IMPROVED STENCIL AND METHOD OF PRODUCING SUCH

Abstract: An improved stencil (1) having one or a plurality of apertures therethrough for enabling a printing medium to be deposited on a receiving surface and including a first and normally lower surface (4) which in use is to be proximate the receiving surface and a second and normally upper surface (5) over which a squeegee or other printing medium applicator means is to pass, characterised by the feature that at least one larger diameter or width first aperture (3') is formed in the first, normally lower surface (4), and a plurality of smaller diameter or width second apertures (6') of smaller diameter or width than said first aperture (3') are formed in the second and normally upper surface (5) and lead into said first aperture (3').

Description: IMPROVED STENCIL AND METHOD OF PRODUCING SUCH The present invention concerns an improved stencil for application of solder paste, adhesive or other printing medium and method of producing such. It is known to screen print solder paste onto a receiving surface using a fabric mesh emulsion stencil on which there has been formed a hardened photo-sensitive resist layer which forms areas through which paste cannot pass and defines the pattern or through-passages of mesh through which the solder paste may be urged by a squeegee to form the requisite paste deposition. It is also known to form stencils from continuous sheets of metal or metal foil which are etched or laser cut or otherwise apertured to provide through passages for solder paste and such stencils are illustrated in, for example, GB 2264460 and PCT/GB96/01722 , and are more durable and enable a high degree of accuracy to be achieved. With metal stencils it is not possible to produce apertures of any relatively significant uninterrupted length since the long slot produced leads to the stencil material being pulled apart when the stencil is tensioned which leads to inaccuracies and distortions. Also, with solder paste deposition in some areas, such as are known as the "quad pack", the paste is to be deposited in fine pitch and to stabilize the deposition it is sometimes necessary to reduce the height of paste in such area and this is achieved by "stepping down" or reducing the thickness of the stencil over the general area of the quad pack which is troublesome and costly. It has also been found with the deposition of glue/adhesive over relatively large areas in known arrangements that the squeegee can scoop some of the glue out of the totally open aperture in a stencil with the result that an uneven deposition of glue is achieved. According to the present invention in broad aspect and as claimed in claim 18 there is provided a stencil having one or more apertures therethrough for the deposition of a fluid material onto a lower surface which includes at least one first and normally lower aperture or recess which is in communication with the normally upper surface via a plurality of second apertures or recesses or passages of smaller diameter or width or cross sectional area than said first normally lower aperture. Also according to the present invention there is provided an improved stencil having one or a plurality of apertures therethrough for enabling a printing medium to be deposited on a receiving surface and including a first and normally lower surface which in use is to be proximate the receiving surface and a second and normally upper surface over which a squeegee or other printing medium applicator means is to pass, characterised by the feature that : - (a) at least one larger diameter or width first aperture is formed in the first, normally lower surface, and at least one but preferably a plurality of smaller diameter or width second aperture of smaller diameter or width than said first aperture is formed in the second and normally upper surface and leads into said first aperture; or (b) at least one recess or aperture is provided in the normally upper surface and in the normally lower surface and a bridging portion is formed between the upper and lower surfaces having at least one but preferably a plurality of apertures therethrough of smaller diameter or width than that of those of the said recesses or apertures in the upper and lower surfaces . It will thus be appreciated that in at least (a) or (b) above the first aperture is in effect partially closed in a normally upper region by a tie or bridging portion or portions of the stencil material preferably of mesh-like appearance extending across the normally upper end of said lower first aperture to resist any stretching or distorting effect when the stencil is subject to tensioning and, where necessary, acts to reduce the overall height of fluid material being deposited relative to other regions of the stencil with apertures without such smaller width apertures . Preferably a plurality of second smaller apertures are provided for each first larger aperture i.e. preferably the bridging portion is provided with a plurality of said smaller apertures forming a mesh- like appearance for the passage of the printing medium, such as solder paste or adhesive, to the normally lower, larger aperture and form a mesh-like or sieve-like appearance. It will be appreciated that by providing such a finely apertured bridging portion with one or more normally upper apertures to form a mesh, the stencil is strengthened or reinforced in such region and the tendency for stretching - especially with long features or long track apertures - is minimised or avoided. The smaller apertures in the or each bridging portion forming the mesh may be of any desired shape such as circular or hexagonal . It is desirable for an arrangement of apertures to be provided which avoids linear bridging portions which may form a blockage or bar. The smaller apertures in the upper surface i.e. the finer apertures in the bridging portion may even cover a slightly greater area than the diameter of each first normally lower aperture. The height or depth of each first or lower larger diameter aperture determines the height of printing medium which is deposited . Also according to one aspect of the present invention, a method of producing a stencil having apertures for passage of printing medium comprises removing material from the normally lower first side of the stencil to form at least one first aperture extending part way through the stencil material, and removing material from the normally upper and opposite second side of the stencil to form at least one second aperture of smaller diameter or width than of said first aperture with said first and second apertures extending into each other. When the material removing process is etching, a first transparency with continuously opaque areas corresponding to the apertures to be provided in the normally lower surface is formed, and a second transparency is formed with at least one or more partially opaque areas generally corresponding in outline to that of said continuously opaque areas - said partially opaque areas being preferably made up from a plurality of smaller opaque areas or dots of circular or hexagonal or other outline - somewhat similar to the printing effect of tint-in or halftone as used in newspaper photograph printing. The transparencies are applied to opposite sides of a metal stencil to which a photo-sensitive resist has been applied and said resist is then hardened by exposure to UV light and the remaining soft resist under the opaque areas is washed away leaving bare metal which is then etched in known manner. Whilst half-etching is possible i.e. the first and second apertures in the opposite sides may be of equal length with 50% etching other ratio or possibilities are envisaged by adjusting the etching techniques. Production using laser techniques is also envisaged as is metal addition and electroplating. It will be appreciated that a preferably metal stencil is produced which has greater structural integrity and strength for resisting stretching and providing greater accuracy and permitting longer lower first printing apertures or cavities. A stencil according to the invention also enables the hitherto not possible formation of circular or other continuous circumference depositions which otherwise would not be possible because such previously would have created an "island" of stencil material which would fall out. Also, a stencil according to the invention enables the deposition of fluid material of a lesser height in selected areas relative to simple apertures elsewhere in the stencil without such different diameter apertures of the invention. It will be appreciated that it is envisaged in (b) above to be within the scope of the present invention for larger diameter recesses or apertures to be formed in the upper and lower surfaces and for a bridging portion with a smaller aperture or apertures to be formed intermediate the upper and lower surfaces. It has been found that with large areas apertured or "meshed" according to the invention and with an equally large first aperture in the lower surface, there is a tendency for the squeegee, which applies the fluid material, to cause the apertured or mesh area to flex as the squeegee passes over which can result in an unevenness of deposition of fluid therebeneath. This is particularly relevant in the deposition of glue or adhesive. It is envisaged in (c) above to be within the scope of the present invention for the through apertures to be tapered from the lower side decreasing in diameter to the upper side or for each through aperture to be formed from two opposing tapered portions each decreasing in diameter/cross section away from the surface it extends from. In accordance with a further development of the stencil according to the invention, there are formed or remain extending from normally lower surface of the tie or bridging portion of mesh-like appearance at least one supporting projection of the stencil material which when the stencil is supported on the deposition surface acts to support the mesh-like bridging portion to resist flexing resultant from the passage of the squeegee over the top surface . Preferably a plurality of said supporting projections are provided and will be formed by forming opaque areas on the normally lower surface of the stencil prior to etching. Whilst the shape/cross section of the supporting projections may be of any desired shape, such of circular cross section have proven satisfactory. Normally the supporting projections and the apertured stencil material from which they extend will be of a dimension equivalent to or approximating the overall thickness of the stencil since such are an original part thereof. It has been found that if the smaller diameter apertures forming the mesh are defined by surfaces of octagonal shape, and if the supporting projections are provided one at each alternate corner, then such is a satisfactory distribution. Of course the shape and/or size and/or arrangement of supporting projections may be selected as desired. It has been found that a large area of fluid can be deposited which initially has small point gaps in the fluid where the projections are but such readily fill-in, in contrast to known glue-dot deposition. The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan of transparency on reduced scale having opaque peripheral tensioning slots and a central region with an opaque configuration which is to mask a photo-sensitive resist layer on a stencil to be etched to produce apertures in the normally lower first side or surface of a sheet of metal to form the stencil; Fig. 2 is a transparency similar to that of Fig. l except instead of the stencilling configuration in the central region being formed of continuously opaque lines, the lines are made up of a plurality of dots each of any desired shape; Fig. 3 is an enlarged detail of the central region of Fig. 2; Fig. 4a is an enlarged fragmentary detail of the etching pattern of the normally upper surface of a stencil and Fig. 4b of the normally lower surface print or etching arrangement ; Fig. 5 is a schematic illustration of the etching process to produce the normally upper and lower apertures of a stencil. Fig. 6 is a fragmentary enlarged cross sectional view through an aperture and apertured bridging portion of a stencil according to the invention; Fig. 7 is an enlarged perspective detail of the normally lower surface of a stencil showing the first aperture and the lower ends of a plurality of second apertures ; Fig. 8 is an enlarged schematic cross section through a modified embodiment of the invention in which supporting projections extend from the lower surface of the mesh portion having smaller apertures; and Fig. 9 is a schematic plan from below showing the mesh-like portions with hexagonal apertures of the modified embodiment . The views of the transparencies of Figs. 1-3 correspond to an inverted plan and plan of a stencil produced therefrom and thus Figs. 1 to 3 will also be described in terms of a stencil produced therefrom since the apertures produced correspond to the dark portions. Thus on such basis there is produced a stencil 1 of metal foil or sheet having a plurality of peripheral slots 2 for receiving tensioning teeth (not shown) , such as of the tensioning frames disclosed in PCT/GB96/01722. In the example, a central apertured region 3 is illustrated in the normally lower surface/side wherein the apertures 3 ' extending part -way through the stencil are of circular and linear configuration. The normally upper surface or side 5 has a central apertured region 6 of corresponding outline to the apertures 3 of Fig. 1 except that the outline of Fig. 2 is not formed of continuous apertures or recesses as in Fig. l but made up of a plurality of much smaller apertures 6' extending part -way through the stencil Fig. 3 is an enlargement of the region 6 of Fig. 2 (and 4a) showing the plurality of smaller second apertures 6' which are preferably of hexagonal cross-section. Figs 4a and 4b are enlarged fragmentary plan of opaque regions of a transparency for use in the process and correspond to apertures produced in the upper and lower surfaces of the stencil. The black portions result in apertured regions in the stencil since no photo-resist layer results there. Fig. 5 is a schematic illustration of the formation of first and second apertures 3 ',6' in a stencil to which a photo resist layer 7 has been applied. It will thus be appreciated that the stencil according to the invention which will be formed from an integral sheet of continuous material which will normally be metal, may be for deposition of solder paste or adhesive or other printing stencil medium or material has in its normally lower surface continuous cavities of a depth requisite for the deposition of the necessary depth of printing medium and that the upper region of said cavities will be bridged or partially closed by a finely apertured portion of the stencil material in the upper region surface to provide for strength and structural integrity whilst enabling the printing medium to be supplied to said cavities for stencilling operations. The diameter of each smaller second aperture may, for example, be equal or substantially equal to the thickness of the material of the stencil, for example, typically 0.15 mm. Also according to the invention a stencil is provided comprising a sheet of metal or other which is apertured or recessed to form the stencil image in which at least one recess is formed in the normally lower surface of a depth to determine the thickness of printing medium to be deposited and is partially closed by a finely apertured mesh-like portion forming part of the normally upper surface of the stencil which has fine apertures for the passage of a printing medium into said at least one lower recess and for deposition in the requisite area on a receiving surface. Fig. 8 is a fragmentary schematic cross section through a stencil 11 forming a modified embodiment according to the invention. Similarly as in Figs. 6 and 7, the stencil 11 has a normally lower surface or side 14 and a normally upper surface or side 15 which has a central apertured region of corresponding outline to the apertures 3 of Fig. 1 except that the outline of Fig. 2 is not formed of continuous apertures or recesses as in Fig. 1 but made up of a plurality of much smaller apertures 16' extending part-way through the stencil and which are of hexagonal outline - as shown in Fig. 1. A larger diameter aperture 13 ' is formed by etching in the normally lower surface of the stencil and at the same time the smaller diameter hexagonal apertures 16' are formed in the upper surface 15 and extend into aperture 13' and form a mesh-like upper portion 17. On the normally underside 18 of the mesh-like portion 17 a plurality of supporting projections 19 are formed by having a photo-resist layer applied at such points prior to etching. The projections 19 are integral portions of the stencil material and of generally circular cross section and will have curved sides as a result of the etching process. The projection 19 extend downwardly and to or substantially to the plane of the lower surface 14 so as to rest on the supporting surface (not shown) during the application of fluid to be stencilled thereonto. In the example with hexagonal apertures 16', the projections 19 are arranged at every alternate corner of the hexagonal so as to provide adequate support to prevent undesirable flexing of the mesh portion 17 whilst at the same time not providing too much interruption of the fluid material being deposited. A better fluid covering effect can be achieved using the stencil and a more even layer provided which in the use of glue increases the strength of the resultant bond.

Claims: CLAIMS 1. An improved stencil having one or a plurality of apertures therethrough for enabling a printing medium to be deposited on a receiving surface and including a first and normally lower surface which in use is to be proximate the receiving surface and a second and normally upper surface over which a squeegee or other printing medium applicator means is to pass, characterised by the feature that : - (a) at least one larger diameter or width first aperture is formed in the first, normally lower surface, and a plurality of smaller diameter or width second apertures of smaller diameter or width than said first aperture are formed in the second and normally upper surface and lead into said first aperture; or (b) at least one recess or aperture is provided in the normally upper surface and in the normally lower surface and a bridging portion is formed between the upper and lower surfaces having a plurality of apertures therethrough of smaller diameter or width than that of those of the said recesses or apertures in the upper and lower surfaces. 2. A stencil as claimed in claim 1, in which in (a) the aperturing results in at least one first aperture being partially closed in a normally upper region by a tie or bridging portion or portions of the stencil material extending across the normally upper end or normally upper region of said lower first aperture to resist any stretching or distortion effect on tensioning and/or to act to reduce the overall height of fluid material being deposited relative to other regions of the stencil with apertures without such smaller width apertures. 3. A stencil as claimed in claim 2, in which the bridging portion or portions is/are of mesh-like appearance . 4. A stencil as claimed in any of claims 1 to 3 , in which a plurality of second smaller apertures are provided for each first larger aperture so that the bridging portion is provided with a plurality of said smaller apertures forming a mesh-like appearance for the passage of the printing medium to the normally lower, larger aperture and form a mesh-like or sieve like appearance. 5. A stencil as claimed in claim 4, in which the provision of a finely apertured bridging portion with a plurality of normally upper apertures to form a mesh, strengthens or reinforces the stencil in such region as would otherwise exist on a simply apertured region, and any otherwise tendency for stretching is minimised or avoided. 6. A stencil as claimed in any of claims 1 to 5, in which the smaller apertures in the or each bridging portion forming the mesh is of any desired shape. 6A. A stencil as claimed in claim 6, in which the shape is circular or hexagonal. 7. A stencil as claimed in any of claims 1 to 6A, in which an arrangement of apertures are provided which avoids linear bridging portions which may form a blockage or bar. 8. A stencil as claimed in any of claims 1 to 7, which the smaller apertures in the upper surface i.e. the finer apertures in the bridging portion, cover a slightly greater area than the diameter of each first normally lower aperture . 9. A stencil as claimed in any of claims 1 to 9, in which a circular or other continuous circumference first aperture is provided. 10. A stencil as claimed in any of claims 1 to 9, in which there are formed or remain extending from normally lower surface or each tie or bridging portion at least one supporting projection of the stencil material which when the stencil is supported on the deposition surface acts to support the mesh-like bridging portion to resist flexing resultant from the passage of the squeegee over the top surface . 11. A stencil as claimed in claim 10, in which a plurality of said supporting projections are formed. 12. A stencil as claimed in claims 10 or 11, in which the supporting projections are of horizontal cross section. 13. A stencil as claimed in any of claims 10 to 12, in which the supporting projections and the apertured stencil material from which they extend are of a dimension equivalent to or approximating the overall thickness of the stencil since such are an original part thereof. 14. A stencil as claimed in any of claims 10 to 13, in which the smaller diameter apertures forming the mesh are defined by surfaces of octagonal shape, and the supporting projections are provided one at each alternate corner. 15. A stencil formed from an integral sheet of continuous material for deposition of solder paste or adhesive or other printing stencil medium or material includes in its normally lower surface one or more continuous or long cavities of a depth requisite for the deposition of the necessary depth of printing medium and the upper region of said cavities are bridged or partially closed by a finely apertured portion of the stencil material in the upper region surface to provide for strength and structural integrity whilst enabling the printing medium to be supplied to said cavities for stencilling operations. 16. A stencil as claimed in any of claims 1 to 15, which the diameter of each smaller second aperture is equal or substantially equal to the thickness of the material of the stencil. 17. A stencil comprising a sheet of metal which is apertured or recessed to form the stencil image in which at least one recess is formed in the normally lower surface of a depth to determine the thickness of printing medium to be deposited and is partially closed by a finely apertured mesh-like portion forming part of the normally upper surface of the stencil which has fine apertures for the passage of a printing medium into said at least one lower recess and for deposition in the requisite area on a receiving surface. 18. A stencil having one or more apertures therethrough for the deposition of a fluid material onto a lower surface includes at least one first and normally lower aperture or recess which is in communication with the normally upper surface via a plurality of second apertures or recesses or passages of smaller diameter or width or cross sectional area than said first normally lower aperture. 19. A stencil substantially as herein described with reference to the accompanying drawings. 20. A method of producing a stencil having apertures for passage of printing medium, comprising removing material from the normally lower first side of the stencil to form at least one first aperture extending part way through the stencil material, and removing material from the normally upper and opposite second side of the stencil to form at least one second aperture of smaller diameter or width than of said first aperture with said first and second apertures extending into each other. 21. A method as claimed in claim 20, in which the material removing process is etching, and wherein a first transparency with continuously opaque areas corresponding to the apertures to be provided in the normally lower surface is formed, and a second transparency is formed with at least one or more partially opaque areas generally corresponding in outline to that of said continuously opaque areas . 22. A method as claimed in claimed 21 in which said partially opaque areas are made up from a plurality of smaller opaque areas or dots of circular or hexagonal or other outline. 23. A method as claimed in claims 21 or 22, in which the transparencies are applied to opposite sides of a metal stencil to which a photo-sensitive resist has been applied and said resist is then hardened by exposure to UV light and the remaining soft resist under the opaque areas is washed away leaving bare metal which is then etched in known manner. 24. A method as claimed in any of claims 20 to 23, in which half-etching is effected i.e. the first and second apertures in the opposite sides may be of equal length with 50% etching or other ratios or possibilities are effected by adjusting the etching techniques.

Inventors: WILLIAMS DAVID GODFREY, WILLIAMS, DAVID, GODFREY
Applicants: MICRO METALLIC LTD, WILLIAMS DAVID GODFREY, MICRO METALLIC LIMITED, WILLIAMS, DAVID, GODFREY
Application year: 1997
Main classification: H05K3
IPC codes: B41C1, B41M1, B41N1, H05K3