During line troubleshooting of Solder Paste cold solder joint defects, we found that under the same stencil, same reflow oven, and same temperature zone parameters, the yield rate remained stable at 99.8% when using Brand A solder paste, while after switching to Brand B, the AOI cold solder joint alarm rate suddenly rose to 3.2%, concentrated at the QFN thermal pad and both ends of 0201 resistors. This indicates that the root cause of the problem does not lie in equipment or fixtures, but in the insufficient activity of the Solder Paste Flux itself, which cannot effectively decompose Cu₂O and Ni-P layer oxides within the set 150℃～170℃ preheating interval.

The activity strength of solder paste Flux directly depends on the acid value of its organic acid system and the release efficiency of halogen ions. When the acid value is lower than 65mgKOH/g and the bromine content is lower than 0.08%, the organic protective film formed on the pad surface of OSP boards is difficult to remove in time. After the solder melts, it can only wet locally. After cooling, the solder joints appear gray-white and matte, the thrust test is lower than 8gf, and X-ray shows continuous voids between the solder and the pad. This typical solder paste cold solder joint phenomenon will be sharply amplified in fast-paced production where the peak reflow temperature is maintained at 235℃ but the preheating time is compressed to 60 seconds.

We once conducted cross-validation on solder paste samples provided by five suppliers. After printing different solder pastes on OSP PCBs of the same batch, they were run through the same JUKI oven. The results showed that only two solder pastes with flux activation onset temperature ≤142℃ and active time maintained at 160℃ ≥90 seconds did not show batch cold solder joints. The other three all showed dewetting at the root of QFP pins. Under a microscope, obvious shrinkage of the solder edge and solder fillet angle >60° can be seen. This indicates that the thermal response window of the solder paste flux must closely match the actual line reflow profile; otherwise, no matter how high the metal content is, it cannot save wetting failure.

The on-site rapid method to determine whether the solder paste flux activity meets the standard is to take 0.3g of solder paste and apply it to a bare copper plate, then place it in a constant temperature oven at 160℃ for 60 seconds. If a uniform silver-bright solder film is formed on the surface without brown residues, the activity is qualified. If mottled shrinkage cavities or brown-yellow coking traces appear, it proves that the organic acid decomposition is incomplete or the halogen carrier has failed. Continuing to use this solder paste at this time will inevitably aggravate the risk of solder paste cold solder joints. Especially under a nitrogen atmosphere, this problem will be 2.3 times worse than in an air environment, because N₂ isolates the synergistic oxidation effect of oxygen on some intermediate active substances.

Before replacing the solder paste, the coupling relationship between the stencil aperture size and the flux volatilization rate must be evaluated synchronously. For example, after increasing the stencil thickness of a 0.3mm pitch QFN from 0.12mm to 0.15mm, although the solder volume increased by 18%, the increased flux residue led to poor exhaust in the early stage of reflow, which instead induced more micro-void type solder paste cold solder joints. Therefore, the solution to insufficient solder paste flux activity is never simply increasing the solder volume, but returning to the dynamic balance of acid value, halogen type, and resin thermal decomposition temperature.