After Solder Paste is left standing on the printer for more than 4 hours, its viscosity increase rate is significantly positively correlated with ambient temperature. When the workshop temperature is higher than 28°C and the relative humidity is lower than 40%, a micro-dry film easily forms on the solder paste surface. In this case, even stirring for 3 minutes cannot restore its original rheological properties, and the solder paste must be scrapped. If the downtime is between 4 and 8 hours, and the ambient temperature and humidity are controlled within the ranges of 22°C ± 2°C and 50% ± 5%, it is necessary to first use a squeegee to sample and test the dispersion uniformity of solder balls, and then combine the printing results of 0201 components with a stencil opening rate greater than 60% to determine whether the dual control indicators of solder joint bridging rate < 0.3% and insufficient solder rate < 0.5% are met. If any indicator exceeds the limit, reuse must be terminated.

We have continuously tracked the performance of downtime solder paste on 12 production lines during the summer high-temperature and high-humidity season. We found that for solder paste of the same brand and same batch number after 6 hours of downtime, Line A experienced intensified Flux volatilization due to failure to turn off the printer's nitrogen protection, resulting in a 37% increase in viscosity, while Line B maintained a nitrogen atmosphere throughout the process, with only a 12% increase. This indicates that the actual state of downtime solder paste depends not only on the duration of downtime, but also on the coupled influence of multiple factors such as equipment airtightness, nitrogen flow stability, and stencil cleanliness. A one-size-fits-all judgment based solely on time is highly prone to misjudgment.

For solder paste that has been opened but not yet loaded onto the machine, if the aluminum foil seal is intact and it is stored in a constant temperature cabinet (23 ± 1°C), it can still be loaded onto the machine for verification even after 72 hours of storage. However, the first 3 boards must be skipped during the first printing for first-article confirmation. The focus should be on checking whether the void rate at the bottom of QFN is > 15% and whether the wetting angle of 0402 resistor terminals is > 45°. Once these two parameters exceed the limits, it indicates that the oxidation degree of the solder paste metal powder has exceeded the process window. In this case, regardless of appearance, the solder paste scrapping process must be initiated.

A common misconception in actual operation is relying on visual inspection of solder paste gloss to decide whether to retain it. However, numerous failure cases show that some solder paste still has an oily surface but has lost its flux activity, resulting in a large number of cold solder joints after reflow. Therefore, the final criteria must be the AOI missed detection rate and X-ray void rate after actual printing + reflow. When the dispersion σ of the solder joint IMC layer thickness for 5 consecutive boards is > 0.8 μm, it can be confirmed that this batch of downtime solder paste has undergone irreversible degradation.

The shelf life provided by the solder paste supplier only applies to the unopened state. Once exposed to air or the stencil, its chemical life immediately enters an accelerated decay phase. Therefore, before each downtime restart, the "three checks" action must be enforced: check the solder paste batch number and opening timestamp, check whether the residue on the back of the stencil is whitish, and check whether the squeegee stringing length after stirring is shorter than 15 mm. If any of the three items is abnormal, new solder paste must be replaced. Under no circumstances should one gamble on the reuse success rate of downtime solder paste to save material costs.