GB2448352A

GB2448352A - Wiper blade for a screen printing machine

Info

Publication number: GB2448352A
Application number: GB0707065A
Authority: GB
Inventors: Boon Hiang Tang
Original assignee: DEK International GmbH
Current assignee: ASM Assembly Systems Switzerland GmbH
Priority date: 2007-04-12
Filing date: 2007-04-12
Publication date: 2008-10-15
Also published as: GB0707065D0; GB2448352A8; WO2008125297A1

Abstract

A wiper blade for use in a screen printing machine which comprises a blade element (25), typically of stainless steel, and a tetrahedral amorphous carbon (ta-C) coating (27) on the blade element (25).

Description

WIPER BLADE FOR A SCREEN PRINTING MACHINE

The present invention relates to a wiper blade, often alternatively referred to as a squeegee, for a screen printing machine, in particular for the screen printing of printing materials, typically viscous materials, such as adhesives, fluxes and solder pastes, through printing screens or stencils onto workpieces, such as printed circuit boards.

Screen printing is widely used in the electronics industry for depositing printing materials, such as adhesives, fluxes and solder pastes, onto printed circuit boards, and many printing materials, in particular thixotropic solder pastes, require constant working to maintain the viscosity at a sufficiently-low level as to allow for printing, where this process is known as shear thining. S...

Figure 1 illustrates the operation of a traditional wiper blade or squeegee 3, where the wiper blade 3 is angled forwards over a printing screen 5 to the direction of movement, such that the forward movement of the wiper blade S..

* 3 causes both a downward pressure on the printing material so as to force the printing material into apertures 7 in the printing screen 5 and cause the S...

printing material to move up the wiper blade 3 and fall onto the printing screen 5 ahead of the front of the printing material, resulting in continuous circulation of the printing material which shear thins the printing material.

Figure 2 illustrates an enclosed screen printing head 11, as disclosed in WO-A-1998/016387, which comprises a pair of wiper blades 13 which are disposed in opposed relation over a printing screen 15 and the trailing wiper blade 13 is angled backwards to the direction of movement, such that the forward movement of the printing head 11 causes the trailing wiper blade 13 to lift the printing material off of the printing screen 15, and the pressure as applied to the printing material causes the printing material both to circulate within the printing head 11, causing shear thinning, and force the printing material into apertures 17 in the printing screen 15. t

Problems can occur with either of these technologies if the printing material adheres to the wiper blade 3, 13, as this adhesion inhibits the movement of the printing material over the wiper blade 3, 13 and reduces the required circulation of the printing material. Within the electronics industry, these problems have been increased by the recent move towards printing lead-free solder pastes.

A particular problem associated with such adhesion of the printing material, known as dog-earring, is illustrated in Figures 3 and 4 in relation to the traditional wiper blade 3, where the adhering printing material creeps up the back edge of the wiper blade 3 and drags printing material out of the aperture 7 in the printing screen 5, resulting in the printing of a defective deposit D. S...

Another problem associated with such adhesion of the printing material is to increase the friction between the wiper blade 3, 13 and the printing screen 5, 15, which leads to increased wear of both the wiper blade 3, 13 and the S..

printing screen 5, 15 and hence shorter operational lifetimes. US.. *5.

Various wiper blade configurations and structures have been developed with a view to providing improved performance.

WO-A-2000/078549 discloses a wiper blade for use with the above-described enclosed screen printing head, which includes a strip, typically of a plastics material, which is fixed along the free edge to the upper, non-screen contacting surface of the main, blade element. This configuration has been found to be effective in maintaining the shearing of the printing material, and thus reducing printing defects, in particular dog-earring.

US-A-5947021 discloses another wiper blade structure in which a metal blade element is coated with a titanium nitride coating. This coating provides a smooth, very hard, low-friction coating, which apparently provides for lower defect rates and extends the lifetime of the wiper blade.

Another wiper blade structure, which comprises a metal blade element having an infused polymer lubricant tip, is available from Transition Automation, Inc. (North Billerica, MA, USA). This lubricant tip apparently provides for lower defect rates and extends the lifetime of the wiper blade.

The present inventor has surprisingly determined that the performance of a wiper blade can be markedly improved by utilizing a tetrahedral amorphous carbon (ta-C) coating.

Accordingly, the present invention provides a wiper blade for use in a screen printing machine which comprises a blade element and a tetrahedral amorphous carbon (ta-C) coating on the blade element. *S.. *..s

In one embodiment the blade element comprises an elongate element. S... S..

* In one embodiment the blade element comprises a sheet element. *..S * S.

In one embodiment the blade element is formed of a metal, preferably stainless steel.

In one embodiment the blade element has a square free edge.

In one embodiment the blade element has a thickness of about 250 pm.

In one embodiment the ta-C coating comprises a PVD coating.

In one embodiment the ta-C coating has a thickness of from about 1 pm to about 1.5 pm.

In one embodiment the ta-C coating has a thickness of about 1.5 pm.

In one embodiment the ta-C coating has a thickness of about 1 pm.

The present invention also extends to a screen printing head which comprises at least one wiper blade as described hereinabove.

The wiper blade of the present invention provides for a significant reduction in the adhesion of printing material to the wiper blade, which results in improved circulation of the printing material and consequently an improved quality of printing. By reducing the adhesion of printing material to the wiper blade, lower print pressures can be used which reduces the bleeding of material between the printing screen and the workpiece as disposed therebeneath, which would result in contamination of the printing screen and bridging between printed deposits, and also allows faster printing speeds which reduces the printing time. *S*.

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A preferred embodiment of the present invention will now be described *S*S.

hereinbelow by way of example only with reference to the accompanying S..

* drawings, in which: *S.. S...

Figure 1 illustrates the operation of a traditional wiper blade or squeegee; Figure 2 illustrates the operation of an enclosed screen printing head; Figures 3 and 4 illustrate the dog-earring effect which results from adhesion of printing material to a wiper blade; Figure 5 illustrates a perspective view of a wiper blade in accordance with a preferred embodiment of the present invention; and Figure 6 illustrates in enlarged scale a vertical sectional view (along section I-I) through the wiper blade of Figure 5.

The wiper blade 23 comprises a blade element 25, in this embodiment an elongate element formed from a sheet material, and a coating 27 of tetrahedral amorphous carbon (ta-C) on the blade element 25.

The blade element 25 is formed from stainless steel sheet, in this embodiment a type 302, full-hard stainless steel.

In preferred embodiments the blade element 25 is cut by laser or chemical etching, but it will be understood that any suitable means of cutting could be employed. Following cutting, the blade element 25 is mechanically processed, in this embodiment by one or both of grinding and lapping, to provide a smooth finish with square edges, and subsequently de-burred and polished, in this embodiment using a soft abrasive sponge, here Norton 3X, 320 ultrafine grit (Saint-Gobain Abrasives, Inc., Worcester, MA, USA), with a light pressure and a forwards-and-backwards movement. *S.. * * * ***

In this embodiment the blade element 25 has a thickness of about 250 pm, but can have a thickness of up to about 3 mm. *

The ta-C coating 27 is formed using a physical vapour deposition (PVD) *S*.

apparatus, here comprising a carbon plasma generator within a vacuum chamber, in the manner as described in WO-A-1996/026531.

In this embodiment the blade element 25, following cleaning, here first in a degreasing bath, then by rinsing with de-lonized water and drying in a hot air oven, is located in a vacuum chamber at a pressure of about lxlO6 Torr, and the plasma generator, here comprising a graphite target, is actuated to provide a plasma of the graphite target, such that carbon ions coat the blade element 25 as ta-C. In this embodiment the blade element 25 is biased to a negative bias, and the coating interval controlled to provide the required coating thickness for the ta-C coating 27.

In this embodiment the ta-C coating 27 has a thickness of about 1 pm, and in preferred embodiments has a thickness of from about 1 pm to about 1.5 pm.

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. S. * S S ** S...

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Claims

1. A wiper blade for use in a screen printing machine which comprises a blade element and a tetrahedral amorphous carbon (ta-C) coating on the blade element.

2. The wiper blade of claim 1, wherein the blade element comprises an elongate element.

3. The wiper blade of claim 2, wherein the blade element comprises a sheet element.

4. The wiper blade of any of claims 1 to 3, wherein the blade element is formed of a metal. S. * . * S..

5. The wiper blade of claim 4, wherein the blade element is formed of stainless steel. *S*.

6. The wiper blade of any of claims 1 to 5, wherein the blade element has a square free edge. *SS. * S. * .

7. The wiper blade of any of claims 1 to 6, wherein the blade element has a thickness of about 250 pm.

8. The wiper blade of any of claims 1 to 7, wherein the ta-C coating comprises a PVD coating.

9. The wiper blade of any of claims 1 to 8, wherein the ta-C coating has a thickness of from about 1 pm to about 1.5 pm.

10. The wiper blade of claim 9, wherein the ta-C coating has a thickness of about 1.5 pm.

11. The wiper blade of claim 9, wherein the ta-C coating has a thickness of about 1 pm.

12. A screen printing head comprising at least one wiper blade of claims 1 to 11.

13. A wiper blade substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings. S. * S * S.. *S*.

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