The tin dipping process uses tin bars for batch terminal tinning, and the solder bath temperature must be stably controlled within the range of 255±3℃. If the temperature is below 252℃, poor wetting and weak solder joints will occur after the terminals are immersed; if it exceeds 258℃, it will accelerate the oxidation of the tin bars, increase the amount of dross, and cause excessive dissolution of the terminal plating. Our actual measurement at an automotive wiring harness factory in Dongguan found that when the tin temperature fluctuated by ±5℃, the defect rate among 200 consecutive terminals soared from 0.8% to 4.6%.

Tin bar selection directly affects dross generation rate and solder joint consistency

Jia Jinyuan tin bars have a tin content of 99.3%, copper content of 0.02%, iron content of 0.005%, and lead content of <0.001%, with the total impurity content strictly controlled within 0.05%. This reduces the surface dross generation rate after melting by approximately 35% compared to conventional tin bars, and the dross structure is denser and less prone to cracking and splashing. The cleanliness of the tin surface can be maintained even after 8 hours of continuous operation. In contrast, using tin bars with excessive impurities will produce a large amount of loose dross within 4 hours, causing dross to adhere to terminals when they are removed from the tin bath and uneven solder climb height on solder joints.

Tin dipping time and lifting speed must be coordinated to match terminal structural characteristics

For crimp terminals with a wire diameter of Φ0.5～1.2mm, the tin dipping time should be set to 3.5±0.5 seconds, and the lifting speed should be controlled at 120±10mm/min. If the time is shortened to 2.8 seconds or the lifting speed is too fast at 140mm/min, missing solder and necking of solder joints are likely to occur at the terminal root. Conversely, if the time is extended to 4.2 seconds or the lifting speed is slower than 100mm/min, obvious tin climb will occur at the edge of the terminal insulation layer, and even thermal damage may result. When we verified with the same model of terminal on a Foshan production line, adjusting the tin dipping time by only 0.3 seconds caused the solder joint pull force value to fluctuate by more than 12N.

Flux selection and preheating method determine the stability of wetting effect

Using a no-clean rosin-based flux with a spray thickness controlled at 8～12μm, combined with a 60±5℃ preheating plate to preheat the terminals for 30 seconds, can stabilize the solder joint wetting angle between 22° and 28°. If the preheating step is skipped or the flux spray is uneven, some terminals will experience local wetting delay, and the solder joints will appear as spherical accumulation. At a customer site in Zhuhai, we once had poor flux atomization due to unstable compressed air pressure, resulting in 17 out of 2400 terminals in the entire batch having incomplete wetting defects.

The focus of daily maintenance is tin surface height monitoring and periodic dross removal

Before each shift starts, a special ruler must be used to confirm that the distance from the tin surface to the furnace opening is 45±3mm. If the liquid level is too low, it will aggravate uneven melting of the tin bars and produce hard particles; if the liquid level is too high, it will easily cause tin overflow during dipping and contaminate the fixtures. At the same time, perform mechanical dross scraping every 2 hours. After scraping off the dross, immediately add 0.8～1.2kg of new tin bars to maintain the stability of the tin composition. Otherwise, after 6 hours of continuous operation, the copper content in the tin will rise above 0.08%, directly leading to dark solder joints and reduced gloss in subsequent processes.