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56 SMT Magazine • December 2017 side of the square and the aperture, to include the form of the aperture. Based on the results, the NF-factor can be calculated as: To calculate the new area ratio the NF-fac- tor will be added to the common area ratio cal- culation: This extended formula applied to conven- tionally low area ratio east-west oriented aper- tures is considered to improve accuracy of area ratio specification for such aperture designs. While the apertures with north-south direction still show better transfer efficiency results com- pared to squares, the correction by the NF im- plementation reduces such bias. This exercise serves as a first idea to rethink common rules and consider new requirements provoked by miniaturization to find new ways of achieving more process adapted stencil designs. Conclusion and Outlook This paper discussed the effect of the aper- ture shape and orientation on solder paste print- ing performance. For the experiments three dif- ferent stencil thicknesses (80 µm, 100 µm and 120 µm), two different solder paste types (Type 4 and 5) and two manufacturers were used. For apertures with identical area ratios a strong influence is induced by a varying aspect ratio and the orientation of these structures to the printing direction. The results focusing on the transfer efficiency and the standard devia- tion show, that the shape of a rectangle and an east-west orientation achieve the best results, as illustrated in Figure 10. Furthermore, the solder paste type can have an influence, especially when relatively narrow apertures are printed. Based on the boundary conditions of the PCB layout within the print- ing tests the thicker stencils have the higher as- pect ratios and achieve better printing results. In general, the printing results show that also area ratios lower than 0.66 can be quite regularly printed above 75% transfer efficien- cy, with dependencies on the shape of the ap- ertures. This leads to the idea to extend the cur- rent calculation of the area ratio by introduc- tion of the leveling factor NF. Using NF for the calculations, the present assumed design limita- THE EFFECT OF AREA SHAPE AND AREA RATIO ON SOLDER PASTE PRINTING PERFORMANCE Figure 10: Mean increase of the transfer efficiency in relation to the aperture shape and orientation.