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34 SMT Magazine • December 2017 show some differences in solder paste volume. Tukey-Kramer Honest Significant Difference (HSD) testing shows that most of these varia- tions are statistically similar. This means that there is very little difference in printed solder paste volume from 10 to 50 mils from the step edge. There is one exception to this (Figure 11). The printed solder paste volume is higher for the 10 and 20 mil distances than the 50 mil distance. This indicates that the squeegee is not able to conform into the step pocket and squee- gee the paste cleanly away from the surface. The same Tukey HSD analysis is true for the uncoat- ed version of this step and aperture size. It is easy to see solder paste residue left near the step wall after printing. This seems to corre- spond to higher printed-paste volumes. Solder Paste Printing Data— Welded Stencil The solder paste volume box plots for the 3.0 mil, 2.5 mil, and 2.0 mil thick welded steps are shown below (Figures 13, 14, and 15). These are broken out by distance from step edge, aper- ture size and nano-coating. Figure 11: Tukey-Kramer HSD analysis for the FPN coated, etched 3.0 mil step and the 9.8 x 35.4 aperture. Figure 12: Etched 3.0 mil step after one print. Figure 15: Solder paste volumes for the 2.0 mil welded step. Figure 14: Solder paste volumes for the 2.5 mil welded step. Figure 13: Solder paste volumes for the 3.0 mil welded step. STEP STENCIL TECHNOLOGIES AND THEIR EFFECT ON THE SMT PRINTING PROCESS