Issue link: https://iconnect007.uberflip.com/i/911509
December 2017 • SMT Magazine 35 The printed solder paste volume did not vary much from 10 mils to 50 mils away from the step edge regardless of welded step thick- ness. This is very similar to the results seen with the etched steps. Tukey-Kramer HSD testing shows some interesting results (Figure 16). In this case, the printed solder paste vol- ume at the 10-mil distance is significantly high- er than the 40 and 50 mil distances. The print- ed solder paste volume is significantly higher at the 20-mil distance than then 50 mil distanc- es. Again, this indicates that the squeegee could not conform down into the step to remove all the solder paste from the surface of the stencil during printing. This was also true for the FPN coated version of the 3.0 mil welded step and the same aperture size. Solder Paste Printing Data— Machined Stencil The solder paste volume box plots for the 3.0 mil, 2.5 mil, and 2.0 mil thick machined steps are shown below (Figures 17, 18, and 19). These are broken out by distance from step edge, aper- ture size and nano-coating. The printed solder paste volume did not vary much from 10 mils to 50 mils away from the step edge regardless of machined step thick- ness. This is very similar to the results seen with the etched and welded steps. Tukey-Kramer HSD testing shows some significant differences in the results (Figure 20). This Tukey HSD analysis shows that the printed solder paste volume is significantly higher at the 10 and 20 mil distances than the 30, 40, and 50 mil distances. This is also true for the 9.8 x 35.4 mil aperture with 2.0 mil and 2.5 mil machined step thicknesses. Conclusions Chemical etching, laser welding, and mi- cro-machining are each valid methods of pro- ducing step stencils. Each process produces Figure 16: Tukey-Kramer HSD analysis for the uncoated, welded 2.5 mil step and the 9.8 x 35.4 aperture. Figure 19: Solder paste volumes for the 2.0 mil machined step. Figure 18: Solder paste volumes for the 2.5 mil machined step. Figure 17: Solder paste volumes for the 3.0 mil machined step. STEP STENCIL TECHNOLOGIES AND THEIR EFFECT ON THE SMT PRINTING PROCESS