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JULY 2019 I SMT007 MAGAZINE 45 Generally speaking, the transfer efficiencies increase with decreasing solder powder size. There are some anomalies in this data that do not follow that trend. The no-clean Type 6 sol- der paste gave lower transfer efficiencies for the 0.30 and 0.35 area ratios than the other solder powder sizes. This was an unexpected result. This same data set is shown as smooth curves in Figure 20. The transfer efficiency of Type 5 solder pastes was generally higher than all of the other sol- der powder sizes. This was an unexpected result. Type 6 solder paste was expected to give higher transfer efficiencies than the other sol- der powder sizes. The water-soluble Type 6 sol- der paste gave some of the lowest TE% values. If we use a rule of 0.60 area ratio for Type 3 solder pastes, then the minimum area ratios for the other solder paste types can be esti- mated based on the TE differences in this study. Table 8 lists these minimum area ratios by solder paste. These minimum area ratio rules are guide- lines based on the print patterns used in this evaluation. More work is needed to evaluate the print performance of Type 6 solder pastes. These rules do not take into account solder joint quality or reliability. Solder joint quality and reliability should be verified before using these rules in a production environment. Print-and-Pause Results and Stencil Life Print-and-pause testing is one way to deter- mine the stability of solder pastes as they sit open to the air in the printer. Drops in transfer efficiency over time are a way to estimate sten- cil life. Figure 21 shows the transfer efficiency over time for each solder paste. The no-clean solder pastes each showed similar performance in print-and-pause test- ing. The transfer efficiency was stable at time zero hours, one hour, two hours, four hours, and eight hours. At a time of 24 hours, there was a significant drop in transfer efficiency. The smaller solder powder sizes showed larger drops in transfer efficiency. The water-solu- ble solder pastes showed similar performance. There was a significant drop in TE% at a time of 24 hours. Table 9 summarizes the TE% drops from the eight-hour time to the 24-hour time. Table 8: Minimum area ratios for each solder paste and solder powder size. Figure 20: Transfer efficiency curves for each solder paste by area ratio.