During production line debugging, we found that when the stencil was not wiped after continuously printing 30 PCBs, obvious Solder Paste stringing and tailing could be observed at more than 65% of the aperture edges under a microscope. For the same stencil, when wiped once every 15 prints with a lint-free cloth dipped in IPA, the solder deficiency rate dropped from 7.2% to 1.8%. However, if the wiping frequency was increased to once every 5 prints, the stencil tension decreased by 12% within 4 hours, resulting in failure of squeegee pressure compensation and a 9% reduction in solder paste filling rate. This indicates that the stencil wiping frequency for solder paste is not simply "the more frequent the better", but must be matched with the rheological properties of the solder paste, ambient temperature and humidity, and the fatigue curve of the stencil material.

In actual operation, we usually use the Tack Life data provided by the solder paste supplier as the benchmark. For example, a certain medium-viscosity solder paste has a nominal Tack Life of 4 hours. Under an environment of 25°C and 55%RH, the measured active maintenance time of the solder paste on the stencil surface is about 3 hours and 20 minutes. At this time, if wiping is still performed according to a fixed number of prints, it is easy to miss the optimal window. We must simultaneously monitor changes in solder paste gloss, residual thickness at the rear edge of the squeegee, and the deviation rate of SPI data between the first and last prints. Once it is found that the solder volume fluctuation of the last print pad exceeds ±15%, the wiping action is triggered immediately, rather than rigidly adhering to the old rule of wiping every 20 prints.

More importantly, solder pastes of different alloy types have extremely different responses to wiping. SAC305 Solder Paste, due to its high silver content and low oxidation tendency, can maintain a stable printing state on the stencil for up to 35 prints. Bismuth-containing solder paste, however, tends to crystallize easily at low temperatures. By the 22nd print, microcrystalline particle accumulation has already appeared at the bottom of the apertures. Wiping at this point not only fails to remove the particles, but also presses the hard particles against the sidewalls of the apertures, causing permanent clogging. Therefore, solder paste control must be modeled separately according to the alloy system, and a unified wiping SOP cannot be issued.

We once used the same DEK printer to conduct a comparative test of three groups of parameters: one group without wiping, one group wiping every 10 prints, and one group wiping every 25 prints. The results showed that only under the preconditions of stable ambient temperature and humidity, consistent solder paste batch, squeegee hardness of 90A, and stencil service life of less than 200 hours, could wiping every 25 prints achieve CPK≥1.33. Once the stencil is used for more than 300 hours, even if wiped every 15 prints, the solder bridging rate still rises to 4.6%. This indicates that wiping frequency is only one link in the solder paste printing defect control chain. It must form a closed loop with stencil life management, Solder Paste Storage conditions, temperature recovery time, and real-time SPI feedback. Any single-point optimization is difficult to continuously and stably produce qualified solder joints.