Hot air leveling is the process of immersing a printed circuit board into molten solder (63SN/37PB), and then using hot air to blow off excess solder from the surface and metalized holes of the printed circuit board, resulting in a smooth, uniform, and bright solder coating layer. The lead tin alloy coating on the surface of the printed circuit board after hot air leveling should be bright, uniform, and complete, with good solderability, no nodules or semi wetting, and the coating should be completely free of exposed copper. Copper exposure on the surface and metalized holes of the solder pad after hot air leveling is an important defect in finished product inspection, and is one of the common causes of hot air leveling rework. There are many reasons that can cause this problem, including the following.

1. Insufficient pre-treatment and poor coarsening.

The quality of the pre-treatment process for PCB hot air leveling has a significant impact on the quality of hot air leveling. This process must thoroughly remove oil stains, impurities, and oxide layers on the solder pads, providing a fresh and solderable copper surface for tin immersion. The commonly used pre-treatment process nowadays is mechanical spraying. Firstly, sulfuric acid hydrogen peroxide micro etching is used, followed by acid immersion. Then, water spraying is used for rinsing, hot air drying is performed, and flux is sprayed. Immediately, hot air leveling is carried out. The phenomenon of exposed copper caused by poor pre-treatment occurs in large quantities regardless of the type and batch. Exposed copper points are often distributed throughout the entire board surface, and are even more severe at the edges. Observing the preprocessed circuit board with a magnifying glass will reveal obvious residual oxidation points and stains on the solder pads. In case of similar situations, chemical analysis should be conducted on the micro etching solution. The second pickling solution should be checked, and the concentration of the solution should be adjusted to replace the solution that has been heavily contaminated due to prolonged use. The spraying system should be checked for smoothness. Extending the processing time appropriately can also improve the processing effect, but attention should be paid to the occurrence of excessive corrosion. After the reworked circuit board is leveled by hot air, the processing line is further treated in a 5% hydrochloric acid solution to remove surface oxides.

2. The surface of the solder pad is unclean and there is residual solder mask contaminating the solder pad.

At present, most manufacturers use liquid photosensitive solder mask ink for full plate screen printing, and then remove excess solder mask through exposure and development to obtain solder mask patterns over time. During this process, poor control of the pre drying process and prolonged exposure to high temperatures can cause difficulties in development. Whether there are defects on the solder mask, whether the composition and temperature of the developing solution are correct, whether the development speed and development point are correct, whether the nozzle is blocked and the pressure of the nozzle is normal, whether the water washing is good, any of these situations will leave residue on the solder pad. The exposed copper formed due to the film is generally more regular, all at the same point. In this case, a magnifying glass can be used to detect residual traces of solder mask material at the exposed copper area. Generally, in PCB design, a post should be set up before the curing process to inspect the interior of the graphics and metalization holes, ensuring that the solder pads and metalization holes of the printed circuit board sent to the next process are clean and free of solder mask ink residue.