The Flux proportion in Solder Paste is not a fixed value but a process window. Our long-term tracking of data from 32 SMT lines in the factory found that when the Flux Content is below 7.5%, 0201 components show obvious edge burrs after printing, severe trailing after the squeegee passes, and the solder joints appear dull after reflow while the bridging rate increases by 1.8 times. When the flux content exceeds 12.5%, the residence time of the solder paste on the stencil shortens, and the fluctuation in solder deposition volume increases. Especially at the 2512 thick-film resistor positions, flux splashing often occurs and adheres to the thermocouple probe, directly interfering with the accuracy of temperature profile collection.

The flux proportion must be adjusted in coordination with the solder powder particle size, rosin type, and thixotropic agent ratio. For example, when using Type 4 solder powder (20–38μm) with a modified rosin system, a flux content of 9.2%±0.3% can balance stencil release rate and post-soldering ionic contamination value. However, if switching to Type 5 powder (15–25μm), the flux content must be reduced to 8.6% under the same formulation; otherwise, the solder paste will spread too quickly on the PCB surface after printing, and the void rate at the bottom of QFN will surge from 12% to 29%.

Ambient temperature and humidity have a very significant impact on the volatilization rate of active flux components. When the workshop temperature reaches 28℃ and relative humidity is 65% in summer, the actual activity of the flux in solder paste that has been left open for 45 minutes has decayed by approximately 22%. If the nominal 10% flux content is still used at this time, the equivalent effective flux function is only 7.8%. Therefore, we require all production lines to re-test viscosity with a viscometer every 2 hours during high-temperature and high-humidity periods, and simultaneously raise the target flux value by 0.4 percentage points as compensation.

A customer once requested reducing the flux content from 9.8% to 8.3% due to cost pressure, which resulted in batch-level head-in-pillow defects in the BGA area of three consecutive batches of motherboards. X-ray sampling inspection found 0.012mm-level gaps between solder balls and pads. The root cause was insufficient flux leading to incomplete removal of the oxide film, preventing atomic-level contact during solder paste melting. This type of defect is completely invisible under conventional AOI and can only be verified by cross-sectioning, with rework costs far exceeding the material savings.

The final determination of flux proportion cannot rely solely on the label on the solder paste bottle; the mass of residual flux per unit area after printing must be measured. We use FTIR infrared scanning combined with the standard copper sheet weighing method. After printing in a 20×20mm stencil opening area, the measured average residual flux in the dry film is 0.047mg/mm2, corresponding to a theoretical flux proportion in the solder paste falling between 9.1% and 9.5%. This value is on average 0.6% lower than the manufacturer's factory test value, indicating slight solvent evaporation during transportation and storage.