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64 SMT007 MAGAZINE I OCTOBER 2020 observed that flux spattering could be reduced by using a thinner stencil as shown in Figure 8. Reducing the solder paste amount will help to reduce flux splattering, but reduced sol- der volume may lead to solder joint reliabil- ity issues. Development of Solder Paste to Reduce Flux Splattering (Paste Product C) Based on the results, a project was initiated to develop solder paste that could suppress flux splattering even when lower preheating temperatures and times were used. From these evaluations, to suppress flux splattering, flux designs that do not leave volatile substances during the melting of solder are needed. It is possible to design resin components, like rosin, and additives that volatilize less dur- ing reflow temperatures. However, in solvents with the largest volatilization amount, increas- ing the volatility by too much causes the pro- gression of volatilization even at room tem- perature, resulting in many problems, such as the drying of the paste and increased viscosity, which has caused printing and assembly chal- lenges. We focused on the behavior of the flux resi- due and worked on how the flux residue can help to suppress flux spattering by developing a new paste material (Paste C) that used a flux system to suppress flux spattering. The solder paste product leaves flux residue on the sur- face of the molten solder during reflow so that flux splattering can be prevented even when gas is discharged (by a capping effect) after the melting of solder, instead of suppressing the generation of gas (Figure 9). As shown in Figure 10, compared with con- ventional paste products, flux splattering of Paste C was suppressed even when using low preheat temperatures and times, allowing sol- dering with low flux splattering under a wide variety of reflow conditions. Figure 11 shows screenshot film images from the reflow simulator using Paste Product C with no flux splattering. Figure 12 shows screenshot film images from the reflow simu- lator using the conventional no-clean Type 4 Paste Product A as a comparison, showing flux splattering. Additional Testing of Paste Product C In addition to the flux splattering tests, Paste Product C was evaluated using various tests to assess its general applicability to electronics manufacturing assembly. Figure 8: The amount of flux splattering based on the variation of stencil thickness. Figure 10: The comparison of flux residue spattering of Paste Product C with conventional Pastes A and B using different reflow profiles. Figure 9: Capping effect by Paste Product C flux residue to reduce flux splattering.