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The water-soluble solder pastes all reacted to the point of being unusable. The no-clean solder pastes showed some stability in this test. The no-clean Type 3 and 4 solder pastes increased in viscosity roughly 40–70% but were still usable. The no-clean Type 5 and 6 solder pastes increased in viscosity by more than 100%. These solder pastes were thick but printable with an increase in blade pressure. The IPC solder balling test was run on the heat aged solder pastes and the results are shown in Table 12. Initially, the fresh solder pastes gave accept- able solder balling for all solder powder sizes except Type 6, which was unacceptable. After heat aging, the solder on Type 5 and 6 no-clean solder pastes gave unacceptable results. Type 4, 5, and 6 water-soluble solder pastes gave unacceptable results. It is clear from this test- ing that the smaller solder powder sizes are more susceptible to reaction over time, which may lead to shorter usable shelf life. The JIS tack force was measured on the heat aged solder pastes. Figure 32 shows the tack force data for the fresh and heat aged solder pastes. The no-clean Type 3 solder paste decreased slightly in tack force after heat aging. Type 4 and 6 no-clean solder pastes increased in tack force with heat aging. The tack force of Type 5 solder pastes was stable with respect to heat aging. The water-soluble Type 3 and 4 solder pastes showed a decrease in tack force with heat aging. The water-soluble Type 5 and 6 solder pastes showed very little change in tack force with heat aging. This shows that tack force is not necessarily affected by heat aging and may not change through the shelf life. Print performance was measured before and after heat aging. The water-soluble solder pastes were too viscous to print, so the data in Figure 33 is only for the no-clean solder pastes. Figure 31: Viscosity T-bar spindle results before and after heat aging. Figure 32: JIS tack force before and after heat aging. Table 12: IPC solder balling performance of solder pastes after heat aging. 50 SMT007 MAGAZINE I JULY 2019