Due to their thinness and flexibility, flexible printed circuit boards (FPCs) are widely used in consumer electronics, automotive electronics, and other fields. However, their surfaces are susceptible to corrosion from environmental factors such as moisture, salt spray, and chemicals, leading to oxidation of the copper foil, decreased conductivity, and even circuit failure. The key to enhancing FPC surface corrosion resistance lies in creating a dense, stable protective layer through surface treatment, which blocks direct contact between corrosive media and the substrate while maintaining a balance between flexibility and conductivity.
Electroless immersion gold (ENIG) is a commonly used surface treatment for FPCs. It deposits a nickel layer on the copper foil through a chemical replacement reaction, followed by a thin layer of gold. The nickel layer acts as a barrier, effectively preventing electrochemical reactions between the copper and the external environment. The gold layer, with its high chemical inertness and strong oxidation resistance, further enhances the surface's corrosion resistance. The gold layer thickness is typically controlled between 0.05 and 0.1 microns, ensuring effective protection without compromising flexibility due to excessive thickness. Furthermore, the electroless gold (EIG) process offers excellent plating uniformity, making it particularly suitable for FPCs with high-density circuits, as it avoids the risk of localized corrosion caused by uneven plating thickness.
Organic solderability protection (OSP) offers another surface treatment solution that balances cost and performance. This process chemically forms an extremely thin organic film, just 0.2-0.5 microns thick, on the copper surface. This film temporarily protects the copper foil from oxidation and decomposes under heat during soldering, without affecting solder wettability. The advantages of the OSP process include its simplicity, low cost, and minimal impact on the FPC's flexibility, making it suitable for applications requiring moderate corrosion resistance and frequent flexing. However, OSP film has relatively limited durability and can break down during long-term storage or harsh environments, exposing the copper foil. Therefore, the choice of OSP film must be considered in conjunction with the packaging process and operating environment.
The electroless nickel-palladium-gold (ENEPIG) process adds a palladium layer to the ENIG process, creating a multilayer structure of copper, nickel, palladium, and gold. The palladium layer, acting as a transition layer between gold and nickel, inhibits nickel diffusion and prevents the formation of brittle intermetallic compounds between the gold and nickel layers, thereby improving the coating's flexibility and reliability. Furthermore, palladium's inherent corrosion resistance further enhances surface protection. The ENEPIG process is particularly suitable for FPCs that are subject to frequent flexing or mechanical stress, such as dynamic connectors in wearable devices. However, this process is also relatively complex and expensive.
For FPCs exposed to high humidity or salt spray environments for extended periods, electroplating hard gold can be considered. This process deposits a hard gold layer up to 1-3 microns thick on the copper surface through electrolytic deposition. Its hardness can reach over HV200, significantly improving wear and corrosion resistance compared to electroless gold. Small amounts of cobalt or nickel are often added to hard gold to enhance its hardness and corrosion resistance. However, hard gold coatings lack flexibility, and excessive flexing can cause cracking. Therefore, a balance between protective performance and mechanical reliability must be considered depending on the application.
The corrosion resistance of the surface treatment process is also closely related to the post-processing steps. For example, after the electroless gold (ICG) or ENEPIG process is completed, residual chemicals must be removed through pure water washing to prevent residual chemicals from causing localized corrosion. Furthermore, the drying temperature and time must be strictly controlled to prevent residual moisture from causing blistering or peeling of the coating. Furthermore, FPC packaging materials must be moisture-resistant and anti-static, and humidity and temperature must be controlled during storage to extend the lifespan of the surface treatment layer.
The selection of an FPC surface treatment process requires a comprehensive consideration of corrosion resistance, flexibility, cost, and process compatibility. By optimizing process parameters and incorporating multi-layer composite structures or novel materials, corrosion resistance can be significantly improved without sacrificing flexibility, thereby meeting the requirements for long-term reliable operation in complex environments.
