During debugging of SMT printing parameters, we found that solder spikes and insufficient solder are often not isolated phenomena, but rather the combined result of a mismatch between squeegee pressure and travel speed. When the pressure exceeds 4.8 kg while the speed remains above 50 mm/s, the Solder Paste is over-extruded and spreads laterally along the stencil apertures, with obvious trailing-like stringing visible under a microscope. Conversely, if the pressure is below 4.2 kg and the speed is below 40 mm/s, the solder paste cannot fully fill the bottom of the apertures, resulting in post-reflow solder joint heights generally below 120 μm, which is especially prominent on 0201 and 0.4 mm pitch QFN-type components.

Actual production line verification of JJY squeegees shows that for every 0.1 kg increase in pressure, the stencil deformation increases by approximately 3.2 μm, while for every 5 mm/s increase in speed, the solder paste shear time is shortened by approximately 18 ms. The combination of these two factors causes an imbalance between the cohesive force and adhesion of the solder paste during the release phase. At this point, even if the stencil tension meets the standard and the Solder Paste Viscosity is qualified, local solder spikes and bridging between adjacent pads will still coexist. This contradictory phenomenon occurs as frequently as 27% when using Type 6 powder solder paste with a 0.1 mm thick laser stencil.

We typically set the initial pressure to 4.5 kg and simultaneously enable speed closed-loop feedback. By automatically identifying the deviation value of the pad edge profile through post-printing AOI, we reverse-correct the parameters. When solder spike length >80 μm occurs on three consecutive boards, we prioritize reducing the pressure by 0.2 kg rather than increasing the speed, because increasing the speed will exacerbate the retention and accumulation of solder paste at the squeegee blade edge, causing the standard deviation of subsequent printing thickness fluctuations to rise to ±9.6 μm. If the insufficient solder area ratio >15% on five consecutive boards, first confirm whether the squeegee blade edge wear exceeds 0.03 mm, then reduce the speed to 42 mm/s and observe the change in the average solder paste volume on the next board.

The carbon fiber substrate of the JJY squeegee maintains a stable thermal deformation within ±0.8 μm under operation at 45 mm/s. However, if the pressure is maintained above 4.9 kg for a long time, its blade edge support structure will undergo irreversible micro-buckling after 72 hours of continuous operation. At this point, even recalibrating the pressure plate cannot restore the original contact angle, and the entire squeegee assembly must be replaced; otherwise, all subsequent parameter adjustments will merely mask the problem rather than solve it.

On-site, a production line once mistakenly increased the pressure abruptly from 4.5 kg to 5.2 kg to address sudden insufficient solder. As a result, the solder spike defect rate soared from 0.18% to 3.7% within two hours, while regular transverse scratches appeared on the bottom of the stencil. Magnifying glass inspection confirmed that micro-chipping had occurred on the squeegee blade edge. This demonstrates that parameter adjustments must follow the principle of gradual change: each pressure adjustment must not exceed 0.2 kg, each speed adjustment must not exceed 3 mm/s, and at least two boards must be spaced between adjustments to confirm the effect.