Issue link: https://iconnect007.uberflip.com/i/745476
30 SMT Magazine • November 2016 Cross Sections Cross-section images showed that vias at the middle of the thermal pad typically had more voids as compared to the vias located closer to the edge of the pad. It was expected because the edge of the component thermal pad would allow more air to be outgassed. The cross-section also revealed the outgassing mechanism and how it would af- fect solder protrusion. As the solder melted and spread, it would get into the via and fill a portion of the via. The outgassing of the air bubble would push the solder into the via and flow onto the sec- ondary side or expand the existing air gap (void) in the via. Figure 19 and Figure 20 showed the cross-section images of 0.22 mm (9 mil) via and 0.25 mm (10 mil) vias from 1.6 mm thick boards. Figure 21 showed the cross-section images of a 0.25 mm via using a 2.4 mm thick board. Conclusions Solder was seen flowing into the PTH via with no solder mask ring during the reflow pro- cess regardless of the via sizes. The smallest via size was 0.20 mm (8 mil) from the study. Small- er vias didn't result in less solder protrusion on the secondary side. However, less solder protru- sion was seen with larger vias. Via size of 0.51 mm (20 mil) with no solder mask ring could re- sult in no solder protrusion, depending on the process condition. Board thickness had no sig- Figure 19: The cross-section of 0.22 mm (9 mil) via using 1.6 mm thick PCB. Figure 20: The cross-section of 0.25 mm (10 mil) via using 1.6 mm thick PCB. Figure 21: The cross-section of 0.25 mm (10 mil) via using 2.4 mm thick PCB. THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY