PCB Technical - Why is there a bad tin eating situation when pcb is designed and manufactured

In the process of PCB design and production, have you ever encountered the bad situation of PCB eating tin? For engineers, once a PCB board suffers from poor tin-eating problems, it often means that it needs to be re-soldered or even re-made, and the consequences are very troublesome. So, what are the reasons for the poor tin-eating of PCB? How can we avoid this problem?
Under normal circumstances, the main reason for the poor tin-eating phenomenon of PCB boards is that part of the surface of the circuit is not stained with tin. This kind of PCB board with bad tin eating situation will usually behave as shown in the following figure in reality:

PCB board that eats bad tin
However, there are many reasons for the bad situation of PCB eating tin, which can usually be summarized as the following aspects.
Grease, impurities and other debris attached to the surface of the, or grinding particles left on the circuit surface during the manufacturing process of the substrate, or residual silicone oil, will cause the PCB to eat poorly. If the above situation occurs during the inspection, you can use a solvent to clean the debris. But if it is silicone oil, it needs to be washed with a special cleaning solvent, otherwise it is not easy to be cleaned.
There is also a situation that can cause poor tin-eating on PCB boards, that PCB board is, the storage time is too long or the environment is humid, and the production process is not rigorous. As a result, the tin surface of the substrate or parts is oxidized and the copper surface is dull. When this happens, switching to flux can no longer solve this problem, and the technician must re-soldering once, so as to improve the PCB's tin-eating effect.
Failure to ensure sufficient temperature or time during the PCB soldering process, or incorrect use of flux, will also lead to poor PCB tin eating. Generally, the operating temperature of soldering tin is 55～80 degree Celsius higher than its melting point temperature. Insufficient preheating time can easily lead to poor tin eating. The amount of flux distribution on the circuit surface is affected by the specific gravity. Checking the specific gravity can also rule out the possibility of misuse of improper flux due to wrong labeling, poor storage conditions and other reasons.
In the process of soldering, the quality of the solder material and the cleanliness of the terminals are directly related to the final result. If there are too many impurities in the solder or the terminals are dirty, it will also cause the PCB to eat badly. When soldering, you can measure the impurities in the solder on time and ensure the cleanliness of each terminal. If the quality of the solder does not meet the regulations, you need to replace the standard solder.
In addition to the above-mentioned situations, there is another problem similar to the bad situation of PCB eating tin, that is, tin stripping. PCB de-tinning mostly occurs on tin-lead-plated substrates, and its specific performance is very similar to that of poor tin-eating. However, when the surface of the tin road to be soldered is separated from the tin wave, most of the solder that has been stuck on it will be pulled back into the tin furnace. Therefore, the situation of tin stripping is more serious than poor tin eating. Resoldering the substrate may not always improve, so once this happens, the engineer must return the PCB board to the factory for repair.