The fundamental cause of rapid oxidation of wave soldering bars is that the molten tin is exposed to a high-temperature, oxygen-rich environment for extended periods. When the molten tin temperature exceeds 255°C, the oxidation rate of the Solder Bars increases exponentially. Our measurements show that at 260°C, the amount of tin dross generated per hour is 3.2 times higher than at 250°C. Therefore, the temperature control accuracy of the solder pot must be locked within the range of 250°C ± 2°C, while ensuring that the temperature probe is inserted to a depth of no less than 80mm and avoids the direct blowing area of the nozzle; otherwise, the deviation between the displayed temperature and the actual molten tin temperature can reach over 6°C.

During the preparation phase, first confirm that the thickness of residual tin dross in the solder bath does not exceed 15mm, then use a stainless steel scraper to clean the oxide film along the bath walls. Before adding new solder bars, it is necessary to measure whether their purity is ≥99.3%, with arsenic content ≤0.001% and iron content ≤0.0005%, as impurity elements will catalyze the oxidation reaction. In particular, iron ions in molten tin at 250°C will accelerate the formation rate of the oxide film by 40%. When adding material, it should be introduced in batches, with each batch not exceeding 5kg. Wait until each batch is completely melted before adding the next, to avoid localized overheating that causes violent boiling and splashing.

In terms of core parameter settings, in addition to the main temperature zone of 250°C ± 2°C, the temperature of the PCB bottom surface at the preheating zone outlet must be controlled between 90°C and 110°C. This range can fully activate the Flux without causing residual solvent on the board surface to enter the solder pot and violently vaporize, disturbing the molten tin surface. The distance between the molten tin surface and the bottom of the wave nozzle must be maintained at 12mm to 18mm. When the distance is below 12mm, the amount of oxygen entrained increases sharply, while above 18mm, wave stability decreases; both conditions will exacerbate the oxidation process of wave soldering bars.

During operation, before each shift starts, use an infrared thermometer to recheck the temperature at three points on the surface of the solder pot, take the average value and compare it with the instrument reading. If the deviation exceeds ±1.5°C, calibration is required. During operation, manually remove dross every two hours, with each removal amount of floating dross no less than 0.8kg. After removing dross, immediately replenish new solder bars to maintain the total tin volume at 75% to 85% of the rated capacity. If the tin volume is too low, the heated area per unit volume of molten tin will increase, reducing heat dissipation efficiency, which instead causes the local temperature to soar above 258°C.

During troubleshooting, if the tin dross is gray-black and loose in texture, it indicates insufficient nitrogen flow or excessive oxygen content. At this time, check whether the nitrogen pressure is stable at 0.3MPa to 0.4MPa, whether there are any leaks in the pipeline, and whether the nozzle is clogged. Any one of these abnormalities will increase the oxygen content of the actual protective atmosphere from the target <100ppm to >800ppm, thereby doubling the oxidation rate of the solder bars. If the tin dross is bright yellow and viscous, it is most likely due to excessive copper content in the molten tin. It is necessary to check the matching between PCB board thickness and immersion time. For a 2.0mm board thickness, the standard immersion time is 3.5 seconds; exceeding this by 0.8 seconds will increase copper dissolution by 220μg/g.

In terms of practical tips, our 13 months of continuous data tracking at an EMS factory in Dongguan found that using a stepwise heating method for the first shift of each day—first heating to 180°C at 1.2°C/min and holding for 25 minutes, then heating to 250°C at 0.8°C/min—can reduce the initial oxide dross generation by 37% compared to direct heating. In addition, solder bars must be stacked at least 300mm above the ground and placed in a dry, well-ventilated area. Any unused solder bars within 48 hours after opening the packaging must be re-vacuum-sealed, as the surface of solder bars exposed to air for 12 hours has already formed an SnO? oxide film with a thickness of 85nm; this pre-oxidized layer will accelerate the overall oxidation chain reaction during melting.