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20 SMT Magazine • November 2016 Six different QFN packages with different pin counts and component body size were in- cluded in the test vehicle. Both single row and dual row QFN components were studied. The QFN pitch varied from 0.4 mm, 0.5 mm to 0.65 mm. The QFN component body size ranged from 3 x 3 mm to 12 x 12 mm. The details of the QFN components is summarized in Table 1. Design Variables Many via variables were designed into the test vehicle, including via size, via pitch and via design. Five different via sizes were investigated. They were 0.20 mm (8 mil), 0.22 mm (9 mil), 0.25 mm (10 mil), 0.30 mm (12 mil), and 0.51 mm (20 mil). Via spacing was 0.5 mm, 1 mm and 1.27 mm. Most through hole vias with no solder mask ring were used while some vias were designed with the solder mask around the via. Table 2 summarizes the via design variables. Process Variables Besides the component, board thickness and via design variables, the study also included two different stencil designs. Window pane aperture opening and 1:1 pad aperture opening stencils were used. For the window pane design, the sol - der paste was printed away from the vias except at the 0.5 mm via pitch locations. For the 1:1 pad design, the paste was printed over the vias. The example of the window pane aperture open- ing is shown in Figure 3. In addition, the boards were reflowed using air and nitrogen, and were reflowed using two different reflow ovens. Reflow Profiles Three similar lead-free reflow profiles were generated for each board thickness. The oven temperature settings were used based on the board thickness. To achieve this, higher oven temperatures were used for thicker board while THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY Table 1: Details of QFN Components. Table 2: Via Design Variables. Figure 3: Example of window pane aperture opening.