A low-humidity workshop is not a natural safe box for Solder Paste protection. When RH falls below 30%, the rosin carrier in the solder paste instead accelerates cross-linking aging, causing the viscosity to rise by more than 18% within 4 hours. A sudden drop in RH below 20% will further induce rupture of the passivation film on the solder powder surface, increasing the oxidation rate by 3 times. Therefore, the entire process of storage, temperature recovery, and use of workshop solder paste must be included in closed-loop control, and reliance solely on the single parameter of ambient humidity is not sufficient.

After the solder paste is taken out of the cold storage, a forced temperature recovery and standing process must be implemented, that is, it must be placed in a constant temperature room at 22±2℃ for more than 120 minutes before unsealing. Otherwise, condensed water will form a micro-water film on the surface of the solder paste, and local oxidation spots will be triggered within 30 minutes after contact with air. This phenomenon is particularly obvious in bismuth-containing Alloy Solder paste. We once scrapped three batches of 01005 device printed boards consecutively due to this problem, and later completely solved it by installing temperature and humidity sensing tags to real-time track the exposure duration of each can of solder paste.

The printing press feeding system needs to be equipped with a nitrogen positive pressure protection module to stably control the oxygen concentration in the solder paste cylinder below 100ppm. At the same time, the squeegee pressure is set to 5.2±0.3kgf, and the printing speed is controlled within the range of 25±3mm/s. The three must be adjusted synchronously. If only the speed is reduced without adjusting the pressure, it will lead to aggravated solder paste tailing; if only the pressure is increased without nitrogen control, the oxidation products will be repeatedly rolled by the squeegee and embedded in the bottom of the stencil openings, causing the blockage cycle to be shortened to 60% of the conventional state.

Before starting each shift, the rotational viscosity value of the first can of workshop solder paste of the day must be actually measured with a standard solder paste viscometer. When the measured value at 48℃ is 15% higher than the nominal value, it must be immediately stopped and the material return process initiated. This threshold is derived from our tracking data of 237 batches of solder paste, which found that after the viscosity exceeds the standard, the coefficient of variation (Cv) of the printing thickness will inevitably exceed 12.7%, far exceeding the 9% upper limit allowed by IPC-7527.

The key to solder paste protection lies in establishing traceable action nodes. For example, each time the lid is opened, the QR code on the can body must be scanned to register the timestamp, and each time the temperature recovery is completed, an infrared thermal imager is required to confirm that the temperature difference on the can surface is ≤0.5℃ before it is allowed to be put on the machine. These actions have been solidified into the Andon system of our company's SMT production line. Any link timeout or parameter drift will automatically lock and stop the printing station and push the abnormal code to the process engineer's mobile phone.