Purpose of Surface Treatment
The primary purpose of surface treatment is to ensure good solderability and electrical performance. Copper naturally oxidizes in air, which makes it unsuitable for long-term use in its raw form. Although flux can remove most copper oxides during assembly, its strong nature is difficult to clean off, so industries generally avoid using aggressive fluxes.
Common Surface Treatment Processes
1. Hot Air Leveling (HAL)
Hot Air Leveling (HAL), also known as Hot Air Solder Leveling (HASL), involves applying molten tin-lead solder to the PCB surface and leveling it with heated compressed air. This process creates a coating that prevents copper oxidation and provides good solderability. The copper and solder form a copper-tin intermetallic compound at the junction. The solder thickness typically ranges from 1 to 2 mils.
The PCB is immersed in molten solder, and an air knife blows liquid solder before it solidifies. This minimizes the solder meniscus and prevents solder bridging. HAL is available in vertical and horizontal configurations, with the horizontal type being preferred for uniform coating and ease of automation. The general process includes: micro-etching → preheating → flux application → spraying → cleaning.
2. Organic Coating
The organic coating process serves as a barrier between copper and air. It is simple, cost-effective, and widely used in the industry. Early coatings used imidazole and benzotriazole to inhibit rust, while modern formulations often use benzimidazole, which bonds chemically with the copper surface.
Multiple layers of organic coating are required to ensure reliable soldering, especially when exposed to multiple reflow soldering cycles. The process typically involves: degreasing → micro-etching → pickling → water cleaning → organic coating → cleaning. The latest organic coatings maintain excellent performance even during lead-free soldering processes.
3. Electroless Nickel Plating / Immersion Gold
The Electroless Nickel / Immersion Gold (ENIG) process involves applying a nickel-gold alloy to the copper surface for long-term protection and improved electrical performance. Unlike organic coatings, this process does not simply serve as a rust barrier, but provides significant durability and environmental tolerance.
The nickel layer prevents diffusion between gold and copper, ensuring that gold does not migrate into the copper over time. This process offers high strength and prevents copper dissolution, making it suitable for lead-free assembly. The general process includes: acid cleaning → micro-etching → pre-dip → activation → electroless nickel plating → chemical immersion gold. This process involves six chemical baths and requires careful process control.
4. Immersion Silver
The immersion silver process is quicker and simpler than ENIG, providing good electrical performance. Silver is an excellent conductor, and while it may tarnish over time, it maintains good solderability even when exposed to heat, humidity, and contaminants.
While immersion silver lacks the physical strength of ENIG due to the absence of a nickel layer, it is a cost-effective option for reliable electrical connections. The process involves a displacement reaction that coats the copper with a thin layer of pure silver. Some organic additives are used to prevent silver corrosion and migration. The thickness of the organic layer is typically less than 1% by weight.
5. Immersion Tin
Immersion tin provides a flat copper-tin intermetallic compound, ensuring excellent solderability similar to Hot Air Leveling but without the issues related to uneven coating. While it can have reliability issues, such as tin whisker formation, recent advancements have included organic additives that help mitigate these problems and improve the thermal stability and weldability of the tin layer.
Immersion tin plating is not suitable for long-term storage and must be assembled shortly after the process. The immersion tin process avoids the copper-tin diffusion issues seen in other methods, offering reliable soldering without the complications associated with ENIG.
6. Other Surface Treatment Processes
Other less common surface treatments include electroplated nickel-gold and electroless palladium. Electroplated nickel-gold was one of the earliest methods used in PCB manufacturing, involving a nickel layer plated onto the copper followed by a gold layer to prevent diffusion. There are two types of electroplated gold: soft gold (used for wire bonding) and hard gold (used for wear-resistant applications in non-soldered areas).
Electroless palladium plating is similar to electroless nickel plating, where palladium ions are reduced onto the PCB surface. This method is known for good solderability, thermal stability, and flatness, making it ideal for high-reliability applications.
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