This invention relates to screen printing and is particularly concerned with 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 substrate.

The 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. Various methods of applying tension to the stencil are known.

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 shown 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 in this arrangement 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.

In a further method, described in our co-pending British Patent Application the stencil is firmly gripped at two opposed ends in a rigid frame, and is then tensioned by cam or other means which move the body of the stencil into a higher or a lower plane whilst its ends remain in the original plane.

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 a stencil so formed as to avoid or minimise strain lines which would otherwise be present when the stencil is placed in tension, and/or to provide improvements generally.

According to the invention there is provided a stencil foil for mounting in tension on a mount therefor, the foil being formed with one or more regions of weakness located in the region of at least one edge of the foil.

In a preferred embodiment, the region of weakness is spaced inwardly from an edge of the foil. The region extends generally parallel to said edge of the foil. The region of weakness is formed by an array of perforations. Alternatively, the region of weakness may be provided by removing some of the material of the foil from a surface thereof, in order to reduce the cross-sectional thickness of the foil in the weakened region. Such thinning of the foil may be provided instead of or in addition the perforations mentioned above.

End regions of the foil may be clamped to the mounting therefore, and the weakened regions of the foil provide strain relief so as to control the tensioning in the foil.

In an embodiment the regions of weakness are located in portions of the foil which are spaced from the portion thereof which is to receive the stencil pattern. Generally, the regions of weakness will be at the periphery of the body portion of the foil which is to receive the stencil pattern,

but for certain applications, there might be a benefit to be obtained by a modified disposition of the zone or zones of weakness, without compromising the function of a known stencil pattern.

Generally, the regions of weakness are disposed so as to cross (for example of an angle of between 45 and 135 degrees) the general direction of the tension set up in the foil during use, between its opposed edges held in the mount therefor. Since the stencil pattern has an effect on the tendency for strain lines to be produced in the foil, it may be beneficial for the region of weakness to be disposed or constructed so as to be related to characteristics such as the disposition, size or shape of the stencil pattern, or indeed with respect to its effect on the tension pattern produced in the foil during use.

The above and other features of the invention will become clear from the following description which is given by way of example with reference to the accompanying drawings in which:

Figure 1 is a perspective view of the stencil foil assembled with respect to the frame in its tensioned state, parts being removed for clarity;

Figure 2 is a cross-section through an assembled frame and stencil member in its untensioned condition;

Figure 3 is a cross-section similar to Figure 2 but in its tensioned condition;

Figure 4 is a detail plan view of part of the assembly showing particularly details of one strain relief system according to the invention.

As seen in the drawings a stencil/frame assembly in accordance with the invention comprises a frame 2 having front and rear frame members 4 and 6 respectively and left and right-hand frame members 8, 10.

As seen clearly in Figure 3 each of the front and rear frame members 4 and 6 comprises a main elongate base member 12 which is attached at its ends to the side members 8, 10. Locating pins 14 project upwardly from the members 12 seen in

Figure 2 and a clamping bar 16 is formed with elongate holes 18 which align with the pins 14. Clamping screws 19 pass through clearance holes 21 in the bars 16 into threaded holes in the base members 12. The clearance holes in the clamping bars are conveniently formed as key-hole slots for rapid release and replacement of the bars.

Elongate bars 17, having cam shaped profile sections, extend the length of the base member 12 and are pivotally mounted by pins 20 which are rotatable about the axis X in holes formed in the side members 8, 10. Handles 22 are fixed to and project from the end portions of the cam shaped bars and recesses 24 are formed in the ends of the base members 12 to accommodate those handles when they are in operative tensioning position.

The stencil foil F is formed with location holes 26 towards its front and rear edges and the centre distances of these holes are coincident with the centre distances of the locating pins 14 in the base member 12. Further holes 28 are formed in the foil to coincide with the clamping screws 19.

To mount the stencil foil F on the frame 2 the clamping screws 19 are first unscrewed by one or two turns after which the clamping bars 16 are moved to the left as seen in Figure 4 and then lifted off their respective base members 12, the heads of the screws 19 passing through the large diameter of the key-hole slots.

At this time the cams 17 are in their generally horizontal position as seen clearly in Figure 2.

The stencil foil F is then positioned on the frame by locating the holes 26 over the pins 14 on both the forward and rearward base members 12 after which the clamping bars are replaced in position and the screws 19 tightened. The foil is thus clamped between the front and rear frames 4 and 6.

In order to place the foil in tension the handles 22 are pushed downwardly to their substantially horizontal position, thereby rocking the cam bars 17 about the axes of their pivot pins 20 to the position shown in Figure 3. This effectively

raises the central part of the foil above its original plane, stretching it to tension the foil and hence rigidify it to a required degree to ensure accuracy of location of the apertures above the area of the printed circuit board to be printed.

The ready location and clamping of the stencil foil means that only the foils themselves need to be stored, rather than the complete assembly of foils and frames generally, thus saving storage space and reducing the risk of damage in moving the assemblies to and from their storage facility.

As seen clearly in Figures 1 and 4 regions of the stencil adjacent its ends are formed with rows of small diameter holes 40. This has the effect of relieving the strain imposed on the main area of the foil. It is found that by providing these strain relief holes, the tendency of strain lines appearing on the foil is eliminated. Alternatively, similar strain relief may be effected by reducing the cross-sectional area of the foil in that vicinity, eg by etching away part of the metal.

This same technique is effective however the foil is tensioned. For example, when a pneumatically operated frame tensioning arrangement is used the array of holes is provided adjacent the endmost portions of the foil to alleviate the formation of strain lines on the tensioned foil.

The holes or other reduced cross-section arrangement may be formed at one or both end portions of the stencil foil. Further, there may be a number of rows of holes or only one row, and the position, shape, and size of the holes may be varied according to the degree of tension required, or the gauge thickness of the foil being used.

Furthermore, although it is preferable that the foil is ^' made of a suitable metal, eg a stainless steel alloy, the invention may be applied to any stencils, no matter which material is used.

It is found that the use of a stencil foil equipped with the strain relief system as described enables accurate

undistorted location of the printing media on to the substrate. This is particularly advantageous in the printing of solder paste on to printed circuit boards.