Why does PCB circuit board oxidize and turn black

　　The fundamental reason for PCB oxidation and blackening (usually referring to copper surface oxidation) is the chemical reaction between copper (Cu) and oxygen, moisture, and other pollutants in the air.

　　The following are the specific reasons, stages, and detailed mechanisms that cause PCB oxidation and blackening:

　　Chemical residues and exposure during the manufacturing process

　　Excessive inter process retention: After etching and other processes, the fresh copper surface has extremely high activity. If surface treatment (such as tin spraying and gold deposition) is not carried out in a timely manner, it is highly prone to oxidation.

　　Incomplete cleaning: After acid pickling or micro etching, if the water is not washed thoroughly, residual acidic or oxidizing chemicals will continue to corrode the copper surface.

　　Poor water quality: The cleaning water contains impurities such as chloride ions and sulfur ions, which can accelerate the corrosion and blackening of copper.

　　Excessive baking: If the baking temperature is too high or the time is too long, it directly causes the copper surface to oxidize at high temperatures.

　　Improper storage and transportation environment

　　Excessive humidity: When the environmental humidity exceeds 60%, moisture forms an electrolytic liquid film on the copper surface, accelerating electrochemical corrosion.

　　Corrosive gases: Hydrogen sulfide, sulfur dioxide, chlorine gas, etc. present in the air can react with copper to form black sulfides or oxides.

　　Improper packaging: Using non moisture resistant sealed packaging cannot block air and moisture.

　　Bare hand contact: Salt and oil in hand sweat can leave corrosion hazards at the contact point, forming fingerprint like black spots.

　　Overdue storage: Even with surface treatment, its protective layer (such as OSP film) will fail over time.

　　Assembly and post process impact

　　Flux residue: Some active flux residues are hygroscopic and corrosive, and if not cleaned thoroughly, they can slowly corrode the solder pads.

　　Corrosion of cleaning agents: mismatched or highly corrosive cleaning agents were used.

　　Composition and hazards of oxide layer

　　Composition: Mainly black copper oxide (CuO), possibly mixed with cuprous oxide (Cu ₂ O) and sulfides, carbonates, etc.

　　harm:

　　Deterioration of weldability: leading to solder rejection and virtual soldering.

　　Decreased electrical performance: Increased contact resistance leads to poor conductivity.

　　Poor appearance: judged as a quality defect.

　　Core prevention ideas

　　The key is to isolate the contact between copper and corrosive media:

　　On the process side: shorten the process interval, ensure sufficient cleaning and drying, and control process parameters.

　　Storage: Control the environmental temperature and humidity (recommended 20-25 ℃, humidity<60%), use vacuum moisture-proof packaging, and follow the first in first out principle.

　　In terms of operation: avoid bare hand contact, wear gloves when working, and complete subsequent assembly in a timely manner.

　　Summary: PCB oxidation blackening is a process control and environmental management issue. Its core lies in "blocking the contact between copper and corrosive media". By optimizing the process, controlling the storage environment, and standardizing operations, this problem can be greatly avoided, ensuring the quality and solderability of the PCB.