Workpiece support assembly

Abstract

A workpiece support assembly (10, Fig.3) for supporting and aligning a plurality of singulated workpieces. The workpiece support assembly comprises an assembly body (11, Fig.3) and a plurality of supp

Description

This invention relates to a workpiece support assembly, a printing machine and a method of aligning a plurality of singulated workpieces. Background and Prior Art Industrial screen-printing machines typically apply a conductive print medium, such as solder paste or conductive ink, onto a planar workpiece, such as a circuit board, by applying the conductive print medium through a pattern of apertures in a printing screen (sometimes referred to as a foil or stencil) using an angled blade or squeegee. Where the area of the pattern is relatively small with respect to the area of the screen, it is possible to include more than one pattern within the screen, thus allowing more than one area of a board, or more than one board, to be printed simultaneously using the same screen. Alternatively, more than one relatively small screen may be used within the same printing machine to enable the more than one area of a board, or more than one board, to be printed simultaneously using respective screens. While such simultaneous printing may clearly be more efficient than sequential printing, there are problems associated with these techniques. i) Printing of more than one area of a board As noted above, it is possible to print a plurality or array of patterns onto respective areas of a single board or panel in a single print operation, to produce a plurality of printed circuit boards (PCBs) which may be subsequently physically separated. This technique is conceptually and technically simple - a panel with a plurality of boards is loaded into a printing machine, correctly aligned and then all the boards of the panel are printed simultaneously. However, with any circuit board there is a risk that at least partofthat board may be defective, which in turn may lead to a defective PCB. This situation is schematically shown in FIG. 1, where three panels 1, 2, 3 are shown, each having a 4x1 array of boards A-D. While the leftmost panel 1 is completely free from defects, the adjacent panel 2 has a defective board 2A, while the right-most panel 3 has a defective board 3B. It is inefficient to pre-check the circuit boards for defects and reject an entire panel if one board is found to be defective. It is also inefficient and problematic to print a pattern onto an identified defective board and reject the separated defective board subsequent to the printing process. One current solution to this problem is to identify defective boards before commencement of the printing operation, and sort the panels into separate batches having similar defects, for example a first batch which is defect-free, a second batch in which the left-most board is defective, a third batch in which the second-left board is defective and so on. A dedicated respective screen may then be used with each batch. For example, a screen having all four aperture patterns would be used for the first batch, while screens having only three aperture patterns would be used for each remaining batch. For the 4x1 array described here, this would result in the use of five different screens per panel to print on a side of the panel. Since each panel will typically be printed on both sides, this could lead to the use often different set-upsfor a single panel type, rather than the optimal two (i.e. one for each side). In addition, the second to fifth batches will only be printing at 75% efficiency. Furthermore, if two or more boards are defective then additional measures must be taken. ii) Printing of more than one board A solution to the above problem is to pre-separate or "singulate" the individual boards before the printing process. Here, any defective boards could be identified before printing and rejected immediately, so that only non-defective boards are printed. While this process is relatively efficient, it introduces complications. In particular, it is difficult both to support and to align individual relatively small boards for simultaneous (or sequential) printing. Various approaches have been developed to overcome these problems. For example, GB 2484373 A describes a method in which individual boards are respectively positioned, but this only permits the sequential printing of one substrate at a time. JP-2009-248551 describes a method in which the position of each board is checked individually, and each board is sequentially repositioned using a repositioning arm. While this technique permits all boards of a panel to be printed on simultaneously, additional apparatus (i.e. the positioning arm) is required, and moving the arm between boards is time-consuming. In addition, the repositioning arm contacts the top of the boards, which is disadvantageous. WO2014 / 166956 describes an alternative apparatus, in which all boards may be aligned simultaneously using a reference webbing (i.e. using 'edge alignment' of the substrates), and then simultaneously printed. This solution works well, though will not be suitable if an incoming unprinted board, and hence board pattern, is positioned too far from its correct position. A workpiece support assembly, capable of supporting and individually aligning a multiplicity of relatively small workpieces (commonly referred to as "singulated" workpieces) has been described in EP3693168A1. FIG. 2 schematically shows an example of such an assembly 4, here including a 2 x 4 array of individual support "towers" 5. Each tower 5 is topped with a support surface 6 upon which a workpiece (not shown) may be supported during a printing operation. Furthermore, each tower 5 is individually actuable to move in orthogonal directions X and Y, which would typically be in the horizontal plane, and also to rotate about an orthogonal Z axis, which would typically extend in the vertical direction to provide so-called theta correction. As described in EP3693168A1, such movement may be advantageously provided through the use of a parallel kinematic actuation system within each tower. Other arrays of greater or smaller dimension are of course possible. This system has been released by ASM under the name "MASS", and provides a very fast and accurate printing solution. A significant advantage of such a system is that it enables 'optical alignment' of the singulated substrates, whereby the position of each substrate is accurately determined using an overhead camera, and the towers may directly move their respective substrates to the required position based on the position determination. In an extension of the MASS methodology, GB2596517A describes how such apparatus may be used to print a plurality of singulated substrates which are arranged at pitches in the transport direction that are smaller than the spacing of individual support towers. In a further extension of the MASS methodology, GB2613575A describes how such apparatus may be used to print a plurality of singulated substrates which are arranged at pitches orthogonal to the transport direction that are smaller than the spacing of individual support towers. In a yet further extension of the MASS methodology, GB2619961A describes how the use of different height support surfaces on each MASS tower can reduce the minimum pitch at which workpieces may be printed in a single print operation, by staggering alignment of the workpieces. The above-described 'MASS' systems provide effective printing of singulated substrates, which may be arrayed at small pitches. Nevertheless, there is a constant demand for yet smaller pitched arrays of singulated substrates to be printed, and the parallel kinematic actuation system provided within each MASS tower, together with the required encoders, is relatively large, and so presents a barrier to further miniaturisation. In addition, while these actuation systems are fast, it would be preferable to provide even faster alignment. The present invention seeks to overcome these problems, and provide an improved MASS-type tower system which retains its associated advantages of enabling optical alignment and direct positioning of substrates, but which may be realised in a smaller form factor and capable of faster operation. In accordance with the present invention this aim is achieved by using shape-memory alloy (SMA) actuators to effect horizontal movement of MASS-type support members within a workpiece support assembly. Shape-memory alloys (SMAs) are alloys that change shape when heated. In particular, even if an SMA material is deformed at temperatures below a critical temperature, subsequently heating the material above that critical temperature will cause the SMA material to revert shape to a "remembered" shape. The "remembered shape" can be set by subjecting the SMA material to a thermal treatment. Well-known SMA materials include nitinol (nickel titanium or NiTi) and copper-aluminium-nickel, although many others exist. One known application of the shape-memory effect is to use such materials as actuators. In particular, an SMA wire may be constructed which shortens in length upon heating. Conveniently, the heating may be achieved by Joule heating, i.e. through the application of an electrical current to the SMA actuator. Such SMA actuators have found uses in optical image stabilisation modules, used in many mobile phone units, where their small size and rapid response times make them ideal actuators. By suitable attachment and control of the SMA actuators to a camera lens or sensor, the camera lens or sensor may be moved within a plane to counteract unintended movements of the phone. However, such applications are unsuitable for use in workpiece support assemblies in which the workpieces are subjected to large forces, such as within printing machines. In particular, camera modules are typically suspended, by SMA wires, from a surrounding frame or cradle. While these work well for their intended use, such structures are unable to withstand the large forces to which a workpiece would be subjected during a printing operation, in which a squeegee forcibly presses printing medium, such as solder paste, through a screen or stencil and onto a surface of an underlying workpiece. The present invention enables SMA actuators to be used, while providing sufficient support for a workpiece to avoid damage to the actuators. Summary of the Invention In accordance with a first aspect of the present invention there is provided a workpiece support assembly for supporting and aligning a plurality of singulated workpieces, the workpiece support assembly comprising an assembly body and a plurality of support members mounted on the assembly body which each support a workpiece in use, each support member comprising: a base mounted on the assembly body, a head unit for supporting a workpiece thereon, the head unit comprising a plurality of anchor points distributed about the head unit, and a horizontal actuation mechanism for moving the head unit relative to the base, wherein the horizontal actuation mechanism comprises a plurality of elongate shapememory alloy actuators, each shape-memory alloy actuator having a first end that is connected to the base and a second, distal, end that is connected to an anchor point, such that selective actuation of shape-memory actuators of the plurality of shape-memory actuators causes the head unit to move relative to the base to cause: a translation of the head unit parallel to a horizontal plane, a rotation of the head unit about an axis orthogonal to the horizontal plane, or a combination of said translation and said rotation. In accordance with a second aspect of the present invention there is provided a printing machine comprising the workpiece support assembly of the first aspect. In accordance with a third aspect of the present invention there is provided a method of aligning a plurality of singulated workpieces, comprising the steps of: a) positioning the workpieces over the workpiece support assembly of the first aspect, such that each workpiece overlies a respective support member; b) lifting the workpiece support assembly so that the support members contact and lift the respective overlying workpieces; and c) aligning each workpiece individually by operation of the respective horizontal actuation mechanisms. Other specific aspects and features of the present invention are set out in the accompanying claims. Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings (not to scale), in which: FIG. 1 schematically shows three panels, each holding four boards; FIG. 2 schematically shows a known multiple tower support system; FIG. 3 schematically shows, from above, a workpiece support assembly in accordance with an embodiment of the present invention; FIG. 4 schematically shows, from above, a support member of the workpiece support assembly of FIG. 3; FIG. 5 schematically shows, from the side, a sectional view of the support member of FIG. 4; FIG. 6 schematically shows, from the side, a sectional view of a support member according to a second embodiment of the present invention; FIG. 7 schematically shows, from the side, a sectional view of a support member according to a third embodiment of the present invention; and FIG. 8 schematically shows, from the side, a sectional view of a support member according to a fourth embodiment of the present invention. Detailed Description of the Preferred Embodiments of the Invention An embodiment of the present invention is shown from above in FIGs. 3, 4 and 5, in which FIG. 3 shows a workpiece support assembly 10 comprising six individual support members 12, while FIGs. 4 and 5 show a single support member 12 of this workpiece support assembly. Referring firstly to FIG. 3, the workpiece support assembly 10 shown comprises six support members 12 arranged in a 3x2 rectangular array, mounted to an assembly body 11. In use, the assembly body may be removably, permanently or semi-permanently mounted to a tooling table (not shown) of a printing machine (not shown). Cartesian coordinate axes are shown for clarity, which reflect the normal understanding of directionality within such a printing machine, and which conform with those shown in FIG. 2 - the Z axis is directed vertically upwards, the X axis is horizontal, and directed parallel to a transport direction of workpieces (not shown) as they travel through a printing machine, and the Y axis is also horizontal. Usually, a print operation will proceed by passing a squeegee over the workpieces in a direction parallel to the Y axis. Also shown is a rotational theta (0) direction, which is directed clockwise about the Z axis as shown. Each of the six support members 12 is of rectangular, here approximately square, footprint, and comprises two main parts: a head unit 13 and a base 14, which is fixed to the assembly body 11. The head unit 13 lies within its respective base 14, with its upper surface projecting above the base 14, and is movable relative to that base 14 in the horizontal (X-Y) plane, as is described in more detail below. Each head unit 13 is used for supporting a workpiece (not shown) thereon, and advantageously a cap (not shown), specifically designed to accommodate a particular workpiece, may be fitted onto a respective head unit 13 in a releasable fashion. The head units 13 comprise mounting features 17 which enable such caps to be releasably mounted thereto. Each workpiece may be held onto the respective support member 12 by applying an at least partial vacuum to the underside of the workpiece, this is applied via a vacuum aperture 16 provided at the upper surface of each head unit 13. Each vacuum aperture 16 is connected to a vacuum source (not shown), such as a vacuum pump, via a fluid pathway extending through the respective support member 12 and into the assembly body 11. So that relative motion between the head unit 13 and base 14 may be accommodated, the fluid pathway preferably comprises flexible tubing 19 (see FIG. 5) which may move with the head unit 13 without incurring damage or leaking. Of course, each cap would also comprise a vacuum path to direct vacuum from the vacuum aperture 16 to the underside of the workpiece itself. Each support member 12 comprises a horizontal actuation mechanism for moving the head unit 13 relative to the base 14. In more detail, each base 14 comprises a plurality of base anchor points A-D (see FIG. 4) distributed about the base 14, while each head unit 13 comprises a plurality of head unit anchor points E-H (see FIG. 4) distributed about the head unit 13. The horizontal actuation mechanism comprises a plurality of elongate shape-memory alloy actuators 15, each shape-memory alloy actuator 15 having a first end that is connected to the base 14, at a base anchor point A-D, and a second, distal, end that is connected to a head unit anchor point E-H. FIG. 4 shows the horizontal actuation mechanism in more detail, including the anchor points A-H of a single support member 12. Each elongate shape-memory alloy actuator 15 is formed as a thin wire of SMA material, such as nitinol or the like. The horizontal actuation mechanism comprises heating means (not shown) for causing actuation, by selectively raising the temperature of each shape-memory actuator 15, causing the selected shape-memory actuators 15 to shorten in length. Using such selective actuation, it is possible to cause the head unit 13 to move relative to the base 14 to cause: a translation of the head unit parallel to the horizontal (X-Y) plane, a rotation of the head unit about an axis (Z) orthogonal to the horizontal (X-Y) plane, or a combination of said translation and said rotation. These movements may be achieved by selective actuation as set out in Table 1 below. Here, for convenience each shape-memory actuator 15 is identified by the two anchor points it is connected to, so, for example, the actuator 15 connected between base anchor point B and the head unit anchor point G is designated "BG". Movement of Head Unit SMA Actuators contracted i+ve X translation i-ve X translation j+ve Y translation i-ve Y translation kve 0 rotation hve 0 rotation Table 1 [BG + AH P+DE Kf+'dg Eh + BE IZJdeTb^^ Combinations of these movements may be achieved by superposing the corresponding actuator contractions. For example, the head unit 13 may be simultaneously rotated in the +ve 0 direction and translated in the -ve X direction by simultaneously contracting actuators CF, DE and AH, or alternatively by simultaneously contracting actuators CF, DE, BE and DG. In each case, the magnitude of translation and / or rotation is controlled by controlling the amount of contraction of the relevant actuators 15, i.e. by controlling the temperature of the individual actuators 15 concerned, through control of the current passing therethrough. This may be controlled by a suitable processor 18 (see FIG. 5) or other control means, computing means or the like in communication with the actuators 15 of each support member 12. It will be understood that each movement outlined above will cause a simultaneous stretching of the opposite pairs of actuators 15. For example, a +ve translation in the Y direction, caused by contracting the actuators AF and DG, will cause a corresponding stretching of actuators CH and BE. However, each actuator 15 is elastic, and so this stretching is readily accommodated, without either damaging the stretched actuators or submitting the head unit 13 to an appreciable opposing movement force. The position of the head unit 13 may advantageously be determined by measuring the resistance on each shape-memory actuator 15, this information being processed by the processor 18 to derive the position. In alternative embodiments (not shown), the head unit 13 and / or the base 14 may comprise at least one encoder which is operative to determine a horizontal position of the head unit 13 relative to the base 14. FIG. 5 schematically shows, from the side, a sectional view of the support member 12 of FIG. 4. In this embodiment, the base 14 is shaped as a tray with upstanding sidewalls, which surround and laterally constrain the head unit 13. The shape-memory actuators 15 are attached between the base 14 and head unit 13 at respective base anchor points and head unit anchor points as described previously. The head unit 13 is supported by a bearing 20 which is located intermediate the head unit 13 and base 14, and permits low-friction sliding of the head unit 13 in the horizontal (X-Y) plane relative to the base 14. Advantageously the bearing 20 may comprise an air bearing, though other bearings are equally possible. Importantly, the bearing 20 is capable of supporting the head unit 13 during a printing operation such that the shape-memory actuators 15 are prevented from undergoing an excessive stretching force during the printing operation. The bearing 20 may be mounted to the underside of head unit 13, or to the base 14, or even be provided as a separate component intermediate the head unit 13 and base 14. It is important that, once moved to its aligned horizontal position, the head unit 13 may be locked at that position, to ensure accurate printing and to avoid damaging the shapememory alloy actuators 15. If an air bearing is used as the bearing 20, then such locking may be conveniently achieved by reversing the air supply to the bearing 20, so that an at least partial vacuum is created between the head unit 13 and base 14. If other forms of bearing are used, then other locking arrangements must be provided. For example, a separate vacuum lock or magnetic lock (neither shown) may be provided for this purpose. In this view, flexible tubing 19 is visible, forming a vacuum path connecting the vacuum aperture 16 to a vacuum source 32 such as a pump. The flexible tubing 19 passes through a port of the base 14, the bearing 20 and the head unit 13, providing an airtight conduit which permits relative horizontal movement of the base 14 and head unit 13. Although not clearly shown in FIG. 5, the vacuum source associated with the body 11 may be connected to each support member 12 of the workpiece support assembly 10 via respective flexible tubings 19, so that an at least partial vacuum may be provided to each head unit 13. The above-described workpiece support assembly is operable to translate and / or rotate a plurality of workpieces within the horizontal (X-Y) plane. In other embodiments of the present invention, the workpiece support assembly may also be operable to translate individual workpieces vertically, i.e. parallel to the Z axis. FIG. 6 schematically shows, from the side, a sectional view of a support member 12' of a workpiece support assembly according to a second embodiment of the present invention, which includes such vertical translation means. Where possible, items common to the first embodiment retain the reference numerals used above. It should be noted that, for clarity only, flexible tubing 19 and vacuum source 32 have been omitted from FIG. 6; it is to be understood that these may be provided similarly to shown in FIG. 5. In this embodiment, the head unit 13, base 14, shape memory actuators 15, processor 18, bearing 20 and locking arrangement are all similar to those of the embodiment shown in FIG. 4, and need not be described further here. In this embodiment however, a vertical actuation means is provided interposed between the bearing 20 and the horizontal inner wall of the base 14. This comprises first and second wedges 21, 22, one orientated inversely to the other so that an upper wedge 21 rests on a lower wedge 22, which wedges are free to slide against each other. Two shape-memory actuators are connected between each wedge 21, 22 and the base 14: upper wedge 21 is connected to the left side of the base 14 as shown by a shape-memory alloy actuator 23, and to the right side of the base 14 by a shape-memory alloy actuator 24, while the lower wedge 22 is connected to the left side of the base 14 as shown by a shape-memory alloy actuator 25, and to the right side of the base 14 by a shapememory alloy actuator 26. Each of the shape-memory actuators 23, 24, 25 and 26 may conveniently be controlled by processor 18. Actuation of the shape-memory actuators 23 and 26 will cause the upper wedge 21 to move to the left and the lower wedge 22 to move to the right, causing the head unit 13 to be displaced vertically upwards, i.e. in the positive Z direction. Conversely, actuation of the shape-memory actuators 24 and 25 will cause the upper wedge 21 to move to the right and the lower wedge 22 to move to the left, causing the head unit 13 to be displaced vertically downwards, i.e. in the negative Z direction. The vertical position of the head unit 13 may advantageously be determined by measuring the resistance on each shape-memory actuator 23, 24, 25, 26, this information being processed by the processor 18 to derive the vertical position. In alternative embodiments (not shown), the head unit 13 and / or the base 14 may comprise at least one encoder which is operative to determine a vertical position of the head unit 13 relative to the base 14. In alternative embodiments (not shown), only one of the wedges could be movable, with the other remaining stationary. In this case, only one pair of shape-memory actuators would be required, fixed to opposite sides of the movable wedge. This design of vertical actuation means retains full support for the head unit 13, with any downward force transmitted through the head unit 13, bearing 20, upper and lower wedges 21, 22 and the base 14, to the body 11. FIG. 7 schematically shows, from the side, a sectional view of a support member 12” of a workpiece support assembly according to a third embodiment of the present invention, which includes an alternative vertical translation means. Where possible, items common to the first and second embodiments retain the reference numerals used above. It should be noted that, for clarity only, flexible tubing 19 and vacuum source 32 have been omitted from FIG. 7; it is to be understood that these may be provided similarly to shown in FIG. 5. In this embodiment, the head unit 13, base 14, shape memory actuators 15, processor 18, bearing 20 and locking arrangement are all similar to those of the embodiment shown in FIG. 6, and need not be described further here. In this embodiment however, an alternative form of vertical actuation means is provided interposed between the bearing 20 and base 14. As shown, two cam rollers 30 and 31, being cylindrical rollers of non-circular cross-section, are located between the horizontal inner wall of the base 14 and the bearing 20, such that the bearing 20, and hence head unit 13, is supported thereon. Each cam roller 30, 31 is connected to a respective side of the base 14 by a respective shape-memory actuator 27, 28 at a point on the upper half of the respective cam roller 30, 31, while another shape memory actuator 29 connects the two cam rollers 30, 31. In FIG. 7, the two cam rollers 30, 31 are shown as ellipses, with their major axes angled to the vertical (Z) axis. As shown therefore, the head unit 13 is at a mid-point in its possible range of vertical movement. If shapememory actuators 27 and 28 are actuated (under the control of processor 18), then the major axes of the cam rollers 30, 31 will rotate to become closer to the parallel with the vertical (Z) axis, and the head unit 13 will be moved upwardly. Conversely, actuation of the shapememory actuator 29 will cause the major axes of the cam rollers 30, 31 to rotate to become closer to the parallel with the horizontal (X) axis, and the head unit 13 will be moved downwardly. Throughout the entire range of vertical travel, the cam rollers 30, 31 fully support the head unit 13. FIG. 8 schematically shows, from the side, a sectional view of a support member 12'” according to a fourth embodiment of the present invention, which includes an alternative vertical translation means. Where possible, items common with previously-described embodiments retain the reference numerals used above. It should be noted that, for clarity only, flexible tubing 19 and vacuum source 32 have been omitted from FIG. 8; it is to be understood that these may be provided similarly to shown in FIG. 5. In this embodiment, the base 14, shape memory actuators 15, processor 18, bearing 20 and locking arrangement are all similar to those of the embodiment shown in FIG. 6, and need not be described further here. In this embodiment however, an alternative form of vertical actuation means is provided interposed between the bearing 20 and base 14. In addition, the head unit 13 is shown as being of smaller horizontal extent than in previous embodiments, as will be described in more detail below. In this embodiment, a vertical actuation means in the form of a scissor mechanism is provided. The bearing 20 and horizontal surface of base 14 are mechanically connected by pivotally mounted arms, with arms 41 and 42 connected in series between the bearing 20 and horizontal surface of base 14, and, in parallel thereto, arms 43 and 44 also connected in series between the bearing 20 and horizontal surface of base 14, with each connected pair of arms 41, 42 or 43, 44 being in an angled configuration with the 'elbow' or pivot point of the pair pointing away from each other. The uppermost arms 41 and 43 are connected to the base 14 by respective shape-memory actuators 44,45. In other embodiments (not shown), these shape memory actuators 44, 45 may be connected directly between the bearing 20 and base 14. A further shape-memory actuator 46 is connected between the elbows or pivot points. All of the shape-memory actuators 15, 44, 45, 46 may be controlled by the processor 18. It can be seen from FIG. 8 that, when shape-memory actuator 46 is actuated, the elbows will be brought closer together causing the arms to straighten, and the bearing 20, and hence head unit 13, will be moved upwardly. Conversely, when shape-memory actuators 44 and 45 are actuated, the bearing 20, and hence head unit 13, will be moved downwardly. Such a system may be used to provide full support to the head unit 13 at each vertical height. However, to reduce strain on the shape-memory actuator 46 and provide additional support to the head unit 13, the interior side walls of base 14 are equipped with a clamp in the form of an air bearing 47, which surrounds the bearing 20. By applying air to the air bearing 47, the bearing 20, and hence head unit 13, may freely move vertically with respect to the base 14, and in this operating state the shape-memory actuators 44, 45, 46 may be actuated as required. Once the head unit 13 is at the correct vertical height, the operating state of the clamp may be changed, such that the air supply to the air bearing 47 is removed. By selecting the horizontal dimensions of the bearing 20 and base 14 so that there is close conformance between them, removing the air supply from the air bearing 47 will cause the bearing 20 to become clamped to the air bearing 47, such that the bearing 20, and hence head unit 13, is fully supported. With such a clamping arrangement, the head unit 13 must be slightly smaller in horizontal extent than the bearing 20, to avoid contact with the air bearing 47 during its horizontal movement under control of the shape-memory actuators 15, with such horizontal movement being effected before, after, or even during any vertical movement of the head unit 13. The above-described workpiece support assemblies enable a method of aligning a plurality of singulated workpieces, which comprises the steps of: transporting a plurality of workpieces, for example located within a carrier, into a printing machine fitted with such a workpiece support assembly; positioning the workpieces over the workpiece support assembly, for example by suitable driving of a conveyor within the printing machine, such that each workpiece overlies a respective support member of the workpiece support assembly; lifting the workpiece support assembly so that the support members contact and lift the respective overlying workpieces, for example by lifting a rising table of the printing machine upon which the assembly is supported; and aligning each workpiece individually by operation of the respective horizontal actuation mechanisms. Once aligned, the respective head unit may be locked, by the locking arrangement, at that horizontal position ready for printing. If a vertical actuation mechanism is also provided, then vertical actuation may take place before, after or during horizontal alignment. Following alignment, a printing operation may be performed, in which printing medium, such as solder paste, is swept through a screen or stencil by a squeegee, onto the workpieces. Because of the support afforded to the head unit by the support member construction, it is possible to avoid damage to any of the actuation mechanisms. Following printing, the workpiece support assembly is lowered, with the printed workpieces, by lowering the rising table. The printed workpieces are thereby returned to the carrier for transport away from the printing machine. The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example, it may be possible to reduce the number of shape-memory alloy actuators by providing resilient members such as springs to return the actuators to their original length. Other mechanisms for vertical actuation are possible and will be apparent to those skilled in the art, for example including hydraulic actuators, rotary screws or air actuators with air bearings similar to that shown in FIG. 8. Reference numerals used: 1, 2, 3 - Panels 1A-3D - Boards 4 - Workpiece support assembly 5 - Towers 6 - Support surfaces 10 - Workpiece support assembly 11 - Assembly body 12,12', 12", 12"' - Support member 13 - Head unit 14 - Base 15 - Shape-memory actuators 16 - Vacuum aperture 17 - Mounting features 18 - Processor 5 19 - Flexible tubing 20 - Bearing 21, 22 - Wedges 23, 24, 25, 26, 27, 28, 29, 44, 45, 46 - Shape-memory actuators 30, 31 - Cam rollers 10 32 - Vacuum source 41, 42, 43, 44 - Arms 47 - Air bearing A, B, C, D - Base anchor points E, F, G, H - Head unit anchor points 15

Claims

1. A workpiece support assembly for supporting and aligning a plurality of singulated workpieces, the workpiece support assembly comprising an assembly body and a plurality of support members mounted on the assembly body which each support a workpiece in use, each support member comprising:a base mounted on the assembly body,a head unit for supporting a workpiece thereon, the head unit comprising a plurality of anchor points distributed about the head unit, anda horizontal actuation mechanism for moving the head unit relative to the base,wherein the horizontal actuation mechanism comprises a plurality of elongate shapememory alloy actuators, each shape-memory alloy actuator having a first end that is connected to the base and a second, distal, end that is connected to an anchor point, such that selective actuation of shape-memory actuators of the plurality of shape-memory actuators causes the head unit to move relative to the base to cause:a translation of the head unit parallel to a horizontal plane,a rotation of the head unit about an axis orthogonal to the horizontal plane, ora combination of said translation and said rotation.

2. The workpiece support assembly of claim 1, wherein the horizontal actuation mechanism comprises heating means for causing actuation, by selectively raising the temperature of shape-memory actuators of the plurality of shape-memory actuators, causing the selected shape-memory actuators to shorten in length.

3. The workpiece support assembly of either of claims 1 and 2, comprising a processor for controlling the horizontal actuation mechanism.

4. The workpiece support assembly of any preceding claim, comprising a locking arrangement to lock the head unit in a horizontal position relative to the base.

5. The workpiece support assembly of any preceding claim, wherein, in each support member, the head unit is supported, optionally directly supported, by the base such thatmovement of the head unit towards the base is prevented while a downward force is applied to the head unit in use.

6. The workpiece support assembly of claim 5, wherein the base comprises a clamp operable to releasably clamp the head unit to the base.

7. The workpiece support assembly of claim 6, wherein the clamp comprises an air bearing, the arrangement being such that in an operating state in which air is supplied to the air bearing, the head unit is relatively moveable with respect to the base, and in an operating state in which air is not supplied to the air bearing, the head unit is clamped to the base.

8. The workpiece support assembly of any preceding claim, wherein each support member comprises a bearing, optionally an air bearing, arranged such that the head unit may slide, parallel to the horizontal plane, relative to the base.

9. The workpiece support assembly of any preceding claim, comprising a vertical actuation mechanism operable to move the head unit relative to the base along an axis orthogonal to the horizontal plane.

10. The workpiece support assembly of claim 9, wherein the vertical actuation mechanism comprises a plurality of elongate shape-memory alloy actuators.

11. The workpiece support assembly of either of claims 9 and 10, wherein the vertical actuation mechanism comprises one of the group consisting of: sliding wedges, cam rollers and scissor mechanisms.

12. The workpiece support assembly of any preceding claim, wherein the head unit or base comprises at least one encoder which is operative to determine a position of the head unit relative to the base.

13. The workpiece support assembly of any preceding claim, wherein the base comprises a port for connection to a vacuum source, the head unit comprises an aperture,and the support member comprises a vacuum path, optionally comprising flexible tubing, extending between the port and the aperture.

14. A printing machine comprising the workpiece support assembly of any preceding5 claim.

15. A method of aligning a plurality of singulated workpieces, comprising the steps of:a) positioning the workpieces over the workpiece support assembly of any of claims10 1 to 13, such that each workpiece overlies a respective support member;b) lifting the workpiece support assembly so that the support members contact and lift the respective overlying workpieces; andc) aligning each workpiece individually by operation of the respective horizontal actuation mechanisms.

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