GB2293291A

GB2293291A - P.C.B Inspection System

Info

Publication number: GB2293291A
Application number: GB9418288A
Authority: GB
Inventors: Andrew Sibley
Original assignee: TASKDISK Ltd
Current assignee: TASKDISK Ltd
Priority date: 1994-09-10
Filing date: 1994-09-10
Publication date: 1996-03-20
Anticipated expiration: 2014-09-10
Also published as: GB2293291B; GB9418288D0

Abstract

The system is for inspecting pieces, having a nominally flat surface with undulated areas thereon e.g. PCB's. It comprises a source 6 for a band of light "L" across each piece 15 passed through the system, optical means 5 responsive to undulated areas of each piece passing through the band of light to detect changes in height or depth from the flat surface to produce an optically derived image of the profile of said piece, video imaging means 4 arranged in a line and directed towards the optically derived image, photoelectric conversion means such as a CCD detector 14 for receiving light images detected by the video imaging means and converting them into electric Signals indicative of any height or depth changes, means 3 for moving the or each piece progressively through the band of light to produce a plurality of profile slices of each piece, and means 9 for integrating the electric signals representative of these slices to produce a complete profile of the undulations and their position over the complete surface of each piece for comparison with a reference piece. <IMAGE>

Description

INSPECTION SYSTEM FOR ELECTRONIC ASSEMBLIES SUCH AS PRINTED CIRCUIT BOARDS This invention relates generally to the inspect ion of pieces having a nominally flat surface where the dimensions and position of undulations, particularly components thereon, are critical.

The invention has particular application to electronic assemblies such as Printed Circuit Boards (PCBs). PCBs for electronic circuits are currently mass produced using machines for solder paste printing; automat-ic placement and soldering. These machines are arranged in a production line, which can sustain a manufacturing rate, even with complex PCBs as large as 508mm x 381mm, of one PCB per minute or less. To maintain a low defect rate, which leads to a reduced cost, the partially constructed PCBs require some form of inspection.

The methods currently used for inspecting production pieces such as PCBs include visual inspection, image processing, and optical and physical: Manual inspection: Even today PCBs are simply examined visually, which is a slow and error prone method, purely reliant on human resources.

Image processing: Consists of a system which captures an image of t.he piece under inspection, eit.her by video camera or by scanning, and then comparing the captured image with a pre-programmed computer test image. The disadvantages of this system of inspection are that the inspection image has to be defined by a laborious manual or semi-manual method and that, consequently a very considerable amount of computer power is required both in terms of the hardware and software to provide the desired result. This in turn means a high purchase cost and slow throughput.

Optical and Physical: There are several methods for determining the height of an object with respect to a known flat surface, by either physical or optical means. However, these methods generally only measure the height and, in some instances, the position of the object with respect to a predetermined origin at a single point. These methods are currently used for manual inspection and metrology, since to produce a complete profile of the piece under inspection, it is necessary to measure each single point in both the X and Y coordinates, resulting in a slow throughput or small area of capture.

An object of the invention is to provide a practical inspection system to determine the position and height and/or depth of a piece which can be compared with a reference piece at a reasonable financial outlay. Another object is to produce a system which will run as fast as the slowest machines on a production line for the pieces to be inspected.

According to the present invention, an inspection system for inspecting pieces having a normally flat surface with undulated areas thereon comprises, illumination means for producing a band of light across each piece passed through the system, video imaging means arranged in a line and directed towards the band of light, optical means associated with said video imaging means and responsive to undulated areas that pass through the band of light to detect any change in height or depth from the flat surface thereof, photoelectric conversion means for receiving detected light images and converting them into electric signals indicative of any changes, means for moving individual pieces progressively through the band of light to produce a plurality of profile slices, and means for integrating the electric signals representative of these slices to produce a complete profile of the undulations and their position over the complete surface of each piece for comparison with a reference piece.

Preferably, the video imaging means comprise a plurality of video cameras arranged side-by-side, each with an associated said optical means.

The optical means may comprise a two-image, or single image, forming optical arrangement comprising an objective lens for focussing incoming reflected light, a separator lens for separating the focussed light into two, or single secondary images respectively and a secondary lens system for focussing the two, or single secondary images onto the photoelectric conversion means, the arrangement being such that, where an undulated area is detected, the secondary images provide either two peaks the distance between which is, or a single peak the distance of which from a fixed point is, proportional to the height or depth of said undulated area above or below the nominal flat surface.

In order that the invention be readily understood and further features made apparent, one embodiment of inspection system for PCBs and a modification thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a somewhat diagramatic perspective view of the system.

Figure 2 is a diagram of one optical arrangement for the system, Figures 3 A to C are diagrams showing the operation of the optical. arrangement of Figure 2, Figure 4 is a diagram of an alternative arrangement for the system, Figures 5 A to C are diagrams showing the operation of the optical arrangement of Figure 4, and Figure 6 is a diagram similar to Figure 1 showing a modification to the system.

With reference to Figure 1, the system basically comprises a conveyor station S through which PCBs can be fed by a conveyor 1. The station comprises a horizontal table 2 which is positioned in a space between aligned runs la, lb of the conveyor and is mounted so as to be capable of horizontal movement normal to the conveyor direction i.e. in the direction of the arrows X-X. A drive 3 is provided whereby the table can be moved in increments with respect to the width of the conveyor 1. A number of video cameras 4 (in this embodiment four) are mounted above the table with their lens arrangements 5 being located "in-line" and directed towards the table as shown.A light source (or sources) 6 is also provided above the table 2 which is arranged to provide a focussed line of light L across the table 2; in this embodiment the line L is shown extending in a direction parallel to the conveyor direction although this line could, if required, be adjusted to be at an angle to the conveyor direction. The lens arrangements 5 of the video cameras 4 are directed towards the line of light "L" and, hence, the line angle of the video cameras can be adjusted into alignment with the line "L". The drive 3 for the table and the outputs from the video cameras are electrically connected via lines 7 and 8 respectively to a computer 9.

Referring to Figure 2, in one optical arrangement the lens assembly 5 of each video camera comprises an objective lens or lens system 10 which focuses images detected and reflected from the line L at a focal plane F, a condenser lens 11 for separating these images into two secondary images and directing them via a double-slit aperture 12 and respective focussing lenses 13a, 13b, onto photoelectric conversion means, preferably in the form of a charge coupled device (CCD) detector 14.

In use, the PCBs 15 to be inspected are fed via the conveyor singly onto and from the table 2, the conveyor runs la, lb being appropriately controlled in relation to each other to provide a delay sufficient for inspection of each PCB to be carried out. When positioned on the table 2, each PCB is inspected by moving said table in small fixed increments in the direction X-X to enable the line of video cameras to scan continuously across the PCB along the line L provided by the illumination source.Referring also to Figure 3A, it will be appreciated that the incremental movement of the table will cause the PCB to be inspected along the line L in slices, the width of each slice being dependent upon each movement of the table and preferably being of the order of 0.05mm. The images produced by the components shown along the line L of Figure 3A are represented in Figure 3B, from which it will be appreciated that the lens assemblies 5 produce secondary images of the position of the components along the line L and are of a width proportional to the height of the components (or depth of cavities) with respect to the flat surface of the PCB.Figure 3C shows a sample profile over the width of a scan image produced, from which it will be noted that the secondary images provide two peaks, the height of the component being determined by the distance between these peaks.

Referring to Figure 4, in an alternative optical arrangement, the condenser 11 produces single secondary images 13 via a single-slit aperture 12 onto the CCD detector 14; it will be noted from Figure 5B and 5C that the secondary images formed of the line illuminated subject, on the CCD detector 14 will consist of a peak of intensity on the detector, on each scan line, where the distance of the peak from a fixed point on said CCD detector is proportional to the height of the subject with respect to a reference line of height.

It will be appreciated that the secondary image profile for each slice will be captured by the CCD detector 14 where it will be digitised and fed to the computer 9 via the line 8 and, thereafter, integrated by said computer to produce a profile image of the complete PCB 15 for subsequent comparison with the profile image of a reference PCB.

Referring to Figure 6, in a modification of the system of Figure 1, the table 2 is mounted for hbrizontal movement parallel to the conveyor direction, as will be noted by the direction arrows X-X. The video cameras 4 are therefore mounted in a line across the width of the table and the light source 6 is arranged to provide the line of light "L" normal to the table direction. It may also be possible to replace the table 2 with other means, such as an endless conveyor belt having an upper horizontal surface in alignment with the conveyor runs la, lb, in which case the table would not need to be returned to its load position for each scan.

The computer 9 can simply be a personal, or desktop computer with appropriate software to enable it to: (1) digitise and capture the output signals of the CCD detector 14; (2) integrate the incoming slices of height information into a complete profile of the PCB 15 under inspection (3) compare this profile with the profile of a reference PCB to check for errors: (4) inform the operator of faulty PCBs and the location of the detected error, or errors, and (5) control the movement of the table 2 and the placement and removal of PCBs via the conveyor runs la and lb.

These functions can be accomplished by providing a plug-in interface board with the requisite number of inputs, for converting incoming digitised signals to a suitable format and storing them for further processing.

It will be appreciated that such a system provides the following advantages.

(i) The speed of throughput can be significantly faster than the known methods discussed hereinbefore since it is dependent on capturing complete slice profiles of the PCB across the whole width of the PCB in increments.

(ii) The hardware requirements are modest, none of the mechanics or electronic circuits being particularly expensive or hard to obtain.

(iii) As the system measures height, depth and positional data, only a moderately simple comparison computer program is required.

(iv) A commercial system would have a higher rate of throughput at a much lower cost than the presently available inspection systems.

(v) The system could be extended to provide an even faster throughput by the use of a further line or lines of video cameras and respective lines of light sources.

Claims

1. An inspection system for inspecting pieces having a nominally flat surface with undulated areas thereon comprising, illumination means for producing a band of light across each piece passed through the system, video imaging means arranged in a line and directed towards the band of light, optical means associated with said video imaging means and responsive to undulated areas that pass through the band of light to detect any change in height or depth from the flat surface thereof, photoelectric conversion means for receiving detected light images and converting them into electric signals indicative of any changes, means for moving individual pieces progressively through the band of light to produce a plurality of profile slices, and means for integrating the electric signals representative of these slices to produce a complete profile of the undulations and their position over the complete surface of each piece for comparison with a reference piece.

2. An inspection system according to Claim 1, wherein the video imaging means comprise a plurality of video cameras arranged side-by-side, each with an associated said optical means.

3. An inspection system according to Claim 1, or 2, wherein a table is located at an inspection station of the system onto which each piece is individually located, and wherein the moving means are operative to cause the table to pass in incremental movements through said band of light for producing said slice profiles.

4. An inspection system according to Claim 3, wherein said table is associated with a conveyor which is operative to feed and remove pieces to and from said table.

5. An inspection system according to any preceding Claim, wherein said optical means comprise a two-image forming optical arrangement comprising an objective lens for focussing incoming reflected light, a separator lens for separating the focussed light into two secondary images and a secondary lens system for focussing the two secondary images onto the photoelectric conversion means, the arrangement being such that, where an undulated area is detected, the secondary images provide two peaks spaced apart by a distance proportional to the height or depth of said undulated area above or below the nominal flat surface.

6. An inspection system according to any one of Claims 1 to 4, wherein said optical means comprises a single-image forming optical arrangement comprising an objective lens for focusing incoming reflected light, a separator lens for separating focussed light into single secondary images and a secondary lens system for focussing the single secondary images onto the photoelectric conversion means, the arrangement being such that, where an undulated area is detected, the secondary images provide a peak of intensity on the detector where the distance of the peak from a fixed point on the detector is proportional to the height or depth of said undulated area above or below the nominal flat surface.

7. An inspection system according to any one of Claims 1 to 6, wherein the photoelectric conversion means is a charge coupled device (CCD).

8. An inspection system according to any one of the preceding Claims, wherein the pieces are electronic assemblies such as PCBs.

9. An inspection system constructed, arranged and adapted for use substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. An inspection system for inspecting pieces having a nominally flat surface with undulated areas thereon comprising, illumination means for producing a band of light across each piece passed through the system, optical means responsive to undulated areas of the or each piece that passes through the band of light to detect any change in height or depth from the flat surface thereof and produce an optically derived image of the profile of said piece, video imaging means arranged in a line and directed towards the optically derived image, photoelectric conversion means for receiving light images detected by the video imaging means and converting them into electric signals indicative of any height or depth changes, means for moving the or each piece progressively through the band of light to produce a plurality of profile slices of each piece, and means for integrating the electric signals representative of these slices to produce a complete profile of the undulations and their position over the complete surface of each piece for comparison with a reference piece.

3. An inspection system according to Claim 1, or 2, wherein a table is located at an inspection station of the system onto which each piece is individually located, and wherein the moving means are operative to cause the table to pass in 1ncrPmental movomants though said band of light for producing said slice profiles.