The selection of stencil thickness must strictly match the height of PCB pads and the particle size of Solder Paste. For example, a 0.1mm pad is usually paired with a 0.12mm stencil. If a 0.15mm stencil is mistakenly used, the support force at the bottom of the aperture decreases, the adhesion of the solder paste becomes unbalanced, and stringing is easily induced at the moment of demolding. The thixotropic index of JJY G600 series solder paste is 4.2 at 25°C, which is extremely sensitive to shear rate. Once the printing speed is increased from 25mm/s to 35mm/s, the peeling work per unit time increases by about 37%, and the probability of stringing directly doubles.

We once continuously tracked three batches of motherboards of the same model on a customer's production line in Dongguan, and found that stringing was concentrated in the ground pad area of the QFN32 package. Under a microscope, the solder paste columns were slender and conical with obvious base shrinkage. Further measurement showed that the actual stencil thickness was 0.138mm, exceeding the upper limit of the ±0.003mm tolerance specified in the process document. At the same time, the squeegee pressure of this line was set to 6.8kg, while the speed remained at a high level of 32mm/s. The combination of these two parameters caused the solder paste to be forcibly torn off before it was fully released from the aperture. The stringing length generally reached 0.15～0.22mm, which has exceeded the 0.1mm limit allowed by IPC-7525B.

On-site adjustment is not simply a matter of reducing the speed, because a speed lower than 20mm/s will lead to insufficient solder paste filling. It is necessary to synchronously calibrate the stencil tension to 45N/cm2, fine-tune the squeegee angle from 60° to 58° to reduce the vertical pressing component, and replace it with trapezoidal chamfered apertures custom-opened by JJY, so that the solder paste can obtain a gentler stress transition at the moment of separation. The measured stringing rate decreased from 18.7% to 0.9%, and the CPK remained stable above 1.42.

It is particularly important to note that after the same stencil has been used for 200,000 times, wear on the aperture edges will locally reduce the effective thickness by 0.005～0.008mm. At this time, even if the original settings are reasonable, stringing will reappear due to local support failure. Therefore, the stencil life must be included in SPC control. After every 5,000 prints, use a coordinate measuring microscope to spot-check the cross-section of 3 apertures. If unilateral wear exceeds 0.004mm, the stencil must be scrapped immediately; visual inspection must not be relied upon.

There is also a hidden situation where PCB pad oxidation or uneven OSP film thickness reduces surface energy, causing solder paste wetting to lag, and forming reverse capillary traction during demolding. This type of stringing is often accompanied by solder beads. It is necessary to cooperate with XRF detection to check whether the phosphorus content on the OSP copper surface is lower than 65μg/cm2; otherwise, adjusting only the equipment parameters will address the symptoms rather than the root cause.