GB2192719A

GB2192719A - Alignment fixture for circuit boards

Info

Publication number: GB2192719A
Application number: GB08702862A
Authority: GB
Inventors: Alan Martin Bishop
Original assignee: DATABASIX Ltd
Current assignee: DATABASIX Ltd
Priority date: 1986-03-06
Filing date: 1987-02-09
Publication date: 1988-01-20
Anticipated expiration: 2007-02-09
Also published as: GB2192719B; GB8702862D0; GB8605565D0

Abstract

A method and apparatus relates to aligning circuit boards with the testing pins of automatic testing equipment. In the testing equipment, vacuum means is used to produce an impression of the testing pins in the deformable surface, eg tinned copper, of a dummy circuit board 5 mounted on a plate. The plate and dummy circuit board are then placed on an alignment fixture and the position of the impression on the dummy circuit board are calibrated by means of an optical microscope illuminated by a fibre optic device. The dummy circuit board is then removed from the plate and is replaced by a circuit board to be automatically tested. The plate and circuit board are then returned to the testing equipment where the circuit board can be engaged, by the said vacuum means, with the testing pins for the testing to be effected. <IMAGE>

Description

SPECIFICATION Automatic testing systems for circuit boards This invention relates to improvements in automatic testing systems particularly to systems for testing surface mounted circuit boards.

One of the systems presently used for testing circuit boards is called automatic testing equipment and consists of a fixture into which circuit boards are inserted and this fixture can then be connected to testing and data processing equipment. The fixtures for this type of system consist of a connector which connects to the testing equipment, a vacuum system and an area of probe pins which are fixed into a pattern determined by the type of circuit board to be tested and known as a pin bed. The circuit board is placed over the pin bed and a seal made between the board, the fixture and the connector is connected to the testing equipment. When the vacuum is created the board is forced on to the pin bed and signals are applied to the components and connectors of the circuit board and the results can be analysed to locate any faults in the board.Although the testing equipment works well for this system the fixtures are bulky and as each type of board requires an associated fixture storage becomes a problem.

In our co-pending UK application number 8520052 it is proposed to overcome the problems with such fixtures by providing equipment in which there is the master type of fixture suitable for several types of circuit boards and which interfaces with the testing equipment, into which could be placed a personality fixture which relates to the individual circuit board type. Fig. la shows a simplified version of the master fixture and Fig 1 b shows the underside of the pin bed which forms part of the personality fixture. The master fixture comprises a connector 1 which connects to the testing equipment and which itself connects to connecting strip 2. A vacuum connection 2 is provided #in a central recess in the master fixture and clamps and rods are provided to hold and locate the personality fixture in place.The personality fixture includes a pin bed, the probes of which are positioned in a pattern corresponding to the type of circuit board to be tested. These are connected by wire to a connecting strip 4 which is on the edge of the pin bed outside a box protecting the rear of the probe pins.

When in use the personality fixture is placed with the boxed under side in the central recess of the master fixture and the two connecting strips are in contact. A seal plate is placed on the top of the pin bed and the circuit board is placed in position aligned by means of the rods in the pin bed and the dowel holes in the circuit board. When a vacuum is created it is only created beneath the seal plate and circuit boards and this pulls the circuit board on to the probes which are con nected to the connector 1 by means of the connecting strips 2 & .

Although this system works successfully for the conventional printed circuit boards problems are encountered when the board to be tested is a large ceramic board with components mounted on one side. The size of these boards is not completely standard and there are no dowel holes as in the printed type of board so there are problems with the alignment with the testing pins. With the ceramic boards there are the additional problems that if the vacuum is applied only to one side of the board the pressure on the pins may not be enough to ensure contact and also the board may bend and fracture.

The object of the invention is to provide a method and apparatus for obtaining an alignable personality fixture for automatic testing equipment for use with surface mounted circuit boards.

According to the invention there is provided a method of producing an aligned fixture for automatic testing equipment for use with surface mounted boards comprising, positioning connector probes relating to one type of cir- cuit board, placing a piece of deformable material on a plate over said probes, QLforcing said material on to the probes so that the probes produce indentation in said material, transferring said material and said plate to an alignment fixture, calibrating the alignment fixture with reference to indentations relating, to known probes, replacing said material by a circuit board, positioning said circuit board on said plate until said circuit board is aligned to said alignment fixture and to the position of said known probes, and locking said board into position.

Preferably said plate can then be placed in position in a personality fixture with said circuit board being positioned accurately with reference to the testing probes.

Further according to the invention there is provided apparatus for producing a fixture for automatic testing equipment comprising, an alignment fixture including a microscope optic, said alignment fixture being pre-aligned with testing probes by means of a dummy board, a plate, a surface mounted circuit board placed on said plate, means for adjusting the position of said board, said adjusting means being such that the board is adjusted to align said board-with said testing probes, and means for securing said board in position.

Preferably said plate is attachable to the top of a personality fixture, said plate being such that a vacuum is created on both sides of the circuit board when a vacuum is created in the fixture.

One embodiment of the invention is now described with reference to the accompanying drawings, Fig. 1 shows a simplified version of the prior art system. Fig. 2 is the personality fixture for surface -mounted boards.

Fig. 3 is the pin bed plate. Fig. 4 is the wa#ffle plate.

Fig. 5 is the alignment fixture.

Fig. 6 shows the alignment fixture in use.

This embodiment of the invention is con cerned with producing a personality fixture for .use-wit.h the master fixture of our co-pending application number 8520052 to test surface mounted ceramic circuit boards. A complete personality fixture is shown in Fig. 2 The board to be tested 5 is mounted on the un derside of a plate 6 (called the waffle plate and shown in. more detail in Fig. 4) and this is placed on bearings. on dowel rods 7 of the pins plate 8 as shown. Fig. 3 shows the con- - struction of the pin plate. The dowel rods 7 are inserted into the pin plate 8 at the positions shown in Fig. 3, these positions being known accurately, and at the centre of the pin plate a pin bed 9 is placed. The pin bed 9 has probe pins inserted in a pattern deter mined by the type of circuit boards to be tested.The positioning of the pins can be# produced by using the CAD used in producing the circuit boards. and thus will be accurate within the board area. The positioning o#f the pins can also be known accurately with refer ence to the dowel rods.

When constructing the personality fixture seal plate 12 and frame 10 are attached to the exterior of the pin plate as shown #in Fig.

2. with a standard seal between the seal plate and the frame. The frame includes a connect ing strip 11 which corresponds to the con necting strip 4 of the prior art system and the probe pins are connected to this strip by means of connecting. wires. The seal plate 12 is held at a distance from the# upperside# of the main part of the frame and a box 13 on the underside which fits into the recess in the master fixture. When in position in the master fixture the connecting strip 11 is connected# to the connecting strip 2 and. there-is a connection made between the probe pins and the testing equipment.

When in position ready for use the waffle plate rests on-the dowel rods and seal plate 12 and sealing caps 14 are placed over the openings for the dowels. There- is a seal between the waffle plate and the seal plate and there may also be a spring between the waffle plate and. the pin plate. The pipe from vacuum pump is inserted through a hole in the frame and a vacuum is created. The waffle plate is drawn down and the seal plate moves down the movement being limited by the dowel and the spring. The presence of the waffle plate ensures that the vacuum load is transformed to the circuit board and that there is enough pressure on the probes to make contact.

The waffle plate is shown in more detail in Fig. 4 which shows the underside of the plate when in position on the personality feature.

Two-spring like items 16 and 17 called snugglers are positioned at two side of the central section and eccentric pins 18-20 at the other two sides. The board is placed against the snugglers and its postion altered by means of the eccentric pins which can be locked when the desired position is achieved.

On initial assembly, when the waffle plate is in place on the personality fixture the board will not be accurately aligned with the pins contact will not be made and correct testing will not be possible. In order to align the boards to be tested with the pins an accu rateiy profiled deformable board such as a tinned copper clad dummy board or some similar device is loaded into the waffle plate and the eccentrics set to a nominal position.

This is then placed on top of the fixture as described previously and a vacuum created so that the dummy board is drawn onto the probes. The probes produce a pattern of in dentation on the dummy board. The waffle plate is then removed with the dummy board in -position an alignment fixture as shown in Fig. 5 is mounted on it, which alignment fixture comprises a stand 21 with slots 22 and 23 running along two sides and a microscope optic 24 which is mounted on mount 26. The optic can be placed over the alignment fixture and moved along the frame to be locked in position by means of screw in the slots when desired. There is a second optic which is not shown in Fig. 5 but which is mounted and used in the same way.

With the dummy board in position as shown in Fig. 6 the optics which include a scale are sighted, onto the indentation of known probes, illumination being provided by (for instance) fibre optics 28. Once the indentation is sighted the microscope optic scale is used to zero onto it and the microscope locked into position. The alignment fixture is thus calibrated to the position of the probes on the pin bed.

Once this calibration is achieved the dummy board is removed and discarded and the board to be tested is placed- on the waffle plate which is then replaced under the alignment fixture. The position of the board is then altered with the eccentric until the known positions on the board (corresponding to the position of the known probes) coincide with the position locked on to by the optics. The eccentrics are then secured in this position and the board is now aligned with the position of the probes on the pin bed.

In order to test the board the waffle plate with the board in position is removed from the alignment fixture and placed with the circuit board underneath on the dowel rods to form the personality fixture. This is placed in the master fixture and a vacuum created. The circuit board is brought into contact with the probes on the pin bed and the increased vacuum area of the waffle board ensures that there is enough pressure on the probes to make contact and that the board does not fracture. The testing can then proceed as usual. After testing the waffle plate and board can be reassembled in the alignment fixture and checked for probe marks on the targets on the board.

Claims

1. A method of producing an aligned fixture for automatic testing equipment for use with surface mounted boards comprising; positioning connector probes relating to one type of circuit board, placing a piece of deformable material on a plate over said probes, forcing said material on to the probes so that the probes produce indentations in said material, transferring said material and said plate to an alignment fixture, calibrating the alignment fixture with reference to indentations relating to known probes, replacing said material by a circuit board, positioning said circuit board on said plate until said circuit board is aligned to said alignment fixture and to the position of said known probes, and locking said board into position.

2. A method as in claim 1 whereby said positioning is such that said plate can be placed in position in a fixture for automatic testing equipment with said circuit board being positioned accurately with reference to the testing probes:

3. Apparatus for producing a fixture for automatic testing equipment comprising, an alignment fixture including a microscope optic, said alignment fixture being pre-aligned with testing probes by means of a dummy board, a plate, a surface mounted circuit board place on said plate, means for securing said board in position.

4. Apparatus as in claim 3 wherein said plate is attachable to the top of a fixture for automatic testing equipment, said plate being such that a vacuum is created on both sides of the circuit board when a vacuum is created in the fixture;

5. Apparatus as in claim 3 wherein said alignment fixture further includes fibre optic means for providing illumination for the microscope optic.

6. Apparatus for producing a fixture for automatic testing equipment substantially as described herein with reference to the accompanying drawings.