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Electroless Nickel Immersion Gold (ENIG) is a type of surface finish used on printed circuit boards (PCBs). It's a two-layer metallic coating where the first layer is electroless nickel and the second is immersion gold.

The purpose of this dual layer is to protect the copper surface of the PCB, while providing a solderable surface for component assembly. Here's a more detailed breakdown of each layer:

Electroless Nickel

The nickel layer acts as a barrier to protect the copper from the solder, which could otherwise migrate into the copper and weaken the adhesion to the board's surface. Electroless nickel is used because it can be deposited evenly over the entire board without the need for an electric current, making it ideal for complex boards with a lot of intricate detail.

Immersion Gold

The gold layer protects the nickel during storage and provides a surface that is resistant to oxidation, ensuring good solderability. It's deposited by a process known as immersion plating, where the PCB is dipped into a bath of gold salts. This gold layer is typically very thin, as it's only intended to protect the nickel layer beneath it and doesn't need to withstand soldering temperatures.

Advantages

ENIG has several advantages, including excellent surface planarity (making it ideal for PCBs with large BGA packages), good oxidation resistance, and a long shelf-life. However, it's also more expensive than other finishes like HASL (Hot Air Solder Leveling), and can sometimes suffer from what is known as "black pad syndrome," a defect where the gold and nickel layers separate, leading to poor solder joint integrity.