GB2204656A - Test fixture and support structure - Google Patents

Abstract

A support structure for a test fixture includes two members 1, 2 one mounted within the other and having respective cam 4, 5 and cam followers 8 such that pressure actuation and release of one member against the other causes relative rotational motion and causes the members to abut in a successive one of a plurality of relative longitudinal positions. A test fixture for testing printed circuit boards includes a plurality of such supports (1) mounted between and spacing parallel plattens (13, 14) Fig. 4 such that successive pressure actuation and release of one platten causes the spacing to be controllably altered so as to bring different numbers of test pins into contact with a board under test. <IMAGE>

Description

Test Fixture and Support Structure This invention relates to a support structure for separating and spacing two planes and which is adapted to alter their spacing sequentially through a plurality of distinct values. In particular but not exclusively it relates to a support for a "bed of nails" type of test fixture in which arrays of pins having different lengths are used to test a circuit board, and the circuit board is raised or lowered depending upon the particular set up of pins in operation.

This type of test fixture is well known as used with automatic test equipment. A printed circuit board is probed by many small test pins which contact sections of the board for testing the board or components placed upon it. There are two main types of testing which can be achieved with such equipment. Firstly in-circuit testing, for testing the functioning of the components and the circuit on the board. In this type of testing many test pins, typically several hundred, of a short length are used to impinge on every circuit node. The second type of testing is functional testing in which only the input and output sections of the circuit in the test are contacted and for this one uses fewer, perhaps one hundred or less, longer pins. It is frequently desired to conduct both the above tests consecutively on a single board.If the circuit board is to be lowered on to the test pins this clearly requires the board to be lowered to two distinct levels.

Conventional techniques to achieve a two stage test apparatus either use a number of pillars mounted alongside the test pins or a two stage vacuum technique in which pressure is lowered to a first value to achieve the first relative position of the circuit board and then to a second value to achieve a second position. Such a system can be expensive and requires good seals against air leakage.

According to the present invention in a first aspect there is provided a support structure including a first member connected to a second member by means of a cooperating cam and cam follower, and resilient means; the support being adapted such that successive pressure actuation and release of the first member against the resilient means causes relative rotational motion of the first and second member and causes the first member to abut in a successive one of a plurality of longitudinal positions with respect to the second member.

In a second aspect the invention provides a test fixture including a first and a second platten aligned substantially parallel to each other and separated by a plurality of support structures as described above; and wherein the first platten is connected to the first member of each support structure and the second platten is connected to the second member thereof such that each successive pressure actuation or release of the support structures causes the relative spacing of the plattens to alter to a successive one of a plurality of fixed values.

Advantageously, the first member is an elongate member having a hollow region at one end and the second member is adapted to fit partially within the hollow region such that the plurality of longitudinal positions correspond to a plurality of positions in which the second member protrudes by different amounts from the end of the first member.

Preferably, the first member includes a circumferential structure of chosen profile projecting into the hollow region and acting as the cam, and the second member includes an outwardly extending projection acting as the cam follower and adapted to follow the profile of teh cam with successive pressure actuation or release.

Preferably the cam and follower structure is provided with three abutment positions.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which; Figures 1 and 2 show an adjustable support assembly in respectively an upper and lower position.

Figure 3 shows the profile of the internal cam within the cylinder of figures 1 and 2, and; Figure 4 shows the sequence of operation of a test fixture provided with supports embodying the present inventions.

Referring to figures 1 and 2 there is shown a cross section of a complete stop assembly which includes a plunger 1, having an upper portion of greater crosssection than its lower portion, co-axially mounted within a cylinder structure having a lower nut 2 and upper bush member 3. A cam structure is mounted to the inside of the bush 3 and comprises a lower cam 4 and upper cam 5 which have the profiles shown in figure 3, as will be further described below. A compression spring 6 is mounted around plunger 1 and abuts against a shoulder portion 7 of nut 2.

The upper end of compression spring 6 is in contact with the ledge formed by the upper part of plunger 1 being of greater cross section than the lower part as is shown in the figure. Hence movement up or down of plunger 1 causes compression or relaxation of spring 6. A pin 8 is mounted through plunger 1 so as to extend, in use, to cooperate with cam structure 4 and 5. A typical carriage plate 9 as used with automatic test equipment is shown systematically on the figure.

In figure 2 the apparatus is shown at its lower stop position and the pin 8 is seen lying in a'direction perpendicular to that of the paper.

Figure 3 is in effect a "Flattened out" development representing the profile of cams 4 and 5. In use, pin 8 may rest at any of the positions shown with a circle at various stages of the mechanism. In the fully unloaded position the compression spring forces plunger 1 up until pin 8 rests in the upper stop chamber 10. This is equivalent to the unloaded position shown in dot-dashed lines of figures 1 and 2.

Depression of the plunger 1 from rest position 10, caused by downward motion of the carriage plate 9 causes the crosspin 8 to partly rotate and to lower until it is located within the lower Vee slot 11. This is systematically shown by the dotted line in figure 3. The plunger will therefore come to rest at this point so long as continuous pressure is applied in direction 9a.

Typically the position of figure 2 may be reached at this stage. Upon release of the pressure the plunger will be forced upwards by the compression spring 6 and caused, by the profile of upper cam 4 to partly rotate until pin 8 lies within the next upper stop chamber.

Depression of plunger 1 again in the direction of arrow 9a will cause the pin to again partly rotate and to lower into the upper Vee slot 12 and hence to come to rest at the position shown in figure 1. Release of pressure on the plunger will again cause the pin to raise and the cycle can then be repeated.

A "bed of nails" type of test fixture may be constructed by using two flat plates separated from one another by, for example, three of the stops above described, equidistantly positioned. An example of such an arrangement is shown in figure 4 in which two parallel carriage plates 13 and 14 are shown separated by stops, according to the present invention. A circuit board 15 may be mounted upon the upper plate and the lower plate is provided with a plurality of resiliently mounted long and short test pins, 16 and 17 respectively, arranged so as to be able to contact particular points on the circuit board, through a plurality of apertures 19 arranged in upper plate 13. Electrical connections from the test pins are fed to a control and test unit 18.

Figure 4a shows the apparatus in its rest or unloaded position in which none of the test pins makes contact with the circuit board. If a force 9a is applied to the upper plate 13 in a downwards direction then the plungers 1 of the stops are depressed and the position shown in figure 4b will be reached, at which position the long test pins 16 are in electrical contact with the circuit board.

Functional testing of the circuit board in which only a few of the contacts are required may then be carried out.

At the completion of the functional testing stage the pressure on upper plate 13 is removed for a sufficient time to allow pins 8 to move up to stop position 10, and then re-applied so that the stops reach their second or lower stop position as shown at figure 4c. At this point the shorter test pins 17 are in contact with the circuit board and the longer test pins 16 also remain in contact since they are resiliently compressed downwards within their mountings. In-circuit testing of circuit board 15 may then be carried out. At the completion of the process pressure is again removed from the upper test plate, all the test pins are freed from the circuit board and electrical contact is broken. The process may then be repeated by the substitution of another circuit board or perhaps by altering the loading or other parameters on the same circuit board.

The adjustable supports of the present invention may of course equally be used with test apparatus in which the pins are moved with the upper test plate and the circuit board is mounted upon the lower test plate.

Embodiments of the present invention may be suitably used with any fixture or mechanism requiring two stage repetitive stops alternating between long and short spacing. It is particularly beneficial for use in test equipment since no driving or actuation force is required other than that which is already fitted to standard test equipment. The stops themselves are of straightforward design and may be manufactured with only six components thus being cheaply made. To mount a stop onto a platten only single hole fixing is required although there must be a clearance underneath the platten for the lower part of the stop to lie in. Typically a stop will have its longest dimension of only a few centimetres or so.

Often, automatic test equipment is required to operate within vacuum conditions for environmental test purposes and the stops of the present invention are beneficial for use in these circumstances since there is no need to seal them against air leakage.

It should be noted that the stops may equally be adapted, by choosing a different type of resilient biassing means, such that their rest, or unloaded, position is when the pin lies, against the bottom cam and pressure is required to raise the internal plunger.

Furthermore, any suitable cam structure could be devised in different embodiments of the invention, which structure need not necessarily include two, upper and lower, cams.

The cam could alternatively be provided upon the plunger, and the pin or other follower mounted internally on the outer structure.

Claims (8)

1. A support structure comprising a first member connected to a second member by means of a co-operating cam and cam follower, and resilient means; the support being adapted such that successive pressure actuation and release of the first member against the resilient means causes relative rotational motion of the first and second member and causes the first member to abut in a successive one of a plurality of longitudinal positions with respect to the second member.

2. A test fixture including a first and a second platten aligned substantially parallel and separated by a plurality of support structures as claimed in claim 1, and wherein the first platten is connected to the first member of each support structure and the second platten is connected to the second member thereof such that each successive pressure actuation or release of the support structures causes the relative spacing of the plattens to alter to a successive one of a plurality of fixed values.

3. A test fixture as claimed in claim 2 wherein the first member is an elongate member having a hollow region at one end and the second member is adapted to fit partially within the hollow region such that the plurality of longitudinal positions corresponds to'a plurality of positions in which the second member protrudes by different amounts from the end of the first member.

4. A test fixture as claimed in claim 3 wherein the first member includes a circumferential structure of chosen profile projecting into the hollow region and acting as the cam, and the second member includes an outwardly extending projection acting as the cam follower and adapted to follow the profile of the cam with successive pressure actuation or release.

5. A test fixture as claimed in any of claims 2 to 4 wherein the cam and follower structure has three abutment positions.

6. A test fixture as claimed in any preceding claim wherein the cam has a profile substantially as shown in a flattened out representation and as described with reference to Figure 3 of the accompanying drawings.

7. A test fixture substantially as hereinbefore described with reference to, and as illustrated by, Figure 4 of the accompanying drawings.

8. A support structure substantially as hereinbefore described with reference to, and as illustrated by, Figures 1 to 3 of the accompanying drawings.