In actual production lines, we have found that the wettability of halogen-free Solder Paste is generally weaker than that of ordinary solder paste. This is not due to differences in metal powder particle size or viscosity, but because after halogen activators are replaced by organic acids, the decrease in Flux surface tension in the 150℃~180℃ range is reduced by approximately 18%, which directly weakens the wetting driving force of molten solder paste on OSP copper pads. Under the same 0.3mm stencil aperture, the volume consistency deviation of solder paste printed with halogen-free solder paste reaches ±9.2%, while that of ordinary solder paste is only ±6.5%. This fluctuation further amplifies the dispersion of solder paste wettability during the reflow stage.

When the reflow peak temperature is set to 235℃, the average spread rate of ordinary solder paste on FR4 boards is 83.6%, while that of halogen-free solder paste stabilizes at around 76.1%. Moreover, the climb height of solder fillets on the side of QFN device pins decreases from 0.28mm to 0.16mm. This phenomenon still exists under a nitrogen atmosphere, indicating that it is not caused by differences in oxidation inhibition effects. We once used two types of solder paste from the same batch on the same PCB batch, and found in AOI inspection that the bridging defect rate of halogen-free solder paste is 0.03% lower than that of ordinary solder paste, but the cold solder joint proportion is 0.17% higher, which is particularly obvious in areas where the temperature difference between pads at both ends of 0402 resistors exceeds 12℃.

The adaptability of the cleaning process also affects the final performance of solder paste wettability. If water-washable halogen-free solder paste is not cleaned within 4 hours after reflow, its residual flux will slowly react with sulfides in the air to form micron-level white flocculent substances, which cover the metal surface of the pads and thus weaken the solder paste wettability during secondary soldering. In contrast, ordinary solder paste residues are mostly rosin-based, which carbonize at high temperatures but do not form an interface barrier layer. Therefore, when selecting halogen-free solder paste, one cannot only look at the wetting angle value in the TDS parameters, but must also comprehensively evaluate the practical effect of solder paste wettability in combination with the actual stencil aperture rules, reflow oven chain speed fluctuation range, and workshop humidity control level of the production line.

A customer experienced a batch of BGA void rate exceeding the standard after switching from ordinary solder paste to halogen-free solder paste. Our on-site investigation found that they increased the stencil thickness from 0.12mm to 0.15mm to compensate for the decrease in solder paste wettability, which instead caused excessive solder paste volume and squeezed the flux volatilization channels. Eventually, the void rate rose from 12% to 29%. After adjusting back to the original thickness and simultaneously extending the soak zone time by 22 seconds, the void rate fell back to 13.8%. This shows that solder paste wettability is not an isolated parameter; it must form a closed-loop match with the three processes of printing, placement, and reflow.