If Solder Paste is taken out of the refrigerator and directly loaded for printing without sufficient temperature recovery, it will lead to abnormally increased viscosity and poor fluidity of the solder paste, which in turn causes missing printing, insufficient solder, or stencil clogging. The root cause is the significant temperature difference between the low-temperature solder paste and the room-temperature environment, which causes the rosin component in the Flux to precipitate and coat the solder powder particles. The practical solution is to let unopened solder paste stand in an environment of 22℃±3℃ for 4 hours, and opened solder paste stand for 6 hours before use, then stir gently with a spatula for 1 minute to restore a homogeneous state. The key preventive measure is that all solder paste must be affixed with temperature and humidity labels, and immediately sealed and returned to storage after each use, with the opening time recorded.

If solder paste is exposed to room temperature for more than 4 hours and still continues to be used, increased burrs on printing edges, a rising defect rate of poor demolding, dull solder joints after reflow, and an excessively large wetting angle will be observed. This is because rosin-based activators continuously volatilize, leading to a decrease in acid value, while low-boiling-point solvents such as ethanol and ethylene glycol butyl ether escape, causing the paste to shrink and harden. The practical solution is to immediately stop using this batch of solder paste, conduct a comparative printing test with new paste of the same model, and confirm that the appearance and thrust of the first-article solder joints meet the standards before small-batch trial production. The key preventive measure is that the production line must be equipped with a solder paste cabinet with a time-lock function, setting the maximum storage time after opening to 8 hours, with automatic alarm and discharge port locking when the time limit is exceeded.

If solder paste is repeatedly frozen and thawed more than twice and then continues to be used, obvious solder residue on the back of the stencil, uneven depth of spatula scratches, and a large number of solder beads scattered at the bottom of resistive and capacitive components after reflow will appear. The core cause is that the oxide film on the surface of the solder powder thickens with drastic temperature changes, while the surfactant in the flux undergoes irreversible agglomeration. The practical solution is to isolate this solder paste separately, mark it as 'disabled', and transfer it to the laboratory for metal particle distribution testing. If the D50 particle size deviation exceeds ±0.5μm or the oxide content is >1.2wt%, the entire batch will be scrapped. The key preventive measure is that different batches of solder paste must be stored in separate zones in the refrigerator, with a two-color label affixed to each box (blue label for unopened, red label for opened), and mixing of solder paste from different dates is strictly prohibited.

If solder paste is stored for a long time in an environment with humidity >60%RH, whitening around the pads after printing, matte gray-black solder joints after reflow, and frequent false 'solder joint void' alarms from AOI will occur. This is because absorbed moisture reacts with zinc chloride-based activators to form basic salt precipitates, which damage the integrity of the flux film and interfere with the spreading of solder liquid. The practical solution is to immediately stop using all solder paste in this cabinet, measure the moisture content of the paste with a rapid moisture analyzer, and if it is >0.08%, hand it over to the supplier for recycling according to the hazardous waste process. The key preventive measure is that the Solder Paste Storage area must be equipped with a constant humidity machine to maintain the ambient humidity at 40%RH±5%RH, and a portable hygrometer must be used for on-site verification before and after each door opening.

If the sealing ring of the solder paste container is aged and deformed or the bottle cap thread is worn, it will cause the paste surface to crust within 3 days after opening, obvious stratification after stirring, and intensified jaggedness at the edges of the printed pattern. The essence is that continuous air infiltration causes rosin polymerization and solder powder sedimentation. The practical solution is to replace the original sealing components of the same specification, take samples of the contaminated solder paste for melt viscosity testing, and determine it as unusable when the apparent viscosity at 25℃ is >900Pa·s. The key preventive measure is that all solder paste bottle caps must be forcibly replaced every quarter, and the sealing ring material must be uniformly made of fluororubber instead of silicone, as the latter is prone to swelling and failure in a rosin environment.