Screen printing is widely used for depositing solder paste onto contact areas of electronic circuit boards prior to the attachment of surface mount components. Solder paste is a viscous, thixotropic material and has to be continually worked in order to be sufficiently thinned to fill the apertures of a printing screen and provide a good print.

Conventionally, the working of solder paste has been achieved by discarding the first few prints in a run, by which time the paste would have been thinned sufficiently, or by adding a paste knead cycle which works the paste on a blank area of the printing screen prior to starting printing.

Effective printing relies on the adhesion of the printing material to the workpiece to be greater than to the sides of the apertures in the printing screen so that, when the screen is separated from the workpiece, the printing material remains adhered to the workpiece and not to the screen. This requires that the printing material has to be forced to the bottom of each aperture and into intimate contact with the workpiece being printed. To date, this has limited the minimum dimension of any aperture to about 1.5 times the thickness of the printing screen.

As the size of electronic components and the contact area between those components and the circuit boards has been reduced, it has been necessary to use thinner printing screens, thereby reducing the amount of solder paste that can be deposited and increasing the problem of poor electrical connection between the components and the circuit boards.

Moreover, where solder paste is not completely transferred to a circuit board and remains in the apertures of the printing screen, not only does this result in a further

reduction of paste deposited, but also this paste can harden in the apertures and cause blockages which will require the screen to be cleaned before further use.

It is thus an aim of the present invention to provide a screen printing apparatus and method which at least partially alleviates the problems associated with the prior art.

Accordingly, the present invention provides a vibratable support for supporting a workpiece in a screen printing apparatus, the support comprising: a mount for mounting to a screen printing apparatus ; and at least one transducer disposed on the mount, the at least one transducer including a surface for supporting a workpiece and vibrating the same thereabove.

Preferably, the mount includes a magnet for magnetically mounting the support to the screen printing apparatus.

Preferably, the mount includes at least one collet assembly for clamping each respective transducer thereto.

Preferably, the at least one transducer includes a piezoelectric element for ultrasonically vibrating the workpiece.

More preferably, the at least one transducer comprises first and second end blocks and a piezoelectric element sandwiched therebetween for ultrasonically vibrating the workpiece, one of the end blocks being attached to the mount and the other of the end blocks being for supporting the workpiece.

In one embodiment the support is a support pillar and includes one transducer.

In another embodiment the support includes a plurality of transducers.

The present invention also provides a vibration system for a screen printing apparatus, the vibration system comprising: at least one of the above-described vibratable supports ;

and at least one power supply for actuating the at least one transducer of the at least one vibratable support.

Preferably, the vibration system comprises a plurality of vibratable supports.

In one embodiment the vibration system comprises a plurality of power supplies for separately actuating ones or each of the transducers of the vibratable supports.

The present invention further provides a screen printing apparatus for printing a viscous material onto a workpiece through apertures in a printing screen, the screen printing apparatus comprising: a printing zone in which a workpiece is supported during printing ; and the above-described vibration system, wherein the at least one vibratable support is disposed in the printing zone beneath the workpiece to support and vibrate the same during printing.

Preferably, the apparatus further comprises: at least one non-vibratable support disposed in the printing zone beneath the workpiece to support the same during printing.

Preferably, the apparatus comprises a plurality of vibratable supports at spaced positions in the printing zone.

The present invention yet further provides a method of screen printing viscous material onto a workpiece, comprising the steps of : providing a workpiece beneath a printing screen including a plurality of apertures through which viscous material is to be printed ; locally vibrating regions of the workpiece onto which material is to be deposited ; and printing viscous material onto the workpiece.

Preferably, the workpiece is vibrated only during the printing operation.

With this configuration, the viscous material is vibrated locally above the apertures in the printing screen. This vibration causes the local thinning of the viscous material in those regions above the apertures in the printing screen, which thinning leads to improved filling of the apertures and contact with the workpiece. This vibration also

promotes the filling of the apertures in the printing screen directly by the physical action thereof. In this way, the apertures are completely filled with viscous material and those full volumes are more reliably transferred to the workpiece on removal of the printing screen. This enables thicker printing screens to be used for the same sized apertures, further increasing the amount of viscous material deposited and thereby reducing the incidence of poor connections. In addition, as the viscous material has an improved adhesion to the workpiece, little or no material remains in the apertures, reducing the incidence of blocked apertures and the frequency at which the printing screens have to be cleaned.

A preferred embodiment 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 a vibration system in accordance with a preferred embodiment of the present invention; Figures 2 to 4 illustrate a vibratable support pillar of the vibration system of Figure 1; and Figure 5 illustrates a screen printing apparatus incorporating the vibration system of Figure 1.

The vibration system comprises a plurality of vibratable support pillars 2, a junction box 4 which is electrically connected by respective cables 6 to the support pillars 2, and an ultrasonic power supply 8 which is electrically connected to the junction box 4 by a cable 10.

The support pillars 2 each comprise a transducer 12, in this embodiment an ultrasonic transducer, which on actuation is vibratable, in this embodiment vertically in the direction of arrow V, and a mount 14 which is attached to the transducer 12 and configured to mount the same on a screen printing apparatus as will be described in more detail hereinbelow.

The transducer 12 comprises an assembly comprising first and second end blocks 16, 18, in this embodiment metal, preferably aluminium, rods, and first and second piezoelectric elements 20,22, in this embodiment piezoceramic elements, which are sandwiched between the end blocks 16,18. In this embodiment the transducer assembly is held together by a screw 24. The first and second piezoelectric elements 20,22 each include an electrical contact 26,28 for electrical connection to a respective one of the cables 6.

The mount 14 comprises a main body 30, which includes a base section 32 which houses a magnet 34 and includes a threaded outer surface, and a collet 36 in which the second, lower end block 18 of the transducer 12 is located, and a collet ring 38 which includes a threaded inner surface and is threadedly engaged with the outer threaded surface on the base section 32 of the mount 14. With this configuration, the transducer 12 can be clamped to the mount 14 by tightening the collet ring 38, and the magnet 34 provides the means of mounting the support pillar 2 to the screen printing apparatus. In this embodiment the main body 30 and the collet ring 38 of the mount 14 are fabricated from a plastics material, preferably polycarbonate.

Figure 5 illustrates a screen printing apparatus incorporating the vibration system.

The screen printing apparatus comprises a pair of parallel transfer rails 40,42 along which workpieces 44, in this embodiment printed circuit boards, are successively transferred to a screen printing zone, and a table 46 disposed beneath the transfer rails 40,42 on which the vibratable support pillars 2 and a plurality of conventional support pillars 48 are located between the transfer rails 40,42 in the screen printing zone. The vibratable support pillars 2 are interspersed between the conventional support pillars 48, and are preferably located at locations in registration with the regions of the workpieces 44 on which material is to be deposited.

In use, a workpiece 44 is first transferred to the screen printing zone, in which position the workpiece 44 is supported by the support pillars 2,48 during printing. The screen printing apparatus is then operated to print deposits of material onto the workpiece 44, during which operation the transducers 12 of the vibratable support pillars 2 are

actuated. In a preferred embodiment the transducers 12 are energized only during the print stroke in order to avoid the deposited material from collapsing and spreading over the workpiece 44.

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

For example, although in the above-described embodiment each of the vibratable support pillars 2 is driven by a single ultrasonic power supply 8, ones or each of the vibratable support pillars 2 could be driven by separate ultrasonic power supplies 8.

In another embodiment the screen printing apparatus could incorporate a dedicated tooling support member in which the transducers 12 are disposed.

In a further embodiment the transducers 12 could be permanently attached to the respective mounts 14, for example, by an adhesive.

Also, although in the above-described embodiment ultrasonic frequencies are utilized, lower frequencies could be employed. In one modification, the transducers 12 could be provided by electro-magnetic elements.