The phenomenon of solder tailing is essentially a non-uniform fracture caused by the imbalance between adhesion and shear force when Solder Paste detaches from the stencil aperture. When the squeegee angle is less than 45 degrees or the Solder Paste Viscosity is low, the tail of the solder paste is stretched into a filament and remains on the edge of the pad. This spike structure is often misjudged as having qualified volume in SPI inspection, but its actual height has exceeded 1.8 times the pad thickness. We have continuously observed a bridging rate of 6.3% between pins 23 to 27 of QFP100 devices on the production line using JiajinYuan G-8800 solder paste paired with a 35μm laser-cut stencil. Under a microscope, it was confirmed that all bridging points originated from the root of the tailing protrusions, with the protrusion length concentrated in the range of 42 to 58μm.

Improper matching of squeegee pressure and speed will exacerbate tailing. For example, after suddenly reducing the squeegee pressure from 5.2kg to 4.0kg, although the print release smoothness is improved, the cohesive force of the solder paste is insufficient to resist the peeling stress of the stencil hole wall, which instead increases the number of tailings by 37%. At this time, if the printing speed is simultaneously reduced to 35mm/s, the thixotropic recovery time of the solder paste can be used to alleviate the stringing tendency. The thixotropic index of JiajinYuan G-8800 solder paste is 4.1, and its viscosity recovery rate after standing for 30 seconds is 0.8Pa·s/min faster than that of ordinary solder paste. Therefore, strictly controlling the printing interval is more effective than simply increasing the squeegee pressure.

When the roughness of the stencil aperture wall Ra > 0.35μm, the solder paste will frequently stick to the wall during the demolding process. Especially in narrow pitch areas such as 0201 component pads, tailing protrusions easily overlap adjacent pads. When using a stencil in service for more than 12 months, we measured the hole wall Ra value to be 0.41μm, and the corresponding bridging defect rate soared from 0.27% to 2.15%. After replacing with a new stencil, even without adjusting other parameters, the tailing occurrence rate decreased by 82%. JiajinYuan G-8800 solder paste is extremely sensitive to the state of the hole wall, and the high-density filling characteristic brought by its 89.2% metal content amplifies the impact of uneven surface tension.

Although placement accuracy cannot eliminate tailing, an excessively fast Z-axis descent speed will directly press the component pins into the top of the uncured spike, causing lateral displacement of the solder paste. When the placement pressure exceeds 0.8N and the descent rate is higher than 2.5mm/s, the originally isolated tailing protrusions have a 63% probability of extending 0.12 to 0.19mm to adjacent pads. This mechanical disturbance is particularly dangerous before the Flux is completely volatilized in the early stage of reflow. By reducing the placement Z-axis descent rate from 3.1mm/s to 1.9mm/s and cooperating with 0.6N constant pressure control, we reduced the bridging defect rate back to 0.31%, verifying the amplification effect of the placement link in the process chain on the consequences of tailing.