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34 SMT007 MAGAZINE I OCTOBER 2026 addendum to ensure standardized measurement practices. The key factors investigated included surface finish (OSP vs. ImSn), nano-coated stencil thickness (4 mil vs. 5 mil), reflow profile (Profile A vs. Profile B), and solder paste. Both overall void percentage and maximum individual void size were used as response variables. The objective of this work is to provide a clear, data-driven ranking of factor importance for voiding control in QFN assemblies. The goal is to reduce voiding, improve assembly reliability, and support robust electronics production for demanding auto- motive environments. In light of voiding concerns, attempts have been implemented to determine if the newly developed mixed alloy solder paste (DFHR: SnAgCuBiSbInNi) could deliver voiding perfor- mance comparable to, or even lower than, SAC305, the most commonly used lead-free solder. 1-7 Solder Pastes The solder pastes selected in the current experi- ment are shown in Table 1. SAC305, Flux A, type 4, 88.5 wt.% solder paste (Paste 1) was used as a control to compare to the selected high-reliability (hi-rel) paste, and DFHR + Flux B, Type 4, 89.0 wt.% solder paste (Paste 2). Two halogen-free flux formulations were evalu- ated in this study. Flux A is a robust formulation designed to enhance stencil transfer efficiency, minimize voiding, and extend stencil life. Flux B is optimized for reduced voiding while maintaining excellent printability and formulation stability. Halogen-free fluxes are the industry's preferred path forward for meeting modern environmental, safety, and reliability requirements, especially in high-reliability or no-clean applications. Halogen- containing fluxes are still used in legacy or specialty cases but carry trade-offs in regulatory exposure. Stencil Design The two test vehicles used stencils with different thicknesses. Even small changes in stencil thick- ness can significantly affect solder volume, despite an identical nine-pane window design. This is because stencil thickness directly influences paste transfer efficiency and the effective aperture area ratio, which, in turn, alters the deposited solder volume and coverage. Both stainless steel stencils had a laser-cut, nano-coated surface and featured the same nine- pane window design, as shown in Figure 1. Each pane consisted of a 2.24 mm (88 mil) square aper- ture spaced 0.50 mm (20 mil) apart, resulting in a center-to-center pitch of 2.74 mm (108 mil). The full window array measured 7.72 mm (304 mil) across. In both cases, the total stencil aperture-to-pad area ratio was 75%. The first stencil was 4 mils (0.102 mm) thick and had an area ratio of 5.5 per aperture. The paste volume was 0.51 mm³ per pane, for a total of 4.59 mm³ across the array. The second stencil was 5 mils (0.127 mm) thick. This reduced the area ratio to 4.4 but increased the paste volume to 0.64 mm³ per pane, totaling 5.76 mm³ for the array. Experimental Procedure Both solder pastes were printed on the PCBs at a speed of 50 mm/s using a print pressure of 5 kg. The stencil separation distance was 2 mm at a speed of 5 mm/s. Print pressure was optimized by printing four setup boards. Any printing defects were observed Figure 1: QFN68 thermal pad stencil design with standard cross-hatch.