The essence of edge slump during thick-board Solder Paste printing is insufficient shear resistance of the solder paste at the moment it detaches from the stencil, combined with lateral flow of the paste caused by PCB warpage and reduced stencil support rigidity. Our production line measurements found that when board thickness exceeds 0.8 mm, the slump rate of the same conventional solder paste on 2.0 mm pitch QFP pads jumps from 1.2% to 6.7%. After switching to thick-board solder paste with a thixotropic index ≥4.8 and viscosity 220 Pa·s (@10 rpm), the slump rate falls back to 0.9%, indicating that the bulk rheological properties of the solder paste play a decisive role in thick-board printing stability.

Stencil apertures must be adjusted synchronously; thin-board parameters cannot simply be reused. We mandate that for thick-board solder paste printing, the aperture area ratio be increased from 0.65 to above 0.72, and the aperture sidewalls polished to Ra≤0.2 μm. Otherwise, even with high-thixotropy solder paste, continuous slump will still appear after the 30th board. This is because rough sidewalls intensify viscous drag between the solder paste and aperture walls, unbalanced release tension distribution, and in turn induce edge paste stringing and collapse.

The squeegee angle must be locked within the range of 58°±1°. For every 0.5° deviation in angle, the edge height variation coefficient after thick-board solder paste printing rises by 11%. This is because an insufficient angle leads to inadequate downward force and incomplete solder paste filling, while an excessive angle results in too high a shear rate that disrupts the internal network structure of the solder paste. Both conditions weaken its ability to resist gravity-induced deformation during release. Therefore, we directly install mechanical limit blocks on the squeegee holder to eliminate manual adjustment errors.

The post-printing standing time must be extended to 65–75 seconds, 25 seconds longer than for conventional boards. This window is not set based on experience; rather, rheometer measurements show that the structure recovery half-life of this thick-board solder paste at 25°C is 58 seconds. Below this value, the paste has not completed network reconstruction; above 80 seconds, the Flux surface layer begins to slightly exude, which instead reduces wetting activity. Therefore, execution must be strictly confined to this range, and the production line has already固化 this parameter into the MES work order automatic countdown.

The pre-reflow AOI inspection point must be moved forward to within 3 minutes after printing, because thick-board solder paste exhibits hidden gradual edge slump during standing—meaning no abnormality is visible to the naked eye, but slow extension of paste at the pad outer edge can be observed under a microscope. This process peaks at 7 minutes; if not intercepted at this point, solder bridging or tombstoning will inevitably form after entering reflow. We have deployed high-speed AOI with Z-axis compensation on Line 3 to ensure judgment is completed before the gradual slump critical point.

All improvement actions must be tied to batch verification; single-board results alone are insufficient. We stipulate that every time a thick-board solder paste model is switched, three sets of data must be continuously tracked for 500 boards: printing CPK, post-reflow ICT open-circuit rate, and X-RAY void rate. Mass production is permitted only when all meet the standards; otherwise, the formulation supplier is required to provide raw rheology reports and DSC thermal analysis spectra, ensuring that solder paste printing improvements are truly rooted in physical essence rather than superficial parameter compromises.