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By T. Sasson, Suron A.C.A Ltd.
Many SMT assemblers use both fixed-frame and universal SMT stencils in the paste printing process. Conventional methods for adhering stencil to frame by conventional adhesives are relatively slow, more expensive and can introduce hazardous materials. One process borrows technology from sewing methods to attach stencils onto frames without adhesives or glues.
Many SMT assemblers require frame-mounted and universal SMT stencils at the same time for the paste printing process. One process that can supply both is based on sewing technology.
Fixed SMT stencils comprise a stainless-steel SMT stencil attached to a screen frame, which holds the stencil under constant strain for long time. This type of layout is usually chosen for production in large quantities, or in some cases for complicated stencils.
Most stencil producers use adhesion techniques to produce fixed SMT stencils, which is done by attaching the stencil edge to the plastic mesh. The main advantage of this process is its simplicity; it does not require special equipment or knowledge, and above all can be applied easily in any production facility. We have in the past used polyurethane-base adhesives to do this. The polyurethane gives good flexibility and is relatively low cost, but this method requires long cure times. A common assembly process could take one hour to prep and four hours to cure. Other adhesive options have shorter cure times, but can be more expensive.
To save time in the production process of fixed SMT stencils, a new assembly process was developed, based on a sewing method. The result is a simple assembly process: low cost and environmentally friendly. The sewing process is also providing an elastic structure between the stencil and the mesh, which is derived from the natural properties of the sewing stitch.
To carry out the sewing process, the stencil is pre-perforated with through holes. This step in the production process facilitates the passage of the sewing needle through the perforation of the stencil. The distances between the perforation holes are uniform to comply with the sewing machine's needle stroke. Then the stencil is centered and glued to the plastic mesh with fast adhesive in few selective points, to prevent movement of the stencil in the sewing process. Then the stencil goes through a conventional sewing machine, which is equipped with Kevlar thread.
ConclusionThe main advantages of this process are that the stencil-to-frame attachment time is relatively quick; the costs of the required materials are relatively low; and there are no hazardous materials are used, as can be the case when adhesives are introduced. Time of production is about half an hour for the typical application. The process is relatively clean and does not require masking and protection of the stencil. Should the stencil need repair, the disassembling process is simple just cut the stitch.