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26 SMT Magazine • November 2016 The study showed that PTH via resulted in less voids and smaller voids than no via and via in pad. It was because PTH via created a chan- nel for air outgassing during the reflow and sol- der joint formation. The voiding X-ray images of the thermal pad designed with no via and via in pad are shown in Figure 14. Breaking the large thermal pad into small- er pads resulted in smaller voids. However, the overall void percentage didn't decrease signifi- cantly (Figure 15). Impact of Board Thickness and Component Type QFN package design played a critical role in voiding formation of the solder joint. The data showed that dual row QFN resulted in more voiding than the single row QFN component. Similar QFN type, but different package design, would result in different voiding level, as shown in the case of QFN132 and QFN156 (Figure 16). Thicker boards didn't result in more voids or less solder protrusion with a similar profile (Fig- ure 16), but higher temperature and hotter pro- file resulted in more voids (Figure 17). Impact of Assembly Process on Voiding In general, slightly more voids and bigger voids were seen when temperature increased. THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY Figure 12: X-ray images of QFN with no solder mask ring around thermal via and with the solder mask ring around the thermal via. a) PTH via with no solder mask, b) PTH via with solder mask ring. Figure 14: Voiding images of QFN thermal pad with no via, and with via in pad. Figure 15: X-ray images of voiding at large pad vs. small pads. Figure 13: Impact of via size on voiding.