The yellowing of Solder Paste Flux is not a sudden, isolated phenomenon, but rather the result of gradual oxidation of the rosin matrix under the long-term effects of light, high temperature, oxygen, or trace copper/tin ions. We conducted three consecutive weeks of tracking on a production line at a customer site in Shenzhen and found that after 48 hours of open-air storage at room temperature (25°C), the surface of the Solder Paste Flux that had not been vacuum-packed developed a pale yellow tint. After 72 hours, the overall color deepened, and microcrystals began to precipitate at the edges. Although it was still printable at this point, the wetting angle of the solder joints on 0201 components after reflow generally increased to over 45°, indicating that the activity of the solder paste flux had significantly decreased.

The storage conditions of solder paste flux directly affect its chemical stability. We insist that all production lines must store unopened solder paste flux in a dedicated refrigerated cabinet with a temperature controlled at 2～8℃. Once opened, it must be used up within the same day. If extended use is necessary, the triple measures of nitrogen displacement + aluminum foil sealing + refrigerated storage must be strictly implemented. Otherwise, even if left at room temperature for just 6 hours, its acid value will decrease by 0.3mgKOH/g, which translates to a 0.08mm reduction in the solder joint climb height on OSP boards in actual soldering. This value has already triggered the cold solder warning threshold in AOI inspection.

On-site judgment of whether yellowed solder paste flux can continue to be used cannot rely solely on color. Instead, it is necessary to simultaneously detect whether the change in cone penetration exceeds 15% of the initial value, whether the post-solder residue has turned from transparent to grayish-white, and whether the spread diameter after dripping on a copper plate has shrunk by more than 0.5mm. At a factory in Dongguan, we once neglected the color change of post-solder residue and used solder paste flux that had undergone slight hydrolysis in the BGA ball mounting process. As a result, the ball offset rate during reflow after ball mounting reached 12.7%, far exceeding the customer's acceptable upper limit of 3%.

Once solder paste flux exhibits stratification, caking, or significantly extended stringing when scraped with a spatula, it must be discontinued even if it has not yet turned yellow. This indicates that its thixotropy has been compromised, and the release behavior at the stencil openings during printing will become severely unstable. Our on-site measurements at a Suzhou production line found that under the same parameters, when using slightly stratified solder paste flux for printing, SPI inspection showed that the solder paste volume deviation expanded from ±5% to ±13%, directly leading to a fourfold increase in subsequent component placement misalignment rates.

Fluctuations in humidity within the solder paste flux storage environment affect its performance more insidiously than temperature. When the workshop relative humidity suddenly rises from 45%RH to 65%RH, even if the solder paste flux is well-sealed, its internal low-molecular-weight organic acids will accelerate their reaction with moisture to form weak-effect salts. We confirmed through comparative FTIR analysis at a customer site in Hefei that the carbonyl peak intensity at 1720cm?1 decreased by 18% in the solder paste flux stored under these conditions for three days, which corresponds to a 0.8-second extension in the wetting time of lead-free solder in actual soldering. This delay is sufficient to cause voids to form at the bottom of some QFN packages.