WO1999013695A1

WO1999013695A1 - Improved stencil or mask for printing fluid material

Info

Publication number: WO1999013695A1
Application number: PCT/GB1998/002698
Authority: WO
Inventors: David Godfrey Williams
Original assignee: Micro Metallic Limited
Priority date: 1997-09-11
Filing date: 1998-09-08
Publication date: 1999-03-18
Also published as: AU8992998A

Abstract

A stencil for the deposition of fluid material comprises a sheet of material having in at least two opposite end regions, means engageable by tensioning means and having at least two apertured regions with the apertures thereof provided for the deposition of fluid therethrough, characterised by the feature that said stencil comprises at least said two separable parts (6, 7) with each part (6, 7) including an apertured region for fluid material deposition and with said parts (6, 7) being detachably interconnectable.

Description

IMPROVED STENCIL OR MASK FOR PRINTING FLUID MATERIAL

The present invention relates to an improved mask or stencil or foil for the deposition of a fluid material such as solder paste or adhesive or other fluid material and, for simplicity, reference will be made herein to such item simply as a "stencil".

In GB Patent 2264460 and Patent 2276589 and International Patent Application PCT/GB96/01722 there are described stencils of the kind with which the present invention is concerned and wherein the stencils are flat and rectangular and are tensionable m a rectangular frame which engages with opposite edges of the stencil and applies force thereto prior to the location of the frame and stencil in a solder paste or other fluid material printing machine.

It is commonplace to use a single, one-piece metal sheet stencil having different fluid-applying arrangements of holes in different regions of the stencil to enable the application of different fluid arrangements to different substrates, such as solder paste to printed circuit boards (PCB's) disposable therebeneath and wnerem the deposition is caused by the movement of a squeegee over the upper surface of the stencil to wipe the fluid paste through the holes. In such arrangements, the substrates are positioned alternately under the different regions. If, for example, it is desired to change the arrangement of fluid applied to one of the substrates/PCB's, it is necessary to provide a completely new stencil with the one part which is not to be changed having the same arrangement of apertures applied thereto and with the other part having the newly required arrangement of apertures. It will be appreciated that a considerable number of permutations of printing apertured parts results which as a consequence involves a considerable number of stencils with considerable expenditure and storage requirement and can lead to delays in having produced a stencil with the necessary combinations of arrangement of apertured regions as required and such constitutes a first disadvantage of known stencils .

Also, whilst it is known to produce stencils by etching stainless steel sheets, it is also known to build up a sheet of stencil material by electroplating onto another substrate, for example, by electroplating nickel onto a stainless steel sheet on which previously electroplating resist material has been deposited in the requisite areas wherein apertures are required in the resultant electroplated sheet i.e. the resist material is applied in areas where holes in the resultant stencil produced by electroplating are required. Subsequently the thus formed/deposited electroplated layer is separated from the original sheet and used as the stencil. Nickel is a relatively expensive material and the process is time consumming .

Stencils used in the deposition of solder paste for printed circuit boards are quite large in area for electroplating techniques and their size results in variations in current density during the electroplating formation of the stencil and such can be difficult to control and as a consequence, different thicknesses in the stencil disadvantageously result and this also forms a second disadvantage of some known stencils. It has been found that the smaller the sheet being formed by electroplating, the easier it is to control the operation and to achieve a resultant sheet of constant thickness.

According to a broad aspect of the present invention an improved foil or stencil, preferably of metal sheet, is comprised of two or more detachably interconnectable parts. Also according to the present invention an improved stencil for the deposition of fluid material comprises a sheet of material having means in at least two opposite edge regions engageable by a tensioning frame and having at least two apertured regions with arrangements of apertures for the deposition of fluid therethrough, characterised by the feature that said stencil comprises at least said two separable parts with each part including an apertured region for fluid deposition and with said parts being detachably interconnectable .

Thus in its simplest form, a stencil for the deposition of fluid material is divided into two halves with a dividing line separating the normally, differently apertured regions and with the two parts being detachably interconnectable by suitable interconnection means. Of course, the stencil may be provided in more than two parts in which case such would normally be generally equal parts and any intermediate parts would thus have two interconnection means along opposite sides interconnectable with adjacent stencil parts (which may or may not include opposite end parts) .

It is desirable that the interconnection means of or between adjacent parts of the stencil be as flat or have as low a profile as possible and such may be provided by a plurality of adjacent generally T-shaped interlocking extensions which inter-digitate with slight relative inclination and interlock to prevent lateral separation of the two adjacent stencil parts. The extensions will normally be identical or very similar and arranged as a row along at least an edge of and in the same plane of the stencil part and have a stem portion extending therefrom with preferably right angled shoulder portions stepping therefrom into a larger generally rectangular in plan head portion with said head portion being of greater width than the spacing between adjacent head portions so as to enable such to inter-digitate and interlock with a stencil part with like interlocking extensions. Thus, when the stencil parts are inter-digitated, by disposing such at about 90° or less and passing the stems of one part between the heads of the other part, and are then laid generally in the same plane and tensioned, the two (or more) parts are held together with the extensions slightly flexed out of the planes of the adjacent parts and overlapping the parts and the inner sides of the heads abutting.

When the stems join the heads with right angled shoulders or steps the load is spread and such is more desirable than with a sloping extension surface which intends to spread the load at laterally in an undesirable manner. Of course any suitable shaping may be selected.

It is envisaged also possible that an additional or intermediate interconnection means may be provided between two adjacent parts with each part having a portion inter-engageable with said part additional interconnection means. For example, the additional interconnection means may have a plurality of offset spaced projections alternately engageable with apertured projecting tangs or projectors from each stencil part.

In order that the interconnection means when interconnecting might provide on as flat as possible interconnection or one with minimal or no projections above the surfaces of the main bodies of the stencil parts, an embodiment of the invention resides in forming a recess or recesses in one stencil part to receive the overlapping portion or portions of the interconnecting means of the adjacent part. Preferably where the interlocking extensions or projections of one stencil overlap a part or parts of another stencil part, said projections and overlapped parts are formed of reduced thickness so as to cooperate together to reduce or avoid any overlapping portions extending above the plane in which the tensioned stencil lies in use. Normally a thinning or reduction in thickness of fifty per cent for each cooperating part will be involved although other possibilities are envisaged.

It is to be appreciated that if a jig-saw like or dove-tail like interconnection were provided with a male and female portion interfitting in the same plane, such would not achieve a suitable interlock since there would be a tendency for the parts to separate unless additional means were provided to maintain the male and female parts in the same horizontal plane when under tension and such is difficult technically to achieve.

Also according to the present invention a part of a stencil comprises means on at least one edge connectable to means for tensioning such and an opposite edge having interconnection means interconnectable with another stencil part having like interconnection means. Said other stencil part may comprise a stencil part having along an edge opposite to that bearing the interconnection means, means for connection to a tensioning frame and/or said other part may comprise an intermediate stencil part having on opposite parallel edges further interconnection means interconnectable with said interconnection means.

It will be appreciated that both the first- and second- mentioned disadvantages are overcome or minimised according to the invention although in respect of solving or minimising the second mentioned disadvantage, the present invention is particularly advantageous if the stencil is formed with at least one detachably interconnectable central part, with the outer, side or edge parts preferably being of a different material.

Thus, also according to second broad aspect of the present invention an improved foil or stencil comprises a first sheet having at least one apertured region with the apertures being for the stencilling deposition of fluid therethrough and with at least two opposite edges of said first sheet each being detachably interconnectable to edges of a further sheet or at least two further sheets with the further sheet or each of the further sheets having mounting means along an opposite edge region for mounting of the further sheet or sheets on a support and tensioning frame for the whole stencil.

Thus the first sheet can be formed such as by being built-up by electroplating, for example of nickel, and be of smaller area than the necessary resultant stencil with frame interconnection means and thus the constancy of the thickness thereof can be more accurately controlled or achieved and/or less overall material for the electroplated first sheet is utilised which can prove of considerable economy where such material is particularly expensive. Also the time required for building up the stencil by electroplating is considerably reduced as a consequence of a smaller area being involved.

As mentioned, the further sheets will preferably be of a different material to that of the first sheet or first sheets. For example, they can be of stainless steel or plastics or other dissimilar material in contrast to the first sheet being of nickel or other more expensive material .

In GB 2264460 a rectangular metal stencil is illustrated wherein interconnection means are provided in two opposite edges each means comprising a row of closely spaced parallel narrow slots provided for interconnection with and tensioning on a mounting frame whilst in GB 2303333 a rectangular stencil is illustrated wherein interconnection means in the form of parallel slots are provided along all four sides of the stencil and the concept of the present invention may be utilised in the construction of similar such stencils by having further said sheets with interconnection means along two opposite sides or along both pairs of opposite sides and m which latter case, whilst the further sheet may, for example, be formed as a continuous rectangular frame, preferably a separate said further sheet will be provided along each of the four sides and thus four said further sheets would be provided where the stencil is rectangular or square and to be tensioned along all four sides.

Where the first sheet material is built up by electroplating, the part of the interconnection means wherein a reduced thickness s required according to a preferred embodiment, may be achieved by etching or otherwise removing part of the thickness of the electroplated material so as to enable it to cooperate with the corresponding reduced portion of the interconnection means of a cooperating further sheet.

When the first and second sheet materials are interconnected, an additional securement by means of adhesive tape may be provided. In all embodiments, a strip of adhesive tape may be provided over the joint for securement and/or smoothness .

The invention will be described further, by way of example, with reference to the accompanying drawings, m which: -

Fig. 1 is a plan of a stencil part according to a first embodiment of the present invention;

Fig. 2 s a plan of a two-part stencil with the interconnection means adjacent but detached;

Fig. 3 is a fragmentary plan of region D of the stencil of Fig. 2 but which has been modified by having reduced thickness regions m the heads of the projections where they overlie the main stencil parts and m said stencil parts where the heads of the projections overlie such ;

Figs. 4 and 5 are enlarged fragmentary details m plan and opposite plan of one stencil half 6,7 and illustrating the reduced thickness regions m black;

Fig. 6 is a perspective view of a projection or tab showing the reduced thickness end;

Figs. 7 and 8 are a plan and elevation of the projection or tab of Fig. 6;

Fig 9 is a section on the line X-X of Fig. 4 showing reduced thickness regions;

Fig. 10 is a schematic plan illustrating a second embodiment of the invention wherein a central, apertured stencil part 4 is detachably interconnectable w th outer mounting parts of a different material to that of the central part; and;

Fig. 11 is a schematic plan of half a stencil similar to that of Fig. 2 but also according to the second embodiment: wherein six outer mounting parts (only two shown) are provided of a different material to the two stencil first sheet parts (only one shown) .

A stencil 1 is illustrated which as regards its mounting, tensioning and flexing is similar to that disclosed in British Patent Specification 2303333 wherein the stencil is generally rectangular and has a plurality of elongate parallel slots 2, 3 and 4, 5 along the opposite edge regions for interconnection with a tensioning frame (not shown) and to be bendable out of the stencilling plane of the stencil. The stencil illustrated is formed m two halves 6, 7 with the uxtaposable edges having generally identical interconnection means 8, 9 each comprising a plurality of generally T-shaped integral projections 8, 9 with the bottoms of the stems 10 of the T being integral with and extending from the stencil sheet material 6, 7 and each by way of two generally right angled or stepped shoulder portions 11 leading into an enlarged cross piece or head 12 of a width w greater than the spacing s between the heads so as to ensure interlocking when the projections of the juxtaposed heads are inclined at slight angles and then pulled together into a horizontal or generally horizontal plane w th the result that the stepped shoulders 11 on each opposite interconnection means abut so as to spread the load and the heads remain slightly inclined and slightly overlap the continuous adjacent edge material of the stencil .

Fig. 2 shows the two stencil parts 6, 7 prior to inter-digitating disposition where that the thin stem portions 10 of one part are moved parallely between the spaces s_. between the enlarged head portions 12 at a slight angle and then the two mam body parts 6, 7 of the stencil are moved into the horizontal position and pulled apart so that interlocking is achieved. It is to be appreciated that at one lateral edge the stencil there is a solid portion 13 whilst a gap 14 is provided at the other so that the two stencil parts 6 , 7 may only be properly brought together one correct position i.e. with the arrangement of apertures the correct position.

In the modification of Figs. 3 to 9, reduced thicknesses of material (preferably half thicknesses) are formed the region where the tips 12 ' of the enlarged head portions 12 overlie the edges 6 ',7' of the mam body of the stencil sheet material 6,7 the region from where the stems 10 extend, and in the said edge regions 6',7', which are overlain by the tips 12', so that when the stencil parts are interlocked and tensioned, the projection of heads above the sheet surface is minimized or avoided altogether. For a stencil part 6 or 7 the reduction of thickness 6', 7' will be on the opposite side to that on which lie the reduction m thickness of the tips 12 ' of the head portions 12. The modification thus enables a considerably smoother interconnection of the two parts to be achieved .

It is envisaged that the stencil may be formed of more than two parts which case an intermediate stencil part would be provided having opposite edges bearing identical ter-engagmg means inter-engageable with each row of mter-engagmg means as illustrated.

It is also envisaged that an intermediate connecting member might be provided engageable with interconnection means on each of the adjacent parts of the stencil .

The embodiment illustrated in Fig. 10 comprises a five part stencil 15 comprising a first central and rectangular sheet part 16 formed of nickel by first electroplating onto a stainless steel sheet (not shown) having resist material to form the stencil deposition apertures m region 17 and then removing the thus formed nickel layer. The sides or edges of part 16 have interconnection means 8 cooperating with interconnecting means 9 on four surrounding edge/side and further parts 18,19,20,21 of stainless steel. The interconnecting means 8 and 9 may be as previously described as are slots 2 , 3 and 4,5 for mounting on a known tensioning frame (not shown) . It will be appreciated that central part 16 formed of nickel (or any other appropriate material) is of a considerably reduced area relative to the whole stencil 15 and thus savings the material and production thereof result. The outer parts 18,19,20 and 21 may be of stainless steel and thus cheaper to produce and may be used with different central parts 16 - thereby achieving further savings .

Fig. 11 illustrates half a stencil forming a further embodiment and to be interconnected with a similar other half (not shown) and is similar to the right hand half of the stencil of Fig. 2 except that the first central part 22 bearing deposition apertures in region 22' has interconnection means 8,9 cooperating with interconnection means 9,8 or further, edge parts 23,24,25. It will be appreciated two central parts 22 will be provided and six outer parts 23,24,25 although any number is envisaged as various possible permutations. The parts are also held together by strips of adhesive tape (not shown) . It is envisaged that instead of two parts 23,23 and 25,25, on each side a single part (23,25) may be provided and to which two central parts 22,22 may be detachably connected each side. Central parts 22,22 may have different apertured regions 22 ' .

In Figs. 10 and 11 the redundant ends and slots of the further end strips may be removed.

In these embodiments, where the first apertured part is formed by electroplating, reduced thickness portions 6 '7' and 12' may be formed by removing material such as by etching.

Claims

1. A foil or stencil comprises two or more detachably interconnectable parts.

2. A stencil as claimed m claim 1 m which at least one part includes an apertured region for the deposition of fluid material.

3. A stencil as claimed claim 1 or 2 , which the stencil is metal sheet.

4. A stencil for the deposition of fluid material comprises a sheet of material having m at least two opposite end regions, means, engageable by tensioning means and having at least two apertured regions with the apertures thereof provided for the deposition of fluid therethrough, characterised by the feature that said stencil comprises at least said two separable parts with each part including an apertured region for fluid material deposition and with said parts being detachably interconnectable .

5. A stencil as claimed m claim 4 in which tne parts are detachably interconnectable by suitable interconnection means and when the stencil is provided in more than two parts, such are normally equal parts and any intermediate parts have two interconnection means along opposite sides interconnectable with adjacent stencil parts (which may or may not include opposite end parts) .

6. A stencil as claimed in any of claims 1 to 5, in which the interconnection means of or between adjacent parts of the stencil are as flat as possible or have an as low a profile as possible.

7. A stencil as claimed m claim 6, in which the interconnection means are provided by a plurality of adjacent generally T-shaped interlocking extensions which inter-digitate and interlock to prevent lateral separation of the two adjacent stencil parts.

8. A stencil as claimed in claim 7, in which the extensions are identical or very similar and arranged as a row along at least an edge of and in the same plane of the stencil part .

9. A stencil as claimed in claim 7 or 8 , in which each extension includes a stem portion extending via right angled shoulder portions stepping therefrom into a larger generally rectangular in plan head portion with said head portion being of greater width than the spacing between adjacent head portions so as to enable such to inter- digitate and interlock with a stencil part with like interlocking extensions.

10. A stencil as claimed in claim 9, in which when the stencil parts are inter-digitated, by disposing such at about 90┬░ or less and passing the stems of one part between the heads of the other part, and when such are then laid generally in the same plane and tensioned, the two (or more) parts are held together.

11. A stencil as claimed in claim 10, in which when tensioned the extensions are slightly flexed out of the planes of the adjacent parts and overlapping the parts and with the inner sides of the heads abutting.

12. A stencil as claimed in any of claims 1 to 11, in which an additional or intermediate interconnection means is provided between two adjacent parts with each part having a portion inter-engageable with said part additional interconnection means.

13. A stencil as claimed in claim 12, in which the additional interconnection means has a plurality of offset spaced projections alternately engageable with apertured projecting tangs or projections from each stencil part.

14. A stencil as claimed in any of claims 6 to 13 , in which in order that the interconnection means when interconnecting can provide on as flat as possible interconnection or one with minimal or no projections above the surfaces of the mam bodies of the stencil parts, a recess for recesses is formed in one stencil part to receive the overlapping portion or portions of the interconnecting means of the adjacent part.

15. A stencil as claimed m claim 14, m which where the interlocking extensions or projections of one stencil overlap a part or parts of another stencil part, said projections and overlapped parts are formed of reduced thickness so as to cooperate together to reduce or avoid any overlapping portions extending above the plane m which the tensioned stencil lies in use.

16. A stencil as claimed m claims 14 or 15 m which a thinning or reduction in thickness of fifty per cent for each cooperating part is provided.

17. A part of a stencil comprises means on at least one edge connectable to means for tensioning such and an opposite edge having interconnection means interconnectable with another stencil part having like interconnection means .

18. Another said stencil as claimed claim 17, m which said other stencil part has along an edge opposite to that bearing the interconnection means, means for connection to a tensioning frame and/or said other part comprises an intermediate stencil part having on opposite parallel edges further interconnection means interconnectable with said interconnection means.

19. A stencil as claimed in any of claims 1 to 18, in which the stencil is formed with at least one detachably interconnectable central part, with the outer, side or edge parts being of a different material.

20. A foil or stencil comprises a first sheet having at least one apertured region with the apertures being for the stencilling deposition of fluid therethrough and with at least two opposite edges of said first sheet each being detachably interconnectable to edges of a further sheet or at least two further sheets with the further sheet or each of the further sheets having mounting means along an opposite edge region for mounting of the further sheet or sheets on a support and tensioning frame for the whole stencil .

21. A stencil as claimed in claim 20, in which the first sheet is formed by being built-up by electroplating and is of smaller area than the necessary resultant stencil with frame interconnection means.

22. A stencil as claimed in claim 20 or 21, in which the further sheets are of a different material to that of the first sheet or first sheets, for example, they are of stainless steel or plastics or other dissimilar material in contrast to the first sheet being of nickel or other more expensive material.

23. A stencil as claimed in claim 21 or 22, in which the first sheet material is built up by electroplating, and the part of the interconnection means wherein a reduced thickness is required is achieved by etching or otherwise removing part of the thickness of the electroplated material so as to enable it to cooperate with the corresponding reduced portion of the interconnection means of a cooperating further sheet.

24. A stencil as claimed in any of claims 1 to 23, in which when the parts or the first and second sheet materials are interconnected, an additional securement thereof by means of adhesive tape is provided.

25. A stencil or stencil part substantially as herein described with reference to the accompanying drawings.