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JULY 2019 I SMT007 MAGAZINE 53 solder pastes. Type 6 no-clean solder paste showed a slight increase in graping due to heat aging. The decrease in wetting and graping perfor- mance with heat aging shows that the solder pastes with the smaller solder powder sizes lose activity more quickly than the larger sol- der powder sizes. The shelf life may be short- ened for solder pastes made with smaller sol- der powder sizes. Conclusions and Recommendations Solder powder size certainly has an effect on solder paste performance. It is well understood that one cannot use every solder powder size with a particular solder paste. The solder paste must be formulated to work properly with the desired solder powder size. Here are recom- mendations for the optimal use of each solder paste and solder powder size based on perfor- mance in this work. Slump (IPC) All of the solder pastes performed well in the IPC slump test except for the no-clean Type 6 and the water-soluble Type 5 and 6 sol- der pastes, which failed the hot slump test. The flux concentration of these solder pastes was significantly higher with the smaller sol- der powder sizes, which changes the rheol- ogy of the solder pastes. This indicates that the smaller solder powder sizes may increase the potential for bridging during reflow. Solder Balling (IPC) Type 3, 4, and 5 water-soluble and no-clean solder pastes gave acceptable solder balling in the IPC test. Both the no-clean and water-sol- uble Type 6 solder pastes failed the IPC sol- der balling test. Type 6 solder powders have a relatively large oxide content compared to the other solder powder sizes. Type 6 solder pastes are more susceptible to solder balling than the other solder pastes. Printing Minimum Area Ratio In general, printed solder paste volume increased with decreasing solder powder size. The transfer efficiencies increased by roughly 5% for each decrease in solder powder size, which corresponded to a 0.05 change in area ratio. Here are the recommendations for mini- mum area ratio for each type of solder paste: • Type 3 no-clean solder paste: 0.60 AR • Type 4 no-clean solder paste: 0.55 AR • Type 5 no-clean solder paste: 0.50 AR • Type 3 and 4 water-soluble solder paste: 0.60 AR • Type 5 water-soluble solder paste: 0.55 AR Please keep in mind that the print parame- ters, stencil technology, and design will affect the printability and these minimum area ratios. Pauses in Printing and Stencil Life The no-clean Type 5 and 6 solder pastes showed the largest drop in transfer effi- ciency after a pause of 16 hours on the sten- cil. The water-soluble Type 5 and 6 solder Figure 36: Graping of the no-clean solder pastes before and after heat aging. Figure 35: Solder balling of the no-clean solder pastes before and after heat aging.