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22 SMT Magazine • February 2014 Summary of round vs. Square aperture Design Round apertures are the process of record for these 0.5 mm microBGA devices. The results of this experiment indicate that the square apertures: A) Provide 20% more solder paste volume B) Increase transfer efficiency from 83% to 87% C) Lower variation from 16% to 11% The effect of the increased paste volume on reflow yields is unknown at this time. The square aperture design will be implemented on a single product and reflow yields will be closely monitored to quantify the aperture's impact on the overall SMT process. experiment #4: Wipe Frequency The production print process for this prod- uct utilizes a dry/vacuum/dry wipe after every print. The 1 print per wipe interval was set by prior experimentation. To test claims of nano- coating extending wipe frequencies, additional 10-print tests with stencils 5 and 7 were per- formed without any wipes at all. Both stencils were the FG SS cut with the POR; stencil 5 used first-generation nanocoating; stencil 7 used sec- ond-generation nanocoating. 1) Print Yields Print yields improved when the wiping step was eliminated from each print. Running 10 consecutive prints without wiping increased the print yields from 80–90% on stencil 5 and from 90–100% on stencil 7, as shown in Figure 16. 2) Transfer Efficiency Stencil prints at the extended wipe interval showed slightly higher transfer efficiency, as shown in Figure 17. 3) Variation In both cases, the processes that extended the wipe intervals showed the least variation. The trend of the second-generation nanocoat- ing to consistently produce less variation than the first continued, as observed in other com- parisons and shown in Figure 18. Follow-up Tests The results of these tests and comparisons show distinct differentiation between experi- mental inputs and consistent trends among its outputs. They appear to serve as good relative indicators of performance. However, a consider- able difference was observed in comparison to the prior round of tests and typical production results. The test vehicle is a production product, and historically runs 98.2% print yields. It also consistently produces about 82% TE with less than 10% CV. The relatively low yield numbers, FEATUrE FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues Figure 16: Effect of extending wipe frequency on print yields. Figure 17: Effect of extending wipe frequency on transfer efficiency.