Solder Paste is a highly sensitive composite material in the SMT process. The spherical tin powder in it has strong surface activity, and the Flux contains organic acids and solvents. If stored at room temperature for more than 24 hours, viscosity increase, thixotropy attenuation and slight oxidation of metal particles will obviously occur, which directly leads to poor stencil release, increased pad residue and excessive void rate after reflow. In our workshop, we once temporarily placed the leftover solder paste on the tool rack next to the temperature recovery zone overnight, and the next day the printing yield dropped sharply from 99.2% to 96.7%. A large number of solder balls and bridges were visible under the microscope, and the root cause was abnormal changes in surface tension after the volatilization balance of the flux was disrupted.

The refrigeration temperature must be stably controlled between 0℃ and 10℃. Below 0℃, some flux components are prone to crystallize and precipitate, while above 10℃, the aging of rosin resin will be accelerated. Our production line uniformly uses medical-grade refrigerators calibrated with dual temperature zones. Before the first article of each shift, an infrared temperature gun is used to actually measure the temperature in the middle of the solder paste tank to ensure that the actual material temperature is no higher than 8℃. If the cold chain is interrupted for more than 30 minutes during transportation, the batch of solder paste must be isolated for inspection and cannot be directly put into production line use, because the microscopic phase separation caused by temperature fluctuations cannot be restored by simple stirring.

Temperature recovery is not simply taking the solder paste out and leaving it for a while. Instead, the outer packaging must be removed first, and then the paste should be placed on a clean workbench with a constant temperature of 22±2℃ and left to stand for at least 4 hours to fully equalize the temperature inside and outside the tank. Otherwise, condensed water on the tank wall will drip into the paste the moment the lid is opened, causing local dilution. Then use a special solder paste mixer to stir at a constant speed of 30rpm for 2 minutes. Note that it must be axial rotation rather than oscillating stirring, otherwise air bubbles will be entrapped to form printing pores. After stirring, immediately scrape a small amount of paste for a stringing test. The qualification standard is to pull out continuous filaments with uniform diameter, with a length of no less than 15mm. Otherwise, it indicates insufficient thixotropic recovery, and additional stirring for 30 seconds is still required.

After each lid opening and material taking, the shift operation must be completed within 2 hours. The remaining solder paste must be resealed and marked with the start time of temperature recovery. Any leftover solder paste that has not been used up after 8 hours must be scrapped. Our production line has a clear solder paste lifecycle dashboard, which is inspected by process technicians every two hours to prevent any over-time temperature-recovered paste from being mixed into normal batches. Once, a new employee mistakenly mixed solder paste that had been temperature-recovered for 6 hours with a newly opened can, resulting in poor wetting of BGA devices in three consecutive reflow ovens. X-ray inspection showed annular flux residue at the edge of solder joints, and the root cause was inconsistent printing thickness caused by differences in the rheological properties of solder paste at different temperature recovery stages.

The refrigeration and temperature recovery of solder paste are not isolated processes. They form a closed-loop control relationship with stencil tension, squeegee pressure and printing speed. For example, when the ambient humidity suddenly rises above 65%RH, even if the temperature recovery meets the standard, the surface layer of the solder paste will quickly absorb moisture. At this time, the squeegee angle must be synchronously increased by 0.5° and the printing speed reduced by 10mm/s, otherwise the paste stringing will break the moment of stencil release. These details are not written in the SOP, but every experienced technician knows them, because the production line will not wait for you to flip through the manual, it only speaks with the first-pass yield.