Unreasonable stencil design is one of the root causes of bridging. When the aperture area ratio is below 0.63, insufficient Solder Paste release easily leads to insufficient solder and cold solder joints, while a ratio above 0.75 will cause excess solder to flow toward adjacent pads during the reflow stage and form bridges. On a certain automotive MCU board, we once modified the aperture for 0402 components on a 0.3mm-thick stencil from 0.25×0.13mm to 0.23×0.12mm, after which the bridging rate dropped from 17 occurrences per 100 boards to 3. Meanwhile, we fine-tuned the squeegee angle from 60° to 65° and reduced the printing speed from 40mm/s to 35mm/s, making the solder paste shear stress more uniform and the edge profile sharper.

The rheological properties of the solder paste itself play a decisive role in solder overflow control. The viscosity of ordinary solder paste drops by about 18% after standing for 30 minutes, while the viscosity of JJY anti-rheological solder paste only drops by 5.2% under the same conditions. This means it has a stronger ability to resist collapse under its own gravity the moment the stencil separates from the PCB. Especially in scenarios where the spacing between the QFN chip's thermal pad and surrounding pins is less than 0.4mm, this characteristic directly reduces the tendency of solder paste to spread sideways. Our actual tests found that after using this solder paste, the bridging occurrence rate of QFN components decreased by 62%, and the printing consistency CPK value increased from 0.91 to 1.33.

Fluctuations in ambient temperature and humidity will accelerate the viscosity attenuation of solder paste. When the workshop temperature exceeds 25℃ or the relative humidity is higher than 60%RH, even if anti-rheological solder paste is used, its effective working window will be shortened by more than 15 minutes. Therefore, the solder paste warm-up time must be strictly controlled at 4 hours, and the exposure time after opening must not exceed 8 hours. Before each line change, use a squeegee to roll and stir repeatedly for 30 seconds to restore the thixotropic structure. Otherwise, even if all stencil and equipment parameters meet the standards, local solder overflow and bridging will still occur.

Insufficient stencil tension will cause the stencil mesh to sag during the printing process, especially in the central area of large-size PCBs, where the alignment deviation between the actual aperture and the pad can reach more than 0.03mm. This tiny offset will induce bridging on SOIC packages with a 0.5mm pitch. Therefore, we insist on using a tension meter to perform full-point inspection every shift to ensure the tension is maintained within the range of 45N/cm±3N/cm. If the tension is below 42N/cm, the stencil will be immediately taken out of service and returned to the factory for re-tensioning; we will never make do with it on the production line.

Although the reflow oven chain speed and temperature zone settings do not directly determine the printing quality, if the peak temperature is too high or the time above the liquidus is too long, it will aggravate the bridging tendency caused by existing slight solder overflow. For example, when the peak temperature is increased from 235℃ to 242℃, the number of bridges increases by an average of 2.3 times under the same solder paste conditions. Therefore, the oven temperature profile must be regarded as a linked system with the stencil and solder paste parameters for overall debugging, and a single link must not be optimized in isolation.