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STENCIL

    Chemically Etched Stencils

    Metal mask and flexible metal mask stencils are etched by chemical milling from both sides using

    two positive images. During this process, etching proceeds not only in the desired vertical direction

    but also laterally. This is called undercutting the openings are larger than desired, causing extra

    solder deposit. Because 50/50 etching proceeds from both sides, it results in almost a straight wall

    tapering to a slight hourglass shape in the center.

    Because electroetched stencil walls may not be smooth, electropolishing, a microetching process,

    is one method for achieving a smooth wall. Another way to achieve smoother side walls in the aperture

    is nickel plating. A polished or smooth surface is good for paste release but may cause the paste to

    skip across the stencil surface rather than roll in front of the squeegee. This problem can be avoided

    by selectively polishing the aperture walls without polishing the stencil surface. Nickel plating further

    improves smoothness and printing performance. However, it does reduce aperture opening and

    requires artwork adjustment.

     

    Laser-cut Stencils

    Laser cutting also is a subtractive process, but it does not have the undercutting problem. The stencil

    is produced directly from the Gerber data, so aperture accuracy is improved. The data can be adjusted

    to change dimensions as necessary. Better process control also improves aperture accuracy.

    Another benefit of laser-cut stencils is that the walls can be tapered. Chemically etched stencils also

    can be tapered if they are etched only from one side, but the aperture size may be too large.

    A tapered aperture with an opening slightly larger on the board side than on the squeegee side

    (0.001 to 0.002" to produce an angle of about 2) is desired for easier paste release.

    Laser cut is capable of producing aperture widths as small as 0.004" with an accuracy of 0.0005",

    so it is very suitable for ultra-fine-pitch component printing. Laser-cut stencils also produce ragged

    edges because the vaporized metal is transformed into metal slag during the cutting process.

    This can cause paste clogging. Smoother walls can be produced by microetching.

    Laser-cut stencils cannot make stepped multilevel stencils without prechemical etching of the areas

    that need to be thinner. The laser cuts each aperture individually, so stencil cost depends upon

    the number of apertures to be cut.




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