Tooling set-up and auto co-planarity adjustment using look-up, look-down distance sensors

Abstract

A printing machine 1 for printing a print medium 2 onto a workpiece W, comprises a stencil support 9 for maintaining a stencil (8,Fig.3) in a horizontal stencil plane P; a workpiece support (18, Fig.3

Description

This invention relates to a printing machine for printing a print medium onto a workpiece, and a method for readying a printing machine for a printing operation in which print medium is printed onto a workpiece. Background and Prior Art Industrial screen-printing machines typically apply a conductive print medium, such as solder paste, silver 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 thin planar layer or mask, such as a stencil (which is a patterned solid material such as stainless steel) or a screen which is a mesh material coated with emulsion. The present invention is equally applicable to both screen and stencil printing, and for convenience the term "stencil" will be used to refer to any such patterned mask throughout the remainder of this document. The print medium is applied using an angled blade or squeegee. The same machines may also be used to print certain non-conductive media, such as glue or other adhesive, onto workpieces. To ensure high quality printing, it is necessary to support the workpiece so that the surface to be printed is parallel to the printing screen, generally horizontal, with the workpiece support being capable of withstanding the pressure placed upon it during the printing operation, especially by the downward pressure applied by the squeegee, while maintaining the correct alignment of the workpiece. The simplest type of support is to use a flat surface or platen on which a workpiece may be placed. However, there are many circumstances where this type of arrangement is not possible, in particular where the underside of a workpiece has previously been printed and equipped with components (for example during a so-called "placement" operation) so that the workpiece is a printed circuit board assembly or "PCBA", and this underside needs to be supported during a printing operation applied to the topside of the workpiece. The presence of components on the underside of the workpiece means that the workpiece will not be flat, and also the components are liable to damage if they are "squashed" during a printing operation. It will be appreciated that workpieces also need support during other processes, for example during a placement operation. To this end, specialist support solutions, known as "tooling", are used. There are currently two common tooling options for providing support for a printed circuit board (PCB) during printing and placement operations: 1) Dedicated tooling blocks - these are blocks whose upper surface is caused, for example by machining, to have a three-dimensional profile designed to accommodate a specific PCB placed thereon. They are relatively expensive, product-specific, and can easily become obsolete when a PCB design changes. 2) Tooling pins - these are thin columns which are positioned to contact the board in use, avoiding contact with any components (or other delicate or critical regions) on the underside. The pins are usually magnetic, i.e. they include either a permanent or an electropermanent magnet within them, to non-permanently attach the pins to a flat, underlying support plate or "tooling table", which may conveniently be made from a magnetically permeable material such as steel. By way of example, ASMPT currently uses simple, low-cost, moulded plastics tooling pins with a single Neodymium permanent magnet in the base of each pin. Since tooling pins can be reused for a wide variety of workpieces, they tend to be cheaper and more convenient than dedicated tooling blocks. When using tooling pins, it is necessary to decide on their placement positions, so that, as outlined previously, they do not contact components on the underside of the workpiece to be printed. This positioning is typically achieved by analysing Gerber data or the like associated with the workpiece, to determine "safe" areas, i.e. sufficiently far from a component, for a pin to be positioned. However, there are various problems associated with such known methods. For example, the Gerber data represents an idealised workpiece, when in fact the components may in practice be placed inaccurately, and such inaccuracies may vary between successive workpieces in the production line. The present invention seeks primarily to overcome this problem. In addition, the present invention also aims to provide methodologies for determining the coplanarity between the tooling table or other supporting apparatus, and hence the workpiece to be printed, and the stencil, so that deviations from co-planarity can be corrected as required. Various other benefits may also be provided by the present invention, as set out in more detail below. In accordance with the present invention this aim is achieved by providing a downward-looking distance sensor, and optionally also an upward-looking distance sensor, movable within a horizontal plane located between a support plane for a workpiece and a stencil plane, which can scan a plurality of regions below the plane. Summary of the Invention In accordance with a first aspect of the present invention there is provided a printing machine for printing a print medium onto a workpiece, comprising: a stencil support for maintaining a stencil in a horizontal stencil plane a workpiece support for supporting the workpiece in a horizontal support plane spaced from the stencil plane, below the stencil in use, a carriage movable within a horizontal carriage plane located between the support plane and the stencil plane, and driving means for moving the carriage within the horizontal carriage plane, wherein the carriage comprises a downward distance sensor for determining a distance of a surface from the downward distance sensor along a downward sensing axis directed downwardly from the downward distance sensor, and wherein the driving means is operative to move the carriage, and hence the downward distance sensor, within the horizontal carriage plane such that the downward distance sensor scans a plurality of regions below the horizontal carriage plane. In accordance with a second aspect of the present invention there is provided a method for readying a printing machine for a printing operation in which print medium is printed onto a workpiece, comprising the steps of: i) providing a printing machine in accordance with the first aspect, ii) moving the carriage within the horizontal carriage plane and scanning with the downward distance sensor a plurality of regions below the horizontal carriage plane and outputting downward distance information for each of the plurality of regions, iii) analysing the downward distance information, and iv) configuring the printing machine in dependence of the downward distance information analysis. 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, in sectional side view, a printing machine in accordance with the present invention; FIG. 2 schematically shows, in perspective view from the side, modules of the printing machine of FIG. 1; FIG. 3 schematically shows, in perspective view, the modules of FIG. 2 used to determine tooling pin locations for a PCBA; and FIG. 4 diagrammatically illustrates how non-planarity of the carriage plane does not unduly affect measurements made using the modules of FIG. 2. Detailed Description of the Preferred Embodiments of the Invention A printing machine 1 in accordance with an embodiment of the present invention, used for printing a print medium 2 onto a workpiece W is schematically shown in FIG. 1. The figure is highly schematic, and for clarity only shows the key modules which are relevant for the purposes of describing the present invention. A workpiece W to be printed, which may for example comprise a board, wafer or PCBA, is supported by a conveyor 3 in a substantially horizontal, i.e. orthogonal to the Z-axis shown, support plane S. The conveyor 3 is also used to transport the workpiece W into the printing machine 1 for printing, and to transport it from the printing machine 1 following completion of a printing operation. Located below the support plane S is a rising table 4, a column which is accurately driveable in the vertical direction, parallel to the Z-axis as shown. The rising table 4 carries at its upper end a tooling table or tooling surface 5, which is an accurately machined flat horizontal surface suitable for supporting a variety of tooling thereon, such as dedicated tooling blocks, columnar tooling for singulated workpieces or, as shown in FIG. 1, a plurality of tooling pins 6. A magazine 7 for storing the tooling pins 6 when not in use is also shown. In FIG. 1, the magazine is located on the tooling surface 5, but may equally be provided elsewhere in the printing machine 1. In use, the rising table 4 is driven upwardly so that the tooling carried by the tooling surface 5 lifts into contact with the underside of the workpiece W and lifts the workpiece W off from the conveyor 3 and towards a stencil 8 (see FIG. 3) held within a stencil frame 9 in a substantially horizontal stencil plane P. The stencil frame 9 is clamped into position by stencil clamps 10 such as mechanical or pneumatic clamps. For the print operation, the rising table 4 is lifted so that the workpiece W actually contacts and is pressed up into the stencil 8, so that there is a good seal between the workpiece W and stencil 8. A squeegee 11, held by a print head 12, is then used to spread print medium 2 across the upper surface of the stencil 8 and through apertures (not shown) provided in the stencil 8 onto the upper surface of the workpiece W. To ensure accurate printing, the workpiece W must be clamped into a fixed position during the print operation. This is achieved by a clamping assembly positioned above the conveyor 3, which clamping assembly is not shown in FIG. 1 for clarity, but is illustrated in FIGs. 2 and 3. The modules and operation as described above are known in the art. In accordance with an embodiment of the present invention, the printing machine 1 is also provided with a carriage 13 provided with driving means for moving the carriage 13 within a substantially horizontal carriage plane C located between the support plane S and the stencil plane P. The driving means shown comprises a gantry 14 on which the carriage 13 is mounted. As will be described in more detail below, the carriage is provided with a downward distance sensor (15, see FIG. 2), and optionally an upward distance sensor 16 (see FIG. 2). The driving means is operative to move the carriage 13 within the carriage plane C with components of movement optionally in both X and Y directions, where the X and Y axes (not shown) are orthogonal both to each other and to the vertical Z axis shown. For example, the gantry 14 may comprise connected beams which extend parallel to the horizontal and orthogonal X and Y axes respectively, with the carriage 13 linearly movable along the X beam in the X direction, and the X beam itself being linearly movable along the Y beam in the Y direction, with the Y beam being fixed within the printing machine 1. It should be noted that in FIG. 1, the gap between the workpiece W and stencil frame 9 is exaggerated, so that the carriage 13 and gantry 14 can be more clearly seen. FIG. 2 schematically shows, in perspective view from the side, modules of the printing machine of FIG. 1. In this figure, the workpiece W has been lifted away from conveyor 3, and is being clamped by the clamping assembly. The clamping assembly comprises first and second workpiece support brackets 17 with respective workpiece clamps 18 provided at the top of the support brackets 17. In known printing machines, the workpiece clamps may grip the workpiece W from the side, or sandwich the workpiece W between the underside of the workpiece clamps 18 and the workpiece support brackets 17. The downward distance sensor 15 is visible in FIG. 2, as is the upward distance sensor 16. Each of these distance sensors is operative to determine a distance of a surface of an object from the sensor along a sensing axis parallel to the vertical Z-axis. In more detail, the downward distance sensor 15 is used for determining a distance of a surface from the downward distance sensor 15 along a downward sensing axis directed downwardly from the downward distance sensor 15, while the upward distance sensor 16 is used for determining a distance of a surface from the upward distance sensor 16 along an upward sensing axis directed upwardly from the upward distance sensor 16. Since the carriage plane C underlies the stencil plane, in practice, the upward distance sensor 16 will detect the distance upwardly to the underside of the stencil 8. The driving means 14 is operative to move the carriage 13, and hence the downward distance sensor 15 and upward distance sensor 16, within the horizontal carriage plane C such that the downward distance sensor 15 scans a plurality of regions below the horizontal carriage plane C. Preferably, and as shown, the downward sensing axis and the upward sensing axis are collinear, so that the upward distance sensor 16 scans regions directly above those scanned by the downward distance sensor 15. FIG. 2 also shows the carriage plane C, stencil plane P and a support plane S. The support plane S here is higher than that shown in FIG. 1. For the purposes of the present invention, this is not relevant - the downward distance sensor 15 is still capable of scanning objects at this plane whichever height the workpiece W is being supported at. The apparatus as shown may be used to ready the printing machine 1 for a printing operation. This may be achieved by: a) moving the carriage 13 within the horizontal carriage plane C and scanning with the downward distance sensor 15 a plurality of regions below the horizontal carriage plane C and outputting downward distance information for each of the plurality of regions, b) analysing the downward distance information, for example by using a control means (not shown) such as a local or remote computer, processor or the like which controls operation of the printing machine 1 to receive the outputted downward distance information and analyse the same, and c) configuring the printing machine 1 in dependence of the downward distance information analysis. As a first specific embodiment, described with reference to FIG. 3, where the workpiece W comprises a printed circuit board assembly (PCBA) having a plurality of components 19, such as electrical components, located on a first, upper, surface 20 thereof, the method may be used to determine optimum tooling pin placement positions as follows: the PCBA may be loaded onto the workpiece support bracket 17 so that the first, upper, surface 20 is parallel to the support plane S and facing upwards; in step a), a plurality of regions of the first surface 20 may then be scanned using the downward distance sensor 15 during movement of the carriage 13; in step b), the location of components 19 located on the first surface may thus be determined from the output downward distance information. Put simply, the regions containing a component 19 will be higher than surrounding regions in which no component 19 is located. The analysis is effectively used to create a three-dimensional topographical map of the PCBA, which can then be filtered by height planes to find suitable locations for tooling pins on the tooling surface 5; and in step c), a plurality of tooling pins 6 may then be placed onto a tooling surface 5 of the printing machine 1 at positions corresponding to locations of the PCBA at which no components 19 are located. So that the placement of the tooling pins 6 is simplified, the PCBA may be removed from the workpiece support bracket 17 prior to the placement in step c). Subsequent to the placement in step c), the PCBA may then be supported on the upper surfaces of the tooling pins 6 in an inverted orientation such that the first surface is facing downwards. The components will not be damaged, since the positioning of the tooling pins specifically avoid their locations, as determined through the downward distance information analysis. As a second specific embodiment, the method may be used to detect damaged or incorrectly-placed tooling pins 6 which have been placed onto a tooling surface 5 (for example if the pin 6 has been placed onto some print medium debris on the tooling surface 5, thus raising the height of the pin 6 above that intended). Here, in step a), at least two of the plurality of regions include respective tooling pins 6 located below the carriage plane C, such that the downward distance information outputted forthese regions is indicative of the height of the tooling pins 6. In such a method, the tooling pins 6 may be located on the tooling surface 5, and in step b) the downward distance information is analysed to identify a tooling pin 6 which is at a height outside a predetermined height range. If the determined pin height is above the height range, this may indicate that the tooling pin may have been placed onto debris. Alternatively, if a tooling pin 6 is determined to be below the height range, then this may indicate a damaged pin, or one that is leaning away from the vertical. In step c), for any tooling pin identified as being at a height outside the predetermined height range, the region associated with this tooling pin may be flagged as non-conforming, and the placing of further tooling pins within the non-conforming region is prevented until a cause of the non-conformance has been identified, and preferably resolved. Such a method may equally be used to analyse tooling pins 6 while they are located in a tooling pin magazine (not shown), as long as the carriage C is operative to travel over the magazine. Alternatively or additionally, in step a), at least one of the plurality of regions may include one of the group comprising: a dedicated tooling block (not shown), a tooling tower (not shown) and a tooling pin magazine (not shown). Such tooling elements all require accurate and consistent tooling element heights, and deviations from the optimum height may be readily detected using the present methodology. As a particular example, where the tooling towers include movable parts (such as "VPT", "TRS" and "MASS" towers sold by ASMPT), the above methodology may be used to measure and ensure that the moving parts are moving and homing correctly. Deviations from correct movement may be flagged for repair or replacement or the affected towers, and maintenance or end-of-life predictions can be made based on the acquired height data. The previously-described embodiments use downward distance sensing through the downward distance sensor 15, but do not necessarily make use of the upward distance sensor 16. In contrast, the following embodiments use the basic methodology set out above, but with a combination of downward and upward distance sensing. In such methods: in step a), the carriage 13 is moved within the carriage plane C and a plurality of upper regions is scanned above the carriage plane C with the upward distance sensor 16. Upward distance information is output for each of the plurality of different regions; in step b), the upward distance information is analysed, for example using the previously-described control system; and in step c), the printing machine 1 is configured in dependence of the upward distance information analysis. This method may for example be used to ensure coplanarity of printing machine elements located above and below the carriage plane C. In this case, step b) may include correlating the downward distance information and the upward distance information to provide information on the coplanarity of printing machine elements located above and below the carriage plane C; and in step c), the printing machine element located below the carriage plane C may be coplaned with the printing machine element located above the carriage plane C. Here, the term "co-plane" means that the elements above and below the carriage plane C are moved into parallel planes. As a particular example, the printing machine element located above the carriage plane C may comprise a stencil 8, and the printing machine element located below the carriage plane C may comprise a tooling surface 5. In such an instance, co-planing may be most easily achieved by adjusting the inclination of the stencil 8 (and for which there are various methods known in the art, see for example the techniques set out in EP 3480015B) while leaving the tooling surface 5 unchanged or stationary, but this need not always be the case. Alternatively, the printing machine element located above the carriage plane C may comprise one of the group consisting of: a conveyor for a workpiece W, a chase (not shown) and a workpiece clamp 18, while the printing machine element located below the carriage plane C may comprise a tooling surface 5. It should be noted that with these examples, the carriage 13 would be arranged differently than shown in FIG. 2 or 3, so that it is below the height of the workpiece W shown. In this respect it should be noted that printing machines are technically complex, and many elements are either independently drivable vertically, are at least movably connected to other vertically-drivable elements. If no workpiece W is present, then the workpiece support and clamping assembly 17,18 may for example be raised above the carriage 13. It should be recognised that the present methodology provides the above benefits even if the carriage plane C is not exactly horizontal, for example if the gantry 14 is misaligned so that the carriage 13 travels out of the true horizontal plane. This is diagrammatically illustrated in FIG. 4, to explain how such non-planarity of the carriage plane C does not unduly affect measurements made using the modules of FIG. 2. As shown, the carriage plane C is quite far off the true horizontal, and slopes upwardly towards the right hand side of the Figure. Nevertheless, the distance between the two planes P and S, as measured by the upward and downward sensors 15,16 is unaffected. The distance between planes P and S measured at the left side of the Figure is determined by summing the sensed upward distance CPI and the sensed downward distance CS1. On the right edge, the distance is determined by summing the sensed upward distance CP2 and the sensed downward distance CS2. Although CPI >CP2, and CS2 >CS1, CPI + CS1 = CP2 + CS2. This effect is particularly true where the sensing axes of the upward and downward distance sensors 15,16 are collinear. This provides a further advantage, that the upward and downward distance sensors 15, 16 may be fitted or retrofitted to existing elements of the printing machine, which need not be particularly accurately arranged. In a preferred embodiment, the carriage 13 comprises a camera module, used in existing printing machines. Alternatively, the upward and downward distance sensors 15,16 may be mounted to an underscreen cleaning module (not shown), currently used to sweep below a stencil and remove print medium debris therefrom. In this case, the module would require some adaptation to be movable in both horizontal direction, since currently only linear horizontal motion is possible. 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: i) In certain printing machines, the height of the board being printed may be measured and / or adjusted in relation to the board clamps (in particular so-called "foil-less clamps". In this way variations in the thickness of boards may be compensated. ii) For tooling tower systems (such as ASMPT's MASS tooling), the system can be used to measure and / or adjust the relative height of the singulated boards being printed in relation to their surround plate. iii) The system could also be used to measure and / or adjust the height and coplanarity of board clamps. Reference numerals used: 1 - Printing machine 2 - Print medium 3 - Conveyor 4 - Rising table 5 - Tooling surface 6 - Tooling pins 7 - Magazine 8 - Stencil 9 - Stencil frame 10 - Stencil clamps 11 - Squeegee 12 - Print head 13 - Carriage 14 - Gantry 15 - Downward distance sensor 16 - Upward distance sensor 17 - Workpiece support bracket 18 - Workpiece clamp 19 - Components 20 - PCBA first surface P - Stencil plane S - Support plane C - Carriage plane W - Workpiece CS1, CS2 - Vertical distance between carriage plane and support plane CPI, CP2 - Vertical distance between carriage plane and stencil plane

Claims

1. A printing machine for printing a print medium onto a workpiece, comprising:a stencil support for maintaining a stencil in a horizontal stencil planea workpiece support for supporting the workpiece in a horizontal support plane spaced from the stencil plane, below the stencil in use,a carriage movable within a horizontal carriage plane located between the support plane and the stencil plane, anddriving means for moving the carriage within the horizontal carriage plane,wherein the carriage comprises a downward distance sensor for determining a distance of a surface from the downward distance sensor along a downward sensing axis directed downwardly from the downward distance sensor, andwherein the driving means is operative to move the carriage, and hence the downward distance sensor, within the horizontal carriage plane such that the downward distance sensor scans a plurality of regions below the horizontal carriage plane.

2. The printing machine of claim 1, wherein the carriage comprises an upward distance sensor for determining a distance of a surface from the upward distance sensoralong an upward sensing axis directed upwardly from the upward distance sensor.

3. The printing machine of claim 2, wherein the downward sensing axis and the upward sensing axis are collinear.

4. A method for readying a printing machine for a printing operation in which print medium is printed onto a workpiece, comprising the steps of:i) providing a printing machine in accordance with claim 1,ii) moving the carriage within the horizontal carriage plane and scanning with the downward distance sensor a plurality of regions below the horizontal carriage plane and outputting downward distance information for each of the plurality of regions,iii) analysing the downward distance information, andiv) configuring the printing machine in dependence of the downward distance information analysis.

5. The method of claim 4, wherein the workpiece comprises a printed circuit board assembly comprising a plurality of components located on a first surface thereof, and wherein: step i) comprises loading the printed circuit board assembly onto the workpiece support so that the first surface is parallel to the horizontal support plane and facing upwards, step ii) comprises scanning a plurality of regions of the first surface,step iii) comprises determining the location of components located on the first surface, andstep iv) comprises placing a plurality of tooling pins onto a tooling surface of the printing machine at positions corresponding to locations of the printed circuit board assembly at which no components are located.

6. The method of claim 5, wherein, prior to step iv), the printed circuit board assembly is removed from the workpiece support, and following the placing of the plurality of tooling pins onto the tooling surface, the printed circuit board assembly is supported on the tooling pins in an inverted orientation such that the first surface is facing downwards.

7. The method of claim 4, wherein in step ii), at least two of the plurality of regions include respective tooling pins located below the horizontal carriage plane, such that the downward distance information outputted for these regions is indicative of the height of the tooling pins.

8. The method of claim 7, wherein the tooling pins are located on a tooling surface, and step iii) comprises analysing the downward distance information to identify a tooling pin which is at a height outside a predetermined height range.

9. The method of claim 8, wherein step iv) comprises, for any tooling pin identified as being at a height outside the predetermined height range, the region associated with this tooling pin is flagged as non-conforming, and the placing of tooling pins within the nonconforming region is prevented until a cause of the non-conformance has been identified.

10. The method of claim 7, wherein the tooling pins are located within a tooling pin magazine.

11. The method of claim 4, wherein in step ii), at least one of the plurality of regions includes one of the group comprising: a dedicated tooling block, a tooling tower and a tooling pin magazine.

12. The method of claim 4, wherein the printing machine comprises a printing machine in accordance with claim 2, and wherein:step ii) comprises moving the carriage within the horizontal carriage plane and scanning with the upward distance sensor a plurality of upper regions above the horizontal carriage plane and outputting upward distance information for each of the plurality of upper regions,step iii) comprises analysing the upward distance information, andstep iv) comprises configuring the printing machine in dependence of the upward distance information analysis.

13. The method of claim 12, wherein step iii) comprises correlating the downward distance information and the upward distance information to provide information on the coplanarity of printing machine elements located above and below the horizontal carriage plane, andstep iv) comprises co-planing the printing machine element located below the horizontal carriage plane with the printing machine element located above the horizontal carriage plane.

14. The method of claim 13, wherein the printing machine element located above the horizontal carriage plane comprises a stencil, and the printing machine element located below the horizontal carriage plane comprises a tooling surface.

15. The method of claim 13, wherein the printing machine element located above the horizontal carriage plane comprises one of the group consisting of: a conveyor for a workpiece, a chase and a workpiece clamp, and the printing machine element located below the horizontal carriage plane comprises a tooling surface.

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