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These transfer efficiency values are statisti- cally the same before and after heat aging. Heat aging increased the viscosity of the no-clean solder pastes but did not affect the printability. By comparison, heat aging increased the vis- cosities of the water-soluble solder pastes to the point where they were not printable. The reflow performance of the no-clean heat-aged solder pastes was measured using the PR test board and compared to the fresh no-clean solder pastes. Figure 34 shows the wetting data. Wetting performance decreased with heat aging for each of the no-clean solder pastes. The largest decreases were with Type 5 and 6 solder pastes. Table 13 shows solder balling performance with respect to heat aging. The no-clean solder pastes with Type 4, 5, and 6 solder powders were not measurable with these criteria and therefore are not shown in this table. The solder balling performance did not change significantly with heat aging of the no-clean solder pastes. Representative sol- der balling pictures are shown in Figure 35. The graping percentages were measured after heat aging and compared to the fresh no- clean solder pastes (Figure 36). The graping percentages were similar for the fresh and heat-aged Type 3, 4, and 5 no-clean Figure 33: Print performance for the no-clean solder pastes before and after heat aging. Figure 34: Wetting for the no-clean solder pastes before and after heat aging. Table 13: Solder balling performance of solder pastes before and after heat aging. 52 SMT007 MAGAZINE I JULY 2019