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48 SMT007 MAGAZINE I MARCH 2018 Further review of Figure 14 shows the nano- coated stencil apertures improved standard deviation for all test conditions except for the 80mm stencil apertures printing on bare Cu. The effect of nano-coating apertures appears to have the greatest performance benefit for the SM Cu pads, improving standard devia- tion by at least 10%. Also, in 3 of 4 cases, the SM pad print uniformity is worse than the LG pad result, despite showing nearly equal aver- age TE values. The main reason for replicat- ing Test 4 (4) with Test 7 (7) was to confirm the occurrence of large standard deviation and significantly insufficient deposits to be repro- ducible. This was indeed proven. Another feature of the graph is the red dashed horizon- tal line indicating the 10% standard deviation level which marks a common threshold iden- tifying print process control. Values above this typically indicate undesirable printing deposit uniformity, which is the case for nearly all our data. The most controlled print volume distributions result from printing on the bare Cu (i.e., smoothest surface). It is somewhat unexpected to see the higher area ratio aper- tures designed on the thinner 50mm step sten- cil producing consistently higher print deposit scatter compared to the low area ratio stencil apertures on the thicker 80mm foil. The expla- nation for this is the inability for the squee- gee blade to effectively wipe the solder paste cleanly inside the step pocket area. As previ- ously indicated in Figure 11 the step size and geometry is not optimal. A favorable aperture area ratio design is not enough to compensate for a poor stencil wipe. Another view of the M0201 data considers the raw print volume distribution results in the boxplots on Figure 15. It was earlier described in Figure 4, a model for estimating the lowest solder volume accomplishing an acceptable soldered termination result. The result in Table 2 indicates a raw print volume of 0.48nl satisfies this requirement and is identified in Figure 13 as the dashed horizontal red line. It is useful to view this threshold against the raw solder paste print data to improve judgment on M0201 printability. The only test condition which supplied enough solder paste on all ten prints is the 50mm thick step stencil nano- coated apertures. This result may actually be enabled by a poor stencil wipe as the side effect of this can be additional paste volume, confirmed by the high TE reported in Figure 13. Another interesting general observation here is the similarity of raw print volume distri- butions when comparing the 50mm thick step stencil data against the full 80mm thick sten- cil. While we may expect to see more physical Table 7: M0201 paste volume standard deviation data. Figure 14: M0201 Paste volume standard deviations. (same legend applies from Figure 13).