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24 SMT Magazine • February 2014 Investigation into the source of the varia- tion among test stencils and their performance differences indicated considerable dissimilarity between the stencil manufacturing processes at the site making the production stencils and the site making the test stencils. The site pro- viding the test stencils had recently undergone an equipment upgrade, which could be the root cause of the observed performance differences, including the overall lower yields and higher volume variations, and the specific issues noted on stencils 1 and 4. It is under investigation by the supplier at the time of publication. SeM analysis Test coupons were cut into each stencil (ex- cept the experimental SS) during their regular manufacturing process for surface roughness analysis. All the laser cut SS stencil walls dem- onstrated high levels of striation. Of particular interest was the comparison of wall topography of the POR stencils. The apertures cut using the same parameters at the different facility demon- strate a much smoother wall finish when viewed at 800X magnification, as shown in Figure 22. It is likely that the lower yields and higher varia- tion are a direct result of the rougher, more highly striated aperture walls. Discussion and Conclusions Despite the experimental noise presumably introduced by the different stencil manufactur- ing site, the trends in the data are consistent throughout the series of tests. FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues FEATUrE Figure 22.