by known methods the prepared substrate with a known resistor thick film having a thickness of atleast 15 microns; (e) drying the resistor coated substrate at a temperature in the range of 60-120°C for atleast 15 mins; (f) firing the said coated substrate at temperature in the range of 800-900°C for a period of 10-20 mins to obtain a thick film resistor; (g) coating the prepared thick film resistor with a cyanometallic compound having a thickness in the range of 10-25 microns;(h) drying the compound coated substrate at a temperature in the range of 60-120°C for atleast 10 mins; and (i) firing the prepared substrate to a temperature in the range of 800-900°C for a period of 10 - 20 mins to obtain an improved thick film resistor useful for making strain gauge with enhanced gauge factor from 0 to 1000. 2. A process as claimed in claim 1 wherein the insulating substrate used is selected from alumina substrate, enamelled steel and other conventional materials. 3. A process as claimed in claims 1 wherein the insulating substrate is cleaned by chemical method, ultrasonic cleaning in acetone, boiling in trylene and followed by rinsing in methanol. 4. A process as claimed in claims 1 wherein the cyanometallic compound used is selected from gold plating solution, potassium ferrocyanide solution. 5. A process as claimed in claims 1 wherein the drying and firing steps are effected in an air atmosphere. 6. A process for the preparation of thick film resistor useful for making strain gauge substantially as herein described with reference to the examples and the drawings accompanying this specification.

Documents:

1101-del-1998-abstract.pdf

1101-del-1998-claims.pdf

1101-del-1998-correspondence-others.pdf

1101-del-1998-correspondence-po.pdf

1101-del-1998-description (complete).pdf

1101-del-1998-drawings.pdf

1101-del-1998-form-1.pdf

1101-del-1998-form-19.pdf

1101-del-1998-form-2.pdf

1101-del-1998-form-3.pdf

1101-del-1998-petition-138.pdf