We have observed in actual production lines that if Solder Paste immediately enters Reflow Soldering after printing, skipping the pre-drying step, solder paste stringing residue at the bottom of the stencil and shiny gel-like accumulation along the squeegee edge often occur. This is actually abnormal adhesion caused by incomplete preliminary solvent migration in the Flux system. Once the chain speed is slowed to 1.2 m/min and the 70°C hot air pre-drying zone is activated, the shape retention of the printed solder paste improves significantly. However, if the temperature rises above 85°C and the dwell time exceeds 45 seconds, some lead-free solder pastes will develop surface skinning and microcracks visible under a microscope, indicating that partial cross-linking of the rosin resin has occurred.

Comparative tests conducted by JiajinYuan at its Dongguan factory showed that using the same SAC305 Printing Solder Paste, under identical stencil tension and squeegee pressure conditions, the coefficient of variation in printing thickness for the non-pre-dried group reached as high as 12.7%, while the coefficient of variation dropped to 6.3% after pre-drying at 75°C for 30 seconds. This difference is directly reflected in the AOI false alarm rate, which was reduced by 38% in the latter group, with particularly significant improvement in pad coverage consistency for 0201 package components. However, the Solder Paste Storage temperature must be adjusted synchronously; otherwise, if the paste is taken out of cold storage and put into production without sufficient temperature equilibration, pre-drying will instead exacerbate the conflict between condensed moisture and flux exudation.

We once neglected the coupling effect between solder paste drying and ambient humidity, and experienced increased burrs along stencil aperture edges after solder paste printing for three consecutive days during the plum rain season, with flux tails appearing serrated under a magnifying glass. Later, we forced the humidity at the pre-drying inlet to be controlled below 45% RH from 65% RH, and fine-tuned the wind speed in the drying section from 1.8 m/s to 1.3 m/s, after which the burr phenomenon disappeared immediately. This shows that solder paste drying is not a simple heating process, but a result of the synergistic interaction among temperature, wind speed, and humidity; imbalance in any one factor will cause the thixotropic recovery time of the printed solder paste to deviate from the process window.

A customer reported a large number of tombstoning defects on QFN components after reflow. During on-site troubleshooting, we found that their pre-drying zone was set to a constant 90°C, and the solder paste had been left at room temperature for more than 6 hours after opening, with a mild oxide film already formed on the paste surface. At this point, high-temperature drying accelerated the passivation of the surface tin powder, causing the printed solder paste to fail to spread uniformly within the stencil apertures, resulting in uneven solder volume distribution. Ultimately, deflection occurred during the early reflow stage driven by surface tension differences. After switching to a 70°C dynamic heating mode and strictly enforcing the rule that opened solder paste must be used up within 2 hours, the tombstoning rate dropped from 1.8% to 0.07%.

During production line debugging, one cannot only focus on the displayed pre-drying temperature value; it is also necessary to observe whether the distance between the hot air nozzles and the PCB surface is maintained at 45±3 mm. A distance deviation exceeding 5 mm will cause local over-drying or under-drying, which in turn leads to inconsistent solder paste drying degrees for BGA and 0402 components on the same board. This non-uniformity manifests in X-ray inspection as a 22% fluctuation in BGA solder ball voidage, while 0402 solder joint wetting angles are generally too large. The root cause lies in the inherent difference in thermal response rates of printed solder paste for pads of different sizes, which must be addressed through wind speed gradient compensation rather than single-point temperature control.