EP-4741170-A1 - FOIL FOR A PRINTING SCREEN, PRINTING SCREEN AND METHODS FOR MAKING THE FOIL AND THE SCREEN

Abstract

A printing screen for SMT applications at least partially comprising graphene, optionally in the form of carbon nanotubes.

Inventors

• WALKER, WILLIAM
• ASHMORE, CLIVE

Assignees

• ASMPT SMT Singapore Pte. Ltd.

Dates

Publication Date

20260513

Application Date

20251011

Claims (15)

1. A foil for a printing screen, comprising graphene.
2. The foil of any claim 1, comprising a sheet of doped or undoped graphene; optionally the sheet is affixed to at least one layer of a base material, the base material being different to graphene; optionally the base material comprises one of the group consisting of nickel, stainless steel and alloys of nickel and stainless steel.
3. The foil of either of claims 1 and 2, wherein the graphene is at least partially in the form of carbon nanotubes.
4. The foil of any preceding claim, wherein the graphene is doped with a dopant material, optionally the dopant material comprises one of the group consisting of fluorine and nitrogen; optionally the dopant is provided at a concentration in the range of 0.5% to 6% by atomic percentage, optionally within the range 2% to 4% by atomic percentage, optionally about 3.5% by atomic percentage.
5. The foil of any preceding claim, comprising an edge member proximate each side of the foil; optionally the edge member comprises jaws which grip an edge of the foil.
6. The foil of claim 5, wherein the foil comprises a plurality of sides, with attachment means proximate each of its sides, each attachment means for connecting with an edge member in use; optionally the attachment means comprises openings formed proximate each of the sides of the foil; optionally the edge member comprises fingers which extend through respective openings.
7. A printing screen comprising the foil of any preceding claim, with at least one aperture formed in the foil and extending therethrough; optionally the printing screen comprises a plurality of apertures formed in a regular array to create a mesh area within the printing screen.
8. The printing screen of claim 7, wherein at least one aperture has a perimeter at one side of the printing screen which is greater than its perimeter at the other side of the printing screen.
9. A method of making a foil for a printing screen, by forming a sheet of material comprising graphene.
10. The method of claim 9, wherein the sheet comprises doped or undoped graphene; optionally the graphene is at least partially in the form of carbon nanotubes; optionally the method comprises doping the graphene with a dopant material; optionally the dopant material comprises one of the group consisting of fluorine and nitrogen.
11. A method of making a printing screen, comprising the steps of: i) making a foil using the method of claim 9; and ii) forming at least one aperture in the sheet.
12. The method of claim 11, wherein step ii) comprises forming the apertures by a process selected from the group consisting of laser-cutting (optionally using a femtolaser), etching and milling.
13. The method of either of claims 11 and 12, wherein step ii) comprises using a laser to form an aperture extending through the sheet, the aperture having a perimeter at one side of the sheet which is greater than its perimeter at the other side of the sheet.
14. The method of any of claims 11 to 13, comprising the steps of forming a first aperture in the sheet having a first size, then adding a layer of material onto the sheet, the layer comprising a second aperture having a second size, different from the first size, so that the first aperture and the second aperture at least partially overlap.
15. The method of any of claims 11 to 14, wherein step ii) comprises forming a plurality of apertures in a regular array to create a mesh area within the sheet; optionally comprising removing material from the mesh area so that the thickness of graphene material at the mesh area is reduced; optionally the method comprises applying an emulsion material to the mesh area, then forming a printing pattern by selectively removing regions of the emulsion material.

Description

This invention relates to a foil for a printing screen, a printing screen, a method of making a foil and a method of making a printing screen. Background and Prior Art Industrial screen-printing machines typically apply a conductive print medium, such as solder paste, silver paste or conductive ink, onto a planar workpiece, such as a circuit board, by applying the conductive print medium through a pattern of apertures in a thin planar layer or mask, such as a stencil or a screen. A stencil is a patterned solid material, such as stainless steel, nickel or, less commonly, plastics material, in the form of a substantially planar sheet which includes apertures which define the pattern to be printed. For the sake of understanding, such a sheet is commonly referred to as a "foil" if it has not yet had apertures formed in it (see below). Stencils are used to print directly onto a PCB (on-contact printing). An image in a stencil consists of apertures (laser-cut, chemically etched, or electroformed) which extend through the sheet. A screen meanwhile is a mesh material comprising a flexible, perforate sheet, for example a woven mesh of polypropylene, polyester or stainless steel strands. This mesh is then coated with emulsion and the image is then exposed with a camera or light source. Screens have a bit of separation (off-contact printing) with the workpiece, allowing the mesh to contact the printed surface as the squeegee passes. Generally, a stencil may produce higher quality print results than a screen, however screens have benefits including reusability and the ability to print features such as long lines (which may be required for solar panel manufacture for example) or annuli, which stencils cannot achieve. The present invention is equally applicable to both screen and stencil printing, and indeed provides for a hybrid of these, and for convenience the term "printing screen" will be used to refer to any such patterned mask throughout the remainder of this document. The term "foil" will be used to describe an unpatterned sheet which may be provided with apertures to form a printing screen. Importantly, the term "foil" will encompass both a continuous sheet of material, and a mesh sheet, for example, but not exclusively, formed by weaving strands together. In both forms it is necessary to hold the printing screen under tension, and conventionally this is achieved by removably attaching the printing screen to a rectangular tensioning frame. The print medium is applied using an angled blade or squeegee. The same machines may also be used to print certain non-conductive media, such as glue or other adhesive, onto workpieces. However, as noted above, while stencils may provide high quality print results, so that very fine features may be printed, it is not possible to use a stencil to print certain types of feature, such as the long lines or annuli previously mentioned. In addition, even current stencils face limits as to the minimum size of features they may reliably print. Generally, the smaller the feature, the thinner the stencil required, which clearly will reduce its strength. The present invention seeks to overcome the above problems, and provide vastly improved printing screens which maintain high strength even at very low thicknesses. Furthermore, the proposed printing screens may be used to combine the features of stencils and screens, producing a "hybrid" printing screen enabling all types of features to be printed, at high quality. In accordance with the present invention this aim is achieved by forming the sheet of the printing screen at least partially from graphene, a material which has not previously been proposed for such applications, nor has it been known how such a material could be used for such an application. Summary of the Invention In accordance with a first aspect of the present invention there is provided a foil for a printing screen, comprising graphene. In accordance with a second aspect of the present invention there is provided a printing screen comprising the foil of the first aspect. In accordance with a third aspect of the present invention there is provided a method of making a foil for a printing screen, by forming a sheet of material comprising graphene. In accordance with a fourth aspect of the present invention there is provided a method of making a printing screen, comprising the steps of: i) making a foil using the method of the third aspect; andii) forming at least one aperture in the foil. Other specific aspects and features of the present invention are set out in the accompanying claims. Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings (not to scale), in which: FIG. 1A schematically shows, in perspective view, a printing screen according to the present invention;FIG. 1B schematically shows, in section, a portion of the printing screen of FIG. 1A;FIG. 2 schematically shows, in section, a portion of a printing