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ENIG (Electroless Nickel Immersion Gold) is the go-to surface finish for high-reliability PCBs in aerospace and medical devices. But is it right for your project? What is the difference between the different surface finish forms?
ELEPCB will give a comprehensive guide to Electroless Nickel Immersion Gold Surfacing (ENIG), its pros and cons, application area, and the steps to coat immersion gold.
What is ENIG in PCB?
The surface finish is to create a smooth, even layer that maximizes the durability and performance of the PCB. ENIG is a two-layer metallic surface finish that includes a thin layer of gold over a layer of nickel. The operations are:
First, the nickel layer is plated on copper pads using an electroless chemical reaction. Next, gold is applied over the nickel layer using an immersion method to coat pads and traces. The gold layer provides excellent conductivity and protects nickel from oxidation.
This ENIG surface finish is used in various industries, including consumer electronics, automotive, aerospace, and medical.
Thickness of ENIG Surface Finish
In any surface finish, the copper pads should be protected from oxidation. The gold layer in ENIG prevents the nickel layer from oxidation. The thickness of the nickel layer ranges from 4 to 7 microns, while the thickness of the gold ranges from 0.05 to 0.23 microns.
The thickness for ENIG surface finish usually follows IPC 4552. This standard requires the thickness:
- The nickel thickness: 100 ~ 250 micro inches
- The immersion gold thickness:2 ~ 8 micro inches
Steps in ENIG Process
The ENIG process involves various steps including coating gold and deposition of Nickel. Let’s check out the steps in detail.
Pretreatment
The PCB goes through an initial cleaning by brushing or sandblasting. It is done to remove any loose particles from the manufacturing process.
Oil Removal
The PCB is immersed in a degreasing solution to remove any oils or grease present on the surface.
Water, Acid & Water Washing
The PCB is rinsed properly with water to remove any residues from the degreasing solution in oil removal. Then it is dipped in an acid solution(HCl), to clean and slightly etch the copper surface. Finally, the board is washed again with water to remove the acid.
Micro-etching
The board is immersed in a micro etching solution (Na(2)S(2)O(8)/H(2)SO(4)) to remove the oxide layer on the copper surface. This helps improve the bond of the electroless nickel layer with the PCB. It is also washed with water to remove any micro-etching solution.
Prepreg (H(2)SO(4))
The board is then dipped in the dilute sulphuric acid to prepare the surface for activation.
Activation(Pd catalyst)
As the copper cannot start the nickel deposition reaction, it is exposed to a palladium(Pd) catalyst, which initiates the electroless nickel plating reaction. In the reaction, the palladium ion is converted to palladium metal and bonded to the copper layer. It is then washed with water to remove any excess palladium catalyst.
Chemical nickel (Ni-P)
The board is immersed in an electroless nickel plating bath. A chemical reaction deposits a uniform layer of nickel-phosphorus (Ni-P) alloy onto the activated copper surface. It helps to create a barrier between copper and gold and diffusion. Then, the board is again washed with water to remove the residual chemicals.
Immersion Gold Plating and Gold Recovery
The PCB is dipped in an immersion gold plating bath during this step. During this displacement reaction, gold ions in the solution are reduced and deposited onto the nickel layer on the other side.
Finally, the PCB is rinsed with water. This helps to remove any remaining chemicals from the gold plating bath, giving a clean, finished surface.
Advantages and Disadvantages of PCB ENIG
Although ENIG is favoured for its outstanding durability in PCB manufacturing, there are some drawbacks. When choosing a surface finish treatment form, consider your needs.
Advantages
Surface planarity: ENIG provides a very flat and plane surface. This is crucial for mounting fine-pitch and SMT components. The uniformity of the ENIG finish also allows for reliable solder joints.
Corrosion resistance: It is well-resistant to oxidation. The nickel and gold layers protect the copper traces, making it RoHS-compliant.
Durability: The ENIG finish makes the PCB durable. The nickel layer provides a hard, wear-resistant surface that can withstand mechanical stress and multiple reflow cycles.
Disadvantages
ENIG has some limitations, which we will discuss below:
- It is expensive compared to other surface finishes, including HASL.
- One of the most common issues in ENIG is black pad syndrome, where the nickel layer corrodes. This leads to poor solder joints and can cause the system or devices to fail.
- The thickness of the gold layer/surface in ENIG affects the circuit board’s performance. Achieving a consistent thickness of this layer is a daunting task.
- The chemicals used in ENIG are harmful, so they need to be disposed of properly.
ENIG vs HASLvs ENEPIG
ENIG differs from other surface finishes in the following types:
Features | ENIG (Electroless Nickel Immersion Gold) | HASL (Hot Air Solder Leveling) | ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) |
Composition | Nickel (3-6 µm) Gold (0.05-0.1 µm) | Solder (Tin-Lead or Lead-Free) | Nickel (3-6 µm) Palladium (0.02-0.1 µm) Gold (0.03-0.05 µm) |
Process | Chemical deposition | Hot air leveling | Chemical deposition |
Surface Planarity | Excellent | Poor | Excellent |
Solderability | Excellent | Good | Excellent |
Durability | High | Medium | High |
Corrosion Resistance | High | Medium | Very High |
Shelf Life | Long | Medium | Long |
Cost | Medium | Low | High |
Environmental Impact | Medium | High (for leaded HASL) | Medium |
Common Issues | Black Pad Syndrome | Uneven surface | High cost |
Applications | High-density PCBs, fine-pitch components | General-purpose PCBs | High-reliability applications |
Thickness Control | Good | Poor | Good |
Thermal Performances | Good | Variable | Excellent |
Conclusion
PCB ENIG plating is essential to protect the copper (exposed) from oxidation. It is one of the best surface finishes for wire bonding because of its highly conductive surface. It offers significant advantages in PCB manufacturing, mainly on high-density boards. Furthermore, it is one of the popular surface findings because of its excellent corrosion resistance. So, if you are looking for a surface finish that provides both effective functionality and environmental compliance, then ENIG is a great choice today.
So, learn and explore more about PCB manufacturing with ENIG surface finish with ELEPCB. Contact us today to learn more about your project requirements and request a quote to know more!
A1: According to IPC-4552 standards, the nickel layer in ENIG ranges from 100-250 micro inches (4-7 μm), while the immersion gold layer is 2-8 micro inches (0.05-0.23 μm) thick. This precise thickness ensures oxidation resistance while maintaining solderability.
A2: ENIG costs 15-30% more than HASL due to its complex two-step chemical process (electroless nickel + immersion gold), higher-priced materials (gold/palladium), and stricter process controls. However, it provides superior flatness for fine-pitch components and RoHS compliance.
A3: Black Pad Syndrome occurs when hyper-corrosion of the nickel-phosphorus (Ni-P) layer creates brittle nickel oxides at the nickel-gold interface. This causes weak solder joints, intermittent connections, and potential device failure. It’s often triggered by improper pH control or contaminated gold baths during processing.
A4: Yes, ENIG is fully RoHS compliant as it uses zero lead in its plating process. Both nickel and gold layers meet EU hazardous substance restrictions (Directive 2011/65/EU), making it ideal for medical, automotive, and consumer electronics.
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