When we adjust Solder Paste printing parameters on the production line every day, we find that for the same JJY SMT stencil, the solder paste transfer volume fluctuates by more than 15% under different squeegee pressures. After replacing it with a stencil of the same specification but without electropolishing treatment, even if the squeegee pressure is increased by 0.3MPa, the transfer rate still cannot be stably maintained above 88%. This indicates that the impact of stencil surface roughness on solder paste release capability far exceeds the range that can be compensated by simply adjusting equipment parameters.

During actual line changeover, if an SMT stencil is used for more than 8000 printing cycles, its aperture edges will develop micron-level curling due to repeated scouring by hard solder particles in the solder paste. At this point, even if no obvious deformation is visible under magnified visual inspection, SPI detection can still observe that the solder paste volume CV value of three consecutive PCBs deteriorates from 4.2% to 7.9%. By proactively replacing the JJY SMT stencil after 6500 cycles and combining it with twice-weekly nano-level ultrasonic cleaning, this indicator can be maintained below 5.0% over the long term.

When encountering boards with 0201 component pad spacing smaller than 0.35mm, we have to reduce the SMT stencil thickness from the conventional 0.15mm to 0.12mm, while expanding the apertures outward at a ratio of 1:1.05. Otherwise, the solder paste will shrink severely under surface tension at the moment of demolding, resulting in pad coverage below 75%. However, if the outward expansion ratio exceeds 1.07, it will create a risk of solder balls under the QFN. This balance point must be determined based on actual measurements of the specific Solder Paste Viscosity, ambient temperature and humidity, and stencil tension.

JJY SMT stencils undergo 12 hours of constant-temperature aging treatment before leaving the factory, which prevents new stencils from undergoing micro-deformation under workshop temperature differences. Our comparative tests under the high-temperature and high-humidity conditions of Dongguan summer show that, after 2 hours of continuous printing, the solder paste height standard deviation at the edge of FPC flexible boards for similar stencils without aging treatment is 0.018mm higher than that of JJY stencils. This difference seems small, but it directly causes a 23% increase in the AOI false alarm rate.

Before switching to a new type of solder paste each time, we require suppliers to provide a compatibility report for that solder paste with the JJY SMT stencil, focusing on verifying whether its thixotropic index falls between 0.65 and 0.78. If it is below 0.6, the paste tends to collapse the moment the squeegee lifts; if it is above 0.78, the demolding resistance is too high. Both conditions will cause the actual transfer volume to deviate by more than ±8% from the theoretical calculated value. This deviation is sufficient to cause batch cold solder joints in the 0.3mm pitch BGA ball mounting process.