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32 SMT Magazine • December 2017 The solder paste volumes were measured us- ing a solder paste inspection system (SPI). The solder paste volume data was analyzed using statistical analysis software and the results are presented in this paper. Results: Step Stencil Thickness Measurements Measurements were taken in each step area for each technology. The 3.5 mil step area was not included due to issues with the measure- ment data. Thickness measurement data is shown below (Table 2). In general, the chemical etching process cre- ated deeper step downs than the nominal val- ue, and the welding and machining processes created steps that are closer to the target depth. The standard deviations of step depth are an in- dication of flatness or roughness in the step ar- eas. The chemical etching process produces a surface that is rougher than the original surface (Figure 4). The welding process involves fixtur- ing a stencil blank into the step area and the blank may not sit perfectly flat as it is welded. The machining process leaves striations on the surface as the cutting tool removes material. Overall, the standard deviations are very similar for each technology. Solder Paste Printing Data —Etched Stencil The solder paste volume box plots for the 3.0 mil, 2.5 mil, and 2.0 mil thick etched steps are shown below (Figures 8, 9, and 10, respec- tively). These are broken out by distance from step edge, aperture size and nano-coating. In general, the larger apertures which are 9.8 x 35.4 mils in size give higher printed sol- der paste volumes. The smaller aperture sizes Table 1: Solder paste printer parameters. Table 2: Step down thicknesses for each step stencil. Figure 10: Solder paste volumes for the 2.0 mil etched step. Figure 9: Solder paste volumes for the 2.5 mil etched step. Figure 8: Solder paste volumes for the 3.0 mil etched step. STEP STENCIL TECHNOLOGIES AND THEIR EFFECT ON THE SMT PRINTING PROCESS