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76 SMT007 MAGAZINE I OCTOBER 2018 For the next stage of the profile the voiding changes noticeably; see Figure 7. Rather than the smaller, yet high frequency voids seen at 155°C, now there are much larger pockets of gas between the component and board. If the solder is allowed to freeze, the pockets will remain in the bulk solder. While the component/solder/board stack up shown in Figure 7 is above the liquidus temperature of the solder, the vapor bubbles are mobile due to Brownian motion forces caused by the heat input from the reflow simulator. If these air pockets reach the outer boundary of the top component, the gas is released and the void disappears. Once the void disappears, there is no source of new gas entrapment (as long as the peak remains below 242°C in the presence of copper). This is how an extended time above liquidus can greatly reduce the amount of voiding under a bottom terminated component. Figure 8 shows this result. Discussion Adjusting the reflow profile is a very easy way to reduce voids. Using a soak pre-heat profile reduces voiding in BGA devices. Keeping the peak temperature below 241°C reduces voiding in BGA and BTC compo - nents. Increasing the time above liquidus reduces voiding in BTCs as well. Experimental Procedure — Chemistry Case Solder paste chemistry is a highly propri- etary subject. However, there is significant experimental evidence that shows the pres- ence or absence of various useful ingredients in solder paste do have a major influence on voiding this example can be seen in Figure 9. The following case looks at two solder paste chemistries and its impact of voiding under the same the process and conditions. Figure 7: Large voids photographed as solder reaches liquidus. Figure 8: Reduced voiding after extended time above liquidus. Figure 9: Difference in solder chemistries vs. voiding.