GB2291624A

GB2291624A - Improved stencil or mask

Info

Publication number: GB2291624A
Application number: GB9414970A
Authority: GB
Inventors: David Godfrey Williams
Original assignee: Micro Metallic Ltd
Current assignee: Micromet Ltd
Priority date: 1994-07-26
Filing date: 1994-07-26
Publication date: 1996-01-31
Also published as: GB9414970D0

Abstract

An improved stencil or mask of sheet material, for mounting on a tensioning support frame and for the application of solder paste, includes a plurality of closely spaced apertures (51) in one or more opposite side regions, and a transverse central axis (x-x) located between the side region. The distal or outermost ends (51) of the apertures against which, in use, interconnection means of the support frame are to abut, are spaced from said transverse central axis at other than a constant distance so as to avoid or minimise any irregularity or distortion in the stencil in the tensioned condition thereof. The distal ends may follow a curved line (8) or spaced straight lines. <IMAGE>

Description

IMPROVED STENCIL OR MASK The present invention relates to an improved stencil or mask for applying solder paste to circuit boards.

In British Patent GB 2264460 in the name of Micro Metallic Limited there is disclosed a stencil or mask and support frame therefor which enables the stencil to be readily located and tensioned with the interconnection means of the stencil and support frame being located out of the plane in which the normally lowermost surface of the stencil lies in its position of use. Furthermore, the stencil may be readily removed from the support frame and replaced by a different stencil. The stencil as illustrated has the unique feature of interconnection means in opposite side regions of the stencil provided by a plurality of elongate parallel slots and the outermost ends of the slots in such a stencil have, hitherto, been at constant spacing from the outermost edges.

It has now been discovered that situations exist where it is necessary to respond to changes in the quality of the material from which the stencil is produced or to respond to changes brought about in the production of the stencil and which become apparent in the tensioned condition of the stencil.

According to the present invention an improved stencil or mask is provided having a plurality of closely spaced apertures in one or more opposite side regions with the distal or remote ends of the slots against which interconnection means of the support frame are to abut in use being spaced from a transverse central axis extending between the side regions at other than a constant distance so as to avoid or minimise any irregularity or distortion in the stencil in the tensioned condition thereof.

Whilst reference is made herein to slots, any suitably shaped aperture is intended to be covered.

Also, the slots will normally although not essentially have parallel, long sides.

The distal ends of the apertures e.g. slots i.e.

the furthest apart ends of the two rows of slots in the opposite side regions are preferably located on a curved line extending across the stencil although may have other dispositions e.g. may be located on straight lines inclined relative to each other or may be located on an undulating line e.g. sinusoidal or other like disposition or only the ends of groups of slots in a side region may be offset, possibly in straight line disposition transversely across the stencil.

If one or each opposite side edge of the stencil is rectilinear i.e. is on a straight line transverse to the transverse central transverse axis, then the ends of the slots in one or each side region will not be regularly spaced therefrom but rather will be spaced at different distances from said edge. In the case where the slot ends lie on an arc or other curve, they will normally be closer to said side edge in the central region than at the outermost lateral side regions. If, however, the opposite edges of the side regions are other than rectilinear e.g. if curved, then the distal ends of the slots may be regularly spaced from the opposite edges. Thus it will be appreciated that it is the variation in the spacings between distal ends of corresponding preferably aligned slots or apertures which is of significance.

It will be appreciated that the present invention enables the stencil manufacturer to respond to the characteristics of a particular material from which a stencil is made and make such adjustments as necessary to ensure optimum performance and an as flat as possible stencil in the tensioned position.

If the ends of the apertures lie on a curve, then the height of the crown should preferably be Cos 450 x the curvature of the crown to calculate the necessary displacement.

The invention will be described further, by way of example, with reference to the accompanying schematic plan view of a stencil.

The drawing is a schematic illustration, not to scale, of a stencil 1 of rectangular sheet material such as of metal, having opposite side edges 2, 3 and a plurality of identical elongate parallel slots 4, 5 in opposite side regions 6, 7. A central larger aligning slot (not shown) may be provided in each side region.

Only a few of the slots are illustrated in the side region but such are to be understood to comprise a plurality of very closely spaced identical elongate parallel slots over the whole of the length of each side of e.g. a width of 1.5 millimetre with a spacing of 1.5 millimetre and a length as desired. The slots 4, 5 in the flat condition of the stencil in each region 6,7 will preferably be parallely disposed relative to each other and to other regions and each slot in region 6 will preferably be aligned with a corresponding slot in region 7.

The slots 4, 5 of each side region have distal or remote ends 4', 5', which are located on other than a straight line extending over the whole of its length at other than a constant distance from a central transverse axis X-X. In the embodiment illustrated the ends 4',5', are each located on an identical curved line 8 such that the distal ends 4',5' of the outermost slots 4", 5'' are spaced at a greater distance a from the edges 2, 3 respectively than in the central region where the spacing is k from the edges 2, 3. The difference (a - b)may be in the region of 1 to 1.5 mm for example. Whilst the innermost proximate ends of slots 4, 5 are illustrated as located on a corresponding curve, this is not absolutely essential.

It will be appreciated that various modifications may be made without departing from the scope of the invention. For example, instead of straight parallel edges 2, 3 such may have, for example, an outwardly curved outline with the ends 4', 5' being correspondingly disposed and the distance a, b from said ends to the edges 2, 3, would be constant. Alternatively, the ends 4', 5' of slots 4, 5 may be located in groups in each region along straight lines located at different distances from the opposite edges 2, 3 in contrast to prior stencils wherein distal ends have all been located along a single straight line extending parallel to each respective edge 2, 3 in a rectangular stencil. Whilst the slots have been described as identical (allowing for their arrangement on a curve) it is also envisaged possible for slots of different lengths to be provided e.g. the inner adjacent ends of the to rows of slots may each be located on a straight line.

Claims

1. An improved stencil or mask of sheet material for mounting on a tensioning support frame and for the application of solder paste, including a plurality of closely spaced apertures in one or more opposite side regions, and a transverse central axis located between the side regions; with the distal or outermost ends of the apertures against which, in use, interconnection means of the support frame are to abut being spaced from said transverse central axis at other than a constant distance so as to avoid or minimise any irregularity or distortion in the stencil in the tensioned condition thereof.

2. A stencil as claimed in claim 1, in which the apertures are slots disposed in two rows and the furthest apart ends of the slots in the two rows of slots are located on a curved line extending across the stencil or located on straight lines inclined relative to each other or stepped relative to each other, or are located on an undulating line or only the ends of groups of slots in a side region are offset.

3. A stencil as claimed in claim 1, in which one or each opposite side edge of the stencil is rectilinear i.e. is on a straight line parallel to the transverse central transverse axis, and the ends of the slots in one or each side region are not regularly spaced from said edge but rather will be spaced at different distances from said edge.

4. A stencil as claimed in claim 1, in which the slot ends lie on an arc or other curve, and are normally closer to said side edge in the central region than at the outermost lateral side regions.

5. A stencil as claimed in claim 1, in which where the opposite edges of the side regions are other than rectilinear (e.g. are curved), the distal ends of the slots are regularly spaced from the opposite edges.

6. A stencil as claimed in any of claims 1 to 5, in which the variation in the spacings between distal ends of apertures in opposite side regions of the stencil is selected to enable even tensioning of the stencil.

7. A stencil as claimed in any of claims 1 to 6, in which, where the ends of the apertures lie on a curve, then the necessary displacement for the height of the crown is calculated from Cos 450 x the curvature of the crown.

8. A stencil substantially as herein described with reference to the accompanying drawings.