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JUNE 2018 I SMT007 MAGAZINE 83 been known to introduce variability to both the print and reflow process due to the increased interaction between the flux medium and solder powder, and smaller mesh sizes often pose increased material cost and availability issues. Therefore, it is recommended that users facing fine pitch printing issues first check their basic process setup, then investigate nanocoat- ing prior to trying T5 solder paste. The benefit of high tension stencil foil mount- ing is less clear than the benefits of nanocoat- ing or finer mesh size. Varying performance improvements were witnessed with higher tension, but they were slight and inconsistent. Users of high-aperture density stencils report incremental improvements in print quality in production settings; however, that was not consistently reproduced in this laboratory test. The benefit of high tensioning may be better realized with a different stencil design, as the test stencil had relatively low aperture density. Similarly, the test stencil was 4 mil (100µm) thick, whereas a thinner stencil—which would be more prone to deflection—may show more benefit from increased foil tension. Finally, it should be noted that the data analyzed in this study was produced in a highly controlled laboratory environment, by professional process engineers, using high-resolution measurement techniques. Similar results should not be anticipated in production environments with typical noise and standard SPI measurement techniques. It would be reasonable to expect similar trends in production, but with somewhat lower TEs and somewhat higher CVs. Therefore, the data should not be interpreted as a direct recommendation of print capability on an assembly line; however, it serves as an excellent basis on which to propose the following guidelines: • Nano-coated stencils should be used whenever area ratios are lower than 0.66 • T4 mesh powder is well suited to print area ratios as low as 0.50 when nano coating is applied • High-tension foils may offer slight benefits, but more study needs to be performed A representative subset of the data is provided within the paper; more details of the extensive analysis are available upon request. SMT007 References 1. O'Neill, T., and Tafoya, C., "The Impact of Reduced Solder Alloy Powder Size on Solder Paste Print Performance," Proceedings of IPC APEX EXPO, Las Vegas, Nevada, 2016. 2. Jabil Solder Paste Evaluation Board, avail- able from Practical Components. 3. O'Neill, T., and Seelig, K., "Squaring the Circle," Circuits Assembly Magazine, February 2016. Editor's Note: This article was originally published in the proceedings of SMTA Interna- tional. Tim O'Neill is the technical marketing manager at AIM Solder. Carlos Tafoya is the international technical support manager at AIM Solder. Gustavo Ramirez is a process application manager at AIM Solder (no image available). The benefit of high tension stencil foil mounting is less clear than the benefits of nanocoating or finer mesh size.

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