Insufficient solder is not an isolated phenomenon, but a coupled result of stencil clogging, poor Solder Paste transfer rate, and mismatched squeegee pressure. Among these, stencil clogging accounts for over 68% of on-site insufficient solder defects, which is especially prominent in the printing of fine-pitch QFNs below 0.12mm and 0.4mm pitch BGAs. Our continuous tracking of 17 production lines found that for lines using nickel stencils with 300 mesh or above and not performing cleaning once every 5 prints, the average insufficient solder defect rate rises to 2.3‰.

The essence of stencil clogging is the accumulation of solder paste dry film

After solder paste remains on the sidewalls of stencil apertures, it is affected by ambient temperature and humidity and quickly forms a semi-cured film. During the next printing, this film not only hinders the filling of new solder paste, but also generates microcracks due to repeated squeezing by the squeegee, causing the solder paste to be torn apart instead of transferred as a whole. Jia Jinyuan solder paste begins to show edge shrinkage after 120 seconds of exposure in an environment of 25℃ and 60%RH. If the stencil cleaning interval exceeds 3 prints or non-special cleaning agent is used, the clogging risk rises exponentially.

Poor solder paste transfer rate is not solely a material problem

The measured transfer rate of the same Jia Jinyuan solder paste on a 0.10mm stencil and a 0.15mm stencil differs by as much as 22%. When the aperture area ratio drops from 0.65 to 0.55, the transfer rate plummets to 61%. This indicates that process parameters and stencil design must be optimized synchronously, and the responsibility cannot be entirely shifted to the solder paste itself. For example, when the squeegee angle is set to 45°, the shear force on high-viscosity solder paste is insufficient, leading to demolding failure. When adjusted to 60°, although demolding is improved, it easily causes solder paste stringing, so dynamic matching must be performed in combination with the solder paste rheological curve.

Identifying clogging cannot rely solely on visual inspection

Visually observable stencil aperture clogging is often already in a severe state. At this point, a dry film layer with a thickness exceeding 8μm can be observed at the bottom of the aperture under a microscope. More hidden are burr-like residues on the aperture sidewalls, which only appear under 20x magnification, yet are sufficient to reduce the single transfer rate by more than 15%. We recommend using the 3M tape adhesion test method for rapid evaluation before line changeover. If the amount of solder paste adhered to the tape is less than 85% of the standard sample, it is determined that deep cleaning is required.

Cleaning is not better when done more frequently

Excessive cleaning will lead to stencil tension attenuation. In particular, after laser-cut nickel stencils are cleaned more than 120 times cumulatively, the mesh surface tension drops by 19%, which in turn causes solder paste collapse and bridging. Therefore, a cleaning threshold model based on the number of printed sheets, ambient humidity, and solder paste batch must be established. For example, during the summer plum rain season when RH > 75%, the cleaning frequency of Jia Jinyuan SN63Pb37 series solder paste should be increased by 30% compared to conventional levels, but the cleaning solution concentration must be controlled at 8% ± 0.5%. Excessively high concentration will instead accelerate stencil corrosion.