Squeegee position

Fix a pattern template to the work-table with tape (position the pattern in the correct place with a screen placed in the printer frame and tape it to the table). Using a sharp probe, punch through the acetate over the holes in the work-table to allow the vacuum to be applied to the substrate. The substrate is surrounded by a piece of Melinex or PVC of the same thickness (taped to the work-table) so that the pressure between the squeegee and the working surface is even throughout the print stroke, and an even thickness of the printed layer is achieved. 

Relative positions of substrate, paste, squeegee, and screen prior to printing. 

The mesh itself is usually based on a plain weave pattern. Some important properties of the screen mesh are the size and density of the strands (usually quoted in terms of lines per inch), the tension, the orientation, and the material, hi addition, the choice of mesh material must ensure that the printed deposit is uniform. The mesh material must be precisely woven and have uniform mesh apertures. The fabric should also be flexible enough to enable good contact all over the substrate. The fabric needs to be resilient so that the mesh returns to its original position after the printing stroke. The squeegee itself is in contact with the fabric for most of the printing stroke, so the finish of the fabric must be slippery and smooth so that the resistance to the squeegee is minimal. The mesh material must also be chemically stable and very resistant to attack... 

In the process in which the paste is transferred to the substrate (green sheet), the squeegee presses down the saeen, and it is important for the screen to come in contact with the green sheet positively. It is also important for the screen to separate from the green sheet positively in the locations where the squeegee passes. 

In printed-circuit board production, stencil printing is the most common process for applying solder paste. A metal stencil is positioned on the blank circuit carrier. The holes in the stencil correspond to the spots where the solder depots will be applied to the board. A squeegee blade passes over the stencil, filling the holes with solder paste. When the stencil is lifted clear, the solder depots are left on the board (Fig. 4.6). All solder-paste depots on the board are applied in a single process step, permitting a high rate of productivity. 

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