GB2156532A

GB2156532A - Apparatus for testing a printed circuit board

Info

Publication number: GB2156532A
Application number: GB08407690A
Authority: GB
Inventors: David Thomas Gwillim
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-03-24
Filing date: 1984-03-24
Publication date: 1985-10-09
Also published as: GB2156532B; GB8407690D0

Abstract

Apparatus for testing a printed circuit board (10) comprises an insulating member (12) with conductors arranged, inter alia, at test point locations (A,B,C) together with a tester printed circuit board (14). The latter has contacts also located at test point locations (A,B,C). Each of these contacts is connected by a printed conductive track to a corresponding connection point at a periphery of the board. By means of edge connection this board may then be connected to automatic test equipment. By means of edge connection this board may then be connected to automatic test equipment. Both this test board, and boards to be tested, may be manufactured by computer aided design. The insulating member may comprise sprung pins set in an insulating plate. Alternatively it may comprise a rubber mat with localised conductive regions. An insulating mask sheet may be interposed between the insulating member (12) and the tester board (14) and this too may be manufactured by computer aided design. Alternatively the tester board may have a patterned resist mask insulation layer over its surface. <IMAGE>

Description

SPECIFICATION Apparatus for testing a printed circuit board This invention relates to apparatus for testing a printed circuit board, hereinafter referred to as a PCB.

A PCB is a thin plate or board on which electronic components are mounted, the board having imprinted on it electrically conducting tracks which interconnect the components. It is necessary to test each PCB to ensure correct functioning and due to the complexity of the PCBs the testing task is usually automated. Testing requires a large number of connections between various points on the PCB and the test equipment.

This is normally achieved by the use of specific test jigs for each type of PCB. These test jigs usually have sprung connecting pins mounted on an insulating plate in position adjacent to the test points on the PCB. The PCB is then located and forced onto the sprung pins by mechanical or pneumatic means. Each type of PCB requires a different jig which has to be manufactured and then wired for correct connection to the test equipment. The supply of these jigs is both time consuming and costly.

The connecting points on PCBs are usually positioned on a fixed grid of typically 0.1 or 0.05 inches and therefore universal test heads can be made having holes at all the points on the grid. All the holes can have connecting pins, in which case selection and connection must be made of specific pins which contact the test points on the PCB to the test equipment. Alternatively, pins can only be inserted in the holes which align with the test points on the PCB. Various schemes have been proposed for overcoming this time consuming problem. One method is to locate a member with holes formed only at the test points between the sprung connecting pins and the PCB under test.Another method is to provide a member with projecting contacts on the opposite side of the test head to the PCB, the connecting pins being slidable in the test head so that the projecting contacts push selected connecting pins into contact with the test points on the PCB.

All these schemes however involve producing an additional member for the jig with holes or contacts which cannot be formed in the member until the details of the PCB and the location of the test points are known.

Thus there is still a period of time after a PCB is made before it can be tested on a proper jig.

It is an object of the present invention therefore to produce apparatus for testing a PCB which will considerably reduce this problem.

According to the present invention apparatus for testing a printed circuit board comprises a tester printed circuit board having a plurality of electrical contacts formed thereon in the same relative positions as the test points on the printed circuit board, the electrical contacts being electrically connected to a plurality of connector points on the tester board for connection to electrical test equipment, an insulating member having a plurality of electrical conductors extending therethrough and means for urging the printed circuit board, the insulating member and the tester board into contact whereby all the test points on the printed circuit board are connected by a number of the electrical conductors in the insulating member to the electrical contacts on the tester board.

Preferably the tester board is produced by the same method of production as the PCB and preferably at substantially the same time.

Thus it is common practice to produce the interconnection design of the PCB automatically by a "Computer Aided Design" system, and preferably the interconnection design from the test points to the connector points on the tester board is also automatically produced when the design of the PCB is finalised.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which, Figure 1 is a perspective view of the three basic components of the apparatus and Figure 2 is a cross-sectional view of the components of the apparatus showing the order of assembly.

Assume a PCB to be tested 10 has 3 connected pads A, B and C which need to be tested for continuity.

Conducting members 1 2 are placed between a tester PCB 14 and the PCB to be tested 10. The conducting members only allow discrete connection between aligned pads between the two PCBs. The tester PCB is then used to connect pads A, B and C to the test equipment for a continuity test between pads A, B and C on the PCB to be tested 10.

Locating dowels 1 6 are used to ensure accurate alignment of all three items. A force is applied to ensure connection between the aligned pads.

The practice of these principles will be effected as follows: The PCB to be tested 10 will vary in size and shape and the test points will vary in position and quantity and may not be on a fixed matrix. The matrix pitch is usually 0. 1 or 0.05 inches.

The PCB may be a bare board which is usually flat and accurate or it may be an assembled board with protrusions and distortions on both surfaces.

The components may be delicate and unable to withstand substantial force.

The conducting members 1 2 may take the form of sprung pins 1 8 set in an insulating plate 20. The pins may be positioned as required or be a standard item with all pins fitted within a standard matrix pitch. This type of sprung pin will allow connection to test points on the PCB with surfaces containing irregularities.

Alternatively a sheet of rubber 22 with localised conducting members 24 through the thickness of the rubber may be used. The density of the conducting members is approximately 1 0/mm2 which will accommodate a matrix pitch of 0.05 inches. This material is unable to accommodate significant surface irregularities and is therefore only applicable when the mating PCB surfaces are flat and accurate as in bare board testing.

The tester PCB 14 is created on a computer aided design (CAD) system in conjunction with the PCB to be tested 10. The test points A, B, C are nominated by the engineer and given a suitable type of blank type, the CAD system automatically routes the test points to connector points A, B, C on the periphery of the board which are then used to connect to the test equipment (not shown).

A force has to be applied to maintain sufficient pressure between the two PCBs 10 and 14 to make contact. The pressure will depend on the method used and the type and density of the conducting members 1 2. Only a low pin density is able to use vacuum techniques and therefore air pressure is more satisfactory.

Mechanical forces would be very high and would require very strong frames on a supporting jig to transmit the force evenly.

Isolation of Test Points If the tracks are on facing sides of the two PCBs and a rubber mat or standard pin matrix head is used then it will be necessary to insulate the track areas from the unused pins or mat areas. This can be achieved by interposing a thin CAD generated insulating sheet either side of the pins with holes positioned over the test pad area so that pin contact can be made in these areas only. Alternatively when using the mat technique a localised resist layer can be formed on the PCBs.

An application of these techniques is shown in Fig. 2.

The main feature of this invention is the method of connecting a multiplicity of test points on a PCB to a test equipment by creating the interconnection automatically on a related "Tester PCB" using a CAD system to produce the link track. Combining this technique with the established multiple pin probes or the special conducting mats produces a system which is unique, can be easily produced and is lower in cost.

Claims

1. Apparatus for testing a printed circuit board, the apparatus comprising: a tester printed circuit board having a plurality of electrical contacts formed thereon, each contact located in position to correspond to a test point on a printed circuit board to be tested, the contacts being electrically connected to respective connector points on the tester printed circuit board for connection to electrical test equipment; an insulating member having a plurality of electrical conductors extending therethrough, located in position over and adjacent to the tester printed circuit board; and, actuation means for urging together the tester printed circuit board, the insulating member, and said printed circuit board to be tested, in manner such that all test points on said board circuit board to be tested are connected by a corresponding number of the conductors in the insulating member to corresponding contacts on the tester printed circuit board.

2. Apparatus, as claimed in claim 1, wherein the insulating member comprises a plurality of sprung pins set in an insulating plate.

3. Apparatus, as claimed in claim 1, wherein the insulating member comprises a sheet of elastic insulating material having localised conductors defined therein.

4. Apparatus, as claimed in claim 3, wherein the sheet is of rubber material of sheet density 1 0/mm2 having conductors spaced at a pitch of 12511 (0.05").

5. Apparatus, as claimed in any one of the preceding claims, wherein the contacts and connector points are connected by conductive tracks arranged on that surface of the tester printed circuit board adjacent to the insulating member there being interposed, between the tester printed circuit board and insulating member, a sheet of insulating material, this sheet being configured to mask areas of the tracks, areas corresponding to all conductors other than those corresponding to test prints.

6. Apparatus, as claimed in claim 5, wherein in place of the sheet of insulating material, a patterned resist layer is formed on said surface of the tester printed circuit board.

7. Apparatus, for testing a printed circuit board, constructed, adapted and arranged to operate substantially as described hereinbefore with reference to and as shown in the accompanying drawings.

8. A combination comprising a set of nominally identical printed circuit boards together with a tester printed circuit board, the latter having a plurality of electrical contacts formed thereon, each contact located in position to correspond to a test point common to all printed circuit boards of the set.

9. A combination as claimed in claim 8 including a complimentary sheet of insulating material.

10. A combination, as claimed in either claim 8 or claim 9, when manufactured by computer aided design.