The rheological behavior of Solder Paste during SMT printing is not a static parameter but a dynamic response. It must simultaneously satisfy the shear-thinning property under squeegee pushing, the anti-sagging ability when residing at stencil openings, and the rapid structural recovery capability at the moment of demolding. If any of these three links is mismatched, it will gradually amplify into aperture clogging, insufficient solder, or solder bridging defects during continuous operation.

We found that the apparent viscosity of JiajinYuan solder paste measured in a 25°C constant-temperature workshop decreased by 62% when the shear rate dropped from 1000s⁻¹ to 10s⁻¹, indicating that its shear-thinning response is sufficiently sensitive. However, if the thixotropic index is lower than 1.6, even with good initial printing, residual solder paste at the bottom of the aperture will accumulate due to insufficient structural rebuilding after continuous operation for 400 pieces, eventually triggering the first clogging around the 600th piece. Conversely, when the thixotropic index exceeds 2.5, although demolding is clear, it is often accompanied by trailing at the rear edge of the squeegee and splashing on the back of the stencil, which is precisely a manifestation of high yield value hindering immediate flow.

During actual parameter adjustment, solder paste data cannot be viewed in isolation; it must be synchronously matched with squeegee hardness (Shore A70), contact angle (58°±2°), pressing depth (0.12mm), and the rheological window of the solder paste. For example, when the aspect ratio of the stencil opening drops to 0.68 (such as a 0.25mm×0.12mm fine-pitch pad), even if the same JiajinYuan solder paste is used, if the squeegee speed remains at 55mm/s, the shortened shear time will prevent the solder paste from fully filling, forming cavities in the middle of the aperture and inducing local dry-type clogging after the third printing.

Warming-up operation seems simple but is extremely prone to hidden risks. JiajinYuan solder paste requires being taken out from refrigeration (4°C) and left to stand in a 25°C environment for 180 minutes before opening and stirring. If opened early or stirred for less than 3 minutes, the internal particle agglomerates of the solder paste remain undissociated, and its dynamic modulus G′ rises abnormally in the low-frequency range. At this time, sudden increase in squeegee traveling resistance and increased solder paste adhesion on the back of the stencil will occur immediately upon machine setup. After continuous printing for 300 pieces, the clogging rate at fine-pitch positions rises to 17%.

Stencil tension attenuation will subtly change the force state of the solder paste. When the new stencil tension is ≥45N/cm, the solder paste is strongly laterally constrained within the aperture, and the demolding deformation is more controllable. After the tension drops below 32N/cm, the demolding rebound delay of the same JiajinYuan solder paste at the 0.3mm opening increases by 0.15 seconds. This tiny lag is cumulatively amplified in high-speed printing, eventually manifesting as decreasing solder volume at the pad edge starting from the 900th piece and uneven residual thickness on the aperture wall visible under a microscope.