WO2002097534A2

WO2002097534A2 - Apparatus and method for aligning an article with a reference object

Info

Publication number: WO2002097534A2
Application number: PCT/GB2002/002519
Authority: WO
Inventors: Ian William Belchamber
Original assignee: Blakell Europlacer Limited
Priority date: 2001-05-31
Filing date: 2002-05-29
Publication date: 2002-12-05
Also published as: GB0113201D0; GB2377908A; WO2002097534A3

Abstract

A screen printer apparatus including an alignment apparatus comprises a lower camera located between a printed circuit board (PCB) and a stencil and an upper camera located above the stencil. A batch setup procedure involves the upper camera determining the positions of one or more stencil apertures which are utilised as alignment features. All stencil apertures have an equivalent PCB pad which is identified during the production sequence. Alternatively, a fiducial mark may be used. The PCB is located on an alignment table which is moved to align the equivalent PCB pad with the stencil aperture prior to a screen printing action.

Description

APPARATUS AND METHOD FOR ALIGNING AN ARTICLE WITH A

REFERENCE OBJECT

Field of the Invention

The invention relates, in general, to alignment apparatus and more particularly to alignment apparatus utilised in the production of electronic equipment. Such an apparatus is used in, for example, screen printers for printing solder paste onto printed circuit boards (PCBs) for surface mount assembly of appropriate electrical components onto the PCB.

Background to the Invention

During manufacture of a printed circuit board for electronic or electrical equipment, the PCB is fed into a screen printer apparatus where solder paste is applied to electrode pads on the PCB where electrical components are to be mounted. The PCB is then conveyed into a surface mount pick and place apparatus where a placement head picks relevant components and then positions the components onto the PCB. The PCB then passes to an oven, where the paste is heated and adheres the components to the pads on the PCB.

In the screen printer apparatus, a stencil (screen) is positioned above the PCB. It is necessary that the apertures of the stencil and the corresponding pads on the PCB are in exact vertical alignment. Thus, when the PCB is brought into contact with the stencil and solder paste is forced through the apertures, the resultant PCB has solder paste only on the correct areas.

However, any misalignment between the stencil and the PCB results in inaccurately positioned (printed) solder paste. Such faulty PCBs are usually cleaned or recycled and the associated loss presents a serious problem for manufacturers. It is known to provide an alignment apparatus, integral to the screen printing apparatus, which utilises a camera located between the stencil and the PCB. The camera locates fiducials (reference marks) on the underside of the stencil and the top of the PCB, and adjustments are made to align the stencil with the PCB (or vice versa), via a control means. An example of such an alignment system is illustrated in US Patent No. 5,752,446.

UK Patent Application GB 2,351 ,258 discloses a screen printer apparatus with an alignment system which utilises two pre-determined positions for a PCB. The PCB is conveyed to a first position which has a misalignment measuring device located above the first position. The PCB is then conveyed to a second position which is within the screen printer apparatus and a stencil is located above the PCB. Information collected by the misalignment measuring device enables the PCB position to be altered prior to screen- printing action.

However, both these prior art systems require specialist calibration to compensate for the usual machine tolerances of the alignment of the bearing systems between the lower position of the PCB, which is where it is first moved below the stencil, and the upper position of the PCB, where it is in contact with the stencil. In order to determine this compensation, the calibration of the machine must be calculated to a very high precision. This calibration process involves positioning a test PCB in the lower position, moving it up to the upper position against a calibration stencil having a special calibration pattern with a large number of calibration points thereon, printing through the stencil onto the test PCB, moving the test print back down to the lower position and then accurately determining the positions of the points on the test print and comparing them with the stencil to provide a calibration. However, in order to perform the calibration effectively, a very high quality test print is needed, and the effects of the print process, such as printing direction, fill, profile, etc. cannot be separated from the mechanical effects of the machine which are being measured. Calibration test prints occasionally fail due to the print not being successful over the entire print area, which, due to the large number of points being measured, may not be noticed for some time, if at all, resulting in print offsets for some products but not for all. Clearly, a calibration stencil and test PCB must be maintained to carry out this calibration, which is a skilled and error-prone operation.

Therefore, it is desirable to overcome, or at least reduce, the problems of the prior art.

Brief Summary of the Invention

Accordingly, a first aspect of the present invention provides a method of aligning an article with a reference object, the method comprising the steps of providing a reference object having at least one feature which is to be aligned with a corresponding feature of the article when the article is substantially in contact with the reference object, determining the position of the at least one feature of the reference object using a first imaging device, moving an article into the second location, determining a position of the corresponding feature of the article using a second imaging device, adjusting the position of the article based on the position of the corresponding feature of the article, and moving the article to the first location, whereby the feature of the reference object and the corresponding feature of the article are aligned.

Preferably, the step of adjusting the position of the article based on the position of the corresponding feature of the article takes place at the second location.

In a preferred embodiment, the method further comprises an initial step of determining a calibration value between a first location in which an article would be substantially in contact with the reference object and a second location in which the article would be generally adjacent the reference object, and wherein the step of adjusting the position of the article is at least partly based on the calibration value.

Preferably, the step of determining a calibration value comprises the steps of providing a test article having alignment features thereon in the second location, determining a position of one or more of said alignment features using said second imaging device, moving the test article to the first location, determining a position of said one or more of said alignment features using said first imaging device, and calculating the calibration value based on the difference between the positions of the alignment feature at the first and second locations.

In one embodiment, the reference object is a stencil and the step of determining the position of the at least one feature of the reference object includes determining the position of at least one aperture of the stencil.

Furthermore, the article may be a printed circuit board and the step of determining a position of the corresponding feature of the article preferably includes determining the position of at least one pad on the printed circuit board corresponding to the feature of the reference object.

Preferably, the first and second imaging devices comprise first and second cameras coupled to a controller for receiving image information from the first and second cameras.

According to a second aspect of the present invention, there is provided a method of aligning a first surface to a second surface, the first surface being located at a first location and the second surface being moved from a second location to the first location where it comes into aligned contact with the first surface, the method comprising the steps of: providing a test surface having a plurality of alignment features thereon at the second location; imaging the test surface to record the position of the plurality of alignment features with respect to a second imaging apparatus; moving the test surface to the first location, imaging the test surface to record the position of the plurality of alignment features with respect to a first imaging apparatus; calculating a calibration value based on the positions of the plurality of alignment features in the second and first locations; providing at the first location the first surface having a plurality of features thereon; imaging the first surface to record the position of the plurality of features with respect to the first imaging apparatus; providing at the second location the second surface having features thereon corresponding to the plurality of features on the first surface; imaging the second surface to record the position of the corresponding features with respect to the second imaging apparatus; adjusting the position of the second surface based on the position of the plurality of features of the first surface, the position of the corresponding features of the second surface, and on the calibration value, so that the corresponding features of the second surface are in alignment with the features of the first surface when the second surface is moved from the second location to the first location, and moving the second surface from the second location to the first location.

According to a third aspect of the present invention, there is provided a method of mounting components on a printed circuit board comprising the steps of providing electrically conductive pads for receiving predetermined components and electrically conductive pathways between predetermined pads on a printed circuit board; aligning the printed circuit board with a stencil utilising the method described above; printing solder paste through the stencil onto the pads; placing predetermined components onto predetermined ones of said pads; heating the solder paste until it melts and solders the components to the pads. Preferably, the first imaging device is positioned above the reference object (stencil) and the second imaging device is positioned above the article (PCB) in the second position, but below the reference object.

According to a fourth aspect of the present invention, there is provided an apparatus for aligning an article with a reference object, the apparatus comprising a first support for receiving a reference object having at least one feature, a second support for receiving an article having a corresponding feature which is to be aligned with the at least one feature of the reference object when the article is substantially in contact with the reference object, means for moving said second support between a second location in which an article on the second support would be generally adjacent the reference object and a first location where the article would be substantially in contact with the reference object, means for adjusting the position of said second support, a first imaging device located adjacent the first support for determining the position of the at least one feature of the reference object on the first support, a second imaging device located adjacent the second support for determining the position of the corresponding feature of the article on the second support at the second location, and control means coupled to the first and second imaging devices, the means for moving said second support and the means for adjusting the position of said second support, wherein, in use, the position of the second support is adjusted based at least partly on a determined position of the at least one feature of the reference object and on a determined position of the corresponding feature of the article.

Preferably, the control means initially receives imaging information from the second imaging device regarding a position of an alignment feature of a test article on the second support at the second location and imaging information from the first imaging device regarding a position of the alignment feature of the test article on the second support at the first location and determines a calibration value therefrom, the calibration value being utilised at least partly to adjust the position of the second support at the second location.

Furthermore, the control means receives imaging information from the first imaging device regarding the position of the at least one feature of the reference object.

Preferably, the control means controls the means for adjusting the position of said second support to adjust the position of the second support based on imaging information from the second imaging device regarding the position of the corresponding feature of the article, pre-acquired information imaging information from the first imaging device regarding the position of the at least one feature of the reference object and the calibration value.

In one embodiment, the first and second imaging devices comprise first and second cameras coupled to the control means for receiving image information from the first and second cameras.

It will be appreciated that the imaging devices, can also be used to provide other functionality to the method and apparatus. For example, the first camera, being above the stencil, can be used to monitor the amount of solder paste available on the stencil and to trigger alarms or other controlling functions, if the amount is below or above predetermined thresholds. This camera can also carry out stencil blockage inspection, while the second camera can check for paste coverage on the PCBs.

According to a still further aspect of the present invention, there is provided apparatus for monitoring the amount of viscous material available on a stencil, the apparatus comprising a support for receiving a stencil, means for dispensing the viscous onto at least part of a top surface of the stencil, an imaging device located above the stencil for monitoring the amount of viscous material dispensed onto the stencil, and control means coupled to the imaging device for triggering an alarm or other controlling function, if the amount is below or above predetermined thresholds.

Preferably, the material further comprising a second support for receiving an article onto which the viscous material is to be printed, and means for moving said second support between a second location in which an article on the second support would be generally adjacent the stencil and a first location where the article would be substantially in contact with the stencil for printing.

The viscous material may be a solder paste and the article may be a printed circuit board.

Brief Description of the Drawings

One embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG.1 is a schematic illustration of a screen printer apparatus including a prior art single camera alignment apparatus; and

FIG.2 is a schematic illustration of the screen printer apparatus including a two camera alignment apparatus of an embodiment of the present invention.

Detailed Description of the Drawings

Thus, FIG.1 shows a known single camera alignment system utilised in a screen printer apparatus 10. A PCB 12 is located on a product transport system, such as a conveyor belt 14 and is approximately positioned below a stencil 16. The stencil 16 comprises a thin metal sheet with an arrangement of apertures 18 therein and, usually, at least one fiducial 20 located on the underside. The PCB 12 includes a number of pads 22 and at least one fiducial 24 located on the topside. The pads 22 and the apertures 18 are in an identical arrangement. A camera 26 is attached to an x-y controllable platform (not illustrated) and the camera 26 is connected to a control means 28. The camera 26 is required to view both the underside of the stencil and the topside of the PCB. This is achieved by utilising a camera with the capability to view in two different directions. For example, a single camera with a beam splitter apparatus may be used, in combination with the use of varied lighting or shutter adjustments. The control means 28 may comprise a microprocessor device. An alignment table 30 supports the stationary PCB 12. The alignment table 30 is provided with mechanical means which move the alignment table 30 in a z-direction, and further mechanical means which move the alignment table 30 in x- and y-directions and rotate it (denoted by theta, (θ) in the x-y plane about the z-axis. For the purposes of this application, the x-direction is parallel to the PCB feed direction and the y- direction is transverse to the PCB feed direction. Further, the alignment table 30 is connected to the control means 28.

In operation, the conveyor belt 14 coarsely positions the PCB 12 below the stencil 16. The camera 26 locates and records the position of a first stencil fiducial 20 and a first PCB fiducial 24. The camera 26 locates and records the positions of a second stencil fiducial 21 and a second PCB fiducial 25. The control means 28 processes this data together with a precalculated adjustment value based on an offset in the x-y plane as the PCB is moved in the z-axis to the stencil and determines a command of x, y and θ adjustments, which adjustments are required to position pads 22 directly below associated apertures 18 when the PCB 12 is moved to the stencil 16. The command is transmitted to the further mechanical means which alters the position of the alignment table 30, and hence the PCB position is altered. The camera 26 (inclusive of any beam splitter apparatus) is removed from between the PCB 12 and the stencil 16, under the control of control means 28, in order to allow the PCB 12 to be moved upwards (in the z-direction) unimpeded by the mechanical means. The PCB 12 stops momentarily when it is in contact with the underside of the stencil 16.

A solder paste dispenser (not illustrated) then applies a predetermined quantity of solder paste to the topside of the stencil 16 and a squeegee (or pumphead) arrangement (not illustrated) operates to force the solder paste evenly through each aperture 18 onto the PCB pads 22 below. Once this printing action is completed, the PCB 12 is lowered to the original position on the conveyor belt 14 and conveyed out of the screen printer apparatus. The next PCB is then conveyed to the coarse position below the stencil 16 and the camera 26 returns to its original position to carry out the adjustment measurement. Thus, the PCB alignment step requires that the camera determine the positions of fiducials on both the underside of the stencil and on the top side of the PCB for every PCB that is to be printed with solder paste.

As mentioned above, prior to use of the screen printer apparatus and alignment system of FIG.1 , it is necessary to perform a system calibration sequence. The system calibration sequence requires a test print to be made onto a rest PCB utilising a calibration stencil. Such a test print must be of very high quality and not, for example, have areas lacking solder paste. This system calibration sequence requires a skilled human operator to be present at the screen printing apparatus.

FIG.2 illustrates a screen printer apparatus 40 including an alignment apparatus. The alignment apparatus comprises, an upper camera 42, a lower camera 44, an alignment table 46, and a control means 48. A PCB 50 is located on a product transport system, such as a conveyor belt 60. A stencil 52 is held in a support means (not illustrated) above the alignment table 46 and the stencil 52 has an arrangement of apertures 54 therein. The PCB 50 includes a number of pads 56 and optionally one or more fiducials 58 may be present on the top of the PCB 50. The pads 56 and the apertures 54 are in an identical arrangement. Both the upper camera 42 and the lower camera 44 are attached to individually controllable x-y platforms (not illustrated). The control means 48 may comprise a microprocessor device and is connected to the alignment table 46, the upper camera 42, the lower camera 44, the controllable x-y platforms and the conveyor belt 60, by wired or wireless means. In combination with the upper camera 42 and the lower camera 44, the control means 48 has the capability to locate and record the position of an alignment feature on the stencil 52 or the PCB 50 or a test PCB (not illustrated). The alignment table 46, which supports the PCB 50 in the screen printer apparatus, is similar to the alignment table of FIG.1.

It is recognised by those skilled in the art that any production equipment is subject to various mechanical tolerances (such as architectural and other engineering imperfections). In the instance of the screen printing apparatus 40 including the alignment apparatus, the result of these engineering tolerances may be an inaccurately positioned solder paste deposit on the PCB 50. Therefore, the system needs to be calibrated to allow the control means to compensate for the inherent tolerances of the machines. The aim of the system calibration sequence is to provide the control means 48 with data specifying the precise positional relationship between the upper camera 42, the lower camera 44, the stencil 52 and the PCB 50. A test-grid PCB (not illustrated), consisting of a grid of calibration features, is provided in the usual PCB position on the alignment table 46 below the lower camera 44. Under the control of the control means 48, the lower camera 44 is moved around over the test-grid PCB, locating specific calibration features and measuring and recording the positions of such features. The lower camera 44 is moved into a park position 62 and the test-grid PCB is raised on the alignment table 46. Under the control of the control means 48, the upper camera 42 is moved around over the test-PCB taking the same measurements as the lower camera 44. The control means 48 processes the information collected during the system calibration sequence to establish a calibration value of the test- PCB present after the transfer from the initial position to the raised (printing) position by the mechanical means. Thus, this calibration value measurement inherently includes any mechanical tolerances and engineering imperfections involved in raising the alignment table (such as a slight rotation of the alignment table about the z-axis). The system calibration sequence does not need to be performed whenever a new stencil is to be utilised, rather it should be performed when the screen printer apparatus is installed and at regular intervals (for example, six monthly) thereafter.

In operation, it is necessary for a brief batch setup sequence to be performed prior to commencing printing with a new stencil or when a stencil is removed and replaced, for example for cleaning. The batch setup sequence requires the control means 48 (or an operator) to initially determine, via the upper camera 42, at least one stencil aperture 54 which is utilised as an alignment feature. Each stencil aperture 54 has an equivalent PCB printed pad 56. The position(s) of the chosen stencil aperture(s) is/are then measured using the upper camera 42 and those positions are stored in the control means 48. This setup sequence may be performed in parallel with the processing of the first PCB of the batch.

During production, the conveyor belt 60 first coarsely positions a PCB 50 below the stencil 52. The control means 48 uses the lower camera 44 to locate pad(s) 56 which are equivalent to the stencil aperture(s) 54 identified as alignment feature(s) in the batch setup sequence. The control means 48 calculates the misalignment, if any, between the alignment features on the stencil 54 and the equivalent pads 56 on the PCB 50. Next, the control means 48 determines an adjustment command of x, y and θ movements, which movements are required to position pads 56 in the correct position below associated apertures 54 so that they are exactly aligned therewith when the PCB is moved into contact with the stencil. In order to determine the adjustment command, the control means utilises data received from the upper camera 42 during the batch setup sequence, data received from the lower camera 44 during the production sequence and data on the calibration value determined during the system calibration sequence. The adjustment command is transmitted to the mechanical means which move the alignment table 46 and the position of the alignment table 46 is altered, and hence the PCB position is altered. Under the control of control means 48, the lower camera 44 is removed from between the PCB 50 and the stencil 52, to a "park" position (which is illustrated by a camera 62 in dashed outline in FIG.2). The PCB 50 is then transported upwards until it is in contact with the underside of the stencil 52.

A solder paste dispenser (not illustrated) then applies a predetermined quantity of solder paste to the topside of the stencil 52 and a squeegee (or pumphead) arrangement (not illustrated) operates to force the solder paste evenly through each aperture 54 onto the PCB pads 56 below. Once this printing action is completed, the PCB 50 is lowered to the original position on the conveyor belt 60 and conveyed out of the screen printer apparatus to a surface mount pick and place machine which picks up appropriate components and places them on the solder paste on particular pads. The PCB is then conveyed into an oven, or other heating apparatus, where the PCB is heated until the solder paste melts and solders the components to the PCB pads.

Further production PCB's may be processed and aligned using identical pads or by the use of fiducial marks 58 on the PCB. The preferred embodiment of the present invention removes the need for alignment position analysis between each PCB and the stencil, as is required in prior art systems.

Advantageously, the present embodiment of the invention does not require a test print to be produced from a test stencil onto a PCB. Furthermore, the present invention does not require the use of stencils which incorporate underside fiducials. This is particularly important for PCB manufacturers that utilise stencils without underside fiducials, for example, where a manufacturer uses a compact fully automatic screen printer apparatus.

Advantage is also gained by using the upper camera for multiple functions. For example, the upper camera can transmit data to the control means concerning the solder paste distribution process on the topside of the stencil. Where the solder paste dispenser dispenses the solder paste in a way that results in an inconsistent amount, for example either too much or too little, being available on the stencil, or where the solder paste dispenser runs out of solder paste such that no solder paste is available on the stencil at all, this is viewed by the upper camera and the control means can activate an alarm to alert the operator. In fact, the upper camera can be used to determine whether the amount of solder paste available on the stencil falls within acceptable parameters irrespective of whether the solder paste is dispensed onto the top surface of the stencil using an automatic paste dispenser or manually. However the solder paste is dispensed, the upper camera can be used to monitor the amount (or complete lack of) solder paste on the stencil prior to the operation of printing the solder paste through the apertures of the stencil, and the control means can activate an alarm if the amount of solder paste, or its distribution on the stencil is incorrect. If an automatic paste dispenser is used, it will further be appreciated that the control means can use the monitoring information from the upper camera to control the automatic paste dispenser to dispense the solder paste on the part or parts of the top surface of the stencil that have a deficient quantity of solder paste available for the printing process.

Furthermore, where one or more apertures of the stencil are blocked, the upper camera can detect this and the control means can activate an alarm to indicate that it is necessary to clean the stencil. It should be noted that after the stencil has been removed, cleaned and replaced, it is necessary to perform the batch setup sequence even though a different batch is not being commenced.

The lower camera can also perform multiple functions. For example, the lower camera can transmit data to the control means concerning the solder paste coverage of the PCB pads before the PCB is ejected from the screen printer apparatus. Since the upper camera is attached to an x-y controllable platform, this same platform may be utilised to perform multiple functions such as supporting the solder paste dispenser, thereby providing a cost saving.

Whilst the invention has been described in respect of a particular embodiment of a PCB screen printer apparatus, it will be appreciated that the present invention is applicable to any system requiring an accurate vertical alignment of two marked entities. Furthermore, it will be appreciated that the above description is given by way of example only and that a person skilled in the art can make modifications and improvements without departing from the scope of the present invention. For example, it should be apparent, in the particular embodiment described above, that other substances can be deposited on a PCB rather than solder paste.

However, it will be apparent to those skilled in the art that fiducials provide more consistent and reliable alignment points than stencil apertures or PCB pads. Therefore, once the batch setup sequence has been completed, the operator can optionally choose to change to use PCB fiducials (as only the lower camera is active during the production sequence) as alignment points. Clearly, this is dependent upon the provision of PCB fiducials.

It is envisaged that the underlying inventive concept of the invention could be applied to an application requiring the alignment of more than two planar objects, where a further camera (connected to the control means) would be required for each further object.

Claims

1. A method of aligning an article with a reference object, the method comprising the steps of: providing a reference object having at least one feature which is to be aligned with a corresponding feature of the article when the article is substantially in contact with the reference object; determining the position of the at least one feature of the reference object using a first imaging device; moving an article into the second location; determining a position of the corresponding feature of the article using a second imaging device; adjusting the position of the article at least partly based on the position of the corresponding feature of the article; and moving the article to the first location, whereby the feature of the reference object and the corresponding feature of the article are aligned.

2. A method of aligning an article with a reference object according to claim 1 , further comprising the preliminary step of: determining a calibration value between a first location in which an article would be substantially in contact with the reference object and a second location in which the article would be generally adjacent the reference object; and wherein the step of adjusting the position of the article is at least partly based on the calibration value.

3. A method of aligning an article with a reference object according to claim 2, wherein the step of determining a calibration value comprises the steps of: providing a test article having alignment features thereon in the second location; determining a position of one or more of said alignment features using said second imaging device; moving the test article to the first location; determining a position of said one or more of said alignment features using said first imaging device; and calculating the calibration value based on the difference between the positions of the alignment feature at the first and second locations.

4. A method of aligning an article with a reference object according to any one of claims 1 , 2 or 3, wherein the reference object is a stencil and the step of determining the position of the at least one feature of the reference object includes determining the position of at least one aperture of the stencil.

5. A method of aligning an article with a reference object according to any preceding claim, wherein the article is a printed circuit board and the step of determining a position of the corresponding feature of the article includes determining the position of at least one pad on the printed circuit board corresponding to the feature of the reference object.

6. A method of aligning an article with a reference object according to any preceding claim, wherein the first and second imaging devices comprise first and second cameras coupled to a controller for receiving image information from the first and second cameras.

7. A method of aligning an article with a reference object according to any preceding claim, wherein the step of adjusting the position of the article based on the position of the corresponding feature of the article takes place at the second location.

8. A method of aligning an article with a reference object according to any preceding claim, wherein the first imaging device is positioned above the reference object and the second imaging device is positioned above the article at the second location, but below the reference object.

9. A method of aligning a first surface to a second surface, the first surface being located at a first location and the second surface being moved from a second location to the first location where it comes into aligned contact with the first surface, the method comprising the steps of: providing a test surface having a plurality of alignment features thereon at the second location; imaging the test surface to record the position of the plurality of alignment features with respect to a second imaging apparatus; moving the test surface to the first location; imaging the test surface to record the position of the plurality of alignment features with respect to a first imaging apparatus; calculating a calibration value based on the positions of the plurality of alignment features in the second and first locations; providing at the first location the first surface having a plurality of features thereon; imaging the first surface to record the position of the plurality of features with respect to the first imaging apparatus; providing at the second location the second surface having features thereon corresponding to the plurality of features on the first surface; imaging the second surface to record the position of the corresponding features with respect to the second imaging apparatus; adjusting the position of the second surface based on the position of the plurality of features of the first surface, the position of the corresponding features of the second surface, and on the calibration value, so that the corresponding features of the second surface are in alignment with the features of the first surface when the second surface is moved from the second location to the first location; and moving the second surface from the second location to the first location.

10. A method of aligning a first surface to a second surface according to claim 9, wherein the first imaging device is positioned above the first surface and the second imaging device is positioned above the second surface at the second location, but below the first surface.

11. A method of aligning a first surface to a second surface according to either claim 9 or claim 10, wherein the step of adjusting the position of the second surface based on the positions of the corresponding features of the second surface takes place at the second location.

12. A method of mounting components on a printed circuit board, comprising the steps of: providing electrically conductive pads for receiving predetermined components and electrically conductive pathways between predetermined pads on a printed circuit board; aligning the printed circuit board with a stencil utilising the method according to any preceding claim; printing solder paste through the stencil onto the pads; placing predetermined components onto predetermined ones of said pads; heating the solder paste until it melts and solders the components to the pads.

13. A method of mounting components on a printed circuit board according to claim 12, further comprising the step of utilising the first imaging device to monitor the amount of solder paste available on the stencil and to trigger alarms or other controlling functions if the amount is below or above predetermined thresholds.

14. A method of mounting components on a printed circuit board according to either claim 12 or claim 13, further comprising the step of utilising the first imaging device to carry out stencil blockage inspection and to trigger alarms or other controlling functions if at least part of the stencil is blocked.

15. A method of mounting components on a printed circuit board according to any one of claims 12, 13 or 14, further comprising the step of utilising the second imaging device to check for paste coverage on the pads and to trigger alarms or other controlling functions if the coverage is below or above predetermined thresholds.

16. Apparatus for aligning an article with a reference object, the apparatus comprising: a first support for receiving a reference object having at least one feature; a second support for receiving an article having a corresponding feature which is to be aligned with at least one feature of the reference object when the article is substantially in contact with the reference object; means for moving said second support between a second location in which an article on the second support would be generally adjacent the reference object and a first location where the article would be substantially in contact with the reference object; means for adjusting the position of said second support; a first imaging device located adjacent the first support for determining the position of the at least one feature of the reference object on the first support; a second imaging device located adjacent the second support for determining the position of the corresponding feature of the article on the second support at the second location; and control means coupled to the first and second imaging devices, the means for moving said second support and the means for adjusting the position of said second support, wherein, in use, the position of the second support is adjusted at least partly based on a determined position of the at least one feature of the reference object and on a determined position of the corresponding feature of the article.

17. Apparatus for aligning an article with a reference object according to claim 16, wherein the position of the second surface is adjusted at the second location.

18. Apparatus for aligning an article with a reference object according to either claim 16 or claim 17, wherein the control means initially receives imaging information from the second imaging device regarding a position of an alignment feature of a test article on the second support at the second location and imaging information from the first imaging device regarding a position of the alignment feature of the test article on the second support at the first location and determines a calibration value therefrom, the calibration value being utilised at least partly to adjust the position of the second support at the second location.

19. Apparatus for aligning an article with a reference object according to claim 18, wherein the control means receives imaging information from the first imaging device regarding the position of the at least one feature of the reference object.

20. Apparatus for aligning an article with a reference object according to claim 19, wherein the control means controls the means for adjusting the position of said second support to adjust the position of the second support based on imaging information from the second imaging device regarding the position of the corresponding feature of the article, pre-acquired information imaging information from the first imaging device regarding the position of the at least one feature of the reference object and the calibration value.

21. Apparatus for aligning an article with a reference object according to any one of claims 16 to 20, wherein the first and second imaging devices comprise first and second cameras coupled to the control means for receiving image information from the first and second cameras.

22. Apparatus for aligning an article with a reference object according to any one of claims 16 to 21 , wherein the first imaging device is utilised to monitor the amount of solder paste available on the stencil and to trigger alarms or other controlling functions, if the amount is below or above predetermined thresholds.

23. Apparatus for aligning an article with a reference object according to any one of claims 16 to 22, wherein the first imaging device is utilised to carry out stencil blockage inspection and to trigger alarms or other controlling functions, if at least part of the stencil is blocked.

24. Apparatus for aligning an article with a reference object according to any one of claims 16 to 23, wherein the second imaging device is utilised to check for paste coverage on the pads and to trigger alarms or other controlling functions if the coverage is below or above predetermined thresholds.

25. Apparatus for monitoring the amount of viscous material available on a stencil for printing through the stencil, the apparatus comprising a support for receiving the stencil, an imaging device located above the stencil support for monitoring an amount of viscous material on the stencil, and control means coupled to the imaging device for triggering an alarm or other controlling function, if the amount is below or above predetermined thresholds.

26. Apparatus for monitoring the amount of viscous material available on a stencil according to claim 25, further comprising means for dispensing the viscous material onto at least part of a top surface of the stencil, when the stencil is positioned on the stencil support.

27. Apparatus for monitoring the amount of viscous material available on a stencil according to claim 26, wherein the control means controls the means for dispensing the viscous material to dispense the viscous material onto at least a part of the top surface of the stencil where the amount of viscous material is below the predetermined threshold.

28. Apparatus for monitoring the amount of viscous material available on a stencil according to any one of claims 25, 26 or 27, further comprising a second support for receiving an article onto which the viscous material is to be printed; and means for moving said second support between a second location in which an article on the second support would be generally adjacent the stencil and a first location where the article would be substantially in contact with the stencil for printing.

29. Apparatus for monitoring the amount of viscous material available on a stencil according to claim 28, wherein the article is a printed circuit board.

30. Apparatus for monitoring the amount of viscous material available on a stencil according to any one of claims 25 to 29, wherein the viscous material is a solder paste.

31. A method of monitoring the amount of viscous material available on a stencil for printing through the stencil, the method comprising the steps of: receiving the stencil onto a support; dispensing the viscous material onto at least part of a top surface of the stencil; utilising an imaging device located above the stencil to monitor the amount of viscous material on the top surface of the stencil; and triggering an alarm or other controlling function, if the amount is below or above predetermined thresholds.

32. A method of monitoring the amount of viscous material available on a stencil according to claim 31 , wherein the step of dispensing the viscous material onto at least part of a top surface of the stencil comprises utilising an automatic dispenser for automatically dispensing the viscous material onto at least part of a top surface of the stencil.

33. A method of monitoring the amount of viscous material available on a stencil according to claim 32, further comprising the step of controlling the automatic dispenser to dispense the viscous material onto at least a part of the top surface of the stencil where the amount of viscous material is below the predetermined threshold.

34. A method of monitoring the amount of viscous material available on a stencil according to claim 31 , wherein the step of dispensing the viscous material onto at least part of a top surface of the stencil comprises manually dispensing the viscous material onto at least part of a top surface of the stencil.

35. A method of monitoring the amount of viscous material available on a stencil according to any one of claims 31 to 34, further comprising the step of, before the step of dispensing the viscous material, moving a second support from a second location in which an article on the second support would be generally adjacent the stencil to a first location where the article would be substantially in contact with at least part of an underside of the stencil.

36. A method of monitoring the amount of viscous material available on a stencil according to claim 35, wherein the article is a printed circuit board.

37. A method of monitoring the amount of viscous material available on a stencil according to any one of claims 31 to 36, wherein the viscous material is a solder paste.