When the wave soldering pot operates continuously at 250°C to 265°C, the surface of the molten solder directly contacts air, triggering irreversible oxidation reactions. Tin oxide (SnO?) continuously accumulates and combines with other impurities such as copper ions and Flux pyrolysis carbides to form loose, porous solder dross. Once this dross detaches from the solder surface, it carries unoxidized molten solder and sinks to the bottom of the pot or adheres to the pump impeller, resulting in reduced effective solder volume, increased risk of nozzle clogging, and decreased wave stability. Our on-site tracking at a customer production line in Dongguan found that when nitrogen coverage in the solder pot is uneven and oxygen content exceeds 800 ppm, the dross removal volume every 4 hours surges from the typical 0.45 kg to 1.32 kg, and free solder beads account for less than 12% of the removed dross, indicating that oxidation has occurred deeply.

A solder pot temperature fluctuation exceeding ±3°C intensifies the convection of the molten solder, causing the enriched copper-tin alloy particles at the bottom to rise, collide with the surface oxide film, and break it apart, subsequently forming a large number of fine dross nuclei. These nuclei then become active sites for new oxidation reactions, creating a vicious cycle. Tests on the JJY FS-320 wave soldering machine found that if excessive wind speed in the preheating section causes the PCB to carry moisture into the solder pot, the instantaneous cooling and subsequent reheating of the local molten solder increases the oxidation rate by 2.3 times, corresponding to a daily average dross increment of 0.9 kg. More critically, if the angle of a conventional dross scraper is set to 52° instead of the JJY-recommended 63°±2°, the disturbance to the solder surface increases by 17%, the tearing frequency of the oxide film rises, and dross generation increases by 11% instead.

Organic ash formed by the carbonization of flux residues at high temperatures not only reduces the surface tension of the molten solder but also hinders the uniform spreading of the nitrogen protective film, prolonging the local exposure time of the solder surface. Our measurements using an infrared thermal imager found that when the surface temperature difference across different areas of the same solder pot reaches 9°C, the dross generation rate in the high-temperature area is 1.8 times that in the low-temperature area. In addition, after the solder pot has been in use for more than 18 months, copper seeps through the micropores in the inner wall plating. When copper ions diffuse into the molten solder and coexist with oxygen, they catalyze the conversion of Sn to SnO?. At this point, even if nitrogen purity reaches 99.99%, the daily dross weight gain is still 0.65 kg higher than that of a new pot. Therefore, the sampling and analysis cycle for the molten solder must be shortened from once a month to once a week, with a focus on whether the Cu content exceeds 0.35 wt%. If it exceeds the limit, solder replacement or the addition of a Sn-Cu adsorbent must be performed immediately.

In daily maintenance, if dross removal relies solely on manual skimming without operating the automatic solder surface cleaning module, the dross will accumulate at the solder pump suction inlet and be repeatedly rolled into dense blocks. This reduces pump efficiency by 7% while extending the oxidation contact time in the molten solder circulation path by 23 seconds, further increasing dross output. Feedback from JJY customers shows that after implementing the dual-control strategy of automatic solder surface scraping plus nitrogen curtain, the weekly average dross weight remained stable at 0.51 kg for three consecutive weeks, a 38.2% reduction compared to before the modification. The solder consumption per board was also reduced by 0.017 g. Based on an output of 800,000 boards per month, the annual solder cost savings are approximately 236,000 RMB.