GB2151361A - Load indicator - Google Patents

Abstract

A load indicator is used to indicate when the force applied to an electronic component to make good electrical and thermal connection thereto reaches a predetermined threshold value. The indicator is a snap action device 15 having two stable states, only one of which is available at loads above the threshold value. The indicator is fitted in a rigid member in the position shown in Fig. 3. Load is applied through the rigid member to a spring which contacts the tang 16 of the indicator. At the predetermined load, the deflection of the spring causes the indicator to snap with an audible "ping" to the second stable state shown in Fig. 2. Subsequent manual inspection may be made to check whether the load exceeds the threshold value by pressing the indicator in the direction 22. <IMAGE>

Description

SPECIFICATION Load indicator This invention relates to a load indicator and is specifically concerned with an indicator which is operative to indicate whether load of a predetermined value is correctly applied to a component. High power semiconductor devices are often held in contact with a heat sink or electrode structure merely by means of application of suitable pressure. The applied pressure value can be critical, as insufficient pressure will result in a poor thermal path and/or a high impedance electrical path, whereas excessive pressure can cause serious damage to the component.

The present invention seeks to provide an improved load indicator.

Accordingly to a first aspect of this invention, a load indicator includes a resiliently deformable member whose deformation is related to the load applied to it; an indicator mechanically coupled to the member and comprising a snap action device having two possible stable positions, the arrangement being such that when a predetermined load is applied, only oneiof the two stable positions is available to the indicator.

Accordingly to a second aspect of this invention, a mounting assembly includes a deformable member which distorts as load is applied to it; and a snap action indicator which moves from a first stable position to a second stable position as the load applied to said member rises through a threshold value.

Preferably, at loads below the threshold value, the snap action indicator is free to adopt both stable positions, but is able to adopt only one of the stable positions at or above the threshold value.

In practice, pressure is transferred from said deformable member to a body of some kind so as to hold it firmly in a required position.

In one embodiment of the invention, pressure is applied to an electronic component so as to mount it firmly on a heat sink. The pressure is transmitted to the component by increasing the curvature applied to the ends of a cantilevered spring member, such that as the ends are moved in one direction, the centre of the member moves in the reverse direction, thereby displacing said snap action indicator from one of its stable positions to the other stable position.

The invention is further described by way of example with reference to the accompanying drawings in which: Figure s is an elevation view of a pressure mounting assembly in accordance with the invention, and Figures 2 and 3 show the snap action indicator in more detail.

Referring to Fig. 1, there is shown therein a pressure mounting assembly which is suitable for mounting a semiconductor device 1 which is subjected to moderate pressure so as to hold it firmly in contact with a heat sink 2.

The device 1 is held firmly in position by means of one or more of parallel laminated leaf springs 3. A number of disc spring washers 4 are located underneath the leaf springs 3 and these washers serve to distribute the load fairly widely on to the upper surface of a busbar insulator 5 against which the washers rest. This insulator 5 bears upon the angled lower end 8 of a busbar 7 and is held in contact with the pole piece of the semiconductor component 1.

Load is applied to the outer ends of the leaf springs 3 by means of a rigid bar 10 which is held in place by bolts 11 which have screw threads engaging with the heat sink 2. The portions of the bolt between the insulator 5 and the heat sink 2 are covered by insulating sleeves 12. This is necessary as the heat sink 2 is itself at anode potential during operation.

The potential difference existing between the two opposing faces of the semiconductor device 1 can be very large, typically several thousand volts, and electrical connection to these faces is made by the heat sink 2, and by the end 8 of the cathode busbar 7 respectively. The busbar insulator 5 and the insulating sleeves 1 2 ensure that the high voltage on the cathode busbar 7 is not allowed to short to the heat sink 2. The heat sink 2 serves the very important task of holding the semiconductor device 1 at a safe operating temperature, and in practice, the heat sink 2 itself is likely to comprise a large finned structure composed of thermal conductive metal.

In order for the semiconductor device 1 to operate satisfactorily, there must be an extremely good thermal path to the heat sink 2 from it, and also very low impedance electrical paths to the heat sink 2 and to the cathode busbar 4. These thermal and electrical paths are achieved by means of a relatively high compressive force which clamps the various components of the structure firmly into contact with the faces of the semiconductor device 1. Although the load can be fairly high, its value is quite critical as above a threshold value damage can easily be imparted to the semiconductor device 1. It is very important that the device 1 is not subjected to too high a compressive force. The necessary pressure is applied to the structure by means of the leaf springs 3 which become curved by the bar 10 which is forced towards heat sink 2 as the bolts 11 are tightened.A snap action indicator 1 5 is mounted on the bar 10 so as to indicate when the semiconductor device 1 is subject to the correct mounting pressure.

The nature of the indicator is illustrated more clearly in Figs. 2 and 3. The indicator 1 5 itself consists of a thin strip of springy material, typically spring steel which is only .3 mm thick. It is shaped as shown, and a short tang 1 6 is formed by stamping out a slot 1 7 from the body of the arm 18, although in practice a symmetrical device could be formed by producing a similar tang from the other arm 1 9 also. In its natural unstressed state the arms 18 and 1 9 are flat and lie in a common plane. The ends of the two arms of the indicator 1 5 engage in opposite grooves 20 formed in the end portions of the indicator bar.The two grooves 20 are space apart by an amount which is slightly less than the relaxed flat length of the indicator strip 1 5.

When the indicator 1 5 is inserted into the grooves 20, it is distorted initially into the position shown in Fig. 3, that is to say, the tang portion 1 6 is displaced in a downwards dierection so that it passes freely through into the aperture 21 formed in the central bridge portion of the bar 10. The downward force, represented by the arrow 22 is caused directly by the inwards force, represented by the arrows 23, exerted by the reaction of the grooves 20. This position is achieved by gentle downward pressure applied manually if necessary to the centre of the indicator. In this position the end of the tang 1 6 protrudes only slightly from the lower surface of the bar 10.As load is increased by tightening the bolts 11, the leaf springs 3 distort to an increasing extent, as the load is applied to the ends of the bar 10 to the very end of the leaf springs 3. The leaf springs 3 tend to pivot about the inner edges of the discs 4 thereby causing the central portion of the leaf springs 3 to rise in the form of a small hump.

Although the hump is extremely small, the extent of the movement is sufficient to bear against the lower end of the tang 1 6 and to move it upwards slightly. Even a slight movement causes the strip 1 5 to pass through an over-centre position and to snap upwardly into the position illustrated in Figs. 1 and 2 when a particular load threshold is reached.

In practice, the load exerted by the leaf springs 3 when the snap action takes place can be easily adjusted by increasing or decreasing the resistance to bending of the leaf spring 3. Thus, if it were desired to increase the value of the load threshold, additional leaf springs 3 would be inserted.

It will be appreciated that when the semiconductor device 1 is under pressure and the indicator 1 5 is forced to adopt its snap-up position as shown in Figs. 1 and 2, it is impossible to return it to its original snapdown position as shown in Fig. 3 because of the close presence of the deflected leaf springs 3. Thus, at any time after assembly, it can be determined whether the mounting load exceeds the threshold value by attempting to press downwardly on the indicator. The manual pressure needed simply to move the indicator from one stable position is very slight, and can easily be achieved by gentle finger pressure.

The invention has the advantage in that the snap action of the indicator from one stable position to the other is accompanied by a distinct audible effect, thereby clearly informing an operator who is tightening the bolts 11 that the required threshold value has been reached. Furthermore, it has been found that the threshold load is relatively insensitive to asymmetrical tightening of the two bolts 11.

This is because the degree of curvature experienced by the leaf springs is relatively independent of the difference in loads applied to its end. That is to say, if one bolt 11 is left relatively statidnary and most of the tightening movement is imparted to the other bolt 11, the overall effect in terms of distortion of the leaf springs and load applied to the spreader discs is not very great. Thus the arrangement is extremely tolerant of operator abuse, and it is relatively easy to obtain a mounting load of the required value. This ensures that the semiconductor device can operate satisfactorily with adequate thermal and electrical conduction without subjecting it to excessie force which might crack or otherwise damage it.

Those parts associated with the indicator are reusable, and cheap to produce. The threshold load for the snap action is not dependent upon variations in thickness of the mounted component.

Claims (11)

1. A load indicator including a resiliently deformable member whose deformation is related to the load applied to it; an indicator mechanically coupled to the member and comprising a snap action device having two possible stable positions, the arrangement being such that when a predetermined load is applied, only one of the two stable positions is available to the indicator.

2. A load mounting assembly including a deformable member which distorts as load is applied to it; and a snap action indicator which moves from a first stable position to a second stable position as the load to said member rises through a threshold value.

3. An assembly as claimed in claim 2 and wherein at loads below the threshold value, the snap action indicator is free to adopt both stable positions, but is able to adopt only one of the stable positions at or above the threshold value.

4. An assembly as claimed in claim 3 or 4 and wherein said deformable member is a spring, deformation of which causes a portion of the spring to bear against a portion of the snap action indicator whilst it is in the first stable position so as to cause it to move to the second stable position.

5. An assembly as claimed in claim 4 and wherein said deformable member is deformed by the action of a rigid member which bears against it, the rigid member also serving to support and locate said snap action indicator.

6. An assembly as claimed in claim 5 and wherein said indicator comprises a thin strip of resilient material which is secured between two opposing recesses in said rigid member, with the two recesses being spaced apart by less than the natural undeformed length of said thin strip.

7. An assembly as claimed in claim 6 and wherein the indicator includes a prong which passes through an aperture in said rigid member so as to be adjacent to said deformable member when in its first stable position.

8. An assembly as claimed in any of the preceding claims 2 to 7, and wherein the threshold value is at least partly dependent upon the resistance to deformation of said deformable member.

9. An assembly as claimed in claim 8 and wherein the plurality of similar deformable members are mounted in contact with each other, each member taking the form of an elongate leaf spring.

10. An assembly as claimed in any of the preceding claims 2 to 9, and wherein there is provided a reaction member in the form of a heat sink against which the assembly is adapted to hold an electronic component.

11. An assembly as claimed in any of the preceding claims 2 to 10 and wherein the adoption by the indicator of the second stable state can be ascertained by unusual and/or manual inspection.

1 2. An assembly as claimed in any of the preceding claims 2 to 11 and wherein said threshold value is independent of the thickness of a component mounted by means of the assembly.

1 3. A mounting assembly substantially as illustrated in and described with reference to thr accompanying drawings.