Issue link: https://iconnect007.uberflip.com/i/1181966
NOVEMBER 2019 I SMT007 MAGAZINE 95 the reference designators 3, 4, 7, and 8 (loca- tions with solder mask doughnut) on the boards assembled with dot apertures stencil. An example is shown in Figure 25. Alcatel/ Alcatel-Lucent/Nokia have been using the sol- der mask doughnuts extensively in volume for more than 15 years, and protrusion problems have essentially been non-existent (many mil- lions of PCBA). A cross-section verification of the PHT via diameter for the smallest via and the high- est thickness board was done, and the data is included in Figure 26 and Table 6. Figure 22: Segmented stencil, via diameter f2 = 0.23 mm, solder mask doughnut around via. Figure 23: Segmented stencil, via diameter f3 = 0.25 mm. Figure 25: Dot stencil, solder mask doughnut around via, via diameter f2 = 0.23 mm (above) and f3 = 0.25 mm (below); no apparent solder trapped in vias. Figure 24: Segmented stencil, via diameter f3 = 0.25 mm, solder mask doughnut around via. Figure 26: Cross-section at reference designator A7 (f1 = 0.2 mm ~0.00787"), PCA#18 (130-mil thickness).