A large number of solder balls appear during the reflow process after Solder Paste printing, mainly due to insufficient volatilization of high-boiling-point solvents in halogen-free solder paste and Flux splashing caused by excessively fast heating rate in the preheating zone. In practice, the heating slope in the 120℃～160℃ section should be reduced from 2.5℃/s to 1.8℃/s, and the 150℃ plateau time should be extended to 90 seconds. The key preventive measure is that the actual temperature profile inside the furnace must be measured with thermocouples for the first piece of each shift, and the displayed values on the equipment panel cannot be relied upon.

The tombstoning phenomenon of chip components occurs frequently. The root cause is that the difference in thermal capacity between the two end pads is amplified, leading to an imbalance in wetting thrust. Halogen-free solder paste is more likely to trigger this defect due to its slightly lower activity. In this case, the end temperature of the soaking zone needs to be fine-tuned from 175℃ to 182℃, and the holding time should be shortened to 60 seconds. At the same time, check whether the stencil apertures are symmetrical and whether the Z-axis pressure of the placement machine is consistent. The key preventive measure is to scan the actual temperature distribution in each zone of the furnace with an infrared thermal imager every week; if the single-point deviation exceeds ±3℃, the machine must be shut down for calibration.

The void rate at the bottom of QFN exceeds the standard and is concentrated in the central area, which is directly related to the slow viscosity decrease of halogen-free solder paste during the reflow peak stage and the obstructed gas escape channels. The solution is to increase the peak temperature from 245℃ to 248℃, reduce the holding time from 45 seconds to 35 seconds, and ensure that the cooling rate in the initial section of the cooling zone is no less than 3.2℃/s. The key preventive measure is that a temperature profile test board must be used for verification before replacing with new solder paste for each batch; mass production is prohibited if the void area proportion exceeds 25%.

Poor wetting on OSP copper pads accompanied by obvious solder dewetting is caused by insufficient activation of weak organic acids in the halogen-free solder paste during the low-temperature stage. The lower temperature limit of the 140℃～170℃ soaking section must be increased to 145℃, the overall range should be broadened to 145℃～175℃ and stably maintained for 100 seconds. In practice, it is necessary to simultaneously confirm that the nitrogen purity is ≥99.995% and the measured oxygen content is ≤50ppm. The key preventive measure is to test the end of the nitrogen pipeline twice per shift with a portable oxygen analyzer; if the value fluctuation exceeds ±10ppm, the joint tightness must be checked.

After reflow, the residue on the PCB surface turns white and is difficult to wipe off, indicating that the rosin-based carrier in the halogen-free solder paste has not been completely thermally decomposed. The root cause lies in the insufficient temperature gradient in the soaking zone and the excessive residence time at the peak temperature. The heating rate in the 160℃～200℃ transition section should be increased from 1.2℃/s to 1.6℃/s, the upper limit of the peak section should be changed from 250℃ to 247℃, and the time should be controlled within 30 seconds. The key preventive measure is to disassemble the furnace heating module monthly to clean carbon deposits; if the glaze layer on the surface of the heating rod peels off, it must be replaced immediately.

The brightness of solder joints in the front and rear sections of the same furnace varies greatly, which is essentially the result of the combined effect of chain speed fluctuations and the increased temperature sensitivity of halogen-free solder paste. The conveyor speed must be locked within the range of 120cm/min±2cm/min. At the same time, an infrared temperature probe should be installed at the furnace entrance to monitor the initial board temperature in real time; if it is lower than 25℃, preheating compensation is automatically triggered. The key preventive measure is to recheck the chain tension with a laser distance meter during each daily shift handover; if the elongation exceeds 0.8mm, the tension wheel must be adjusted.