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76 SMT007 MAGAZINE I JUNE 2018 with a coated and uncoated stencil using T4 solder paste at Time 0. Figure 7 shows that once the area ratios drop below 0.63 the limitations of the uncoated stencil are apparent as the CV falls below the prescribed 10% and TE is only border line at 0.56 AR. However, the nanocoated sten- cil exceeds the benchmark as low as an AR of 0.50, or an 8mil aperture in a 4mil foil. These figures represent a very robust and repeatable print process. Figure 8 shows the test results for the T5 solder paste, using the same best-case scenario of square, mask defined pads at Time 0. The trends established by the nanocoated stencil are repeated with the T5 paste in that it meets the acceptability criteria all the way down to the 0.5 AR. On the uncoated stencil, the T5 paste does offer some improvements in overall transfer efficiency and on coefficient of varia- tion on AR of less than 0.63, but only meets the acceptability criteria at 0.56 AR. In other words, the T5 paste can bring some process improvement, but not at the same level as the nanocoating. It is noteworthy that the measurement threshold on the SPI equipment was lowered to 15 µm in the laboratory, rather than 40 µm, which is the typical threshold in production environments. The process improvements documented within this study may not be as perceptible when testing using production inspection parameters. Figures 9 and 10 represent the worst-case scenario—circular, copper-defined pads—at Time 0 with Type 4 and Type 5 pastes, respec- tively. As detailed in the previous study, circu- lar apertures have lower volume as opposed to square apertures and generally poorer release characteristics. It is believed that the circular apertures exhibit inferior release characteris- tics due to equal surface tension of the solder paste around the apertures' periphery. Square apertures have unequal forces acting on the solder paste/stencil wall interface, which helps facilitate paste release. As is plainly demonstrated in Figure 9 the release characteristics of an uncoated stencil with sub-optimal conditions are simply unac- ceptable. An AR of 0.69, which is considered easily accomplished, cannot be repeatedly printed with an uncoated stencil in this scenario. It is important to emphasize that the apertures are 1:1 with specified pad size, and small pad sizes are commonly over-etched, resulting in smaller pads and poor gasketing. The nano - coated stencil, however, again produced accept- able prints all the way down to the 0.56 AR. Figure 9: Print performance of Type 4 powder with worst-case scenario pad design.