WO1999038706A1

WO1999038706A1 - Flexible screen suitable for use in screen printing and method of making same

Info

Publication number: WO1999038706A1
Application number: PCT/GB1999/000295
Authority: WO
Inventors: John Michael Lowe
Original assignee: John Michael Lowe
Priority date: 1998-01-29
Filing date: 1999-01-28
Publication date: 1999-08-05
Also published as: GB9801881D0; GB2333742A; AU2290499A

Abstract

A flexible screen (10) suitable for use in screen printing comprising a flexible sheet material having a plurality of first holes (12) formed therein positioned to define a pattern which is to be screen printed using the screen. An array (22) of second holes (24) is also formed in the flexible sheet material and a layer of flexible material covers the second holes (24) but leaves the first holes (12) exposed. A method of making a screen is also described.

Description

1 FLEXIBLE SCREEN SUITABLE FOR USE IN SCREEN PRINTING

AND METHOD OF MAKING SAME

This invention is concerned with improvement in or relating to flexible screens suitable for use in screen printing and to methods of making such flexible screens.

Screens commonly used in the well-known screen printing technique commonly comprise a base fabric woven from strong yarns, for example silk, and the screen is commonly mounted in a frame under tension, for example of 15 newtons per cm-deflection. To keep the screen substantially flat and to avoid distortion systems have been developed allowing for tensions up to 100 Newtons per centimetre deflection. The woven fabric base is commonly coated with a gelatine composition of a suitable thickness, for example about 25 μm and by a photographic technique well known to those skilled in die art a pattern is formed in the gelatine layer, by removing the gelatine in those regions of the screen through which ink is to be printed.

In the use of the screen in screen printing, the screen is positioned in the screen printing press with the gelatine layer facing downwardly towards a substrate to which a pattern is to be applied by screen printing and the screen is positioned in register with the substrate in very close proximity to the substrate. The screen is flooded with ink which flows into the interstitial spaces between the woven yarns of the fabric of the screen in the regions of the pattern which are left exposed by the removal of the gelatine in those regions. A squeegee is pressed onto the upper surface of the screen to force the screen, where it is pressed by the squeegee, against the substrate into firm contact with the substrate and the 2 squeegee is drawn along the screen forcing the screen against the substrate as the squeegee travels along the screen. Passage of the squeegee forces the pockets of ink in the interstitial spaces between the yarns into contact with the substrate and at the same time, separates the pockets of ink in the interstitial spaces from the reservoir of screen printing ink on the screen. As the squeegee passes, the screen, which is still under high tension, lifts off the substrate, leaving behind on the substrate the ink from the pockets between the yarns of the woven fabric. The small columns of ink thus deposited on the substrate collapse and coalesce to form a pattern of ink on the substrate corresponding with the pattern on the screen.

Such a screen printing technique is used in many fields including the production of printed circuits, in which appropriately patterned screens are used to print both conductive patterns and patterns of insulating material to produce a desired printed circuit on a substrate. The patterns which are printed onto a substrate for the purpose of making printed circuits often need to be very compact to condense the circuitry into as small a space as is possible and it is therefore necessary to ensure that all of the screens which may be used to build up a multi layer printed circuit are both accurately located in proper register with one another on the printed circuit substrate but also that the printed areas are sharply defined. For the printing of conductive tracks it is also necessary that the track are continuous, that is that the columns of ink deposited on the substrate are sufficient to coalesce making a continuous film of ink on the substrate in the desired pattern without any breaks: a break in a conductive pattern will lead to a break in the desired electrical circuit. It is also essential that insulating patterns are continuous - a break in any insulating pattern can lead to short circuiting and other problems. 3 By way of example, in a square woven base for a typical screen, yarns may be, for example 40 microns in diameter and spaced apart to leave a 60 μm gap between each of the fibres so that the maximum area of each pocket of ink in a pattern will be a square having sides each 60 μm and the depths of the column of ink deposited from such a pocket will be a maximum of about 40 μm (the diameter of the yarns of the screen) depending on to some extent also on the thickness of the gelatine layer. It can be seen that the amount of ink deposited is defined by the thickness of the screen and the open area in the screen between the yarns as well as the thickness of the yarns themselves. In practice, this imposes a practical minimum limit on the width of lines which can be successfully applied to a substrate by practical heretofore known screen printing techniques. This is especially noticeable where the lines to be printed extend generally diagonally across the woven mesh of the screen base, when considered relative to the directions of the woven yarns; there is a substantial probability that at some point a printed track will pass across the region at which warp and weft yarns intersect one another which can seriously deplete the amount of ink which might be deposited at that region, leading to a potential break in the printed track Similar problems could arise should a printed track substantially coincide with one of the yarns either in the warp or weft direction.

One of the various objects of the present invention is to provide an improved flexible screen suitable for use in screen printing. Another of the objects of the present invention is to provide an improved method of making a screen suitable for use in screen printing.

In one aspect the invention may be considered to provide a flexible screen suitable for use in screen printing comprising a flexible sheet 4 material having a plurality of first holes formed therein positioned to define a pattern which is to be screen printed using the screen.

A flexible sheet material suitable for use in die manufacture of the flexible screen as set out in the last preceding paragraph must have sufficient tensile strength that the screen can be tensioned to a relatively high tension without stretching and must have sufficient flexibility for use in commonly available screen printing machines.

One suitable flexible sheet material is stainless steel sheet which has sufficiently high tensile strength, but other sheet materials may be satisfactory, for example fibre reinforced plastics sheet materials, reinforced with high tensile strength fibres, for example carbon fibres.

A preferred flexible screen has a layer of flexible material on the face of said flexible sheet material which is intended to be pressed against a substrate on which a pattern is to be printed during screen printing, said layer having regions therein from which the flexible material is absent, the regions defining a pattern corresponding with the pattern to be printed and in register with the pattern defined by the first holes, thereby leaving the first holes exposed.

It is, of course, necessary that a screen in accordance with the invention is sufficiently flexible and stainless steel sheets or sheets of suitable other materials, of sufficient thickness to provide the necessary strength may prove to be too rigid. In a preferred sheet in accordance with the invention, therefore, an array of second holes is formed in the flexible sheet material, the array of second holes being covered by said layer of flexible material. The array of second holes increases the 5 flexibility of the flexible sheet material when compared witih the same material without these second holes formed in it. Commonly, the first and second holes have different areas. In certain screens in accordance with the invention each second hole may have a larger surface area than the first holes; in a preferred sheet material all of me second holes have the same surface area and they are conveniently arranged in a regular array.

In another screen in accordance with the invention the first holes in a first region of the pattern have a different area from the first holes in a second region of the pattern.

In another aspect the invention may be considered to provide a method of making a screen suitable for use in screen printing comprising the steps of:- (a) procuring a sheet of flexible sheet material; and

(b) perforating the sheet to form a plurality of first holes which define a desired pattern which is to be screen printed using the screen.

The flexible sheet material used in carrying out a method in accordance with the invention may be any convenient material but a preferred flexible sheet material is high tensile strength stainless steel and, in that case, the first holes are formed in said stainless steel sheet material in the desired pattern using photo-etching.

In a preferred method in accordance with the invention comprises the further step of:-

(c) forming a layer of flexible material on the sheet with a pattern in the layer which leaves the first holes exposed. The layer of flexible 6 material may suitably be formed by applying a coating of gelatine of suitable thickness, for example 25 μm. The pattern is suitably formed in the layer of flexible sheet material by similar photographic techniques to those used in the manufacture of previously known screens and is well- known to those skilled in the art.

A preferred method in accordance with the invention further comprises the step of:-

(d) before carrying step (c) forming an array of second holes in the sheet in regions of the sheet which will remain covered when the layer is formed in carrying out the step (c). In the preferred method using a stainless steel sheet, the array of second holes is conveniently formed by photo-etching; preferably the first and second holes are formed in a single etching operation.

Although in carrying out a method in accordance with the invention, using a stainless steel flexible sheet material, the first and second holes are formed by photo-etching, in methods in accordance with the invention, the first and second holes may be formed by any suitable technique - for example with certain flexible sheet materials, it may be possible to perforate the sheet material to form the first and second holes by use of lasers.

In carrying out a preferred method where first and second holes are made by etching a stainless steel sheet material, both the first and second holes may be etched to any desired size. The size of the second holes is chosen depending on the flexibility needed and the size of the first holes depending on the dimensions of the tracks which are to be formed on the substrate. Furthermore, the first holes can be made of a variety of 7 different shapes depending on die requirements, to give good ink-flow so diat the printed ink coalesces to form a continuous track where necessary.

In addition, it would be possible to make a screen configured to facilitate printing around the position at which a circuit component would be placed. Additionally, using a stainless steel screen, due to the inherent tensile strength of the stainless steel it would be possible to make a screen capable of printing on a curved surface (provided that the printing machine was adapted to cause me squeegee to follow an appropriate curved path). In addition it would be possible to etch die surface of the screen that comes into contact with the substrate to accommodate regular variations in the topography of the substrate.

There now follows a detailed description to be read with reference to the accompanying drawings, of a flexible screen embodying the invention and of a method of making me screen also embodying the invention in its method aspects.

In me accompanying drawings :-

Figure 1 is a plan view showing a first screen embodying me invention;

Figure 2 is a diagrammatic side view showing the use of a screen in a screen printing machine;

Figure 3 is a plan view of a very small part of the screen shown in Figure 1 , enlarged in comparison with Figure 1 ; 8

Figure 4 is a diagrammatic view of part of a different second screen embodying the invention showing a part of the pattern to be printed; and

Figure 5 is a plan view of part of the screen shown in Figure 1 with part broken away, overlying a substrate and on a scale intermediate mat of Figure 1 and that of Figure 3.

The illustrative flexible screen 10 shown in Figure 1 is suitable for use in screen printing. It comprises a flexible high tensile stainless steel sheet material having a plurality of first holes 12 formed therein, positioned to define a pattern 14 which is to be printed using the screen.

In the embodiment shown me pattern 14 is a circle.

The screen has a layer (not shown) of flexible material on the face 16 of the flexible sheet material which is intended to be pressed against a substrate 18 on which the pattern 14 is to be printed during screen printing. The layer has regions 20 from which the layer of flexible material is absent, the regions 20 defining a pattern corresponding with but somewhat larger than die pattern 14 to be printed and in register with the pattern 14 defined by the first holes 12, diereby leaving the first holes 12 exposed.

The screen further comprises an array 22 of second holes 24 formed in the flexible sheet material, the array of second holes 24 being covered by the layer of flexible material. As can be seen viewing especially Figure 3, the first and second holes 12, 24 are circular in plan view and have different areas, each second hole 24 having a larger surface area than any first hole. The second holes 24 all have the same area and are arranged in a regular array, as can be seen from Figures 1 and 5, with 9 the exception of the region in which the first holes are formed. In the illustrative screen shown in Figure 1, the first holes 12 all have the same diameter and therefore the same area.

In a screen embodying me invention otherwise similar to the first illustrative screen, different patterns may be formed at different parts of the screen, wherein me patterns include lines of different widms; the first holes for printing those patterns which have lines which are wider will utilise first holes having a greater surface area than the surface area of the first holes intended to print narrower lines.

The first holes may be of any suitable shape and in Figure 4, in the second illustrative screen first holes 26 are shown which are not circular and interlock with one another. It will be seen that the second holes 24 in Figure 4 are of smaller surface area than the first holes 26.

The surface area of the second holes 24 in a screen in accordance with the invention depend on d e flexibility which is to be imparted to the base flexible sheet material as does their number and position. The second holes 24 of the illustrative screens are shown in the drawings as circular but they may be of any suitable shape.

The illustrative screens 10 are made from a sheet of flexible stainless steel sheet material by perforating a sheet of the stainless steel material to form a plurality of first holes 12 which are positioned to define a desired pattern which is to be screen printed using the screen. In carrying out the illustrative method, the first holes 12 and the second holes 24 are formed in the base sheet material in the desired pattern using photo- etching techniques in which a layer of photosensitive etch-resistant 10 material is coated on the face 16 of die sheet material 10 which is intended to be pressed against the substrate 18, and a mask is applied over me layer of flexible material carrying the necessary pattern of first and second holes 12, 24. The mask may be provided by a suitable photo-transparency. The thus formed assembly is exposed to suitable radiation, for example ultraviolet radiation, and the layer is cured to a stable condition throughout the layer except the regions of the layer protected by the mask, corresponding with die first and second holes which remain in a soft, uncured state. The layer of etch-resistant material is then washed to remove the soft material thus leaving holes in the layer of flexible material corresponding with the holes which are to be formed in me base sheet material.

The base sheet material then has a suitable etching material applied to the surface carrying the etch-resistant layer and the etching material acts upon the exposed regions of stainless steel sheet material to etch the first and second holes 12, 14.

The etch resistant layer is then removed from' the stainless steel sheet which is now perforated witii an array of second holes 24 and an array of first holes 12 defining the desired pattern. A layer of flexible material, for example gelatine, is applied to the face 16 and this layer also is treated by a photographic technique which cures the flexible layer on the region of the sheet carrying the second holes 24 but leaves the material uncured in a pattern corresponding with the pattern formed by the first holes 12. The uncured material is then washed away providing a pattern in the layer which leaves the first holes 12 exposed. This pattern may correspond precisely with the first holes but that requires very precise registration of the pattern and first holes. In Figure 4 the boundaries 28 of 11 me flexible layer defining die pattern in die flexible layer are indicated as parallel lines, the region between die boundaries 28 in which the first holes 26 are formed being free of the layer of flexible sheet material; as can be seen, the array of second holes 24 in me sheet remain covered by the layer of flexible sheet material.

In etching the holes and in providing the pattern in the layer of flexible sheet material, it is extremely important that the registration between the etched holes and die pattern in die layer of flexible material is exact. Any suitable means for achieving registration may be used but in carrying out the illustrative method, it is preferred to use a system similar to that described in my copending European patent application publication number 0574459 to position die masks used in forming the appropriate pattern in the layer of flexible material, in precise register with die stainless steel sheet. In this instance me flexible stainless steel sheet material will provide the substrate and the screen referred to in our copending application is provided by the photographic mask.

The illustrative screen may be used in carrying out screen printing and is especially useful in die manufacture of compact printed circuits, allowing finer and more closely spaced lines to be printed man have himerto been reliably manufactured by screen printing techniques. The screen may be used in apparatus as described in my aforementioned European patent application and a screen printing operation is shown diagrammatically in Figure 2. The substrate 18 may be a printed circuit board and the illustrative screen 10 is urged against the substrate 18 by a squeegee 30 travelling lengthwise of the screen 10 in a screen printing operation in known manner. In the rest condition the screen 10 is shown in dashed line on Figure 2 and it can be seen that the screen 10 is closely 12 adjacent the surface of die substrate 18 but spaced slightly dierefrom, being urged into engagement wim the substrate 18 by the squeegee 20.

Claims

13CLAIMS

1. A flexible screen suitable for use in screen printing comprising a flexible sheet material having a plurality of first holes formed therein positioned to define a pattern which is to be screen printed using the screen.

2. A screen according to claim 1 having a layer of flexible material on the face of said flexible sheet material which is intended to be pressed against a substrate on which a pattern is to be printed during screen printing, said layer having regions therein from which the flexible material is absent, die regions defining a pattern corresponding with the pattern to be printed and in register with me pattern defined by the first holes, thereby leaving me first holes exposed.

3. A screen according to claim 2 wherein an array of second holes is formed in die flexible sheet material, said array of second holes being covered by said layer of flexible material.

4. A screen according to claim 3 wherein the first and second holes have different areas.

5. A screen according to claim 4 wherein each second hole has a larger surface area than any first hole.

6. A screen according to any one of claims 3 to 5 wherein the second holes all have the same surface area. 14

7. A screen according to any one of claims 3 to 6 wherein die second holes are arranged in a regular array.

8. A screen according to any one of the preceding claims wherein said flexible sheet material is a stainless steel sheet.

9. A screen according to any one of the preceding claims wherein the first holes in a first region of the pattern have a different area from those in a second region of the pattern.

10. A screen substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of making a screen suitable for use in screen printing comprising the steps of

(a) procuring a sheet of flexible sheet material; and

(b) perforating the sheet to form a plurality of first holes which define a desired pattern which is to be screen printed using me screen.

12. A method according to claim 11 wherein the first holes are formed in said sheet material in the desired pattern using photo-etching.

13. A method according to either one claims 11 and 12 comprising the step of

(c) forming a layer of flexible material on the sheet with a pattern in the layer which leaves the first holes exposed. 15

14. A method according to claim 13 comprising the step of

(d) before carrying out step (c) forming an array of second holes in the sheet in regions of the sheet which will remain covered when the layer is formed in step (c).

15. A mediod according to claim 14 wherein die array of second holes is formed by photo-etching.