GB2351258A

GB2351258A - Apparatus and method for aligning two objects

Info

Publication number: GB2351258A
Application number: GB9914501A
Authority: GB
Inventors: Ian William Belchamber
Original assignee: BLAKELL EUROPLACER Ltd
Current assignee: BLAKELL EUROPLACER Ltd
Priority date: 1999-06-22
Filing date: 1999-06-22
Publication date: 2000-12-27
Also published as: GB9914501D0

Abstract

Apparatus and method for sequentially locating an article for example one or more circuit boards comprising feed means to deliver successive articles from a supply, first and second support means for supporting the article in respective first and second positions, a reference object in spaced alignment with the second support means, a conveyor for conveying each article between the first and second support means, a misalignment measuring device positioned above the first support means for determining the misalignment between each article and the reference object and adjustment means for adjusting each article as required at the second support means relative to the reference object. The reference object may be a stencil in a screen printer used for printing solder paste to circuit boards.

Description

1 2351258 APPARATUS AND METHOD OF ALIGNING TWO OBJECTS This invention

relates to alignment apparatus and a method for use in aligning two objects, in accordance with reference marks on the surface of the objects. Such an apparatus is applied, for example, in screen printers for printing solder paste onto printed circuit boards for surface mount assembly.

In screen printers for printing solder paste, a stencil (or screen) is positioned over a circuit board to be printed and the solder paste is applied to the board through apertures in the stencil. It is important for the stencil to be accurately aligned with the board prior to printing. Commonly, the stencil and each board are provided with a number of reference marks, or "fiduciais", and an alignment apparatus is employed to determine the relative positions of the board and stencil "fiducials" and to determine whether they are correctly aligned. The board or stencil is adjusted by the alignment apparatus to eliminate any misalignment before the printing process. The prior art makes use of cameras between the stencil and the circuit board to assist in locating the fiducials and a control system to make any adjustment so that the board and stencil are aligned, often referred to as a "single roving camera" system.

One example of such an alignment system is illustrated in US 5, 752, 446, in which a camera is located between the board and the print screen to locate the reference points and control means is used to make necessary adjustments to the board to align it with the screen. Other prior art screen printers with vision require a test print to be made during the batch set up.

When using cameras with prior art apparatus, they are required to move between the fiducial positions during each print sequence which slows down the cycle time to print each board.

Furthermore, in order to move the cameras with a sufficient degree of accuracy, it is desirable to use a high precision camera and to move the camera at high speed which increases the cost of the equipment.

2 The present invention seeks to overcome or at least mitigate the problems of the prior art. In essence, the present invention adopts a different approach by moving an object to a desired position repeatably.

One aspect of the invention provides apparatus for sequentially locating an article, for example one or more circuit boards, which apparatus comprises feed means to deliver successive articles from a supply, first and second support means for supporting the article in respective first and second positions, a reference object in spaced alignment with the second support means, a conveyor for conveying each article between the first and second support means, a misalignment measuring device positioned above the first support means for determining the misalignment between each article and the stencil and adjustment means for adjusting each article as required at the second support means relative to the object.

According to an optional feature of this aspect of the invention each article may have at least one reference mark on a surface thereof and the misalignment measuring device comprises imaging means for acquiring an image of an area of the article containing the reference marks and a controller for determining the location of the reference marks in the image, comparing the reference marks with a calibrated position and determining therefrom the extent of misalignment. Preferably, the adjustment means may be controlled by the misalignment measuring device.

In one class of embodiment the adjustment means may cause the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the second support means. Additionally or alternatively, the adjustment means may cause the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the reference object.

According to another optional feature of this aspect of the invention the article may include two reference marks and the adjustment means may comprise means to adjust each article or, as the case may be, the reference object in a first plane and means to adjust each article or, as 3 the case may be, the reference object in a second plane substantially perpendicular said first plane. Preferably, the adjustment means may ftirther comprise rotary means for rotating the second support means to move the article relative the reference object.

According to a further optional feature of this aspect of the invention the imaging means may comprise a pair of cameras mounted for movement between the first and second support means. Alternatively, the imaging means may comprise two pairs of cameras wherein one pair of cameras is mounted above the first support means and the other pair of cameras is mounted above the second support means.

According to another optional feature of this aspect of the invention first and second support means may comprise a clamp assembly for retaining the article in the respective first and second positions. Preferably, the clamp assembly may comprise first and second rails in a spaced parallel arrangement, said first rail is movable relative to said second rail, between a release position and a clamping position whereby an edge pressure is applied to the article to clamp it in position.

According to yet another optional feature of this aspect of the invention the apparatus may ftu-ther comprise a device for determining the calibrated position of the article at the first support means and the misalignment between the article at the second support means and the reference object prior to printing a batch of articles comprising at least one article. Optionally, the article and the reference object may each have at least one reference mark on a surface thereof and the misalignment calibrating device may comprise imaging means for acquiring images of an area of the article at the second support means, the reference object surface and an area of the article at the first support means containing the reference marks and a controller for determining the location of the reference marks in the images and determining therefrom the extent of misalignment of the article and reference object and the calibrated position.

4 A second aspect of the invention provides an apparatus to hold an object, for example a circuit board in a substantially horizontal plane, comprising a support assembly for supporting the board and one or more clamping elements movable between release and clamp positions, wherein the one or more clamping elements are so constructed and arranged as to exert a downward pressure on the board in its clamping position to reduce bowing of the board. Preferably, the one or more clamping elements may be moved between said release and clamp positions by a cam and a cam follower connected to the clamping element.

According to an optional feature of the second aspect of the invention the one or more clamping elements may be movable between stowed and operating positions, wherein each clamping element is stowed in a recess of the support assembly and, in its operative position, the clamping element protrudes over an outer portion of the circuit board. Preferably, the clamping element may be movable by a rack and pinion arrangement which imparts a rotary movement to the clamping element between its stowed and operative positions.

A third aspect of the invention provides a method for determining a fixed reference point to be used in aligning an article with a reference object each having at least one alignment reference mark on the surface thereof, the method comprising the following steps: - (i) supporting the article in two successive positions by first and second support means; (ii) locating imaging means above the article which imaging means views the or each alignment reference mark of the article positioned in the second support means; (iii) loading the reference object and locating and viewing through the imaging means the corresponding alignment reference marks on the reference object; (iv) determining the locations of each reference mark in the images and determining therefrom the extent of misalignment of the article and reference object; (v) locating the article at the first position; and (vi) locating the imaging means to view one or more offset reference marks on the article at the first position and determining the locations of each reference mark to be used as a fixed reference point.

A fourth aspect of the invention provides a method of aligning an article and a reference object, the article having one or more offset reference marks comprising the steps of (i) supporting the article in two successive positions by first and second support means; (ii) locating imaging means above the article which imaging means views each offset reference mark of the article positioned in the first support means; (iii) determining the location of each offset reference marks in the image, comparing each reference mark with a fixed reference point and determining therefrom the extent of misalignment of the article and reference object; and (iv) adjusting the second support means or reference object to align the article and the reference ob ect.

j According to an optional feature of the third or fourth aspects of the invention there may further comprise the step of adjusting the second support means or reference object to minimise the offset of the article between its successive positions.

According to another optional feature of the third or fourth aspects of the invention the article may be a circuit board and the reference object may be a stencil of a screen printer for printing the circuit board.

A fifth aspect of the invention provides apparatus for determining the extent of the offset of an object, for example a circuit board, mounted in successive first and second positions, whereby the position of the object is determined by using a reference mark on the surface of the object, the apparatus comprising first and second support means for supporting the object in the respective first and second positions, each support means comprising a clamp assembly for retaining the object on the support means, and for positioning the object at the predetermined position, wherein the apparatus further comprises an offset calibrating device for identifying the reference mark and for determining the offset of the object between the first and second positions.

According to an optional feature of the fifth aspect of the invention the apparatus may further comprise a conveyor for conveying the object between the first and second positions.

6 According to another optional feature of the fifth aspect of the invention there may further comprise means for adjusting the second support means relative the first support means to reduce the offset. Preferably, the adjustment means may be controlled by the offset calibrating device. 5 According to a further optional feature of the fifth aspect of the invention the offset calibrating device may comprise imaging means for acquiring images of an area of the object surface containing a reference mark and control means for identifying the location of the reference mark in the images and determining therefrom the extent of the offset. Preferably, the control means may cause the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the second support means.

Exemplary embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIGURE I is a perspective view of a preferred embodiment of the invention; FIGURE 2 is a plan view of the first and second support means of the first aspect of the invention; and FIGURES 3a to 3d are perspective views of the clamping apparatus according to another aspect of the invention.

Referring now to the drawings and more particularly to Figure I thereof, there is shown a perspective view of an exemplary embodiment of one aspect of the invention. Apparatus 10 for locating an article for example a batch of circuit boards 11, is shown which apparatus comprises feed means (not shown) to deliver successive circuit boards from a supply (not shown), first and second support means 12, 14 for supporting the circuit board 11 in respective first and second positions 16, 18. There ftirther comprises a reference object, for example a stencil 54 mounted in a screen printer 20 which reference object is in spaced 7 alignment with the second support means 18 for printing the article. A conveyor 22 is provided for conveying the circuit board I I between the first and second support means. A misalignment measuring device 24 is positioned above the first support means 12 for determining the misalignment between the article and the reference object and there is adjustment means (not shown) for adjusting the position of the article at the second support means 14 relative to the reference object.

Turning to the construction of the first and second support means 12, 14 each comprises an assembly 26, 29 which is identical in construction in this embodiment and, therefore only the first support assembly 26 shall be described in any greater detail.

In the embodiment shown in Figure 1, the first support assembly 26 comprises front and rear guide rails 28, 30 that are adapted to support the circuit board. The rear rail 30 is adjustable relative to the front rail 28 for board size adjustment and to push the circuit board 11 held in the assembly towards the front rail 28. There may further comprise suitable drive means 32, for example a pneumatic cylinder, to drive the rear rail 30 forward towards the front rail 28 thereby to clamp the board in position, by engaging the opposed side edges 34, 36 of the circuit board 11 adjacent the respective rails 28, 30. Apparatus for locating the precise position of the circuit board on the support assembly is also provided, which in this example is provided by leading and trailing lugs 38, 40 that locate the leading and trailing edges of the circuit board, shown in Figure 2. The leading lug 3 8 is at a fixed position relative the front rail 28 and the trailing lug 40 can be moved to engage the circuit board 11 thereby to push it towards the leading lug 38. The circuit board is thereby accurately located between leading and trailing lugs 38, 40 and between front and rear rails 28, 30. Other arrangements for positioning the board are envisaged, for example a fixed frame, and the invention is not limited to the illustrated embodiment.

Preferably, the misalignment measuring device 24 comprises imaging means 42, for example two pairs of cameras 44 mounted above the first and second support means 12, 14 8 respectively. Optionally, a single pair of cameras is provided each of which is capable of moving between the two positions.

The circuit board 11 is provided with one or more reference marks 13, known as "fiducials", which are used to identify the precise position of the board so that the offset can be determined, described in more detail below. Preferably, there comprises at least two reference marks 13 usually at opposite comers of the circuit board as shown in Figure 1.

The imaging means 42 acquires images of an area of the circuit board containing the fiducials and the device 24 further comprises a controller for determining the location of the reference mark 13 in each image and determining the extent of misalignment of the board compared to a known calibrated position that has been pre-programmed or derived from the procedure outlined below.

Turning again to the construction of the assembly, a conveyor 22 is provided that is powered by suitable drive means 46, for example a motor, to convey the board between the first position 16 at the first support assembly and the second position 18 at the second support assembly.

The screen printer 20 is of a type known in the art that is capable of being moved between operative and inoperative positions. Figure I illustrates the printer in its inoperative position, but it can be moved in the X direction to be placed above the stencil 54. An underscreen cleaner 48 may be provided to clean the stencil. The apparatus 10 also includes a stencil frame 50 for holding a stencil above the second board support assembly 29, preferably aligned with the assembly. The second support assembly 29 and/or the stencil frame 54 is movable so that the board I I can be moved relative to the stencil 54. Adjustment means may be provided to move the absolute position of either the board I I or the stencil 13 or both. The adjustment means may be provided by a suitable drive means to cause the assembly or frame to move in the X and/or Y directions and/or to rotate about the X, Y and Z axes, R, RI, R2, shown in Figures I and 2 to provide adjustment for the object or article in all three 9 planes. A lift table (not shown) may be provided to move the second support assembly 29 in the Z plane. A controller controls the adjustment of the stencil and/or the board.

A controller is provided to control the various functions of the apparatus. More particularly, the controller may be a series of dedicated processors that individually control the various elements of the apparatus or may be a control means for the apparatus. Alternatively, the controller may be a known programmable control system.

The controller has a central processor, manual input means and a display which indicates useful information to the operator.

The controller controls the positions of the movable components as well as the speed of movement of variable speed components. For example the controller controls the motors which drive the conveyor 22, and the drive means moving the rear rails 30 and trailing lugs 40 of each support assembly. Furthermore, the controller controls the adjustment means through programmed and accurate control of the motors for moving the stencil frame and/or second support assembly in the X and Y directions and/or rotating the assembly relative the frame in directions R, RI and/or R2. The lifting table is controlled by a drive means to move the table holding the second support assembly in the Z plane. The positions and speed of the drive means, second support assembly, stencil frame and/or lift table can be input manually or a specific pre-written programme can be loaded into the central processor for control of the screen printer apparatus. Also, for a controlled changeover of the machine from one batch type or size to another can be the result of a pre-written programme or manual input signal to perform the calibration procedure discussed below.

In order to screen print a circuit board, a suitable feed system is provided which intermittently feeds boards I I from a supply to the infeed of the apparatus 10. The boards I I are then conveyed forward to the leading lug which locates the leading edge at the predetermined position in the first support assembly 26. The trailing lug 40 moves the board into abutment with the leading lug 38.

The rear rail 30 then moves forward towards the front rail 28 to apply an edge pressure to the board to hold it securely between the rails and between the leading and trailing lugs, as shown in Figure 1. The board is then in the first position 16 and misalignment measuring device 24 is employed whereby the imaging means 42 is moved to locate the fiducials 13.

This procedure can be carried out automatically and is controlled by the controller. The position of the fiducials is recorded by the controller and compared to the known calibration position (described below) to determine the misalignment or "offset" i.e. the difference between the calibration position and the first position.

The board I I is released from the first support assembly 12 by moving the rear rail 30 away from the front rail 28 to release the board. Thereafter, the leading and trailing lugs 38, 40 are moved to inoperative positions and the board is conveyed forward to the printing station 52.

At the same time the next successive board enters the first support assembly 26 and misalignment measuring procedure is performed on that board as it was for the immediately preceding board.

At the printing station 52 the board 11 is secured to the second support assembly 29 in the same manner as described above. The controller then causes the adjustment means to adjust the board or the stencil 54 relative to each other. The adjustment is made in the X and Y directions and/or by rotation R, RI, R2 of the second support means 29 to compensate for the offset between the board 11 being printed and the calibrated position For example, linear motors may drive the lift table in these planes, and there may ftirther comprise a rotary drive means to cause the table to rotate. The adjustment is also made to align the board and the stencil, which information was derived at the start of the production run, described below.

Thereafter, the board I I is printed whereby the lift table (not shown) raises it to the print height in the Z direction, shown in Figure 1, and the printer 20 prints the board with the required substance, for example solder paste, as is well known in the art.

I I This apparatus 10 causes the board I I to move into the desired position with known repeatability, so that absolute accuracy at the printing station is not as important, because the required adjustment is achieved by calibrating the apparatus.

In order to calibrate the position of the printer apparatus, the following calibration procedure is employed at the start of each batch of circuit boards to be printed. First, a circuit board I I is loaded and secured to the second support assembly 29, in the same manner as described above, and it is raised by the lift table in the Z direction, shown in Figure 1, to the printing height, before the stencil 54 is loaded.

Imaging means 25 shown in Figure 2 positioned over the board 11 locate the reference marks 13, or other suitable reference points, for example pads on the board and the information is recorded by the controller. The board is lowered to its original position and the stencil 54 is then loaded. The imaging means 25 locates the corresponding reference marks (not shown), or corresponding reference points on the stencil, i.e. the apertures in the stencil, which information is recorded by the controller. The controller determines the initial misalignment between the stencil and the board at the second support means by comparing the images and calculates the required adjustment in the X and Y directions and/or the degree of rotation.

In this embodiment, the stencil is fixed during a production run, but its position may be controlled during the calibration sequence to get a correct coarse position prior to the run.

The board is then moved back to the left hand side of the conveyor 22 to be held by the first support apparatus 26 and the imaging means 24 above the first support apparatus 26 records the position of the same or other reference marks, for example "fiducials" 13 marked on the board. The controller uses the fiducial positions as the calibrated position or "zero point' '. Thus, the control system knows the required adjustment between the board and stencil in the printing station for that calibrated position.

12 The production run commences and the corresponding fiducial positions in each board are measured against this zero point. If there is a difference between the fiducial positions and the zero point referred to in this specification as an "offset", the controller calculates the adjustment required to be applied to that board in its second support apparatus to minimise 5 the "offset".

It is envisaged that the calibration and movement of the second support means and lift table is totally automatic and requires an operator to load the stencil and circuit with the remaining movements and calculations controlled by the controller.

In essence, the present invention is counter intuitive to conventional thought by organising the alignment and printing processes as parallel processes and by making use of the information collected at the start of each production run.

Beneficially, the invention described above does not require use of the cameras between the stencil and circuit board to calculate the offset each time a board enters the stencil, which results in reduced machine movement and no camera movement and contributes to improved accuracy.

As cameras are not required the distance between the board and the stencil is reduced. In this embodiment, the board is required to move to the print height by about 1 Omm only, so reducing a possible source of error in the apparatus and reducing transfer time.

Furthermore, the time taken to print the board is reduced. In the case of a known "single roving camera" print sequence the cycle time is approximately 13.5 seconds. The method and apparatus of the present invention will reduce the cycle time, thereby increasing the throughput of the board through the machine. Camera "offset" calibration of the kind with the single roving camera of the prior art is no longer needed thereby removing a skilled, error prone and time consuming task. Beneficially, the apparatus and method described above is safer to operators and the product because there are fewer moving parts.

13 It is envisaged that the invention can be applied to new machinery or fitted to existing machinery on a retro fit basis.

Another aspect of the invention is provided by an assembly 100 which is used to exert a downward pressure on a bowed object 116 to flatten it and/or clamp the object in a desired position as part of an object aligning apparatus 10. Turning again to the assembly 100 disclosed in this embodiment, it is provided by a plurality of elements 102 mounted in the front and rear parts of the assembly e.g. front and rear rails 28, 30 shown in Figure 1. 10 The construction of the assembly 100 is illustrated in more detail in Figures 3a, b, c and d. There comprises a series of elements 102 positioned along each rail 28, 30 in co-linear manner; each element 102 is constructed in identical manner and therefore only one of the clamps shall be described in any greater detail with reference to Figure 3d. The element 102, 15 comprises a member 104, which is preferably planar and, is connected to a rotating shaft 106. In this embodiment, the member 104 is mounted eccentrically on the rotating shaft 106, however, in alternative classes of embodiments, any suitable element may be used. The shaft 106 is driven by suitable drive means. In this embodiment a rack and pinion mechanism 108, 110 is used to impart the rotary motion, although it is envisaged that other suitable known 20 drive means could be used without departing from the scope of invention. The element 102 is also movable in a vertical plane by second drive means. In this embodiment, vertical movement is provided by a cam 112 and cam follower 114. Optionally, the cam 112 is linear and has a ramp configuration corresponding to each 25 follower 32, the follower 114 is connected to the shaft 106. It is envisaged that other cam configurations could be employed, for example the cam could be configured on the shaft 106, to impart vertical movement during rotation. In order to engage an object 116, the elements 102 are moved from an inoperative position 30 117 shown in Figure 3a to an operative position 119, shown in Figure 3d. In their inoperative 14 position 117 the members 104 are stowed in recesses 118 formed in the rails 30. As the object is transported along the rail 30 by a conveyor 120 to the operative position, the lifting cam 112 is moved to the left L shown in Figures 3 a and 3 b which causes the rotary shaft 106 to move upwards. Thereafter, the member 104 is rotated through 90' using the rack and pinion mechanism 108, 110 by moving the rack 110 towards the right Ra shown in Figure 3c.

Thus the members 104 protrude inwardly towards and above the outer portion of the object 116. The lifting cam 112 is then caused to move in the opposite direction i.e. to the right Rb in Figures 3a to 3d to lower theelements onto the object edge. A downward force is applied by this action to the object which may be augmented by biasing means to flatten it or to reduce bowing or flexing. If the assembly 100 is acting as a clamp then downward force would continue to be applied to the object to prevent unwanted lateral movement.

If the elements 102 are intended to be used purely to flatten the object 116, then it may be necessary to provide other means for holding the object 116 in place laterally. In a preferred embodiment, a "snugging" movement of the front rail towards the rear rail as indicated by arrows X in Figure I whilst the two rails remain parallel provides the lateral holding means.

This movement causes an axial force to be applied to the edges of the object 116 creating sufficient frictional resistance to hold the object 116 in place. In some embodiments, the elements may be employed to conteract bowing induced the the "snugging" movement. One or more sensors may be used to determine when the necessary axial force has been achieved.

In other preferred embodiments, the rear rail, or both rails may move to effect the "snugging". In alternative classes of embodiments, holding means may be provided by other arrangements, for example, a vacuum being applied to the underside of the object.

In order to release the assembly 100, the procedure is reversed and the members 104 are returned to their stowed position 117. In this embodiment, the assembly 100 is stowed prior to printing the board. When used in connection with the circuit board described above in relation to Figures 1 and 2, the assembly 100 helps to reduce bowing of the board which is one cause of inaccurate printing in a screen printer. When the assembly 100 is incorporated into the screen printer apparatus of Figure 1, the movement of the elements 102 is preferably controlled by the controller. In a second application, the assembly clamps an object in a desired position to prevent unwanted movement thereof, which is another cause of inaccurate printing.

It is envisaged that the assembly 100 could be applied to other types of screen printer, or to any other device where accurate and repeatable clamping or positioning of (planar) objects is required and is not limited to the embodiments described above.

Of course, the terms "left", "right", "front" and "rear" used in the preceding paragraphs are 10 relative and the apparatus may be re-oriented as necessary or as desired.

Modifications may be incorporated without departing from the scope of the present invention as defined in the accompanying claims.

uk9708-w 16

Claims

1. Apparatus for sequentially locating an article, for example one or more circuit boards, which apparatus comprises feed means to deliver successive articles from a supply, first and second support means for supporting the article in respective first and second positions, a reference object in spaced alignment with the second support means, a conveyor for conveying each article between the first and second support means, a misalignment measuring device positioned above the first support means for determining the misalignment between each article and the reference object and adjustment means for adjusting each article as required at the second support means relative to the object.

2. The apparatus as claimed in claim I wherein each article has at least one reference mark on a surface thereof and the misalignment measuring device comprises imaging means for acquiring an image of an area of the article containing the reference marks and a controller for determining the location of the reference marks in the image, comparing the reference marks with a calibrated position and determining therefrom the extent of misalignment.

3. The apparatus as claimed in claim 2 wherein the adjustment means is controlled by the misalignment measuring device.

4. The apparatus as claimed in claim 2 or claim 3 wherein the adjustment means causes the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the second support means.

5. The apparatus as claimed in any of claims 2 to 4 wherein the adjustment means causes the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the reference object.

17

6. The apparatus as claimed in any of claims 2 to 5 wherein the article includes two reference marks and the adjustment means comprises means to adjust each article or, as the case may be, the reference ob ect in a first lane and means to adjust each article or, as the i p case may be, the reference ob ect in a second plane substantially perpendicular said first plane.

7. The apparatus as claimed in claim 6 wherein the adjustment means ftirther comprises rotary means for rotating the second support means to move the article relative the reference obJ ect.

8. The apparatus as claimed in any one of claims 2 to 7 wherein the imaging means comprises a pair of cameras mounted for movement between the first and second support means.

9. The apparatus as claimed in any one of claims 2 to 7 wherein the imaging means comprises two pairs of cameras wherein one pair of cameras is mounted above the first support means and the other pair of cameras is mounted above the second support means.

10. The apparatus as claimed in any of claims I to 9 wherein first and second support means comprises a clamp assembly for retaining the article in the respective first and second positions.

11. The apparatus as claimed in claim 10 wherein the clamp assembly comprises first and second rails in a spaced parallel arrangement, said first rail is movable relative to said second rail, between a release position and a clamping position whereby an edge pressure is applied to the article to clamp it in position.

12. The apparatus as claimed in any one of the preceding claims wherein the apparatus ftirther comprises a device for determining the calibrated position of the article at the first 18 support means and the misalignment between the article at the second support means and the reference object prior to printing a batch of articles comprising at least one article.

13. The apparatus as claimed in claim 12 wherein the article and the reference object each have at least one reference mark on a surface thereof and the misalignment calibrating device comprises imaging means for acquiring images of an area of the article at the second support means, the reference object surface and an area of the article at the first support means containing the reference marks and controller for determining the location of the reference marks in the images and determining therefrom the extent of misalignment of the article and reference object and the calibrated position.

14. A screen printer for printing a batch of circuit boards incorporating apparatus as claimed in any of claims I to 13.

15. A method for determining a fixed reference point to be used in aligning an article with a reference object each having at least one alignment reference mark on the surface thereof, the method comprising the following steps: - (i) supporting the article in two successive positions by first and second support means; (ii) locating imaging means above the article which imaging means views the or each alignment reference mark of the article positioned in the second support means; (iii) loading the reference object and locating and viewing through the imaging means the corresponding alignment reference marks on the reference object; (iv) determining the locations of the or each reference mark in the images and determining therefrom the extent of misalignment of the article and the reference object; (v) locating the article at the first position; and (vi) locating the imaging means to view one or more offset reference marks on the article at the first position and determining the locations of the or each reference mark to be used as a fixed reference point.

16. A method of aligning an article and a reference ob. ect, the article having one or more I offset reference marks comprising the steps of (i) supporting the article in two successive positions by first and second support means; (ii) locating imaging means above the article 19 which imaging means views each offset reference mark of the article positioned in the first support means; (iii) determining the location of the offset reference mark in the image, comparing each reference mark with a fixed reference point and determining therefrom the extent of misalignment of the article and reference object; and (iv) adjusting the second 5 support means or reference object to align the article and the reference object.

17. The method of aligning an article and a reference object as claimed in claim 16 wherein the fixed reference mark is derived from the method of claim 15.

18. The method as claimed in claim 16 or claim 17 further comprising the step of adjusting the second support means or reference object to minimise the offset of the article between its successive positions.

19. The method as claimed in any one of claims 15 to 18 wherein the article is a circuit board and the reference object is a stencil of a screen printer for printing the circuit board.

20. Apparatus for determining the extent of the offset of an object, for example a circuit board, mounted in successive first and second positions, whereby the position of the object is determined by using at least one reference mark on the surface of the object, the apparatus comprising first and second support means for supporting the object in the respective first and second positions, each support means comprising a clamp assembly for retaining the object on the support means, and for positioning the object at the predetermined position, wherein the apparatus further comprises an offset calibrating device for identifying the reference mark and for determining the offset of the object between the first and second positions.

21. The apparatus as claimed in claim 20 wherein the apparatus further comprises a conveyor for conveying the object between the first and second positions.

22. Apparatus as claimed in claim 20 or claim 21 further comprising means for adjusting the second support means relative the first support means to reduce the offset.

23. The apparatus as claimed in claim 22 wherein the adjustment means is controlled by 5 the offset calibrating device.

24. The apparatus as claimed in any of claims 20 to 23 wherein the offset calibrating device comprises imaging means for acquiring images of an area of the object surface containing a reference mark and control means for identifying the location of the reference mark in the images and determining therefrom the extent of the offset.

25. The apparatus as claimed in claim 24 wherein the control means causes the relative position of the second support means and the first support means to be adjusted by changing the absolute position of the second support means. 15

26. The apparatus substantially as hereinbefore described.

27. The apparatus substantially as hereinbefore, described by reference to or as illustrated in Figures 1, 2 or 3. 20

28. The method of aligning a circuit board to be printed and a stencil of a screen printer substantially as hereinbefore, described.