Selecting wave soldering bars is not a simple matter of comparing composition tables; it requires dynamic matching based on the actual load of the production line, PCB thickness, and component density. For example, high-density fine-pitch boards must use Sn63/Pb37 Solder Bars with low surface tension and a residue rate below 0.15%. Its solidus temperature is 183°C and liquidus temperature is 183°C; the eutectic characteristics ensure consistent solder joint formation. For lead-free processes using Sn99.3/Cu0.7, the solder pot temperature must be stably controlled at 260±2°C; otherwise, the precipitation of liquid copper will accelerate nozzle clogging.

Preparatory work before melting solder must be thorough. First, confirm that the residual solder amount in the pot is no less than 65% of the rated capacity. Then check whether the heating rod power reaches above 92% of the nominal value. Next, use an infrared thermometer to measure the temperature deviation at three points on the pot wall, which must not exceed ±3°C. When adding new solder bars, feed them in batches with an interval of no less than 90 seconds between batches to avoid thermal stress cracks caused by local sudden cooling. At the same time, turn on the nitrogen protection flow meter and reduce the oxygen concentration to below 500ppm; otherwise, the oxidation rate will surge by more than three times.

The core of the dross removal process is to suppress the continuous generation of mixed oxides of SnO and SnO₂. We found that when the solder surface remains stationary for more than 22 minutes, the oxide film thickness will exceed the critical value of 8μm. At this point, the scraper system must be activated. The scraper material is uniformly made of SUS316L stainless steel, with the edge passivated to R0.8μm, the operating speed set to 12mm/s, and the inclination angle strictly controlled at 5.5°±0.3°. An excessively large angle easily carries out liquid solder and causes waste, while an excessively small angle cannot cut the oxide film bridging structure. After each scraping, immediately use a special titanium alloy shovel to clean the accumulated dross in the flow guide groove; the residual thickness must not be greater than 0.3mm.

Abnormality troubleshooting should reverse-infer the root cause from the phenomenon. For example, solder joint spikes are often accompanied by solder dross particles with a diameter >120μm, indicating insufficient dross removal frequency or decreased nitrogen purity. Solder hanging on the bottom of the PCB is mostly caused by scraper pressure fluctuation exceeding ±1.2N; at this time, it is necessary to calibrate whether the feedback voltage of the pneumatic actuator is maintained at 4.82V±0.05V. If three consecutive batches show solder joints that are dull and have no metallic luster, it can basically be determined that the Cu content in the solder bars has exceeded the standard to 0.085wt%, and the machine must be shut down for sampling and spectral analysis.

Old production lines often ignore the subtle differences between solder bar batches. The As content of the same model from different furnace numbers can fluctuate by up to 0.002wt%. Although it does not exceed the standard, it will significantly affect the wetting spreading time. We require that each batch of incoming goods must undergo a 24-hour high-temperature aging test to observe the degree of wrinkling on the solder bar surface at a constant temperature of 180°C; if the wrinkle depth is >0.15mm, the batch will be rejected. In addition, solder dross cannot be directly returned to the furnace; it must go through two processes: centrifugal separation + acid pickling passivation. Otherwise, residual Cl⁻ ions will induce leakage in ICT testing during the next soldering. Measured data shows that when the Cl⁻ concentration is >8.3μg/g, the defect rate increases by 47%.

Nitrogen protection is not a decoration. Measured data shows that when the inlet pressure drops from 0.3MPa to 0.26MPa, the daily increase in solder dross will rise by 34%. Therefore, the first thing during each shift handover is to check the reading of the pressure gauge at the outlet of the pressure reducing valve; if the error exceeds ±0.01MPa, it must be replaced. Another easily overlooked point is the stacking height of solder bars. When a standard pallet is loaded with 15kg of solder bars, the vertical distance from the top layer to the pot opening should be maintained at 380mm±5mm. An excessively high height will intensify splashing due to feeding impact, while an excessively low height will easily cause unmelted solder blocks to get stuck in the feeding port and form cold solder nodules.