GB2215064A

GB2215064A - Testing printed circuit boards

Info

Publication number: GB2215064A
Application number: GB8802077A
Authority: GB
Inventors: Ian James Forster
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1988-01-30
Filing date: 1988-01-30
Publication date: 1989-09-13
Anticipated expiration: 2008-01-30
Also published as: GB8802077D0; GB2215064B

Abstract

To test a printed circuit board (1), a sheet of resistive material (6) is put into electrical contact with exposed parts (4) of tracks on the board, and measurements, e.g. of resistance, between test points (A-H) on the sheet are compared with measurements with a good board. The sheet may be of flexible, conductive rubber or polymer, for deformation purposes, or it may be rigid and have pins therein for contacting the tracks of the PCB. <IMAGE>

Description

TESTING PRINTED CIRCUIT BOARDS This invention relates to testing printed circuit boards.

Printed circuit boards consist of conducting tracks on an insulating substrate and, in order to apply solder to the various connecting points, printed circuit boards are often coated with solder resist after being fabricated in order that the solder can be applied to the whole substrate but will only adhere to those connecting points, to which components are to be connected.

Typical printed circuit boards may have hundreds of tracks on one substrate, and in order to test for faults, i.e. short circuits and open circuits, it is necessary to test for connection between each connecting point in turn and every other connecting point. Because such a test is so time-consuming, only a small percentage of the output of a production line can be tested.

The invention provides a method of testing printed circuit boards, which comprises measuring electrical signals at a plurality of test points in a sheet of resistive material that is in electrical contact with exposed parts of the tracks.

The electrical signals will be characteristic of the bridging paths created by the tracks and hence will be characteristic of a particular pattern of tracks: the signals can be compared with reference values for a known good printed circuit board to check whether the board under test is defective or not. In the case of a board coated with solder resist, the exposed parts of the tracks will be those free of solder resist, to which connections are to be made.

The sheet of resistive material may be in electrical contact with the exposed parts of the tracks by virtue of conducting pins connected to the sheet of resistive material that register with the exposed parts of the tracks but, preferebly, the sheet is placed in contact with the printed circuit board to make the electrical contact. The sheet may be of conductive rubber or polymer.

In the case of printed circuit boards having conductive tracks on both sides, a second sheet of resistive material is put in electrical contact with the exposed parts of the tracks on the second side of the board, and electrical signals are measured at a plurality of test points in a peripheral region of the sheet.

The electrical signals may be measurements of resistance between each test point and each of the other test points. This is particularly suitable for a bare printed circuit board i.e. one to which no components have been connected. As an alternative, however, the electrical signals which are measured may be voltages.

These may be produced when a known voltage is applied to a particular point on the printed circuit board. In order for the sheet of resistive material to make electrical contact with the exposed parts of the tracks, the sheet may have appropriately arranged conducting pins connected to it.

The invention also provides apparatus for testing printed circuit boards, which comprises a sheet of resistive material arranged to be placed in electrical contact with exposed parts of the tracks, and means for measuring electrical signals at a plurality of test points in the sheet.

The invention will now be described in greater detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of apparatus for testing printed circuit boards, partly cut-away; Figure 2 is a section through lines 2-2 of Figure 1; and Figure 3 is a plan view of one of the sheets of resistive material.

Referring to the drawings, Figure 1 is partially cut away to show the surface of a printed circuit board 1 which has on it a number of conducting tracks, one of which is indicated by the reference numeral 2. Each end of the track 2 has a pad 3, 4 secured to it, which has an aperture in it in line with an aperture in the board itself to receive component leads. The board has been prepared for the soldering operation, in which components are mounted on the reverse side of the board and leads are inserted through apertures as appropriate, by virtue of the fact that it has been coated with solder resist 5.

When the components have been assembled to the board, the solder-resist-coated surface may be coated with solder, and it will only adhere to the regions where the solder resist is not present, such as apertures 5a, 5b, which expose the pads 3, 4 of the track 2.

To test such a bare printed circuit board hitherto, it would be necessary to place a probe on pad 3, for example, and then to apply another probe to pad 4 and to all other exposed connecting points in turn. A resistance measurement should of course indicate that there is only a connection between pad 3 and pad 4. The same procedure would then have to be followed for each other connecting point or pad.

In accordance with the invention, a sheet of resistive flexible material 6 is set in electrical contact with the exposed parts of the tracks. A nonconductive backing 7 which supports the resistive material 6, is placed into contact with the top face of the printed circuit board; and an identical assembly is placed in contact with the lower side of the printed circuit board. It will be apparent that the uniform resistance of the resistive material will be modified by the various short circuits produced by the various joined conducting pads.

Referring to Figure 3, it will be seen that the sheet of resistive material 6 has eight terminals A to H connected to its periphery.

In use, the sheet of resistive material 6 on its backing 7 is pressed against the top surface of the printed circuit board and an identical assembly is pressed against the lower surface of the printed circuit board.

The backing layers are pressed together at a pre-set pressure which is determined by measuring the resistance between terminals on opposite sides of each resistive sheet. (The resistance of the resistive sheet varies with pressure).

At the pre-determined pressure, the resistance between point A and, respectively, point B to H is separately measured, as well as the resistance between point A and the eight terminals on the lower resistive sheet. This procedure is then followed for each of the other terminals on the upper and lower sheets. This entails a total of 120 measurements.

The read-out of these measurements is then compared with a reference series of measurements prepared by taking an identical board which has been tested by conventional methods and measuring it in the same way as described.

Any short circuit or open circuit will show up as a difference between the series of measurements for. the board being tested and those for the reference board.

With this method, it is possible to test every bare, solder-resist-coated board, which comes off the production line.

The resistive material may be a conductive rubber or polymer, for example, carbon loaded silicon rubber and the resistivity of the material may be in the region of 10K to lOOK per square.

Of course, various modifications may be made without departing from the scope of the invention. Thus, the conductive sheet need not be pressed into electrical contact with exposed parts of the tracks, but could be separated therefrom by a number of pins which align with those exposed parts of the tracks. The series of measurements of resistance, however, is the same as described hereabove.

The technique may be used on one side only of a printed circuit board, in which only 28 measurements are required.

The test points A to H could be distributed over the area of the board instead of being arranged around the periphery, and a different number of test points could be used.

Instead of measuring the resistance between all combinations of pairs of points, the resistance between one point only and each of the other points in turn may be measured, before comparing the result with reference values from a good board. Naturally, this will not provide such an accurate characterisation of the board as by repeating the series of measurements for each point in turn, but it may nevertheless be accurate enough for some situations.

Instead of measuring resistances, each test point could be connected to ground via aknown impedance, a known voltage could be applied to one test point, and the potential difference between that point and each of the other test points in turn could be measured ; the measurements could then be repeated for each of the other test points, if the information obtained from one set of measurements was not sufficient. In each case, the values are compared with reference values produced with a good board.

Either voltage or resistance measuring method is also applicable to a populated board eg. a board bearing logic components on one side, in which case the opposite side of the board only is tested, but the voltage measuring method is particularly suitable in this case. For example, a particular voltage may be applied to a test point on a component, and the potential difference between that point and each of the test points (suitable grounded through predetermined resistances) measured in turn and compared with reference values.

Claims

1. A method of testing printed circuit boards, which comprises measuring electrical signals at a plurality of test points in a sheet of resistive material that is in electrical contact with exposed parts of the tracks.

2. A method as claimed in claim 1, in which the sheet is placed in contact with the printed circuit board to make electrical contact with the exposed parts of the tracks.

3. A method as claimed in claim 2, in which the sheet is flexible.

4. A method as claimed in claim 3, in which the sheet is of conductive rubber or polymer.

5. A method as claimed in anyone of claims 1 to 4, in which the test points are in a peripheral region of the sheet.

6. A method as claimed in any one of claims 1 to 5, in which the electrical signals are measurements of the resistance between each test point and each of the other test points.

7. A method as claimed in any one of claims 1 to 6, in which the electrical signals are voltages.

8. A method substantially herein described with reference to the accompanying drawings.

9. Apparatus for testing printed circuit boards, which comprises a sheet of resistive material arranged to be placed in electrical contact with exposed parts of the tracks, and means for measuring electrical signals at a plurality of test points in the sheet.

10. Apparatus for testing printed circuit boards substantially as herein described with reference to the accompanying drawings.