What is ENIG PCB Surface Finish?

Immersion gold, also known as Electroless Nickel Immersion Gold (ENIG), is a common type of printed circuit board (PCB) surface finish. It involves depositing a thin layer of gold on the PCB's copper traces and pads using a chemical process that does not require an electrical current.

The thickness of an Electroless Nickel Immersion Gold (ENIG) coating on a printed circuit board (PCB) typically ranges from 0.05 to 0.2 microns for the nickel layer and 0.05 to 0.15 microns for the gold layer.

Immersion Gold (ENIG) PCB Coating

The thickness of the ENIG coating is critical to ensure optimal performance and reliability of the PCB. The nickel layer thickness is important for preventing the gold layer from diffusing into the copper, while the gold layer thickness is critical for ensuring good solderability and wire bonding. Immersion gold PCB surface finish is a popular choice for high-reliability PCB applications, such as aerospace, defense, medical, and telecommunications industries, where performance and reliability are critical.

PCB surface finishes protect exposed copper on a PCB from oxidation and ensure a reliable connection between components and the board. They are crucial for the performance, reliability, and lifespan of the PCB, and impact manufacturing quality. They also impact the manufacturing process, including the quality of the soldering and assembly.

In this article, we will explore the concept of Immersion Gold (ENIG) as a PCB surface finish, what is it, and the benefits and applications of using it in your PCB.

Steps Involved in Coating Immersion Gold

The process of coating immersion gold to a printed circuit board (PCB) involves several steps:

Layers of Gold and Nickel deposited over a PCB

1. Cleaning: The PCB is first cleaned to remove any dirt, oil, or other contaminants from the surface. This is typically done using a combination of mechanical and chemical cleaning processes.
2. Etching: The PCB is then etched to remove any unwanted copper from the surface. This is typically done using an acid solution, which selectively dissolves the copper and leaves the remaining circuitry intact.
3. Electroless Nickel Deposition: The PCB is then immersed in a bath of electroless nickel solution. This solution contains a reducing agent and a source of nickel ions, which react to deposit a thin layer of nickel onto the exposed copper surfaces. The nickel layer serves as a barrier between the copper and the gold, preventing the gold from diffusing into the copper.
4. Rinse: The PCB is then rinsed to remove any excess nickel solution from the surface.
5. Immersion Gold Deposition: The PCB is then immersed in a bath of gold solution, which contains a reducing agent and a source of gold ions. The gold ions react with the nickel layer to deposit a thin layer of gold onto the surface of the PCB. The gold layer provides a reliable and durable surface for soldering and wire bonding.
6. Final Rinse: The PCB is then rinsed again to remove any excess gold solution from the surface.
7. Drying and Inspection: The PCB is then dried and inspected for any defects or irregularities in the coating.

Advantages/Disadvantages of ENIG Surface Finish

Here's some information on the advantages and disadvantages of using ENIG as a surface finish:

Advantages of ENIG:

• Good Corrosion Resistance: The nickel layer in ENIG provides excellent corrosion resistance, which is important for applications where the PCB may be exposed to harsh environments.
• Uniform Thickness: ENIG provides a uniform layer thickness, which is essential for fine-pitch surface mount technology. This ensures that there is no variation in the thickness of the coating, which can cause problems during assembly.
• Excellent for Wire Bonding: ENIG is an excellent surface finish for wire bonding applications because it provides a highly conductive surface that is easy to bond to.
• Flat Surface: ENIG provides a flat surface that is suitable for chip-scale packages (CSPs) and other applications where the surface must be flat.
• Suitable for Multiple Reflow Cycles: ENIG is suitable for multiple reflow cycles, making it a good choice for applications that require repeated assembly.

Disadvantages of ENIG:

• Higher Cost: ENIG is more expensive than other surface finishes, such as HASL. The additional cost is due to the use of gold in the plating process.
• Sensitive to Handling: ENIG requires careful handling during the manufacturing process, as nickel is sensitive to oxidation. Any oxidation on the nickel layer can cause adhesion problems during assembly.
• Limited Thermal Shock Resistance: ENIG is susceptible to thermal shock, which can cause the nickel layer to crack and peel. This can lead to poor adhesion and other problems.
• Difficult to Inspect: The nickel layer of ENIG is difficult to inspect visually, which can make it challenging to detect defects during the manufacturing process.
• Not suitable for high-temperature applications: The nickel-gold intermetallic compound in ENIG can break down at elevated temperatures. This makes ENIG unsuitable for applications that require high-temperature resistance.

Applications of Immersion Gold (ENIG) PCB Surface Finish

Following are some applications of immersion gold surface finish in different industries:

1. Electronics Industry: The electronics industry is one of the biggest users of immersion gold surface finish. This surface finish is widely used in electronic components such as printed circuit boards (PCBs), integrated circuits (ICs), and connectors. Immersion gold provides excellent corrosion resistance, flat surface, and uniform thickness, which are crucial for electronic components that require high reliability, especially in harsh environments.
2. Medical Industry: The medical industry also uses immersion gold surface finish in its products. Medical devices, such as pacemakers, heart monitors, and defibrillators, require high reliability and long-term performance. Immersion gold provides excellent corrosion resistance and flat surfaces, which are necessary for these devices.
3. Aerospace Industry: The aerospace industry also uses immersion gold surface finish in its products. The aerospace industry demands high reliability, durability, and safety in its products. Immersion gold provides excellent corrosion resistance and uniform thickness, which are essential for aerospace products that operate in harsh environments and require long-term performance.
4. Telecommunications Industry: The telecommunications industry also uses immersion gold surface finish in its products. Telecommunications products, such as routers, switches, and servers, require high reliability, durability, and performance. Immersion gold provides a uniform layer thickness, which is critical for fine-pitch surface mount technology, and excellent wire bonding properties, which are necessary for high-speed data transmission.
5. Automotive Industry: The automotive industry also uses immersion gold surface finish in its products. Automotive electronics, such as engine control units (ECUs), airbag sensors, and anti-lock brake systems (ABS), require high reliability, durability, and safety. Immersion gold provides excellent corrosion resistance, flat surfaces, and uniform thickness, which are necessary for automotive electronics that operate in harsh environments.

Conclusion

In conclusion, PCB surface finishes are crucial for protecting exposed copper on a PCB from oxidation, ensuring a reliable connection between components and the board. Electroless Nickel Immersion Gold (ENIG) is a common and popular type of PCB surface finish in the electronics industry, widely used in high-reliability PCB applications, such as aerospace, defense, medical, and telecommunications industries. ENIG provides good corrosion resistance, and uniform thickness, and is an excellent surface finish for wire bonding applications because it provides a highly conductive surface that is easy to bond to.

However, ENIG is more expensive than other surface finishes and requires careful handling during the manufacturing process. It is also not suitable for high-temperature applications, and the nickel layer is difficult to inspect visually, which can make it challenging to detect defects during the manufacturing process.

ENIG (Electroless Nickel Immersion Gold) is a surface plating that is applied over the copper pads on a Printed Circuit Board to protect them from corrosion and other abnormalities. Initially, the copper pad is covered by a Nickel (Ni) layer followed by a thin immersion Gold (Au) layer. ENIG provides good oxidation resistance, excellent surface planarity and allows for easy soldering which results in excellent electrical performance of the PCB board.

ENIG is RoHS compliant and is, therefore, one of the most used PCB surface finishes even though it is more complex and expensive when compared to other PCB plating processes like HASL.

ENIG is a two-layer metallic coating – Nickle is the barrier to the copper pad and is also the material to which components are soldered. Gold on the other hand protects the Nickle during storage and also provides low contact resistance. Typical Nickle thickness varies from 4 – 7 µm and Gold thickness varies from 0.05 – 0 23 µm. ENIG requires a processing temperature of around 80 °C.

Advantages of ENIG Surface Finishes:

• It provides impressive wettability, surface planarity, coplanarity, and long shelf life (up to 12 months) to the PCB board since immersion gold has strong chemical properties. 
• In ENIG, the nickel layer acts as a barrier and stops the interfusion between gold and copper. It also produces an intermetallic compound (IMC) Ni3Sn4 to provide good solderability after reacting with Tin.
• It has low contact resistance, high strength, reduces oxidation, and provides anti-friction. Overall, it enhances circuit conductivity requirements.
• It provides good plating over the copper pads and via holes.
• Its excellent surface planarity allows the components to be soldered flat onto the pad, making it ideal for BGA pads and other fine-pitch components.
• ENIG complies with all RoHS requirements.

Limitations of ENIG Surface Finishes:

• ENIG is an expensive surface finish technology
• It has undesirable magnetic properties
• Not good for rework and makes PCB repair very difficult