Our on-line production testing found that for Solder Paste from the same can after opening, the viscosity values measured at three sampling points after continuous stirring for 5 minutes, 10 minutes, and 15 minutes were 680Pa·s, 620Pa·s, and 710Pa·s respectively. This indicates that stirring solder paste is not a case where longer time yields better results. In the initial stage, shear force disrupts the internal thixotropic network structure of the solder paste, reducing its viscosity and thereby improving demolding performance. However, beyond the critical stirring duration, the light components of the Flux accelerate volatilization, while the oxide film on the solder powder surface slightly increases, directly weakening the lubricity between particles, ultimately manifesting as a rebound increase in Solder Paste Viscosity.

In practical operation, if following the standard process of stirring for 3 minutes each time, allowing it to stand for 2 hours before repeating stirring, after three consecutive cycles, the solder paste viscosity only increases by 4% compared to the initial value. However, if a single stirring exceeds 8 minutes without temperature control, the viscosity increase can reach 18%. At this point, the squeegee pressure during printing needs to be synchronously increased by 0.2MPa to maintain thickness consistency; otherwise, stringing and bridging are prone to occur at the edges of stencil openings, especially in areas with 0201 packaged devices where the defect rate rises significantly.

We have compared the response differences of three mainstream solder paste models under the same stirring conditions. Among them, solder paste containing rosin-based flux shows the fastest viscosity recovery after the 4th stirring, while water-white no-clean solder paste, due to fewer volatile components, maintains viscosity fluctuation within ±3% during the first 6 stirrings. This suggests that when selecting solder paste, one cannot only look at the nominal initial viscosity value, but must also pay attention to its tolerance to the stirring process window.

On-site, a production line once set the stirring time uniformly to 12 minutes in pursuit of efficiency, resulting in the proportion of insufficient solder on QFP device pads soaring from 1.2% to 6.7% for three consecutive days. After shutdown inspection, it was found that the solder paste viscosity had risen to 790Pa·s, far exceeding the equipment recommended upper limit of 720Pa·s. After replacing with a new batch and restoring the 3+2 minute segmented stirring, the printing CPK value recovered from 1.03 to 1.32. This indicates that solder paste viscosity change is not a slow drift but has a clear inflection point, which must be combined with real-time measurement rather than empirical estimation.

The setting of stirrer speed is equally critical. When the speed is increased from 800rpm to 1200rpm, the temperature rise of the solder paste accelerates by 2.3℃ within the same time. Every 5℃ increase in temperature is equivalent to the viscosity attenuation effect of stirring for an additional 2 minutes. Therefore, when the summer workshop ambient temperature reaches 32℃, the stirring duration must be compressed to within 2 minutes, and stirring equipment with a heat dissipation air duct should be prioritized; otherwise, the risk of solder paste viscosity instability will multiply.