GB2038489A - Arrangement for testing electric circuits on printed wiring boards - Google Patents

Abstract

A number of test probes 9 are each carried and positioned by a respective linear motor 4 are arranged to make contact with conductors 8 of a circuit board under test by means of actuators 11. The linear motor probe carriers are positioned with respect to a wiring board by means of a horizontal table 3 which forms the static part of the linear motor, the wiring board being supported in a horizontal plane either above or below the table. The use of a small number of probes 9 avoids the problems of strain on the circuit board and stray impedance introduced by unwanted contacts in the usual 'bed-of-nails' test arrangement. <IMAGE>

Description

SPECIFICATION Arrangements for testing electric circuits on printed wiring boards The present invention relates to arrangements for testing electric circuits on printed wiring boards.

In prior testing arrangements individual test jigs have been constructed which incorporate all the electrically conductive probes necessary to make the desired test connections to any particular circuit board, these jigs sometimes being referred to as "bed of nails" jigs. Such jigs have to be stored and maintained carefully, and in use they subject the boards under test to considerable forces, and the pins and wiring not actually in use during any one test contribute unnecessary stray capacitances and inductances to the electric circuits on the boards.

According to one aspect of the present invention an arrangement for testing electric circuits on printed circuit boards comprises support means for holding a printed circuit board to be tested with its plane substantially horizontal, substantially horizontal table means adjacent said support means, and a plurality of probe means each comprising a carrier which is supported on and independently movable with respect to said table means in response to electric signals supplied to said carrier, the movement being in a plane substantially parallel to the plane of said circuit board to be tested, electrical contactmaking means, and actuator means operable to bring said contact-making means into contact with an electrical conductor on said circuit board.

Preferably each said carrier comprises a moving part of a linear electric motor arrangement and said table means incorporates the cooperating static part of said motor arrangement, the static part cooperating in common with all of the respective moving parts. The linear electric motor arrangement may be of the kind described in United States Patents Nos. 3,376,578 and 3,457,482.

The support means may comprise a jig arranged to locate a particular design of circuit board in a predetermined position with respect to said table means.

An arrangement for testing electric circuits on printed circuit boards, the arrangement being in accordance with the present invention, will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows an isometric view of part of the arrangement, and Figure 2 shows a sectional view of part of the arrangement.

Referring first to Figure 1 the testing arrangement comprises a supporting frame 1 on the upper surface of which may be placed a circuit board to be tested (not shown) or a generally planar jig (not shown) for locating a circuit board to be tested, the circuit board or the jig being located accurately with respect to the supporting frame 1 by means of pegs 2.

Within the frame 1 there is provided a table 3 which extends generally horizontally beneath the upper surface of the frame 1, the table 3 serving to support a plurality of probe carriers 4, of which six are shown in Figure 1. The surface of the table 3 incorporates a regular array of zones (not shown) of magnetic material, the spaces between the zones being filled with non-magnetic material to form a generally smooth surface. The zones may be formed for example as raised areas on a substrate of the magnetic material. The array forms the stationary part of a linear electric motor arrangement, with the probe carriers 4 each including a corresponding moving part of the motor arrangement. Linear electric motor arrangements of this kind are described for example in United States Patents Nos. 3,376,578 and 3,457,482.

The moving parts of the motor arrangement each comprise a combination of permanent magnets and electromagnets (not shown), the latter being energised selectively by electric signals supplied by way of a respective trailing lead 5 from a control unit (not shown) whereby the respective probe carrier 4 may be moved over the table 3 and brought accurately to a required position. Each of the probe carriers 4 may be positioned independently of the others within the limitations imposed by the presence of the other carriers 4.

Referring now to Figure 2 a jig plate 6 is secured to the upper surface of the frame 1, and this plate 6 is arranged to support and locate a printed wiring board 7.

Some at least of printed conductors 8, disposed on the under surface of the board 7, lie over the table 3, and a probe 9 supported on a pivoted arm 10 mounted on the top of a respective carrier 4 may be brought into contact with a selected conductor 8 by operation of a solenoid 1 The mounting for the pivotted arm 10 may be rotatable about a vertical axis with respect to the base of the carrier 4 such that two or more probes 9 on adjacent carriers 4 may be brought into contact with closely spaced adjacent conductors 8 or for continuity testing, with the same conductor 8.

Each carrier 4 may be arranged to float on a film of air, or an 'air cushion', at least during periods when it is required to move over the table 3, the air being introduced under pressure between the surface of the table 3 and the underside of the carriers 4 by way of a pipe coextending with the lead 5, or by way of small holes in the surface of the table 3. Alternatively the carriers 4 may be supported for easy movement over the surface of the table 3 on ball bearings.

In an alternative arrangement the positions of the integers shown in the drawings may be inverted, such that the carriers 4 are suspended beneath the table 3 and above a circuit board to be tested. The carriers 4 would then be attracted to the table 3 by magnetic attraction and being pushed downwards by the air cushion.

Claims (6)

1. An arrangement for testing electric circuits on printed circuit boards comprising support means for holding a printed circuit board to be tested with its plane substantially horizontal, substantially horizontal table means adjacent said support means, and a plurality of probe means each comprising a carrier which is supported on and independently movable with respect to said table means in response to electric signals supplied to said carrier, the movement being in a plane substantially parallel to the plane of said circuit board to be tested, electrical contactmaking means, and actuator means operable to bring said contact-making means into contact with an electrical conductor on said circuit board.

2. An arrangement in accordance with Claim 1 wherein each said carrier comprises a moving part of a linear electric motor arrangement and said table means incorporates the cooperating static part of said motor arrangement, the static part cooperating in common with all of the respective moving parts.

3. An arrangement in accordance with Claim 1 or Claim 2 wherein the support means comprises a jig arranged to locate a particular design of circuit board in a predetermined position with respect to said table means.

4. An arrangement in accordance with Claim 1 wherein said support means is located above said table.

5. An arrangement in accordance with Claim 1 wherein said support means is located beneath said table.

6. An arrangement for testing electric circuit substantially as hereinbefore described with reference to the accompanying drawings.