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JUNE 2018 I SMT007 MAGAZINE 79 indicating more consistent release and better process control. Figure 12 shows an example of dog ears from the SPI machine. Foil Mounting Tension The final variable analyzed is one of the latest stencil innovations that promise to help improve print quality: the tension of the foil mounted in the frame. As previously mentioned, standard stencil tension is typically 35-40 N/cm and high tension ~50+ N/cm. The concept is that when mounted with higher tension, the foil will provide a more stable print platform with less deflection or reverber- ation and produce better results. High tension foils require reinforced mesh and frames to withstand in increased forces and justify a cost premium. This experiment tested uncoated stencils at standard and high tensions, with both T4 and T5 solder pastes. Uncoated sten- cils were used to isolate the effect of tension- ing. The stencils were tensioned as foils pre- mounted on their frames before cutting. The 0.5 and 0.4 mm BGAs were analyzed first. The performance of both stencils, includ- ing pause times of 0, 30, 60 and 90 minutes, met the goals of ≥80% TE and ≤10% CV, for both BGAs and both paste sizes. The TEs and CVs were similar for both the high and standard tension foils. Given the unremarkable nature of the BGA tests, the data was further mined to investigate differences for smaller apertures or pause times. In the best-case scenario of square, mask-defined pads, the higher tension demonstrated no distinct advantages, as shown in Figure 13. Both tensions printed well down to a 0.62 AR, but just missed the accep- tance criteria at 0.56. Type 5 paste opened the window slightly by enabling a 0.56 AR (Figure Figure 11: Average heights of rectangular deposits. Figure 12: Solder paste peaks or dog ears.