Issue link: https://iconnect007.uberflip.com/i/745476
24 SMT Magazine • November 2016 al hole size measurements are shown in Tables 6-8, respectively. Impact of Via Design on Solder Protrusion Solder flow down to the via was seen for all the PTH via sizes tested in this study. Solder protrusion onto the secondary size was seen for most of the via sizes with no solder mask ring. 0.2 mm (8 mil) via with the actual hole size of ~ 0.10–0.13 mm (4-5 mil) hole didn't eliminate solder protrusion. More solder protrusion was seen for smaller via sizes. On the other hand, 0.51 mm (20 mil) via with window pane stencil design resulted in no solder protrusion for 1.6 mm and 2.4 mm thick boards. Thicker boards didn't prevent the solder protrusion onto the secondary side. In fact, solder protrusion onto the secondary side was seen at 0.51 mm (20 mil) via of a 3.2 mm thick board. It was prob- ably because the higher oven temperature set- ting of the 3.2 mm thick board allowed the sol- der to spread more than the lower temperature settings used for the thinner boards. The sol- der images of different via sizes at the second- ary side of 1.6 mm thick, 2.4 mm thick and 3.2 mm thick boards are shown in Figures 8, 9 and 10 respectively. The solder mask ring helped to prevent the solder from getting into the via. No solder pro- trusion was seen in the via designed with the solder mask ring for 12 mil via using window pane stencil design, as shown in Figure 11. Impact of Via and Pad Design on Voiding On the contrary, the through hole via with the solder mask ring showed more voiding area as compared to the through hole via with no solder mask ring (Figure 12). Voiding differ- ence due to different via size was insignificant (Figure 13). THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY Table 5: Drill size measurement for 3.2 mm thick board. Table 7: Actual hole size measurement for 2.4 mm thick board. Table 6: Actual hole size measurement for 1.6 mm thick board. Table 8: Hole size measurement for 3.2 mm thick board.