PCB surface finishes determine the durability and solderability of electronic components. Among the various surface finish options available today, ENIG (Electroless Nickel Immersion Gold) has become one of the most widely adopted solutions.
Understanding ENIG Surface Finish
ENIG is a double-layer metallic coating applied to the exposed copper areas of a PCB. It involves depositing a layer of nickel through an electroless chemical reaction, followed by depositing a thin layer of gold through an immersion plating process. The nickel layer is typically 3 to 6 microns thick, while the gold layer is much thinner, usually between 0.05 and 0.23 microns. This combination forms a robust surface that protects the underlying copper from oxidation.
The nickel layer acts as the primary barrier between the copper substrate and the external environment, preventing copper migration to the surface and providing a base for component connections. The gold layer acts as a protective layer, preventing the nickel from oxidizing during storage and handling. Gold also possesses excellent contact resistance characteristics, making ENIG well-suited for applications requiring long-term electrical connections.
ENIG Process Flow
The implementation of the ENIG surface finish begins with thorough cleaning to remove contaminants from the copper surface. After cleaning, the circuit board undergoes a micro-etching process to slightly roughen the copper surface, improving the adhesion of the nickel layer.
The electroless nickel plating process begins when the cleaned PCB is immersed in a chemical bath containing nickel salts and reducing agents. Unlike electroplating, which requires an external current, electroless plating relies on a controlled chemical reaction to deposit nickel uniformly onto all exposed copper surfaces. The reaction continues until the desired nickel thickness is achieved, taking 20 to 40 minutes depending on the chemical bath composition and temperature. After nickel deposition, the circuit board undergoes a brief activation process before entering the immersion gold bath. In the immersion gold bath, a displacement reaction occurs, where gold ions in the solution replace a small number of nickel atoms on the surface. This forms a thin, uniform layer of gold that completely covers the nickel layer.
Advantages of ENIG Surface Finish
The flat surface of ENIG is ideal for fine-pitch components and Ball Grid Array (BGA) components, and it maintains the precise pad geometry required for surface mount technology. As component spacing shrinks and assembly tolerances tighten, this flatness becomes increasingly important.
The excellent solderability of ENIG surface-treated circuit boards is another significant advantage. During the reflow soldering process, the gold layer quickly dissolves into the solder, allowing the solder to form a strong metallurgical bond with the underlying nickel layer. This results in reliable solder joints with good mechanical strength and conductivity. ENIG circuit boards can typically be stored for extended periods without degradation of solderability, often maintaining excellent performance for a year or more under proper storage conditions.
Compared to bare copper or other surface treatment processes, ENIG offers superior corrosion resistance. The nickel-gold combination forms an effective barrier against moisture, chemicals, and environmental factors. This makes ENIG particularly suitable for products operating in harsh environments. This surface treatment process also exhibits good thermal cycling resistance, maintaining its integrity even after repeated heating and cooling during assembly and operation.
ENIG's wire bonding capability is another important characteristic. The gold surface provides an excellent base for aluminum wires, making ENIG suitable for chip-on-board applications and other direct chip attachment processes. The soft, pure gold surface treatment allows for strong bond formation without damaging fragile bond wires. This versatility makes electroless nickel immersion gold (ENIG) an ideal choice for hybrid technology components that combine surface-mount components and wire-bonded chips.
Email: all@poe-pcba.com
Whatsapp: 85292069596
Tel: 0755-25312250/ +8613798543496
Factory Address: Floor 3, Jinyuan Industrial Park, No. 56, Tangtou Avenue, Shiyan Town, Baoan District, Shenzhen China
