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JULY 2019 I SMT007 MAGAZINE 49 The wetting for the freshly printed no-clean solder pastes and the no-clean solder pastes stored for 24 hours were similar for every sol- der powder size except Type 6. The no-clean Type 6 solder paste showed a decrease in wet- ting when stored open to the air for 24 hours. The water-soluble pastes showed similar wet- ting when fresh and after 24 hours except for Type 3 solder paste, which showed a drop in wetting with storage. Solder balling was measured using the over- print patterns in the PR test board with freshly printed solder paste and again with printed boards stored for 24 hours (Table 11). The overprint percentages decreased as the printed solder pastes were stored over 24 hours. This indicates a general increase in random solder balling with storage. The increase in random solder balling was worse for the smaller solder powder sizes. Repre - sentative pictures of the solder balling before and after the 24-hour storage are shown in Figure 29. The graping percentages were measured before and after 24-hour storage (Figure 30). The no-clean solder paste showed increases in graping for Type 3, 4, and 6 solder powder sizes when stored for 24 hours. The increases in graping were 6% for Type 3, 16% for Type 4, and 13% for Type 6. Type 5 solder pastes were unaffected by 24-hour storage. The water-sol- uble solder paste showed the same levels of graping before and after the 24-hour hold time. The graping with the no-clean solder paste shows increased sensitivity to hold times with the smaller solder powder sizes. Stability of the Solder Pastes: Heat Aging The solder pastes were sealed in their con- tainers and heat aged in an oven at 50°C (122°F) for three days. After heat aging vis- cosity, IPC solder balling, tack force, and print and reflow performance were measured. These results were compared to the results from the fresh solder pastes. Figure 31 shows the viscos- ity results before and after heat aging. The viscosity of the water-soluble solder pastes increased dramatically with heat aging. Table 11: Solder balling performance of solder pastes before and after 24-hour storage. Figure 30: Graping results of solder pastes before and after 24-hour storage. Figure 29: Solder balling of solder pastes before and after 24-hour storage.