After being removed from cold storage, Solder Paste must be placed in a constant temperature room with a temperature of 22℃±3℃ and humidity of 45%RH±10%RH for temperature recovery. The recovery time must be no less than 4 hours and no more than 72 hours. Insufficient recovery will result in a temperature gradient inside the solder paste, which may easily cause Flux precipitation and sudden viscosity changes due to local condensation after opening, leading to stencil clogging and poor release; although excessive recovery time will not cause failure, it will accelerate the formation of oxide film, especially for high-reliability solder paste with silver content ≥3%.

Before opening the lid, it is necessary to visually confirm that there are no water droplets or frost-like condensates on the surface of the solder paste, and wipe the outer edge of the can mouth with a clean dust-free cloth before opening. Otherwise, the condensation water mixed into the solder paste will directly damage the active layer on the surface of the metal particles, causing the post-printing collapse rate to increase by more than 15%; after opening, immediately use a scraper to slowly press down along the can wall and stir for 30 seconds to re-disperse the settled alloy powder evenly. High-speed rotation or violent impact on the can body is prohibited during stirring to prevent introducing bubbles and aggravating oxidation.

The opened recovered solder paste must be temporarily stored in a solder paste mixer under nitrogen protection. The cumulative exposure time in air must not exceed 8 hours, and it must be scrapped if the time limit is exceeded. There was a case on site where the solder paste was left for 11 hours after opening and continued to be used, which resulted in the soldering void rate of two consecutive batches of BGA devices exceeding the standard to 32%, and the rework cost reached 28 yuan per board; after each use of solder paste, the sealing lid must be tightened immediately, and the opening time, operator and shift number must be handwritten on the label. The production line with a 67% omission rate of this action has 4.2 times more solder paste abnormal complaints than the production line with standardized execution.

During the use of solder paste, it is strictly prohibited to mix solder paste of different batches, models or suppliers into the same stencil feeder. There was a production line that mixed low-temperature and high-temperature solder paste to save line change time, which led to a reflow peak temperature fluctuation of ±12℃, and ultimately caused the tombstoning rate of 0201 resistors to soar to 9.7%; before replacing new solder paste each time, the residual paste in the feeder must be thoroughly cleaned. A residual amount exceeding 0.3g may cause component cross-contamination, and conventional air gun purging can only remove about 62% of the residue, which must be matched with mechanical wiping with alcohol cotton pads to meet the standard.

The batch information, recovery start and end time, opening time, first use time and last use time of all recovered solder paste must be recorded in the solder paste control module of the MES system. The paper records and electronic records must be completely consistent, and a deviation of more than 1 minute will trigger a quality alarm. A factory caused gray solder joint spots on a whole batch of PCBA at the customer end due to a 23-minute omission in manual copying, and the final recall loss reached 1.37 million yuan. This kind of problem is not within the scope of equipment accuracy at all, but purely a direct consequence of out-of-control solder paste usage process.