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26 SMT Magazine • February 2014 experiment #1: effect of Cutting Parameters and Nanocoating Originally, the laser cutters at the two dif- ferent manufacturing sites were assumed to produce similar results. The considerable dif- ferences between their outputs were not known until the print test results were calculated and walls were examined at high magnification. The goal of Experiment #1—to refine the cut- ting parameters to optimize stencil print per- formance on the assembly line—was obviously not reached. Even comparisons within the da- taset for this manufacturing facility were ham- pered by stencil manufacturing issues on two of the four test sets; however, one trend was abundantly clear. The stencils treated with the first-generation nanocoating consistently pro- duced better yields and better print volume variation. The nanocoated stencils demon- strated slightly lower transfer efficiencies than untreated stencils. experiment #2: New Stencil Materials The experimental materials treated with the second-generation of nanocoating produced the highest yields and best print volume repeat- ability. The FG and EF stencil foils were tested with both first- and second-generation nano- coatings, and in both cases, the second-gener- ation product provided better volume repeat- ability. experiment #3: MicrobGa aperture Shape Square apertures provided better release, better repeatability, and higher print volumes than round apertures of the same major dimen- sion (diameter = side of square). experiment #4: Wipe Frequency Achieving 100% yields at 10 prints per wipe is a considerable achievement. Prior to execut- ing this test, the concept of running this PCB to 10 prints without wiping was completely unre- alistic. The production process wipes after every print. Previous tests on the original test vehicle were able to successfully achieve wipe frequen- cies of 3 prints per wipe using wet wipes with solvents that were chemically matched to the solder paste 3 . Volume repeatability also improved with the extended wipe interval. The influence of under wiping on a stencil treated with the Nano2 is now the subject of a current investigation. General Comments The stencil materials test compared current state-of-the art materials with developmental ones, and the results were extremely encourag- ing. Continued research and development of more sophisticated materials and manufactur- ing processes will help drive continued advance- ments in stencil printing technology and enable better economics in the drive for miniaturization. FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues FEATUrE Figure 23: Effect of stencil nanocoating treatment on flux spread on underside of stencil, ubgA. Figure 24: Effect of nanocoating treatment on flux spread on underside of stencil, QFN.