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36 SMT007 MAGAZINE I JULY 2019 of solder powder oxidation and oxide removal by the flux tends to thicken the solder paste. Over time, this can lead to clogged stencil aper- tures and cause the solder paste to stick to the squeegee blades. The usable stencil life of the solder paste may be shortened by the smaller solder powders. The good news is that sol- der paste fluxes are made with ingredients to protect the solder powder, which significantly slows this oxidation process. Oxidation of the solder powder also occurs during reflow. The solder paste flux reacts with and removes the oxides from the solder pow- der during reflow. As the solder powder size decreases, more flux is required to deal with these oxides. When reflowing solder pastes made with smaller solder powder sizes, the flux may run out of activity; then, oxides are left on the solder powder which interferes with the proper coalescence of the solder. Solder pastes made with smaller solder powders are susceptible to potential issues like random sol- der balling and graping (Figure 4). The shelf life for solder pastes made with smaller solder powders may also be shorter than solder pastes made with larger solder powders. During storage, the flux can react with the solder metal, creating metal salts. The flux activity is depleted through this reaction over time, and this reaction is faster for smaller solder powder sizes. As more reactive solder pastes age, the solder paste may thicken and a change from a smooth and creamy appearance to more of a dull grainy appearance (Figure 5). Print and reflow characteristics may degrade over time if the solder paste is too reactive. Solder pastes are formulated to prevent or slow this process. Storing the solder paste in a refrigerator also helps to slow this process and preserve the intended performance character- istics. Proper storage is especially important to prolong the shelf life of solder pastes made with smaller solder powder sizes. Materials and Test Methods The circuit board used for this experimen- tation is shown in Figure 1. This circuit board is made of FR-4 and is 0.062 inches thick with etched copper pads and electroless nickel immersion gold (ENIG) surface finish. This print and reflow test board has patterns used to quantify printed solder paste volume, wetting or spread, random solder balling, grap- ing, and voiding (Figure 7). Solder Pastes Eight solder pastes were made for this study, including four water-soluble solder pastes and four no-clean solder pastes. The no-clean flux has an IPC J-STD-004 classification of ROL0, and the water-soluble flux is classified as ORH1. The solder alloy chosen was SAC305 (Sn/Ag 3.0%/Cu 0.5%). The solder paste metal concentrations were varied based on sol- der powder size (Table 4). Figure 4: Random solder balling (L) and graping (R). Figure 5: Fresh solder paste (L) versus aged solder paste (R). Figure 6: Print and reflow test circuit board.