Solder Paste is not a simple consumable in SMT quality control, but a key variable determining first-pass yield and first article pass rate. JJY solder paste has been calibrated for metal content, viscosity and slump in accordance with the J-STD-005 standard before leaving the factory. However, if the storage temperature exceeds 10°C for more than 2 hours, its Flux activity will irreversibly decay, leading to blackened solder joints and poor wetting after reflow; freezing below 0°C will damage the uniformity of the oxide film on the solder powder surface, and agglomeration and stratification are prone to occur during thawing.

Our production line enforces dual-temperature-zone refrigerator management: the upper layer at 2~4°C is dedicated to storing unopened solder paste, while the lower layer at 6~8°C stores opened paste ready for use. A calibrated electronic temperature and humidity recorder must be installed inside the refrigerator, with data recorded three times per shift and signed for confirmation. Once a temperature fluctuation exceeding ±0.5°C lasting more than 15 minutes is detected, the current batch of solder paste shall be immediately isolated for review and shall not be directly put into production. If opened solder paste is exposed to room temperature (23±3°C) for more than 8 hours without stirring, solvent volatilization on its surface layer will cause viscosity to rise by more than 15%, resulting in obvious squeegee tailing during printing and insufficient filling of stencil apertures.

Thawing must follow a step-by-step procedure: after taking the whole tube of solder paste out of cold storage, first place it on a 23°C standing platform for 2 hours, then transfer it to a preheating fixture of a constant-temperature mixer to slowly warm up to 25°C. Hot water immersion or direct hot air blowing is strictly prohibited throughout the process; otherwise, a temperature difference exceeding 3°C inside the solder paste will induce micro-phase separation, and solder powder agglomeration and flux precipitation can be observed under a microscope. JJY stipulates that the total thawing time shall be no less than 4 hours, and stirring must be completed within 2 hours after thawing, otherwise the timing restarts. Because when the ambient humidity is >60%RH, the solder paste surface will quickly absorb moisture, increasing the probability of reflow solder spattering by 3 times.

The stirring action itself must meet triple verification requirements. Manual stirring requires a dedicated squeegee to rotate at a constant speed in the same direction for more than 120 seconds, with no pauses or reversals midway; visually confirm no wall adhesion, no bubbles, and the paste presents a mirror-like reflective state. For mechanical stirring, set the rotation speed to 250rpm, time to 120 seconds, and torque to 0.8N·m. Before each start, a standard viscometer must be used to actually measure whether the current solder paste flow-out time is within the 18~22 second range; if it exceeds the range, the stirring is judged as invalid. After stirring is completed, the solder paste must be put on the machine for printing within 30 minutes. Solder paste that is not used up after the time limit shall not be mixed with new paste; it must be sealed separately, marked with the activation time, and used up within 48 hours, otherwise it shall be discarded.

Before the first shift starts production every day, a solder paste printing tensile test must be conducted: take 3 test printed boards, mount 0402 resistors, and pass them through reflow; use a push-pull force gauge to detect whether the solder joint shear force is ≥0.15N. If two consecutive boards fail, trace back the storage temperature and humidity records, thaw start and end times, and stirring parameters of this batch of solder paste, simultaneously suspend the production line and notify the process engineer to conduct on-site re-evaluation. This closed loop is not for filling out forms to cope with audits, but to prevent sudden batch cold solder joints at 3 a.m. one day without finding the source.