Excess solder is never an isolated phenomenon in actual production lines; it is a concentrated manifestation of the mismatch among stencil design, squeegee process, and Solder Paste rheological properties. Especially in the printing of fine-pitch components such as 0402 and 0603, once the aperture size exceeds 105% of the theoretical pad area, the solder paste will tail and string at the moment of release due to insufficient cohesion, thereby forming excess solder between adjacent pads.
Stencil aperture dimensions must strictly match the geometric characteristics of the pads
JiajinYuan's on-site testing at its Dongguan factory found that, for the same 0.3mm-thick laser-cut stencil, when a 1.8mm×1.8mm aperture was used for the thermal pad of a QFN32 package (the actual pad size is 1.7mm×1.7mm), post-printing SPI inspection showed that the average solder paste volume on this pad reached 125±8nL, far exceeding the 95~105nL range recommended by IPC-7527. After reducing the aperture to 1.72mm×1.72mm, the volume fell back to 101±5nL, and the excess solder rate dropped sharply from 17.3% to 2.1%.
The squeegee parameter combination determines the shear and release behavior of the solder paste
If the squeegee angle is below 55°, the lateral shear force exerted on the solder paste within the stencil apertures weakens, leading to insufficient filling and easy residue during release. Conversely, an angle above 65° will exacerbate the solder paste extrusion effect, which is more pronounced especially when using medium-to-low viscosity solder paste with a viscosity <650Pa·s. JiajinYuan's synchronous tests showed that when the squeegee pressure was set to 7.2kg, the speed to 45mm/s, and the angle to 62°, the excess solder rate for 0.4mm pitch QFP components reached as high as 23.6%. However, after reducing the pressure to 5.8kg, adjusting the angle back to 58°, and fine-tuning the speed to 40mm/s, this rate immediately dropped to 3.4%.
Stencil tension and support status affect the actual equivalent size of the apertures
Even if the aperture drawings are fully compliant, if the stencil tension is below 35N/cm or there is local collapse in the support base, stencil sag during printing will cause the actual projected area of the apertures to increase. Especially in the central area of large stencils with a span >50mm, this deformation can additionally increase the equivalent aperture area by 4%~7%. At this point, even if the squeegee parameters are within the conventional range, periodic excess solder will still occur. JiajinYuan's maintenance team once used a laser displacement meter to measure that the sag in the middle of a 736mm×736mm stencil reached 0.028mm, corresponding to an aperture magnification of approximately 5.3%. This abnormality disappeared after replacing it with a high-tension stencil.
All parameter adjustments must use SPI first-article data as the sole criterion; empirical visual judgment cannot be relied upon, and excess solder must not be misjudged as overly high solder paste activity or inadequate stencil cleaning. JiajinYuan's internal standards explicitly require that if excess solder occurs at the same position on 3 consecutive PCBs, the line must be stopped immediately to check the stencil aperture drawings, the real-time squeegee pressure curve, and the stencil tension verification records. Any action that skips these three items will cause the problem to be exponentially amplified in subsequent processes.
