Printing – Stenciling – Stencils
Patent
1995-06-28
1997-03-04
Burr, Edgar S.
Printing
Stenciling
Stencils
101127, B05C 1708
Patent
active
056069120
DESCRIPTION:
BRIEF SUMMARY
This invention relates to screen printing and is particularly concerned with a mounting arrangement for screen stencils used in such a process.
Screen printing is well-known whereby ink, paint or other media is passed through apertures formed in a stencil onto the surface of a chosen material.
This same process is used in the preparation of printed circuit boards where an amount of solder paste is deposited on the stencil and is then squeegeed through the apertures to the surface of a printed circuit board positioned therebelow. The stencil is usually made of thin stainless steel or other suitable metal foil.
Because of the demands for extreme accuracy in solder printing of circuit boards, the stencil is precisely positioned within a frame under tension and the frame is mounted in position on parts of the printing machine.
In one current practice a pre-tensioned mesh is first bonded to a rigid framework, the perimeter of the stencil foil is bonded to the mesh and then the mesh is removed from the printing area. Thus the tension present in the mesh is transmitted to the stencil foil. This system is complicated, time consuming and therefore costly. Furthermore the stencil foil and the frame become a semi-permanent assembly which demands substantial storage space, each stencil requiring a separate frame.
In an attempt to overcome that problem an arrangement has been devised and is described in PCT patent specification WO 92/08616 whereby edgemost portions of the stencil foil are formed with slots which are located over studs on the frame. The frame is then placed in tension pneumatically to rigidify the foil to some degree. However it is found that this arrangement is expensive to manufacture, requires connection to a source of pneumatic power and furthermore, because the pneumatic device can only act on the lateral ends of the frame member, the tension is unevenly distributed across the foil giving rise to so-called "strain lines" with consequent distortion of the apertures and inaccurate deposition of solder paste onto the printed circuit boards.
An object of the invention is to overcome or mitigate one or more of the shortcomings of previous systems and devices, and/or to provide improvements generally.
According to the invention there is provided a method of mounting a stencil foil under tension on a mounting therefor, wherein the foil is tensioned by mechanical means located to act on the foil at least at one end thereof. The invention also provides a method of screen printing comprising the step of mounting a stencil foil under tension, as defined above.
The mechanical means may act directly on the foil. The mechanical means may have a progressive action, and/or an action produced by angular movement of a mechanical member. Preferably, the mechanical means is in the form of cam means. The mechanical means may be disposed so as to act on the foil lengthwise in a region extending transverse to the direction in which the foil is tensioned. The foil may lie in one plane in its untensioned state, and in another plane in its tensioned state.
Opposite ends of the foil may be clamped for tensioning purposes. In the region of the ends, the foil may be weakened to provide strain relief in order to improve the uniformity of tensioning of the foil. Preferably, such weakening is effected by the provision of one or more arrays of perforations in the foil.
In an embodiment the tensioning means for the foil comprises a tensioning member located inwardly of the mounting for the foil, that is to say between opposite sides of the mounting, and positioned for direct engagement with the foil. Actuation of the tensioning member, for example by cam means, moves the tensioning member to set up or increase tension in the foil.
The tensioning member has an elongate surface for engagement with the foil. The elongate surface serves to apply a loading to the foil uniformly along the length of the tensioning member, thereby avoiding the regions of varying tension inherent in systems in which tension is applied at specific locations,
REFERENCES:
patent: 2925774 (1960-02-01), Scheeler
patent: 3971312 (1976-07-01), Carli
patent: 5044306 (1991-09-01), Erdmann
Burr Edgar S.
Colilla Daniel J.
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