PCB copper oxidation refers to the process in which the copper layer (etching circuit, pads, etc.) on the printed circuit board (PCB) reacts chemically with oxygen, moisture or other environmental factors to form compounds such as copper oxide (CuO or Cu₂O) or copper sulfide (CuS). This phenomenon directly affects the PCB electrical performance and welding reliability , and is an issue that requires special attention when manufacturing, storing and using PCBs.
1. Environmental factors
Humidity: Moisture in the air is one of the key factors that trigger oxidation. When the relative humidity is high, water molecules will adsorb on the PCB surface to form a thin layer of water film. This layer of water film provides an electrolyte environment for the oxidation reaction, accelerates the electrochemical reaction between the copper and oxygen, and thus promotes the occurrence of oxidation. For example, in humid coastal areas or the rainy season, PCBs are more prone to oxidation.
Temperature: Increased temperature will accelerate the rate of chemical reactions, including oxidation reactions. In a high temperature environment, the activity of copper atoms is enhanced, and it is easier to react with oxygen to form oxides. Moreover, high temperature may also destroy the protective layer on the PCB surface , causing it to lose its protective effect on the PCB copper, further aggravating oxidation.
Air pollution: Pollutants in the air, such as SO₂ and NOx, will combine with moisture in the air to form acidic solutions and adhere to the PCB surface. These acidic substances will corrode the copper and accelerate the oxidation process. In addition, dust particles may also be adsorbed on the surface of the PCB, and some of their components may catalyze the oxidation reaction on PCB Copper.
2. PCB Manufacturing process Factors
Incomplete cleaning: During the PCB manufacturing process, if the cleaning process is not thorough, residual chemicals, oil, dust and other impurities will adhere to the circuit board surface . These impurities will destroy the natural oxide layer on the copper surface of the board, making it more likely to react with oxygen in the air, leading to the PCB copper oxidation.
Improper surface treatment: The process of the PCB surface treatment has an important impact on its antioxidant properties. For example, if the tinning process is improper, the thickness of the tin layer is uneven or there are pores, the underlying copper will be exposed to the air and prone to copper oxidation. In addition, some surface treatment layers may be damaged during storage or transportation, which will reduce their protective effect on copper and cause copper oxidation.
3. Storage and transportation factors
Excessive storage time: As the storage time increases, the contact time between PCB and oxygen, moisture, etc. in the air also increases accordingly, and the possibility of copper oxidation increases. Even in a relatively dry and clean environment, long-term storage may cause the PCB surface to gradually oxidize.
Improper selection of packaging materials: If the packaging material does not have good moisture-proof and anti-oxidation properties, moisture and oxygen in the air can easily enter the interior of the package, contact the PCB and cause oxidation. For example, using ordinary plastic bags to package PCBs cannot effectively block moisture and oxygen. In contrast, vacuum packaging or aluminum foil packaging with moisture-proof and anti-oxidation functions can better protect the PCB.
Vibration and friction during transportation: During transportation, the PCB may be subject to vibration and friction. These external forces may damage the pcb surface protective layer, exposing the copper inside, thereby increasing the risk of copper oxidation.
1.Degradation of electrical performance:
The oxide layer will increase the contact resistance, resulting in signal attenuation or power loss.
Also,It may cause open circuit, short circuit or signal interference.
2.Welding problems:
Oxidation of the pcb pads will lead to poor welding (false solder joints, cracked solder joints),affecting the quality of reflow soldering or manual soldering.
3.Degradation of long-term reliability:
PCB copper Oxidation may gradually spread, causing more severe corrosion and eventually leading to PCB failure.
Visual inspection:
Surface treatment Color: The surface of a normal PCB copper foil generally presents a uniform copper metallic luster and is bright in color. When oxidation occurs, the surface of the copper foil will change color. When slightly oxidized, the surface may appear light brown; as the degree of oxidation deepens, the color will gradually change to dark brown or even black. For example, if dark and uneven patches appear in the originally bright copper foil area under a magnifying glass, it is likely that copper oxidation has occurred.
Surface texture change: Copper Oxidation will change the microscopic texture of the PCB surface. The surface of a normal copper foil is relatively smooth and flat, while the oxidized surface may become rough, granular or have tiny bumps. This change in surface texture can be detected by touch or observation under a microscope. If you gently touch the surface of the PCB with your hand and feel a noticeable roughness, which is different from the normal area, this may be a sign of copper oxidation.
Electrical performance detection:
Resistance measurement: Use a multimeter or other instrument to measure the resistance of the PCB circuit. Since oxidation forms a layer of oxide with high resistance on the surface of the copper foil, when the circuit is oxidized, its resistance value will increase significantly. If the resistance value of a certain section of the circuit is much higher than the design value or the normal reference value, after excluding other possible reasons, it is likely that the part of the circuit has been oxidized.
Electrical test: Use equipment such as flying probe tester or test fixture to perform continuity test on PCB. Oxidation may cause the circuit to be open or short-circuited. When the test results show that the connection between certain circuits is abnormal, the circuit that should be turned on is not turned on, or the circuit that should not be turned on is turned on, this may be due to oxidation causing damage to the circuit or reduced insulation performance.
Surface treatment process:
OSP (Organic Solderability Preservative): Forms a protective layer on the copper surface, but the validity period is short (usually 6 months).
ENIG: Nickel + gold layer, strong oxidation resistance, suitable for high reliability scenarios.
HASL: Tin layer protects the copper surface, low cost.
Immersion silver/silver plating: Good oxidation resistance, but sulfurization problems may occur.
Storage and packaging:
Store in a dry, low-humidity environment (relative humidity <40%).
Use vacuum packaging or add desiccant.
Avoid long-term exposure to air and use as soon as possible after opening.
Design optimization:
Reduce the design of exposed copper and increase soldermask coverage.
Use gold plating in easily oxidized areas (such as edge connectors).
Mild oxydation:
Gently wipe the oxidized surface with an eraser or fine sandpaper.
Wash with a special cleaner (such as isopropyl alcohol).
Serious oxydation:
Re-surface treatment.
Replace damaged pads or wires.
Welding repair:
Use a more active solder paste (containing flux) to assist welding.
Partially tin the oxidized pads.
PCB oxidation is a problem that needs to be vigilant in electronic manufacturing process,While some amount of copper oxidation is inevitable, excessive oxidation can lead to poor electrical conductivity and weaken the integrity of the PCB. In conclusion, copper oxidation on PCB is a common occurrence that can be managed through proper preventative measures. By understanding the process of oxidation and taking steps to protect the copper traces or Pads, the longevity and performance of the PCB can be preserved.