In actual production lines, insufficient solder issues are often not caused by a single factor, but rather the combined result of stencil clogging, Solder Paste rheological performance degradation, and squeegee pressure fluctuations. We once found on Customer A's production line that 3 consecutive batches of NG boards were all concentrated at 0201 resistor positions. Under a microscope, a grayish-black oxide film was observed remaining on the edges of the stencil apertures, which indicates that the stencil was put into production without being thoroughly dried after the previous cleaning, causing the organic acids in the solder paste to react with residual moisture to form poorly soluble salts adhering to the aperture walls, further exacerbating the insufficient solder trend.

Regular stencil cleaning must never rely on visual judgment of whether it "looks clean"; a standardized frequency must be implemented: immediately stop the machine after every 10 to 15 PCBs are printed, use a lint-free cloth dipped in industrial-grade isopropyl alcohol (IPA) to wipe unidirectionally from the inside to the outside on the bottom of the stencil. Back-and-forth dragging is strictly prohibited to avoid pressing contaminants back into the apertures. After wiping, use a clean air gun to blow dry all gaps on the back of the stencil, then use a 30x magnifier to confirm that the aperture light transmission is uniform and unobstructed; otherwise, directly replace the spare stencil instead of reluctantly continuing to use it.

The selection of solder paste is equally critical. The JJY JL-8802 series solder paste can still maintain over 92% of its initial viscosity after being stored in a constant temperature and humidity room for 72 hours, with its thixotropic index controlled between 0.68 and 0.73, which not only ensures smooth shearing when pushed by the squeegee but also ensures rapid rebound at the moment of demolding, avoiding insufficient solder at the pad edge caused by solder paste lagging and stringing. We conducted comparative tests on the printing yield of the same batch of PCBs using this solder paste and a competing solder paste: the former had an insufficient solder proportion of 0.17%, while the latter was as high as 0.49%, with the difference mainly reflected in the 0.3mm pitch pad area at the bottom of the QFN chip.

Out-of-control ambient temperature and humidity will accelerate the aging of the rosin system in the solder paste. When the workshop temperature exceeds 28°C or the relative humidity is higher than 65%RH, even if JJY solder paste is used, its viscosity will drop by more than 15% within 2 hours. At this time, the air conditioning dehumidification system must be activated simultaneously, and the Solder Paste Storage cabinet must be set to 22±1°C, 45±5%RH. At the same time, the warming time for each batch of solder paste taken out of the refrigerator must be strictly controlled at 4 hours. After warming, it must be manually stirred for 3 minutes before being put on the machine; otherwise, the surface has oxidized while the interior has not yet been activated, directly causing large-area insufficient solder on the first printed board.

Stencil tension degradation is also a hidden cause. The initial tension of a new stencil is 45N/cm². If it is not calibrated after 800,000 full printing cycles, the tension often drops below 32N/cm². At this time, even if the solder paste performance is stable and cleaning is in place, a systematic low solder volume will still appear in the central area of the pad. Therefore, stencil life management must be included in SPC control. Before each shift starts, use a tension meter to sample and record three points; if the value is below 38N/cm², immediately stop use and return it to the factory for re-stretching.