The squeegee pressure is not a single variable adjusted in isolation; it must be coordinated and matched with the squeegee speed, squeegee angle, stencil tension, and the rheological behavior of the Printing Solder Paste itself. For example, when the thixotropic index in the Solder Paste parameters is low, even if the pressure is increased from 80N to 100N, uneven solder paste shrinkage at the pad edges may still occur. In this case, blindly increasing the pressure will instead exacerbate solder paste extrusion at the bottom of the stencil, causing bridging.

We usually set the squeegee pressure to 90N, the squeegee speed to 50mm/s, and the squeegee angle to 60° during the first-article verification stage, then use a 20x magnifier to check whether the side profile of the solder paste on the pads is trapezoidal rather than inverted trapezoidal. If obvious top collapse is found and there is a continuous silver-gray trailing at the bottom of the stencil, it indicates that the pressure has exceeded the limit. The pressure should be immediately adjusted back to 75N, and it should be simultaneously confirmed whether the solder paste has remained on the stencil for more than 4 hours without stirring.

Printing solder pastes from different batches have extremely large differences in pressure sensitivity. One day, when using a certain brand of 3# powder solder paste, under the same parameters, the pressure was increased from 85N to 95N, and SPI inspection showed that the solder paste volume fluctuation of QFN pads expanded from ±8% to ±15%. After switching to 4# powder solder paste from the same manufacturer, the same pressure increase only caused ±6% fluctuation. This shows that the powder particle size distribution and sphericity in the solder paste parameters have a decisive influence on the pressure tolerance window.

When the production line encounters printing defects of fine-pitch ICs, we do not directly adjust the pressure. Instead, we first lightly stick the back of the stencil with tape to confirm whether there is solder paste residue. If there is residue, it indicates that the pressure is too high, causing the solder paste to be forcibly pressed into the side wall gaps of the stencil openings. At this time, even if the pressure is reduced to 60N, if the stencil is not thoroughly cleaned and new solder paste is not replaced, continuous insufficient solder will still occur subsequently.

In actual debugging, pressure changes must be fine-tuned with 0.5N as the minimum step unit, because the feedback accuracy of the servo motor is limited. If the operation is performed in 5N jumps, the optimal window is often missed. Especially when printing devices with a pitch below 0.3mm, a pressure deviation exceeding 1.2N will reduce the SPI pass rate by more than 12 percentage points. This quantitative relationship comes from the cumulative data of 234 parameter reviews across 17 production lines over three years.

Each time the solder paste model is changed, a pressure baseline test must be redone. Even for two solder pastes that both meet the J-STD-005 standard, a difference of 0.8Pa in their yield values will turn the original 90N pressure into an overload state. If historical parameters are relied upon for operation at this time, systematic solder balls will form at the solder ball positions at the bottom of the BGA. This phenomenon cannot be identified during the AOI stage, and only X-RAY sampling inspection can reveal it.