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December 2014 • SMT Magazine 73 The paste height gets progressively higher for bricks closer to the step edge. Location 6, the one closest to the step edge, provided the high- est paste volume and the largest standard de- viation for all four blade types, with the notch blade showing the highest volume and standard deviation at that location. A more complete picture of the edge effect in location 6 is obtained by analyzing the solder paste volume data for all five runs and all four squeegee blades. Figure 4 shows the solder paste volume for location 6 broken down into the eight columns of the 64 array pattern of the flex circuit pad sites. Column 8 is closest to the step edge. The paste volume increases from column 1 to column 8 as well as the deviation. Column 8 has more paste volume due to paste being left rachel Miller-Short is vice president of sales and marketing at photo Stencil llC. To read past columns or to contact the author, click here. ArTiClE PrINTING TWO-LeveL PcbS IN ONe STeP WITH A 3d eLecTrOFOrm STeNcIL continues on the top side of the stencil. The metal and rubber blades provide a cleaner wipe near the edge leaving less paste on top. The paste vol- ume data is split by print level: • Notch blade was much better on the bottom level than the top level • Slit blade had large deviations on both levels • Rubber blade had low volume on both levels • Straight blade gave best overall results conclusion For two-level printing, printing with a 3D electroform single thickness stencil on two lev- els of a PCB is possible. However, the squeegee blade used in the printer makes a significant difference in the application of the solder paste and the results obtained. The solder paste vol- ume is slightly higher on the top surface for all four blade types tested. The straight metal blade provided the lowest paste volume standard de- viation of all the tested blades. Edge effect due to the raised edge near the step edge caused higher paste deviation near the step edge. SmT Figure 3: Six sites. Figure 4: Solder paste volume shown by rows 1–8 of the FC array for location 6 (8 is closest to step-edge).