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36 SMT Magazine • December 2017 different step surfaces. Regardless of the tech- nology used to produce the step, solder paste volumes for the QFN apertures tend to be high- er 10 to 20 mils from the step edge than 30 to 50 mils away. These increased solder paste vol- umes could lead to shorts with aperture designs for the tested QFN component designs. It would be possible to place apertures this size 30 mils from the step edge and obtain an acceptable solder paste volume. The smaller 8, 8 x 9, and 10 mil apertures gave statistically similar solder paste volumes from 10 to 50 mils away from the step edge. Although further investigation is needed, the data shows that small aperture components can be placed as close as 10 mils from the step edge and still obtain acceptable print volumes. The FPN coating showed a slight increase in volume across all apertures measured. When printing these small apertures, it is recommend- ed to apply a FPN coating. It is apparent that these step stencil technologies bear further in- vestigation to differentiate between them. Future Work Testing is ongoing with these three step stencil technologies. Solder paste volumes at the center of the step area will be compared to volumes near the step edge. The solder paste volumes from a single level stencil of the same thickness as the step-down area will be com- pared and contrasted to the volumes in the step etched stencils. The printed solder paste vol- ume from apertures oriented horizontal to the squeegee will be compared to apertures orient- ed vertical to the squeegee within step areas. Squeegee pressure and speed will be varied and the effects on solder paste volume will be stud- ied. Finally, we plan to increase the number of boards printed to obtain a larger set of data to expand on these findings. Acknowledgments We appreciate the support of MET for providing the welded step stencils for this investigation. We also appreciate the support of Fine Line Stencil, who provided the etched and machined step stencils and the nano- coating. SMT Editor's Note: This article was originally published in the proceedings of SMTA International. Greg Smith is the manager of stencil technology at BlueRing Stencils. Bill Kunkle is a technical service manager at BlueRing Stencils. Tony Lentz is a chemist and field application engineer at FCT Assembly. Figure 20: Tukey-Kramer HSD analysis for the uncoated, machined 2.0 mil step and the 8 x 9 aperture. STEP STENCIL TECHNOLOGIES AND THEIR EFFECT ON THE SMT PRINTING PROCESS