ENIG, or Electroless Nickel Immersion Gold, is a popular surface finish for multilayer printed circuit boards (PCBs). This process is favored for its excellent surface planarity, good solderability, and wire bondability. The ENIG process involves depositing a layer of nickel followed by a thin layer of gold on the copper surface of the PCB.

　　The process begins with the cleaning of the copper surface to remove any contaminants. A micro-etching step is then performed to ensure proper adhesion of the nickel layer. The board is then immersed in a solution containing nickel ions, where a chemical reaction deposits a uniform layer of nickel onto the copper surface. This nickel layer typically ranges from 3 to 6 microns in thickness.

　　Following the nickel deposition, the board is rinsed and then immersed in a gold solution. The gold layer, which is usually between 0.05 and 0.2 microns thick, is deposited onto the nickel layer through a chemical reduction process. The gold layer provides a protective barrier against oxidation and ensures good solderability.

　　After the gold deposition, the board undergoes a final rinse and drying process. The resulting surface is smooth, flat, and highly resistant to oxidation, making it ideal for fine-pitch components and surface mount technology (SMT).

　　ENIG is particularly useful for multilayer PCBs because it provides a reliable and consistent surface finish across all layers. This is crucial for maintaining the integrity of electrical connections and ensuring the proper functioning of complex electronic devices.