Stencil technology is one of the keys to successful printing, along with the solder paste and printer. Most engineers know there are different options for stencils: etched brass or stainless steel, laser-cut stainless steel, polyimide or electroformed nickel. In the early days of SMT, poor control of the etching process and incorrect material specification resulted in center-aperture stencil reductions. Today, many etched stencils remain in use, though not all with hourglass aperture reduction. FIGURE 1 shows the reduction in aperture size in the middle of the stencil foil, which will have restricted clean paste separation and lead to variability in paste volume. FIGURE 2 shows no hourglass shape on the side of the stencil wall. As stated, never assume the stencil is what you expect. Always check, as it's the process yield we are trying to improve.
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