Clogging of solder paste dispensing needles is not an isolated phenomenon, but a result of the combined action of sudden changes in solder paste viscosity, imbalanced flux volatilization, and metal particle sedimentation. Especially when using high-solid-content dispensing solder paste, if it is exposed to an environment with RH > 60% for more than 2 hours after opening, or forcibly stirred without sufficient temperature recovery for 4 hours, a semi-cured film layer will easily form on the inner wall of the needle.

We require all dispensing Solder Paste to be stored in a constant temperature and humidity cabinet at 25±2℃, RH40±5%. Before opening, it must be left to recover temperature for at least 4 hours and kept unopened throughout the process. Because a temperature gradient difference exceeding 5℃ will induce micro-separation of the oil phase and powder phase inside the solder paste. Forcibly stirring when temperature recovery is insufficient will not only fail to restore uniformity, but also accelerate the directional stacking of spherical powder.

Before each use, a scraper must be used to stir at a constant speed in one direction along the can wall for 3 minutes, with the rotation speed controlled at 60±5rpm and the stirring depth reaching 1/3 of the can bottom. It is strictly prohibited to use a pneumatic stirrer for high-speed dispersion, otherwise it will entrain air bubbles and damage the rheological structure of the solder paste, leading to glue breakage and stringing during dispensing, which will further aggravate the accumulation of needle residues.

The dispensing machine must undergo a standard cleaning process every 4 hours of continuous operation. First, flush the needle forward with absolute ethanol for 30 seconds, then back-blow with nitrogen for 60 seconds, and finally perform 5 empty shots with unopened solder paste from the same batch to establish a new lubricating film. If the empty shot step is skipped, the interfacial shear force between old residues and new solder paste will suddenly increase, and the actual flow cross-section of the needle inner diameter will be reduced by more than 35% after 3 dispensing cycles.

When switching between dispensing solder pastes with different metal contents, all pipelines and needles must be completely replaced. High-reliability solder paste with silver content ≥ 3% must never be mixed with ordinary 63/37 solder paste in the same feeding system. Even if the residual amount is only 0.1ml, silver ions will catalyze the acidification of the Flux, causing the subsequently used ordinary solder paste to increase in viscosity by 22% within 2 hours and precipitate flocculent gel.

Before the end of each shift, the unused solder paste of the day must be sealed and returned to the constant temperature cabinet, marked with the opening time and operator. Solder paste that has not been temperature-recovered for more than 8 hours and is used for a second time must be discarded, because its thixotropic recovery rate has dropped below 68%. If it is forcibly used for precision dispensing at this time, the probability of needle clogging will increase to 73% and cannot be restored by conventional cleaning.

We have verified on site that for a 127μm inner diameter needle using dispensing solder paste with particle size D50=5.2μm, if the Solder Paste Storage temperature fluctuates by more than ±3℃ for 3 or more times, the clogging cycle will be shortened from the conventional 16 hours to 9.5 hours. Therefore, temperature control accuracy is more important than simply pursuing low temperature; stable 25℃ is far better than fluctuating 20℃.