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48 SMT007 MAGAZINE I JULY 2019 The no-clean solder paste showed statisti- cally identical voiding behavior for each solder powder size. The water-soluble solder paste showed some differences in voiding by solder powder size. The largest solder powder (Type 3) gave the lowest voiding while the highest voiding was generated by the no-clean Type 6 solder paste. One possible explanation for this is the difference in rosin content of the no- clean and water-soluble solder pastes. The no- clean solder paste contains rosins, which help to protect the solder powder from oxidation during reflow. It is theorized that the byprod- ucts of the reaction of the flux with the sol- der oxides can lead to voiding. With the added protection that rosins give, the amount of oxide generated during reflow is much lower for the no-clean than for the water-soluble sol- der paste. This may explain the voiding behav- ior observed in this work. Stability of the Solder Pastes: Tack Force Over Time The tack force was measured for each solder paste using the JIS method [11] . The tack force coupons were printed and stored in a cham- ber at room temperature and 50–55% relative humidity over a 72-hour period. Tack force was measured with the freshly printed solder paste after 24, 48, and 72 hours (Figure 27). In general, the tack force decreases over time for each type of solder paste and each solder powder size. The tack force drops sig - nificantly at 72 hours for most of the solder pastes. This is not true for the water-soluble Type 5 and 6 solder pastes. The water-soluble Type 5 solder paste showed relatively stable tack force over the 72-hour time period. The water-soluble Type 6 solder paste showed an increase in tack force at the 72-hour time. This difference in the water-soluble paste performance is likely related to the increased flux content for the smaller solder powder types. Stability of the Solder Pastes: Reflow Performance After a 24-hour Hold Time The reflow performance of each solder paste was measured with freshly printed PR test boards and again with test boards that were printed and stored open to the air for 24 hours. Storing the printed solder paste open to the air can increase the solder oxide levels and deplete the activity of the solder paste. This test is one way to show the stability of the solder paste and to determine if that stability is lessened with smaller solder powders. Figure 28 shows the wetting results. Figure 27: Tack force of no-clean (L) and water-soluble (R) solder pastes over time. Figure 28: Wetting results of solder pastes before and after 24-hour storage.