GB2364119A

GB2364119A - Circuit board testing

Info

Publication number: GB2364119A
Application number: GB0102436A
Authority: GB
Inventors: David Hall
Original assignee: NOTIONLIGHT Ltd
Current assignee: NOTIONLIGHT Ltd
Priority date: 2000-02-01
Filing date: 2001-01-31
Publication date: 2002-01-16
Also published as: GB0002325D0; GB0102436D0

Abstract

Testing a printed circuit board 11 for faults in component mounting. The board 11 is taken at a stage where components having connector pins have been positioned on the board 11 with the pins projecting through holes in the board 11. A laser beam 13 is directed across the projecting pins. The region of the board including the pins is scanned with a video camera 14 at an angle to the laser beam 13. The resulting height profile is determined by a convener 48, and the profile is analysed to determine whether there is a fault in the component mounting. The method may be carried out before solder has been applied to the pins, detecting whether all pins are present. Alternatively, it may be applied after soldering, discriminating between solder blobs which have formed over empty holes and solder blobs which have formed around pins.

Description

2364119 Circuit Board Testing S The present invention relates to testing

printed circuit boards (PC Bs), and more specifically to determining the presence of component pins through a board, though it also has firther applications such as the detection of debris on a board.

There are two basic techniques for testing printed circuit boards The board may be 1 o checked optically for the presence of the correct components, and electrically, for correct operation Optical checking and testing generally involves scanning the board (automatically), or selected areas of it, and comparing the resulting image with a reference image obtained from a board which is known to be sound (the reference image may be obtained from a physical reference board but is more usually a stored image) Electrical is testing may be performed by exercising the board by applying a sequence of test signals to it via its connector(s), or by applying test signals to various selected regions or components on the board by means of a "bed of nails" tester (or, in principle, by a combination of the two).

As with any testing, PCB testing involves striking a balance between time and cost on the one hand and thoroughness on the other The more detailed the testing, the longer and costlier it becomes, and the more stringently the criteria are set, the greater the chance that a sound PCB will fail and be rejected.

Hitherto, it has not generally been the practice to test PC Bs during the process of manufacture, for the reasons just outlined In particular, testing during manufacture introduces extra manufacturing steps, and has been regarded as adding little value since any faults should become apparent during testing of the PCB in its final manufactured state.

The general object of the present invention is to provide an improved technique for testing printed circuit boards.

According to its main feature, the invention provides a method of testing a printed circuit board comprising taking the board at a stage where components having connector pins have been positioned on the board with the pins projecting through holes in the board, directing a laser beam across the projecting pins, scanning the region of the board including the pins with a video camera at an angle to the laser beam, determining the resulting height profile, and analysing the profile to determine whether there is a fault in the component 1 o mounting The invention also provides apparatus for carrying out the method.

The method may be carried out before solder has been applied to the pins, in which case it will detect whether all pins are present, ie have passed correctly through the board, before soldering Preferably however the method is applied after soldering, in which case it will discriminate between solder blobs which have formed over empty holes which the connector pins have not passed through and those blobs formed around pins which have correctly passed through the board.

The production of a PCB generally involves the production of the basic board itself, with the tracks on it and holes in it, then the placement of the various components on it (generally on one side), with their connector pins projecting through the holes in the board, and then the soldering of the pins to the tracks by a soldering step If the present technique is applied after the placement of the components but before the soldering, then, since the placement of the components is normally performed by one machine and the soldering by another, and the board normally has to be moved between these two machines, the interpolation of the testing step does not cause significant disruption.

A component such as an integrated circuit typically consists of a flat rectangular body with two rows of connectors along two opposite edges The connectors are formed to stand at right angles to the body, so that when the component is offered to the board, the connector pins should pass through holes in the board and project out on the other side; when the board proceeds to soldering, these pins are soldered to tracks on that other side ending at the holes.

It may happen that a connector is slightly displaced from its correct angle, or the component is not accurately aligned with the board when it is being placed on the board.

As a result, the pin may catch against the board and get bent and so fail to pass through the hole An optical inspection of the board after soldering is likely to fail to detect this, as the lo pins themselves are concealed within their solder bumps and a solder bump is likely to be formed over the missing pin.

In the past, detection of this situation has therefore had to be achieved by electrical testing of the finished PCB However, the testing of all pin connections for missing pins 1 S involves a considerable number of tests More significantly, the devising of suitable test conditions for the detection of such faults requires detailed knowledge of the functionality of the components involved and considerable skill and effort in devising test conditions which will allow the detection of such faults The present invention obviates the need for that, by providing a simple and direct way of detecting missing pins before the soldering step.

A PCB testing apparatus embodying the invention will now be described, by way of example, with reference to the drawings, in which:

Fig 1 is a general diagrammatic view of the apparatus; and Fig 2 is more detailed view of the scanning of a line of pins.

Referring to Fig 1, a PCB carrier 10 has the PCB 11 to be analysed mounted on it.

This carrier can be moved in the X and Y directions to bring any desired region of the PCB into the operating area A laser 12 is mounted above the carrier, and produces a beam which is automatically scanned across the PCB 10 at high speed to produce in effect a fan- shaped beam 13 A video camera 14 is also mounted above the carrier, and is directed towards the region on which the laser beam fan falls The laser beam fan lies in a plane which is perpendicular to the carrier and PCB, and the camera is mounted away from this s plane so that it views the region illuminated by the laser beam fan, ie the region below the laser, at an angle.

Fig 2 illustrates the operation of the apparatus Fig 2 A is a perspective view of part of a row of connector pins 21, 22, etc on the board 11 This row of pins is aligned beneath the laser beam fan 13, so that the laser beam follows along the path 23, passing along the surface of the board 11 between the pins and rising up over the pins 21, 22, etc as shown Since the pins protrude through the board from the component side, the board must effectively be upside down for this (or alternatively the laser and camera are mounted beneath the board).

The camera 14 views the region of the board from in front of and above the line of connector pins; in fact, Fig 2 A can be regarded as a part of the image as seen by the camera (the pins 21 and 22 being to the left of the centre of the field of view, so that their inner edges can also be seen) With a camera with a typical resolution, eg 512 x 512 pixels, the region scanned may conveniently be chosen to include around 10 connector pins The laser should be focussed so that its scan beam is significantly narrower than the width of the connector pins.

The laser fan beam is shown as substantially perpendicular to the board and the camera as viewing the board at roughly 45 Other angles can obviously be used; the laser fan beam and the camera viewing angle must not be too close to each other, and neither angle should be so small that the illumination or viewing of the scan line would be in danger of being obscured by other components such as an adjacent line of connector pins.

The camera (and, if necessary, the laser fan beam) can be moved to successive positions by suitable X and Y positioning units (and/or the board may be similarly moved).

The present system may be include in a general board optical monitoring system, and the cameras of that system may be used to locate the laser fan beam and/or camera of the present system accurately The laser brightness can readily be made considerably higher than the brightness of any general illumination, so that the present system will still operate correctly in the presence of general background illumination.

Fig 2 B shows the corresponding part of the scanning pattern of the camera, which lo will see an image as shown The image is of a square wave extending across several scan lines, and is geometrically identical to the laser track 23 as displayed on the paper of Fig.

2 A.

The height profile of the line of connector pins has thus been converted into a 2- dimensional camera image This image is converted to a waveform profile or graph signal in a converter 38, and the resulting signal is fed to an analysis circuit 36 (Fig 1), where it is compared with a stored signal from a memory 35 A signal is produced at 37 indicating whether the profile obtained by the camera matches the stored profile If a connector has failed to pass through the PCB, the profile from the camera will not match the stored profile, and the signal will indicate a mismatch or failure.

The image processing of blocks 35-38 may conveniently be performed by a suitably programmed computer.

As noted above, it may be preferable to apply the method after the board has been through the soldering process The soldering process will produce bumps or beads of different shapes depending on whether they are formed on connectors projecting through the board or empty holes in the board The resulting waveform profiles will therefore be different, and can be used to discriminate between good and bad connections.

The present technique can readily be extended to the scanning of the board, or selected regions thereof, for debris For this, the preferred time to performing the scanning is after the soldering step, in order to detect stray blobs of solder.

Claims

1 A method of testing a printed circuit board comprising taking the board at a stage where components having connector pins have been positioned on the board with the pins projecting through holes in the board, directing a laser beam across the projecting pins, scanning the region of the board including the pins with a video camera at an angle to the laser beam, determining the resulting height profile, and analysing the profile to determine o 10 whether there is a fault in the component mounting.

2 A method according to claim 1 wherein it is carried out before solder has been applied to the pins.

3 A method according to claim 1 wherein it is carried out after soldering.

4 A method of testing a printed circuit board substantially as herein described and illustrated.

5 Apparatus for carrying out the method of any previous claim, comprising means for directing a laser beam across the projecting pins, means for scanning the region of the board including the pins with a video camera at an angle to the laser beam, means for determining the resulting height profile, and means for analysing the profile to determine whether there is a fault in the component mounting

6 Apparatus for testing a printed circuit board substantially as herein described and illustrated.

7 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4 H of the International Convention (Paris Convention).