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18 SMT Magazine • November 2016 the thermal pad without any solder mask [1] . The solder mask around the via can keep the solder away from the via and prevent it from flowing into the via. However, the solder mask ring tends to create more voids or unsoldered areas at the thermal pad. On the other hand, the solder can flow into the thermal vias if there is no solder mask ring and result in solder loss and solder protrusion onto the secondary side, which can interfere with the assembly pro- cess and become a quality issue. In this paper, we will discuss the impact of via design, board design and process parameters on solder protru- sion at the thermal pad's vias. QFN voiding is a known industry challenge with many publica- tions [3–6] . The influence of via design and pro- cesses on voiding will also be presented in the paper. EXPERIMENTAL DETAILS Test Vehicle and Components A QFN test vehicle was designed for this study. The test vehicle had the dimension of 177 x 177 mm. The board surface finish was immer- sion silver (I-Ag). Three different board thick- nesses of 1.6 mm (62 mil), 2.4 mm (93 mil) and 3.2 mm (125 mil) were investigated. The image of the test vehicle is shown in Figure 2. THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY Figure 1: Cross-section of QFN and PCB structure. Figure 2: Flex QFN test vehicle Rev 2.