Every morning before starting the line, we must use a rotational viscometer to measure the apparent viscosity of the Solder Paste batch used for the shift, and simultaneously verify whether the alloy content in the incoming material report falls within the tolerance range of Sn96.5/Ag3.0/Cu0.5±0.15%. If a batch of solder paste is found to have a viscosity more than 8% lower than the previous batch or a silver content deviation exceeding 0.2%, it must be immediately taken out of service and the isolation process initiated, because such minor fluctuations will cause edge drawing and stencil residue during 0402 component printing, which in turn leads to an increase in solder joint void rate by more than 12% after reflow.

The solder paste used on the production line must be left to stand unsealed for 2 hours in an environment with a constant temperature of 18°C to 22°C and humidity of 45%RH to 55%RH before being loaded onto the machine. Otherwise, the internal stress of the solder paste will not be released, causing instantaneous shear thinning when the squeegee presses down. At this time, even if the printing parameters completely replicate the historical optimal values, the inconsistent rheological properties of the solder paste batch will cause a 7% to 10% decrease in solder paste transfer rate, especially in the 0.3mm fine-pitch QFN aperture area, where bridging defects are extremely likely to form.

Every time a new batch of solder paste is replaced, three sets of comparative verification must be conducted. The first set runs 10 boards with standard parameters, the second set reduces the squeegee pressure by 0.2kgf, and the third set increases the demolding speed by 15mm/s. Then, the standard deviation of volume deviation, area coverage, and position offset in the SPI data are collected synchronously. Only when the standard deviations of the three sets of data are all less than 1.8% and there are no 5 consecutive boards exceeding the control limit of CPK≥1.33, is the batch of solder paste allowed to enter mass production; otherwise, it is returned to the supplier with the measured viscosity curve and particle size distribution chart attached.

We found that for solder paste batches from the same manufacturer with different production dates, after being stored under the same storage conditions for 30 days, their thixotropic recovery rate will decrease by 22% to 28%. This means that the paste's rebound ability during the demolding stage is weakened, which is directly manifested as uneven shrinkage of the solder paste edges. Therefore, all batches of solder paste are now labeled with a time tag upon warehousing, and it is mandatory that they be used up within 48 hours after opening. Any batch of solder paste that is not used up after the time limit is scrapped and must not be mixed into the next shift's use, to avoid uncontrollable fluctuations in solder paste quality caused by aging.

Process technicians must carry a portable XRF instrument during daily inspections to spot-check the alloy ratio at two points, focusing on whether selective oxidation and precipitation of silver elements occur. Because some batches of solder paste experience multiple condensation and dew formation during transportation, it will cause local failure of the surface Flux layer and accelerate silver particle agglomeration. This phenomenon is visible as obvious gray-white flocculent matter under a microscope. At this time, even if the viscosity meets the standard, the wetting angle of the solder joint after printing will increase by more than 8°, directly affecting the IMC growth thickness and shear strength.