1. When the downtime is less than 2 hours, the Solder Paste remains within the working temperature window, but a slight dry film has begun to appear at the edge of the stencil aperture. Before powering on, use a lint-free cloth dipped in an appropriate amount of solder paste cleaner to wipe the bottom of the stencil, then use a nitrogen gun to blow obliquely along the aperture direction for more than 3 seconds, then perform 3 idle prints and visually inspect whether the solder deposition profile on the pads is continuous and uniform; otherwise, recalibrate the squeegee pressure and release speed.

2. After 2 to 4 hours of downtime, the viscosity of the solder paste increases by about 15% to 20%, leading to a decrease in printing transfer efficiency. The original set squeegee pressure must be increased by 0.2 to 0.3 kgf, while the printing speed is reduced by 10 mm/s. Before printing the first piece, manually stir the solder paste for more than 60 seconds. The stirring direction must maintain unidirectional rotation to avoid introducing air bubbles. After stirring, let it stand for 1 minute before loading it onto the machine.

3. When the downtime exceeds 4 hours and the workshop temperature is higher than 28°C, the rosin carrier in the solder paste is prone to local crystallization. At this time, the original solder paste cannot be used directly. All residual solder paste on the stencil should be recovered into a sealed can, and a newly opened batch should be replaced, confirming that its production date is no more than 6 months ago. Before loading the new solder paste onto the machine, it must be warmed at room temperature for more than 2 hours, and the cover film must not be torn during the warming process.

4. The stencil tension will attenuate by 0.5 to 1.2 N/cm² due to temperature differences during downtime, leading to printing offset of fine-pitch devices. Before powering on, a tension meter must be used to measure the four corners and the center five points of the stencil. If any point is lower than 40 N/cm², the stencil must be re-tensioned. After re-tensioning, at least 10 idle prints must be made, and an SPI device must be used to scan and verify that the CPK value is greater than 1.33.

5. If tiny particles or stringing appear on the surface of the warmed solder paste, it indicates that Flux delamination has occurred. At this time, this can of solder paste must be discarded, because the probability of splashing during soldering will increase by more than 3 times, and the void rate at the bottom of QFN will exceed 35%. Even if the SPI inspection passes, the reliability after reflow cannot be guaranteed.

6. Within the first 3 hours of the first shift after powering on, 1 PCB's 0201 and 0.4 mm pitch QFP pads must be sampled and inspected under a microscope every 15 minutes, focusing on observing whether the edge of the solder paste is a smooth arc. If serrated burrs or collapse appear, immediately stop the line to check whether the squeegee wear exceeds 0.08 mm and whether there are scratch marks on the stencil aperture wall.

7. For a nitrogen-protected printing machine, when the humidity in the cavity rises above 55% RH after downtime, the oxidation rate of the solder paste accelerates. At this time, the nitrogen replacement mode must be turned on and run for 8 minutes before powering on. After the humidity in the cavity stabilizes below 30% RH, the solder paste is introduced; otherwise, the solder joint wetting angle of the first 20 boards will generally increase by 8° to 12°.

8. All solder paste printing parameters after downtime and restart must be saved separately as the 'RESTART' version, and must not overwrite the daily production parameters. Because the squeegee angle compensation value needs to be increased by an additional 0.3° after downtime to offset the slight stencil deformation, while the release distance must be shortened by 0.05 mm to prevent sticking.

9. If the ambient temperature fluctuates by more than ±3°C during downtime, the rheological curve of the solder paste must be re-measured, focusing on confirming whether the shear thinning index still maintains the range of 0.72 to 0.78. If it is lower than this range, the solder paste cannot recover its shape in time after printing, which will increase the probability of bridging adjacent pads by 40%.

10. The last item is not a suggestion but an ironclad rule: any first-piece board after downtime that has not been verified by full SPI testing, no matter how perfect it looks visually, is prohibited from flowing into the placement process. Because solder paste printing defects will be covered by components after placement, and when they are discovered only during reflow, the rework cost for the entire batch is more than 7 times that of interception in advance.