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76 SMT Magazine • December 2017 to squeeze out. Several other stencils exhibited light slag, referred to as "streaks," which is be- lieved to be related to the relative movement of the laser and table during cutting. The average roughness and bottom slag observation for all are listed in Table 4. The small streaks of metal measured less than five microns high, and would not be ex- pected to have a major impact on volume. Analysis of Solder Paste Inspection (SPI) Data Transfer efficiency (TE) and coefficient of variation (CV%) are excellent indicative met- rics in assessment of overall stencil print per- formance. In the present study these calcula- tions were performed using inline SPI, mainly at miniature component sizes of interest: 0.3 mm BGA, 0.4 mm BGA and 0201H locations on all six cards printed by each stencil. The average print volume and standard deviation calcula- tions of SPI data from six prints on each device type provide statistical indices for the compari- son of each stencil's release characteristics, such as paste deposit and print repeatability. EVALUATION OF STENCIL TECHNOLOGY FOR MINIATURIZATION The aperture size at 0.3 mm BGA, 0.4 mm BGA and 0201H locations were measured di- rectly on each stencil. The theoretical volume for each aperture was calculated. The specifica- tion defined area ratios for 0.3 mm BGA-square, 0.4 mm BGA-square and 0201s—circular shape features for 4 mil stencil thickness are 0.45, 0.55 and 0.70, respectively. The actual area ratios for these features not calculated in present work are slightly different on many stencils due to mea- sured variation in actually cut aperture sizes. A similar study showed aperture size variation was generally within 2% on any given stencil, but as much as 22% different from stencil to stencil, which can have a considerable impact on the SMT process. 3 Transfer efficiency (TE) is calculated as the ratio of average deposit volume to the aperture volume to obtain a measure of stencil's print transferability. The coefficient of variation (CV %) is calculated as the ratio of standard devia- tion of paste volume to average paste volume and it serves as a measure of paste deposit re- peatability from print to print. A generally ac- Figure 6a: Bottom side slag on C11. Figure 6b: Bottom side streaks on stencil C11. Table 4: Stencil coupon roughness and observations.