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28 SMT Magazine • November 2016 Figure 17 shows the voiding of a QFN compo- nent reflowed at low and high temperature us- ing air and nitrogen reflow. Smaller voids were observed with air reflow, and less voiding was seen with higher temperature profile. This phe- nomenon is opposite to the BGA voiding in which less voiding is seen with nitrogen reflow and lower temperature. Impact of Assembly Process on Solder Protrusion The main effects plot for solder protrusion from samples reflowed in air and nitrogen en- vironment is shown in Figure 18. The data showed that less solder protrusion was seen when the sample was reflowed in air as com- pared to nitrogen atmosphere. One explanation is that the solder didn't spread as much in the air atmosphere and resulted in less solder flow- ing into the via. Figure 16: Impact of board thickness and package type on voiding. Figure 17: Reflow temperature and atmosphere impact on voiding QFN88. a) low profile in air; b) high profile in air; c) low profile in N2; d) high profile in N2. Figure 18: Impact of reflow atmosphere on solder protrusion. THE IMPACT OF VIA AND PAD DESIGN ON QFN ASSEMBLY