SMT Magazine

SMT-Dec2017

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72 SMT Magazine • December 2017 release would provide an indication of stencil performance. Test Design The stencil analysis included: • 6 different stencil suppliers • 9 different foil materials • 5 different manufacturing processes • 7 different nanocoatings The experimental design was not a full fac- torial. Each supplier provided stencils using technologies that were either their top per- formers (high end), developmental technolo- gies that they wanted to learn more about (sup- plier choices #1 and #2), or lower cost stencils commonly ordered (AR>0.70). One to four sten- cils were submitted by each supplier. A total of 18 stencils were print tested. All were created using the same Gerber file, and all were spec- ified at 0.0040" thick with identical apertures depending on the feature. The stencil test ma- trix is shown in Table 1. EVALUATION OF STENCIL TECHNOLOGY FOR MINIATURIZATION The Benchmark test vehicles were printed at the Benchmark Electronic Inc., Nashua, New Hampshire facility on a DEK 265 screen print- er utilizing 18 stencils over a duration of three days. The solder paste was Pb-free, SAC305 no clean, Type 4 mesh. For each stencil, a total of six boards with a 10-minute delay between prints were printed by the same operator, using identical and common machine print parame- ters. The cards were printed in the same order and the underside of stencil was dry wiped after each print. All boards were measured with a Koh Young solder paste inspection (SPI) platform. The 1st and 6th boards were photographed to provide a visual assessment of print repeatabil- ity; these images appear in Appendix I. Each stencil was photographed after completion of the dry wipe following the 6th print. These im- ages also appear in Appendix I and provide a relative comparison of paste release. Specifical- ly, stencil apertures with the less amounts of paste indicate better release. Stencil Assessment Criteria This study incorporated multiple parame- ters to assess stencil performance: • Dimensional accuracy of the aperture opening or size accuracy • Visual assessment of the print and stencil after the final print to determine the amount of paste remaining in the apertures • Topography of the aperture walls • Analysis of SPI data for transfer efficiency and repeatability Size Accuracy of the Apertures Miniature components require apertures that are within specification. Apertures that are too small increase the risk of insufficient solder de- fects due to poor paste release and apertures that are too large increase the risk of bridging and solder balls. Inconsistent apertures on 01005 or 0201 components increase the risk of tombston- ing. The stencil providers were asked to include four small coupons outside the print area that could be removed and measured for accuracy and topography assessment. The latter is a destructive test. A stencil coupon is shown in Figure 2. Table 1: Test stencil submissions.

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