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40 SMT007 MAGAZINE I JULY 2019 are 0.4 mm wide (15.7 mils) and spacing ranges from 0.1–0.4 mm (3.9–15.7 mils). During reflow, the solder paste bricks spread together down each line. A wetting or spread percentage is calculated by counting the number of solder paste bricks that bridged together and dividing by the total possible number of bricks. Ideal sol - der paste performance is 100% wetting. Solder balling is measured using solder ball- ing/pullback patterns. The solder paste is over- printed onto the solder mask. When the sol- der paste reflows, it pulls back into a central sphere of solder. Random solder balls are left behind in the flux pools (Figure 12). The overprint percentages range from 500– 1,250% over the pad size. Solder balling is evaluated by the largest overprint percentage that has: zero solder balls, less than five solder balls, and less than 10 solder balls. Ideal sol- der paste performance is 1,250% overprint in each category. Graping is measured using graping pat- terns. These patterns include square and circu- lar, solder mask defined and non-solder mask defined pads. The pad sizes range from 0.18 mm (7 mils) to 0.30 mm (12 mils), and the cor- responding stencil aperture area ratios range from 0.35–0.60 respectively (Figure 13). After reflow, the solder deposits are evalu- ated for graping. Graping typically occurs at the smaller solder paste deposits (Figure 13). A graping percentage is calculated by dividing the total number of solder deposits with grap- ing by the total possible. Ideal solder paste per- formance is 0% graping. Voiding The larger quad-flat no-lead (QFN) thermal pads were used for void measurements. The QFN components have 68 perimeter leads on a 0.5-mm pitch, 10-mm body size, and matte tin finish (Figure 14). The stencil design was identical for each QFN location (Figure 15). In each case, the sol- der paste coverage was approximately 65% of the thermal pad area. Figure 12: PR test board solder balling/pullback patterns. Figure 14: QFN (MLF68) dummy component. Figure 13: PR test board graping patterns.