We frequently encounter oily residues on PCB edges or non-pad areas after Solder Paste printing on the production line. Under a microscope, the Flux is seen to disperse in a mist and adhere to the solder mask. This Solder Paste Flux spatter phenomenon not only causes false optical inspection results, but also induces ionic contamination and CAF risks after reflow. The root cause of the problem often lies not in the equipment accuracy itself, but in the failure of the dynamic matching among solder paste, stencil and squeegee.

The selection of solder paste must take into account both the flux system and the metal powder particle size distribution. When using Type 4 (20–38μm) solder paste with a highly volatile rosin-based flux, if the printing pressure exceeds 0.45MPa and the squeegee speed is higher than 40mm/s, the flux is highly likely to break through the solder paste body due to shear thinning and form spatter. In this case, switching to Type 5 (15–25μm) solder paste with a medium-volatility no-clean flux can instead improve thixotropic stability, provided that the stencil thickness is reduced synchronously from 0.12mm to 0.10mm; otherwise, a decrease in the aperture area ratio will lead to poor release and further aggravate spatter.

Stencil apertures cannot be simply replicated 1:1 according to pad dimensions. For pads of small components 0402 and above, the aperture must be inset by 8–10μm and treated with a trapezoidal chamfer. This not only reduces the pulling stress at the moment of solder paste release, but also keeps the flux confined within the pad outline during the printing process. For BGA areas, a 0.05mm wide anti-overflow groove must be added around the aperture. Although this structure increases laser cutting costs, it can intercept more than 90% of the lateral migration paths of the flux.

The standing time after printing is often overlooked. Actual measurements show that at a room temperature of 25℃, if the interval between solder paste printing completion and component placement is less than 60 seconds, the flux has not yet established surface tension equilibrium. The downward pressing action of the placement head will directly disturb the unstable flux film, causing it to creep along the PCB surface via capillary action. After extending the standing time to 90–120 seconds, the spatter rate decreases by about 70%. However, this measure must be linked with workshop humidity control. When the relative humidity is below 40%RH, excessively long standing time will instead cause cracking of the flux surface film.

Airflow disturbance in the entry section of the reflow oven is also a hidden cause. We once found that spatter complaints surged on a certain production line after the fresh air system was replaced. It was finally located that the top air velocity in the first heating zone of the preheat section reached 0.8m/s, far exceeding the safety threshold of 0.3m/s. After adjusting the deflector angle and installing low-turbulence baffles, the flux atomization phenomenon completely disappeared. This shows that root cause analysis of solder paste defects must not be separated from the coupled environment of the entire line; any single-point optimization may be reversed and undermined by downstream processes.