Issue link: https://iconnect007.uberflip.com/i/989774
60 SMT007 MAGAZINE I JUNE 2018 shown in Figure 7. The resultant voiding was approximately in 22% average range. ImmSn board finish and the high soak profile marginally drove to lesser voiding. Reflow atmosphere had very little impact on the void- ing results overall. Unplugged vias were the most problematic of the three vias design in this study. Focusing on the results for PF+paste config- uration only, an average voiding of 9% was measured. The overall average voiding for PF+paste configuration was more than two times lower relative to the SPO configuration. Figure 8 shows the main effect of various parameters on the voiding of the PF+Paste configuration. Unlike the SPO configuration, the primary contributor to voiding in the PF+Paste configuration was the reflow atmosphere. Reflow in nitrogen reduces voiding nearly to 5%. Clear corre- lation with component size and voiding perfor- mance was observed. Void- ing levels of about 12% were observed on the larger QFN component with thermal pad 5.4 x 5.4 mm. Reduction in the size of the thermal pad down to 3.6 x 3.6 mm reduced voiding level down to around 9%. As expected, the smallest package with the thermal pad of 2.1 x 2.1 mm showed the lowest voiding level and it was about 5%. Like SPO configuration, assemblies with unplugged vias were the most problematic and produced higher void- ing levels compared to another pad design. Earlier conducted studies were undertaken to understand the effects of micro-flux coated solder preforms and voiding for bottom termi- nation components (BTC). In the last several years a lot of advancement was achieved in flux chemistry development to improve void- ing performance significantly. In the previous voiding studies [3,4] it was identified that the higher preform-to-paste volume ratio on the pad contributed to lower voiding, specifically in nitrogen. It is evident that the increased Figure 7: The main effects for SPO configuration. Figure 8: The main effects for PF+paste configuration.