WO2003103965A1

WO2003103965A1 - Screen printing

Info

Publication number: WO2003103965A1
Application number: PCT/GB2003/002404
Authority: WO
Inventors: Andrew Edmonds; Tom Falcon; Andrew Charles Ricketts
Original assignee: Dek International Gmbh
Priority date: 2002-06-05
Filing date: 2003-06-02
Publication date: 2003-12-18
Also published as: AU2003240066A1; GB0212792D0

Abstract

A screen spacing element (3) for spacing a printing screen from at least one workpiece (11) in a screen printing apparatus, and a printing screen for a screen printing apparatus. The screen spacing element comprises a spacing sheet (7) including a plurality of printing apertures (9) which allow for printing of deposits of printing material onto the at least one workpiece, which printing apertures are separated by ribs (15) which act normally to space the printing screen from the at least one workpiece (11) and provide for the printing screen to be brought onto contact with the at least one workpiece at the printing apertures in a printing operation.

Description

SCREEN PRINTING

The present invention relates to a screen spacing element for a screen printing apparatus, a screen spacing unit for a screen printing apparatus, a printing screen (often referred to as a stencil) for a screen printing apparatus, a printing screen unit for a screen printing apparatus, a screen printing apparatus incorporating the same, and a method of screen printing deposits of printing material onto workpieces, in particular substrates, through a printing screen.

The present invention finds particular application in the electronics field in printing fine deposits of printing material, such as solder paste, onto workpieces for the placement of electronic components.

The separation of a printing screen and a workpiece is a critical factor in achieving a high-quality print. Currently, numerous techniques are employed in an attempt to control the spacing of printing screens and workpieces.

hi one technique, the printing screen is held clear of a workpiece and forced into contact with the workpiece by the printing head only at the point of contact, with the printing screen peeling away from the workpiece behind the printing head.

hi another technique, the printing screen is maintained in contact with a workpiece during printing, and subsequently separated from the workpiece in a precisely-controlled fashion, in particular at a controlled speed.

These existing techniques do suffer, however, from a number of disadvantages.

In the first-described technique, where the printing screen is held clear of a workpiece by attachment at the periphery thereof, the peel angle varies as the printing head moves between the periphery and the center of the printing screen, causing a variation in the printed deposits of printing material. This variation in the peel angle can be accommodated by the provision of a mechanism which raises the printing screen at a constant rate behind the printing head, but the required mechanism is complex. h the second-described technique, where the separation of the printing screen from a workpiece is controlled, such control is frequently difficult to maintain in the central region of the printing screen.

It is an aim of the present invention to provide a screen spacing element for a screen printing apparatus, a screen spacing unit for a screen printing apparatus, an improved printing screen for a screen printing apparatus, an improved printing screen unit for a screen printing apparatus, a screen printing apparatus incorporating the same, and an improved method of screen printing deposits of printing material onto workpieces .

In one aspect the present invention provides a screen spacing element for spacing a printing screen from at least one workpiece in a screen printing apparatus, the screen spacing element comprising a spacing sheet including a plurality of printing apertures which allow for printing of deposits of printing material onto the at least one workpiece, which printing apertures are separated by ribs which act to space the printing screen from the at least one workpiece.

In another aspect the present invention provides a screen spacing element for spacing a printing screen from at least one workpiece in a screen printing apparatus, the screen spacing element comprising a spacing sheet including a plurality of printing apertures which allow for printing of deposits of printing material onto the at least one workpiece, which printing apertures are separated by ribs which act normally to space the printing screen from the at least one workpiece and provide for the printing screen to be brought into contact with the at least one workpiece at the printing apertures in a printing operation.

Preferably, the spacing sheet is formed of a metal.

Preferably, the spacing sheet comprises a foil.

Preferably, the printing apertures comprise elongate apertures which extend substantially orthogonally to a direction of printing. More preferably, the printing apertures comprise substantially rectangular apertures.

Preferably, the printing apertures each have the same dimensions in a direction of printing.

Preferably, the printing apertures are spaced uniformly in a direction of printing.

Preferably, the printing screen includes a plurality of arrays of screen apertures through which arrays of deposits of printing material are in use printed as print regions onto the at least one workpiece, and the printing apertures have dimensions in a direction of printing which provides borders adjacent leading and trailing edges of respective ones of the arrays of screen apertures.

Preferably, the borders extend at least about 2 mm.

More preferably, the borders extend at least about 5 mm.

Preferably, the ribs comprise elongate members.

More preferably, the ribs comprise linear members.

Preferably, the screen spacing element further comprises a frame to which the spacing sheet is attached.

The present invention also extends to a screen spacing unit for a screen printing apparatus, comprising: the above-described screen spacing element; and a mounting assembly to which the screen spacing element is mounted.

Preferably, the screen spacing element is detachably mounted to the mounting assembly.

The present invention further extends to a screen printing apparatus including the above- described screen spacing unit. The present invention yet further extends to a screen printing apparatus, including: a printing screen including a plurality of screen apertures through which deposits of printing material are in use printed; a workpiece support for supporting at least one workpiece at a printing zone beneath the printing screen; and the above-described screen spacing unit disposed between the printing screen and the workpiece support; wherein the printing screen, the workpiece support and the screen spacing unit are movable relative to one another between a workpiece-receiving configuration in which at least one workpiece can be provided to the workpiece support and a workpiece-printing configuration where the spacing sheet of the screen spacing element is disposed at a lower surface of the printing screen and the at least one workpiece is disposed at a lower surface of the spacing sheet of the screen spacing element.

Preferably, the screen printing apparatus further includes: a printing head for traversing the printing screen in a direction of printing to force printing material into the screen apertures in the printing screen; wherein the printing apertures each have a length in a direction orthogonal to the direction of printing which is at least about 4 mm greater than that of a screen contact surface of the printing head.

More preferably, the printing apertures each have a length in a direction orthogonal to the direction of printing which is at least about 10 mm greater than that of the screen contact surface of the printing head.

hi a further aspect the present invention provides a method of screen printing deposits of printing material onto at least one workpiece, comprising the steps of: providing a printing screen including a plurality of screen apertures through which deposits of printing material are printed onto at least one workpiece; disposing a screen spacing element adjacent a lower surface of the printing screen; disposing at least one workpiece adjacent a lower surface of the screen spacing element; and traversing a printing head over an upper surface of the printing screen to force printing material into the screen apertures in the printing screen and print deposits of printing material onto the at least one workpiece; whereby the screen spacing element acts to constrain deflection of the printing screen on traversal of the printing head over the printing screen. In a yet further aspect the present invention provides a method of screen printing deposits of printing material onto at least one workpiece, comprising the steps of: providing a printing screen including a plurality of screen apertures through which deposits of printing material are printed onto at least one workpiece; disposing a screen spacing element adjacent a lower surface of the printing screen; disposing at least one workpiece adjacent a lower surface of the screen spacing element; and traversing a printing head over an upper surface of the printing screen to force printing material into the screen apertures in the printing screen and print deposits of printing material onto the at least one workpiece; whereby the screen spacing element acts normally to space the printing screen from the at least one workpiece and provides for the printing screen to be brought into contact with the at least one workpiece at the printing apertures in the screen spacing element on traversal of the printing head over the printing screen.

In a still further aspect the present invention provides a printing screen for a screen printing apparatus, the printing screen comprising a screen sheet including a plurality of arrays of screen apertures through which arrays of deposits of printing material are in use printed as print regions onto at least one workpiece, and a plurality of ribs on a lower surface of the screen sheet, the ribs being spaced in a direction of printing such as to define printing recesses between adjacent ones thereof and bound respective ones of the arrays of screen apertures.

Preferably, the screen sheet is formed of a metal.

Preferably, the screen sheet comprises a foil.

Preferably, the ribs are formed of a metal.

Preferably, the screen sheet and the ribs are formed from a single sheet.

Preferably, the printing recesses comprise elongate recesses which extend substantially orthogonally to a direction of printing. More preferably, the printing recesses comprise substantially rectangular recesses.

Preferably, the printing recesses each have the same dimensions in a direction of printing.

Preferably, the printing recesses are spaced uniformly in a direction of printing.

Preferably, the printing recesses have a dimension in a direction of printing which provides borders, having no screen apertures therein, adjacent the ribs.

Preferably, the borders extend at least about 2 mm.

More preferably, the borders extend at least about 5 mm.

Preferably, the ribs comprise elongate members.

More preferably, the ribs comprise linear members.

Preferably, the ribs comprise continuous members.

Preferably, the ribs act normally to space the screen sheet from the at least one workpiece and provide for the screen sheet to be brought into contact with the at least one workpiece at the printing recesses in a screen printing operation.

The present invention also extends to a printing screen unit, comprising: the above- described printing screen; and a frame to which the printing screen is attached.

The present invention further extends to a screen printing apparatus including the above- described printing screen unit.

Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Figure 1 illustrates an exploded perspective view of a screen spacing unit for a screen printing apparatus in accordance with a first embodiment of the present invention;

Figure 2 illustrates a vertical sectional view (along section I-I in Figure 1) of the screen spacing unit of Figure 1 as fitted to a screen printing apparatus, with the screen printing apparatus in the workpiece-transport configuration;

Figure 3 illustrates a vertical sectional view (along section I-I in Figure 1) of the screen spacing unit of Figure 1 as fitted to a screen printing apparatus, with the screen spacing unit in the lowered, workpiece-holding position;

Figure 4 illustrates a vertical sectional view (along section I-I in Figure 1) of the screen spacing unit of Figure 1 as fitted to a screen printing apparatus, with the tooling support in a first raised position engaging the transport rails;

Figure 5 illustrates a vertical sectional view (along section I-I in Figure 1) of the screen spacing unit of Figure 1 as fitted to a screen printing apparatus, with the tooling support in a second raised position, where the screen spacing unit and the transport rails, between which the at least one workpiece is clamped, are positioned such that the spacing sheet of the screen spacing unit contacts the lower surface of the printing screen, in which position the screen printing apparatus is in the workpiece-printing configuration;

Figure 6 illustrates a vertical sectional view (along section I-I in Figure 1) of the screen spacing unit of Figure 1 as fitted to a screen printing apparatus, with the screen printing apparatus in the workpiece-printing configuration and the printing head traversing the printing screen in a printing stroke;

Figure 7 illustrates region A of Figure 6 in enlarged scale;

Figure 8 illustrates a perspective view from above of a printing screen unit in accordance with a second embodiment of the present invention; Figure 9 illustrates a perspective view from below of the printing screen unit of Figure 8;

Figure 10 illustrates in enlarged scale a fragmentary vertical sectional view (along section II-II in Figure 8) of the printing screen of the printing screen unit of Figure 8;

Figure 11 illustrates a vertical sectional view (along section II-II in Figure 8) of the printing screen unit of Figure 8 as fitted to a screen printing apparatus, with the screen printing apparatus in the workpiece-printing configuration and the printing head traversing the printing screen in a printing stroke; and

Figure 12 illustrates region B of Figure 11 in enlarged scale.

Figure 1 illustrates a screen spacing unit 1 for a screen printing apparatus in accordance with a first embodiment of the present invention.

The screen spacing unit 1 comprises a screen spacing element 3 and a mounting assembly 5 to which the screen spacing element 3 is mounted.

The screen spacing element 3 comprises a spacing sheet 7, in this embodiment a foil, which includes a plurality of first, printing apertures 9 which allow for the printing of deposits of printing material onto at least one workpiece 11 supported by a tooling support 13 beneath the spacing sheet 7, with adjacent ones of the printing apertures 9 being separated by respective ones of a plurality of ribs 15, and a plurality of second, clearance apertures 17 which provide an optical path through the spacing sheet 7.

In this embodiment the spacing sheet 7 comprises a metal foil, hi another embodiment the spacing sheet 7 could comprise a plastics foil. The thickness of the spacing sheet 7 is dependent upon a number of factors which include the characteristics of the printing material, and the sizes of the printing apertures 9 and the ribs 15. Typically, the spacing sheet 7 has a thickness of about 0.15 mm.

hi this embodiment the printing apertures 9 comprise parallel, elongate apertures which extend orthogonally to the printing direction, with each of the printing apertures 9 bounding at least one print region within which deposits of printing material are to be printed onto at least one workpiece 11. In this embodiment the printing apertures 9 have a length /, that is, a dimension in a direction orthogonal to the printing direction, which is about 4 mm longer than the lateral dimension of the printing head to be used for printing, and a width w, that is, a dimension in the printing direction, which is such as to provide borders of about 2 mm adjacent the leading and trailing edges of the print regions. In a preferred embodiment the printing apertures 9 have a length / which is about 10 mm longer than the lateral dimension of the printing head to be used for printing, and a width w which is such as to provide borders of about 5 mm adjacent the leading and trailing edges of the print regions.

In this embodiment the printing apertures 9 are of uniform size and shape in relation to the print regions of material deposits to be printed. By configuring the printing apertures 9 to have substantially the same relationship to each print region to be printed, the separation characteristics of a printing screen located above the spacing sheet 7 in a printing operation are substantially identical, thereby providing for consistent print quality.

The screen spacing unit 1 of this embodiment is particularly suited to the printing of repeating arrays of material deposits onto a plurality of workpieces 11, such as separable panels in a single, large board, or separate workpieces 11, such as ball grid array (BGA) substrates or pin grid array (PGA) substrates, as the print regions are identical and the printing apertures 9 can have the same size, shape and position in relation to the print regions, thereby providing for identical separation characteristics. Where a plurality of separate workpieces 11 are to be printed, the workpieces 11 are supported in a carrier.

The screen spacing element 3 further comprises a frame 19, in this embodiment a rectangular frame, to which the spacing sheet 7 is attached. In this embodiment the spacing sheet 7 is bonded to the frame 19 with an adhesive. It will be appreciated that other forms of attachment could be used, such as, for example, welding. For the purposes of illustration, one of the corners of the spacing sheet 7 is cut away to reveal the frame 19. The frame 19 includes a plurality of recesses 21 in the members thereof which are disposed laterally to the workpiece-fransport direction, which recesses 21, as will become apparent hereinbelow, provide for the fixing of the screen spacing element 3. In this embodiment the frame 19 includes four recesses 21, one at each end of the members of the frame 19 disposed laterally to the workpiece-fransport direction. In this embodiment the recesses 21 are outwardly tapered from the lower surface of the frame 19, which taper provides for the biasing of the frame 19 onto the mounting assembly 5, as will become apparent hereinbelow.

The mounting assembly 5 comprises first and second mounting units 23, 25 which are disposed adjacent and outwardly of the transport rails 27, 29 of the screen printing apparatus.

The transport rails 27, 29 each comprise a driven belt 31, which engages a longitudinal edge of the at least one workpiece 11 in the workpiece-fransport direction, and a support rail 33 which supports the respective driven belt 31. The support rails 33, 33 each include first and second recesses 35, 37 at positions located upstream and downstream of the printing zone for receiving the respective ones of the members of the frame 19 which extend over the transport rails 27, 29 when the mounting assembly 5 is lowered in relation to the transport rails 27, 29.

The mounting units 23, 25 each comprise an elongate bar 39 which includes first and second recesses 41, 43 for receiving the respective ones of the members of the frame 19 which extend orthogonally to the workpiece-fransport direction.

The mounting units 23, 25 each further comprise a plurality of, in this embodiment four, detents 45, in this embodiment resilient elements, in particular spring-loaded catches, which are disposed such as to engage respective ones of the recesses 21 in the frame 19. With this configuration, the detents 45 act on the tapered recesses 21 in the frame 19 to bias the frame 19 downwardly into contact with the elongate bars 39, 39 of the mounting units 23, 25. Operation of a screen printing apparatus incorporating the screen spacing unit 1 will now be described hereinbelow with reference to Figures 2 to 7.

Referring particularly to Figure 7, the screen printing apparatus includes a printing screen 47, which includes a plurality of screen apertures 49 through which deposits 50 of printing material are printed onto at least one workpiece 11, and a printing head 51 for forcing printing material into the screen apertures 49 in the printing screen 47.

In this embodiment the screen apertures 49 are grouped in repeating arrays and located in registration with ones of the printing apertures 9 in the spacing sheet 7 of the screen spacing element 3.

In this embodiment the printing head 51 comprises an elongate main body 53, and first and second wiper blades 55, 57, here elongate elements, which are attached to a lower surface of the main body 53. The wiper blades 55, 57 are inwardly and downwardly directed and define an outlet aperture 59 through which printing material is delivered, and together with the main body 53 define a chamber 61 which contains printing material. In this embodiment the printing head 51 has a length which is about 4 mm shorter than the length / of the printing apertures 9 in the spacing sheet 7 of the screen spacing element 3, such as to allow the printing head 51 to fall into the respective ones of the printing apertures 9 and provide for uniform contact of the printing screen 47 on the upper surface of the at least one workpiece 11.

Referring to Figure 2, with the screen spacing unit 1, the tooling support 13 and the transport rails 27, 29 in the respective workpiece-receiving positions, at least one workpiece 11 is transported on the driven belts 31, 31 of the transport rails 27, 29 to the printing zone beneath the spacing sheet 7 of the screen spacing unit 1 and above the tooling support 13. As illustrated in Figure 2, with the screen spacing unit 1, the tooling support 13 and the transport rails 27, 29 in the workpiece-receiving positions, the spacing sheet 7 of the screen spacing unit 1 is spaced from the lower surface of the printing screen 47 of the screen printing apparatus such as to allow an underscreen cleaning unit 63 to be passed over the lower surface of the printing screen 47. hi having . a planar lower surface, the printing screen 47 can be efficiently cleaned. Referring to Figure 3, with the at least one workpiece 11 located at the printing zone, the screen spacing unit 1 is lowered to the workpiece-holding position, such as to clamp the at least one workpiece 11 between the transport rails 27, 29 and the spacing sheet 7 of the screen spacing unit 1.

With the screen spacing unit 1 in the lowered workpiece-holding position, the tooling support 13 is then raised, hi a first phase, as illustrated in Figure 4, the tooling support 13 is raised to engage the fransport rails 27, 29, at which point of engagement, the upper surface of the tooling support 13 supports the lower surface of the at least one workpiece 11; the tooling support 13 being configured such that the upper, supporting surface thereof is positioned precisely relative to the lower surface of the at least one workpiece 11 on engagement of the transport rails 27, 29. hi a second phase, as illustrated in Figure 5, the tooling support 13 is raised further, such as to raise the screen spacing unit 1 and the transport rails 27, 29, between which the at least one workpiece 11 is clamped, to the workpiece-printing position in which the spacing sheet 7 of the screen spacing unit 1 is in contact with the printing screen 47.

Referring to Figure 6, with the screen spacing unit 1 and the transport rails 27, 29, between which the at least one workpiece 11 is clamped, and the tooling support 13 in the raised, workpiece-printing position, the printing head 51 is then driven across the upper surface of the printing screen 47 in a printing stroke.

In the printing stroke, the ribs 15 of the spacing sheet 7 of the screen spacing unit 1 act normally to hold the printing screen 47 clear of the at least one workpiece 11, with the printing screen 47 being brought into contact with the at least one workpiece 11 only on the application of a force E acting through the printing head 51 at locations corresponding to the printing apertures 9 in the spacing sheet 7.

On passing over each of the printing apertures 9 in the spacing sheet 7, the printing head 51 pushes the printing screen 47 downwards through the respective printing aperture 9 and into contact with the at least one workpiece 11. In so doing, the screen apertures 49 in the printing screen 47 are filled with printing material while the printing screen 47 is in contact with the at least one workpiece 11, and, as the trailing edge of the printing head 51 is advanced, the downward force on the printing screen 47 rearward of the trailing edge decreases, causing the separation of the printing screen 47 from the at least one workpiece 11. With this separation of the printing screen 47 from the at least one workpiece 11, material deposits 50 corresponding to ones of an array of screen apertures 49 in the printing screen 47 remain on the at least one workpiece 11 such that, once the printing head 51 has passed over the respective printing aperture 9, an array of material deposits 50 is printed onto the at least one workpiece 11 corresponding to the array of screen apertures 49 in the printing screen 47. It should be appreciated that the thicknesses of the spacing sheet 7 of the screen spacing unit 1 and the printing screen 47 have been expanded so as to be out of proportion for ease of illustration. In practice, the movement of the printing head 51 in dropping into and climbing out of the printing apertures 9 in the spacing sheet 7 is relatively small, with deflection of the wiper blades 55, 57 in the main accommodating for the movement of the printing screen 47 and maintaining contact with the upper surface of the printing screen 47.

Following traversal of the printing head 51 over the printing screen 47, the above- described operation sequence is reversed such that the at least one workpiece 11, as printed, is undamped from between the spacing sheet 7 of the screen spacing unit 1 and the transport rails 27, 29, and transported downstream for further operation.

The above-described operation sequence is then repeated for another at least one workpiece 11.

Figures 8 to 10 illustrate a printing screen unit 101 for a screen printing apparatus in accordance with a second embodiment of the present invention.

The printing screen unit 101 comprises a printing screen 103 and a frame 104, in this embodiment a rectangular frame, to which the printing screen 103 is attached. In this embodiment the printing screen 103 is bonded to the frame 104 with an adhesive. It will be appreciated that other forms of attachment could be used, such as, for example, welding. The printing screen 103 comprises a screen sheet 107, which has a planar upper surface 109 over which a printing head of a screen printing apparatus is passed in a printing stroke and includes a plurality of screen apertures 110 which define the pattern of deposits of printing material to be printed, and a plurality of ribs 111 on a lower surface 113 of the screen sheet 107. In this embodiment the screen apertures 110 are grouped in repeating arrays, which arrays of screen apertures 110 define arrays of material deposits to be printed as print regions. In this embodiment the printing screen 103 is formed of a metal.

The ribs 111 comprise elongate elements, in this embodiment continuous elements, which extend orthogonally to the printing direction, which ribs 111 define printing recesses 115 therebetween and act to space the screen sheet 107 from at least one workpiece when disposed therebeneath. The height of the ribs 111 is dependent upon a number of factors which include the characteristics of the printing material, and the widths of the ribs 111 and the printing recesses 115. Typically, the ribs 111 have a thickness of about 0.15 mm.

In this embodiment the printing recesses 115 comprise parallel, elongate recesses which extend orthogonally to the printing direction, with each of the printing recesses 115 bounding a print region within which deposits of printing material are to be printed onto at least one workpiece. hi this embodiment the printing recesses 115 have a width w, that is, a dimension in the printing direction, which is such as to provide borders of about 2 mm adjacent the leading and trailing edges of the print regions. In a preferred embodiment the printing recesses 115 have a width w which is such as to provide borders of about 5 mm adjacent the leading and trailing edges of the print regions.

In this embodiment the printing recesses 115 are of uniform size and shape in relation to the print regions of material deposits to be printed. By configuring the printing recesses 115 to have substantially the same relationship to each print region to be printed, the separation characteristics of the screen sheet 107 in a printing operation are substantially identical, thereby providing for consistent print quality. The printing screen 103 of this embodiment is particularly suited to the printing of repeating arrays of material deposits onto a plurality of workpieces, such as separable panels in a single, large board, or separate workpieces, such as ball grid array (BGA) substrates or pin grid array (PGA) substrates, as the print regions are identical and the printing recesses 115 can have the same size, shape and position in relation to the print regions, thereby providing for identical separation characteristics. Where a plurality of separate workpieces are to be printed, the workpieces are supported in a carrier.

Operation of a screen printing apparatus incorporating the printing screen unit 101 will now be described hereinbelow with reference to Figures 11 and 12.

The screen printing apparatus comprises a transport mechanism which includes a pair of transport rails 121, 123 along which at least one workpiece 125 is transported to a printing zone. The transport rails 121, 123 each comprise a driven belt 127, which engages a longitudinal edge of the at least one workpiece 125 in the workpiece-fransport direction, and a support rail 129 which supports the respective driven belt 127.

The screen printing apparatus further comprises a tooling support 131 for supporting a lower surface of the at least one workpiece 125 in the workpiece-printing position.

The screen printing apparatus further comprises a printing head 133 for forcing printing material into the screen apertures 110 in the screen sheet 107 of the printing screen 103. The printing head 133 comprises an elongate main body 135, and first and second wiper blades 137, 139, here elongate elements, which are attached to a lower surface of the main body 135. The wiper blades 137, 139 are inwardly and downwardly directed and define an outlet aperture 141 through which printing material is delivered, and together with the main body 135 define a chamber 143 which contains printing material. In this embodiment the printing head 133 has a width which is such as to allow the printing head 133 to fall into the respective ones of the printing recesses 115 and provide for uniform contact of the screen sheet 107 on the upper surface of the at least one workpiece 125, though in one embodiment the movement of the printing head 133 in dropping into and climbing out of the printing recesses 115 is relatively small, and deflection of the wiper blades 137, 139 can accommodate for the movement of the screen sheet 107 and maintain contact with the upper surface 109 of the screen sheet 107.

Figures 11 and 12 illustrate the printing head 133 being driven across the upper surface 109 of the printing screen 103 in a printing stroke.

In the printing stroke, the ribs 111 on the lower surface 113 of the screen sheet 107 act normally to hold the screen sheet 107 clear of the at least one workpiece 125, with the screen sheet 107 being brought into contact with the at least one workpiece 125 only on the application of a force F acting through the printing head 133 at locations corresponding to the printing recesses 115 as defined between the ribs 111.

On passing over each of the printing recesses 115, the printing head 133 pushes the screen sheet 107 downwards into contact with the at least one workpiece 125. In so doing, the screen apertures 110 in the screen sheet 107 are filled with printing material while the screen sheet 107 is in contact with the at least one workpiece 125, and, as the trailing edge of the printing head 133 is advanced, the downward force on the screen sheet 107 rearward of the trailing edge decreases, causing the separation of the screen sheet 107 from the at least one workpiece 125. With this separation of the screen sheet 107 from the at least one workpiece 125, material deposits 145 corresponding to ones of an array of screen apertures 110 in the screen sheet 107 remain on the at least one workpiece 125, such that, once the printing head 133 has passed over the respective printing recess 115, an array of material deposits 145 is printed onto the at least one workpiece 125 corresponding to the array of screen apertures 110 in the screen sheet 107. It should be appreciated that the thickness of the screen sheet 107 and the height of the ribs 111 have been expanded so as to be out of proportion for ease of illustration. In practice, as mentioned hereinabove, the movement of the printing head 133 in dropping into and climbing out of the printing recesses 115 is relatively small, with deflection of the wiper blades 137, 139 in the main accommodating for the movement of the screen sheet 107 and maintaining contact with the upper surface 109 of the screen sheet 107.

Following traversal of the printing head 133 over the printing screen 103, the tooling support 131 is lowered to lower the at least one workpiece 125 from the printing screen 103, allowing the at least one workpiece 125, as printed, to be transported downstream for further operation.

The above-described operation sequence is then repeated for another at least one workpiece 125.

In this embodiment the lower surface 113 of the screen sheet 107 and the ribs 111 can be cleaned using a screen cleaning unit (not illustrated) which can be introduced when the tooling support 131 is lowered to the workpiece-receiving position. The ribs 111 do binder efficient cleaning, particularly when using a conventional underscreen cleaning unit which is driven in the printing direction P, but improved cleaning can be achieved by providing an underscreen cleaning unit which is driven in a direction orthogonal to the printing direction P, that is, in a direction parallel to the ribs 111. This alternative mode of cleaning would allow for the use of a shaped cleaning element, and thereby allow for improved cleaning of the lower surface 113 of the screen sheet 107.

Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

For example, the printing heads 51, 133 of the above-described embodiments could be replaced by any printing head which has a construction capable of forcing printing material into the screen apertures 49, 110 in the printing screens 47, 103, and indeed could be a conventional squeegee.

Claims

1. A screen spacing element for spacing a printing screen from at least one workpiece in a screen printing apparatus, the screen spacing element comprising a spacing sheet including a plurality of printing apertures which allow for printing of deposits of printing material onto the at least one workpiece, which printing apertures are separated by ribs which act normally to space the printing screen from the at least one workpiece and provide for the printing screen to be brought into contact with the at least one workpiece at the printing apertures in a printing operation.

2. The screen spacing element of claim 1, wherein the spacing sheet is formed of a metal.

3. The screen spacing element of claim 1 or 2, wherein the spacing sheet comprises a foil.

4. The screen spacing element of any of claims 1 to 3, wherein the printing apertures comprise elongate apertures which extend substantially orthogonally to a direction of printing.

5. The screen spacing element of claim 4, wherein the printing apertures comprise substantially rectangular apertures.

6. The screen spacing element of any of claims 1 to 5, wherein the printing apertures each have the same dimensions in a direction of printing.

7. The screen spacing element of any of claims 1 to 6, wherein the printing apertures are spaced uniformly in a direction of printing.

The screen spacing element of any of claims 1 to 7, wherein the printing screen includes a plurality of arrays of screen apertures through which arrays of deposits of printing material are in use printed as print regions onto the at least one workpiece, and the printing apertures have dimensions in a direction of printing which provides borders adjacent leading and trailing edges of respective ones of the arrays of screen apertures.

9. The screen spacing element of claim 8, wherein the borders extend at least about 2 mm.

10. The screen spacing element of claim 9, wherein the borders extend at least about 5 mm.

11. The screen spacing element of any of claims 1 to 10, wherein the ribs comprise elongate members.

12. The screen spacing element of claim 11, wherein the ribs comprise linear members.

13. The screen spacing element of any of claims 1 to 12, further comprising a frame to which the spacing sheet is attached.

14. A screen spacing unit for a screen printing apparatus, comprising: the screen spacing element of any of claims 1 to 13; and a mounting assembly to which the screen spacing element is mounted.

15. The screen spacing unit of claim 14, wherein the screen spacing element is detachably mounted to the mounting assembly.

16. A screen printing apparatus including the screen spacing unit of claim 14 or 15.

17. A screen printing apparatus, including: a printing screen including a plurality of screen apertures through which deposits of printing material are in use printed; a workpiece support for supporting at least one workpiece at a printing zone beneath the printing screen; and the screen spacing unit of claim 14 or 15 disposed between the printing screen and the workpiece support; wherein the printing screen, the workpiece support and the screen spacing unit are movable relative to one another between a workpiece-receiving configuration in which at least one workpiece can be provided to the workpiece support and a workpiece-printing configuration where the spacing sheet of the screen spacing element is disposed at a lower surface of the printing screen and the at least one workpiece is disposed at a lower surface of the spacing sheet of the screen spacing element.

18. The screen printing apparatus of claim 16 or 17, further including: a printing head for traversing the printing screen in a direction of printing to force printing material into the screen apertures in the printing screen; wherein the printing apertures each have a length in a direction orthogonal to the direction of printing which is at least about 4 mm greater than that of a screen contact surface of the printing head.

19. The screen printing apparatus of claim 18, wherein the printing apertures each have a length in a direction orthogonal to the direction of printing which is at least about 10 mm greater than that of the screen contact surface of the printing head.

20. A method of screen printing deposits of printing material onto at least one workpiece, comprising the steps of: providing a printing screen including a plurality of screen apertures through which deposits of printing material are printed onto at least one workpiece; disposing a screen spacing element adjacent a lower surface of the printing screen; disposing at least one workpiece adjacent a lower surface of the screen spacing element; and traversing a printing head over an upper surface of the printing screen to force printing material into the screen apertures in the printing screen and print deposits of printing material onto the at least one workpiece; whereby the screen spacing element acts normally to space the printing screen from the at least one workpiece and provides for the printing screen to be brought into contact with the at least one workpiece at the printing apertures in the screen spacing element on traversal of the printing head over the printing screen.

21. A printing screen for a screen printing apparatus, the printing screen comprising a screen sheet including a plurality of arrays of screen apertures through which arrays of deposits of printing material are in use printed as print regions onto at least one workpiece, and a plurality of ribs on a lower surface of the screen sheet, the ribs being spaced in a direction of printing such as to define printing recesses between adjacent ones thereof and bound respective ones of the arrays of screen apertures.

22. The printing screen of claim 21 , wherein the screen sheet is formed of a metal.

23. The printing screen of claim 21 or 22, wherein the screen sheet comprises a foil.

24. The printing screen of any of claims 21 to 23, wherein the ribs are formed of a metal.

25. The printing screen of any of claims 21 to 24, wherein the screen sheet and the ribs are foπned from a single sheet.

26. The printing screen of any of claims 21 to 25, wherein the printing recesses comprise elongate recesses which extend substantially orthogonally to a direction of printing.

27. The printing screen of claim 26, wherein the printing recesses comprise substantially rectangular recesses.

28. The printing screen of any of claims 21 to 27, wherein the printing recesses each have the same dimensions in a direction of printing.

29. The printing screen of any of claims 21 to 28, wherein the printing recesses are spaced uniformly in a direction of printing.

30. The printing screen of any of claims 21 to 29, wherein the printing recesses have a dimension in a direction of printing which provides borders, having no screen apertures therein, adjacent the ribs.

31. The printing screen of claim 30, wherein the borders extend at least about 2 mm.

32. The printing screen of claim 31, wherein the borders extend at least about 5 mm.

33. The printing screen of any of claims 21 to 32, wherein the ribs comprise elongate members.

34. The printing screen of claim 33, wherein the ribs comprise linear members.

35. The printing screen of claim 33 or 34, wherein the ribs comprise continuous members.

36. The printing screen of any of claims 21 to 35, wherein the ribs act normally to space the screen sheet from the at least one workpiece and provide for the screen sheet to be brought into contact with the at least one workpiece at the printing recesses in a screen printing operation.

37. A printing screen unit, comprising: the printing screen of any of claims 21 to 36; and a frame to which the printing screen is attached.

38. A screen printing apparatus including the printing screen unit of claim 37.