Most of the tiny solder beads appearing after Solder Paste printing do not come from free solder powder inherently contained in the solder paste, but are the combined result of Flux bumping, metal powder agglomerate rupture, solder spattering, and gas release from local oxide film rupture during the solder paste reflow process. When the proportion of ultra-fine powder below 25 μm in the solder paste exceeds 8%, excessively high surface energy in the early reflow stage can easily induce non-uniform melting, thereby forming a large amount of solder paste residue with solder beads smaller than 0.2 mm in diameter.

We found that for the same solder paste, the number of solder beads can differ by more than 3 times under different stencil aperture sizes. The root cause is that when the solder paste is forced to undergo shear thinning under squeegee pressure, if the flux system responds too quickly to the shear rate, edge collapse will occur at the moment of demolding, entraining air into the pad gaps. These trapped micro-bubbles expand and rupture sharply after the solder paste reflow temperature rises to 150°C, directly spraying incompletely wetted solder powder particles onto the solder mask to form isolated solder beads.

To address this phenomenon, we focused the solder paste optimization on the matching between the flux resin softening point and the solvent boiling range. When the rosin derivative softening point is controlled at 68–72°C and the initial boiling point of the ethanol/propylene glycol mixed solvent falls within the 112–116°C range, the solder paste can maintain sufficient adhesion before placement to prevent edge collapse, and achieve gentle exhaust during the reflow preheating stage to avoid sudden gas surges. This synergistic design reduced the solder bead occurrence rate from an average of 4.7 beads per square centimeter to below 0.9 beads.

In addition, the silver content in solder paste is not the higher the better. When the Ag content is increased from 0.3wt% to 0.7wt%, although the anti-collapse ability is enhanced, the intensified silver phase segregation tendency instead promotes the enrichment of solder bead solder paste at the pad edges, which is especially obvious around 0402 and smaller package devices. Therefore, while keeping the basic Sn96.5Ag3.0Cu0.5 ratio unchanged, we introduced part of the silver into the flux carrier in the form of nano-scale alumina-coated particles, which both improves high-temperature viscosity and does not disrupt wetting balance.

On-site verification shows that after switching to the optimized solder paste, there is no need to adjust the existing reflow oven temperature zone settings; only fine-tuning the chain speed by ±0.15 m/min is required to adapt to the melting window changes of the new solder paste. All production lines achieved CPK > 1.33 on the first day of switching, and there was no increase in AOI false alarms related to solder beads for 72 consecutive hours, indicating that the solder paste reflow behavior has formed stable coupling with the equipment thermal inertia.