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22 SMT Magazine • November 2016 RESULTS AND DISCUSSION Via Measurement The via diameter of different via sizes was measured before the assembly. Both drill size and hole size were measured according to Figure 7. The measured drill sizes were consistent and close to the theoretical drill sizes. The hole size was smaller than the drill size. It was expect- ed because of the copper plating at the via. The drill size measurements of different board thick- nesses are shown in Tables 3-5, and the actu- THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY Figure 6: Reflow profile of 3.2 mm thick board. Figure 7: Drill size and hole size definition. Table 4: Drill size measurement for 2.4 mm thick board. Table 3: Drill size measurement for 1.6 mm thick board.