GB2234365A

GB2234365A - Strain gauge encapsulation process

Info

Publication number: GB2234365A
Application number: GB9015264A
Authority: GB
Inventors: Wilmot Hancock
Original assignee: GEC Avery Technology Ltd
Current assignee: GEC Avery Technology Ltd
Priority date: 1989-07-27
Filing date: 1990-07-11
Publication date: 1991-01-30
Also published as: GB8917191D0; GB9015264D0

Abstract

A process for encapsulating strain gauges comprising placing a sheet carrying a plurality of strain gauges 10 onto a processing plate, depositing a coat of an imide resin in liquid form over the sheet and spinning the processing plate to spread the resin evenly, baking the resin at 90 DEG for 60 minutes, coating the resin with photo-resist, exposing the photo-resist coated sheet to ultra-violet light using a mask which defines a pair of pads (11) for each of the strain gauges, developing to remove both the photoresist and imide resin, and fully curing the resin by baking at 135 DEG C or above before soldering wires (12) onto the pads. <IMAGE>

Description

STRAIN GAUGE ENCAPSULATION PROCESS.

The present invention concerns the manufacture of strain gauges. These are used in making force-measurement, and particularly, in weighing applications. Essentially a strain gauge comprises a strip of conductive material the resistance of which varies in accordance with strains applied to the strip.

This variation in resistance is measured by monitoring a current passed through the strip.

Strain gauges are generally small and made from thin metal films attached to a flexible base. It is accordingly normal to protect them from ambient conditions in their operational environment by encapsulating them in a suitable epoxy resin. The encapsulation processes currently used all have a number of drawbacks.

These are mainly caused by the necessity of providing the encapsulated strain gauge with pads to which electric leads can be connected.

In one known process the pads are masked prior to encapsulation and in another the pads are simply left out of the encapsulation process.

As the performance of a strain gauge is influenced by the method of lead attachment, i.e. amount of solder used in making the joint or position of the joint on the solder pad, either of the aforesaid encapsulation methods cause the critical soldering operation to be very much operator dependant. The second method also has the disadvantage that rows of gauges have to be treated separately thus slowing down the encapsulation process.

The present invention has for an object an encapsulation process for strain gauges which at least alleviates these drawbacks.

Accordingly from one aspect the present invention comprises a process for encapsulating strain gauges comprising placing a sheet carrying a plurality of strain gauges onto a processing plate, depositing a coat of an epoxy resin in liquid form over the sheet and spinning the processing plate to spread the resin evenly, baking the resin, coating the resin with photo-resist, exposing the photo-resist coated sheet to ultra-violet light using a mask which defines a pair of pads for each of the strain gauges, and developing and etching the sheet.

From a second aspect the invention comprises a strain gauge encapsulated by the aforesaid process.

In order that the present invention may be more readily understood, a method of strain gauge encapsulation will now be described by way of example. The only figure of the accompanying drawings shows three simular strain gauges 10. As can be seen each strain gauge has two terminal pads 11, with pads 12 being soldered to one of the straingauges. The main body of each strain gauge provides a conductive zig-zag path.

The initial manufacture of strain gauges is carried out to produce a rectangular sheet approximately 5 inches by 4 inches containing an array pattern of the devices. However the present invention is concerned with encapsulating the gauges so formed.

Accordingly the sheet of strain gauges so formed is placed on a processing plate and firmly taped to it. The sheet is covered with a coating of Polyimide approximately 7 #u.m. thick by depositing the Polyimide in liquid form on the plate and spinning the latter at approximately 1600 rpm. The Polyimide-coated sheet is then baked in an oven for approximaely 60 minutes at 900C. The time and temperature for this process are critical as they will affect the etch rates. Increased temperature and time will more completely imidise the Polyimide, slowing the etch rate. This will tend to result in reduced quality of the soldering area.

When the sheet has regained room temperature it is ready for coating with photo-resist. This is done in a manner similar to the Polyimide coating on a table spinning at approximately 1600 rpm. The newly coated sheet is stored at room temperature for approximately 10 minutes and then baked at 900C for a period of 30 minutes.

Once the sheet has returned to room temperature it is exposed to ultra-violet light using a photomask which defines the pads which are to be exposed so that the necessary leads can be soldered to them. A typical exposure time is 15 seconds. The exposed resist should be left at room temperature for a minimum of 15 minutes before development.

All positive photo resist developers will etch Polyimides in their soft baked condition. As the photo resist on the sheet of devices is on top of the soft baked encapsulent, development of the resist will be simultaneous to the etching of the Polyimide.

Development and etching is carried out by total immersion in positive resist developer at room temperature using continuous agitation. Development/etching is complete when terminal pads are observed to be clear of encapsulent taking approximately 4i minutes. Etching is followed by a spray wash in tap water then spin dry in preparation for resist removal.

Photo resist removal is carried out by total immersion in a solution of 60% Butyl Acetate, 40% Isopropanol at room temperature for a period of 3# minutes. The etched sheet is then spin-dried.

Full cure of the Polyimide encapsulent is now carried out by using the standard baking times of 1350C for 15 minutes and then elevating the temperature setting to 3000C, and after a further 170 minutes remomoving the sheet from the oven.

Residues and oxide from the exposed terminal pads can then be removed by polishing and washing.

The sheet of strain gauges can now be cut into rows ready for mass soldering leads to the individual devices. The rows of devices are thoroughly cleaned using solvents and ultrasonics after which the rows are cut into portions each having an individual, encapsulated strain gauge with leads attached.

Claims

1. A process for encapsulating strain gauges comprising placing a sheet carrying a plurality of strain gauges onto a processing plate, depositing a coat of an epoxy resin in liquid form over the sheet and spinning the processing plate to spread the resin evenly, baking the resin, coating the resin with photo-resist, exposing the photo-resist coated sheet to ultra-violet light using a mask which defines a pair of pads for each of the strain gauges, and developing and etching the sheet.

2. A process as claimed in Claim 1, wherein the resin is Polyimide.

3. A process as claimed in Claim 1 or Claim 2, wherein the photo-resist is applied to the sheet in the same manner as the resin.

4. A process as claimed in Claim 2 or Claim 3 when dependent on Claim 1, wherein the processing plate is spun at approximately 1600 rpm during the formation of the coat of resin, the final coat being approximately 7AJ.m. thick.

5. A process as claimed in any one of the preceding claims wherein photo-resist removal is carried out by total immersion in a solution of 60% Butyl Acetate, 40% Isopropanol.

6. A process as claimed in Claim 5, wherein the immersion is for a period of approximately 3# minutes.

7. A process for encapsulating strain gauges substantially as hereinbefore described.

8. A strain gauge encapsulated by the process as claimed in any one of the preceding claims.