Aug22
What is ENIG Surface Finished for Circuit Boards? What Are Its Advantages and Disadvantages?

What is ENIG Surface Treatment for Circuit Boards? What Are Its Advantages and Disadvantages?

ENIG (Electroless Nickel Immersion Gold), also known as immersion gold (Au), chemical Ni/Au, or soft gold, is a type of surface finished for printed circuit boards (PCBs). It is widely used in the assembly of mobile phone circuit boards and some BGA package substrates may also use ENIG or NEPIG (Chemical Nickel Palladium Gold).

ENIG is easy to recognize by appearance. If you see a printed circuit board with gold-colored pads (as shown in the image at the beginning of the article), it is likely ENIG. There is another type of “electroplated nickel gold” PCB that also has gold-colored pads, but due to the high cost of gold, it is rarely used unless there is a special need for a thicker gold layer or hard gold.

Compared to electroplated nickel gold, ENIG uses chemical displacement and deposition processes that do not require electrical plating on the circuit board or connecting wires to each pad to plate nickel gold. Therefore, the ENIG process is relatively simpler, and its production volume is much higher than electroplated nickel gold, making it more cost-effective.

However, ENIG surface finished has its drawbacks and issues. For example, the soldering strength is relatively lower compared to copper-based PCBs (OSP, HASL), and it is prone to the black pad problem, which is a common quality issue.

Nickel and Gold Layer Thickness Requirements for ENIG

According to IPC-4552 (2002), the recommended gold layer thickness for standard ENIG printed circuit boards should be between 2µ” to 5µ” (0.05µm to 0.125µm), and the chemical nickel layer thickness should be between 3µm (118µ”) to 6µm (236µ”).

As the price of gold continues to rise, PCB manufacturers often try to reduce costs by skimping on the gold layer. It is recommended to strictly control the thickness of the gold layer, especially for large solder pads. However, thicker gold layers also mean higher costs. Based on Workingbear’s past experience, we have relaxed the gold layer thickness to as thin as 1.2µ” and it still worked normally.

The Production Process of Electroless Nickel Immersion Gold

The general production process of chemical nickel gold is as follows:

Pre-treatment → De-greasing → Water Rinse → Acid Wash → Water Rinse → Micro-etching → Water Rinse → Pre-dipping (H2SO4) → Activation (Pd Catalyst) → Water Rinse → Chemical Nickel (Ni/P) → Water Rinse → Immersion Gold → Gold Recovery → Water Rinse → Drying

  1. Pre-treatment: The goal is to remove oxides from the copper surface by brushing or sandblasting and to roughen the copper surface to increase the adhesion of the subsequent nickel alloy.

  2. Micro-etching: Sodium persulfate/sulfuric acid is used to remove the oxide layer from the copper surface and to reduce the depth of scratches caused by pre-treatment brushing. Deep scratches often contribute to the immersion gold attacking the nickel layer.

  3. Activation: Since the copper surface cannot directly initiate the deposition reaction of chemical nickel, a layer of palladium (Pd) must be applied to the copper surface as a catalyst for the chemical nickel deposition reaction. Using the principle that Cu is more active than Pd, palladium ions are reduced to palladium metal and adhere to the copper surface.

  4. Chemical Nickel: Ni/P’s main function is to prevent migration and diffusion between copper and gold, and it acts as an element that forms an intermetallic compound (IMC) during subsequent soldering with tin.

  5. Immersion Gold: The main purpose of gold is to protect and prevent the nickel layer from oxidizing. Gold does not participate in the chemical reaction during the soldering process. Too much gold can actually hinder solder strength, so the gold layer only needs to be thick enough to cover the nickel layer and prevent oxidation. If it is for COB (Chip On Board) wire bonding, the gold layer must be sufficiently thick.

Related Reading:

  • The Relationship Between Component Drop and Gold Plating Thickness on Circuit Boards
  • The Role of Immersion Gold and Electroplated Gold in Circuit Board Soldering

If you want to learn more details about the entire ENIG process, you can refer to this article and watch the video to deepen your understanding: [Video] PCB Production Process.

Advantages of ENIG (Electroless Nickel Immersion Gold) Surface Finished for Circuit Boards:

  • The surface finished can be used as the base metal for COB wire bonding.

  • It can undergo multiple reflows; typically, it must withstand at least three high-temperature soldering processes while maintaining consistent quality.

  • It has excellent conductivity and can be used as gold finger circuits for button contacts, providing high reliability.

  • Gold has low reactivity and does not easily react with atmospheric components, offering some oxidation and rust resistance. Therefore, ENIG circuit boards generally have a shelf life exceeding six months. Sometimes, even if stored in a warehouse for over a year, as long as they are well-preserved without rust issues, the boards can still be used after baking to remove moisture and passing solderability tests. (Note: Due to the increasing cost of gold, many PCB manufacturers reduce the gold layer thickness to save costs. However, if the gold layer is too thin, it may not fully cover and protect the underlying nickel metal, leading to potential oxidation over time. It is recommended to read “The Relationship Between Component Drop and Gold Plating Thickness on Circuit Boards” and “The Role of Immersion Gold and Electroplated Gold in Circuit Board Soldering” for further understanding of gold layer thickness requirements.)

  • Gold is also not easily oxidized when exposed to the air, allowing for the design of large exposed pads for heat dissipation.

  • It can be used as the contact surface for blade connectors, which requires a thicker gold plating layer, typically hard gold. (For more information, read “What Are Hard Gold, Soft Gold, Electroplated Gold, Chemical Gold, and Flash Gold?”)

  • ENIG has a flat surface, which provides good solder paste printing and easy soldering, making it very suitable for fine-pitch and small components, such as BGA and flip-chip components.

Disadvantages of ENIG (Electroless Nickel Immersion Gold) Surface Finished for Circuit Boards:

  • Generally, the solder joint strength of Ni3Sn4 is not as strong as Cu6Sn5. Some components with special soldering strength requirements may not withstand large external impacts and may fall off.

  • The cost is relatively higher compared to OSP surface finished due to the rising price of gold.

  • There is a risk of “black pad” or “black nickel” formation. Once black pad occurs, it significantly reduces solder joint strength. Black pad is a complex NixOy chemical composition, primarily caused by excessive oxidation during the immersion gold process. Large gold atoms (atomic radius 144pm) irregularly deposit, forming rough, porous grain structures, meaning the gold layer does not completely cover the underlying nickel layer, allowing nickel to continue reacting with air and forming rust underneath the gold layer, ultimately hindering soldering. A process called “ENEPIG” (Electroless Nickel Electroless Palladium Immersion Gold) effectively solves the black pad issue, but it is still relatively expensive and mainly used by high-end boards, CSP (Chip Scale Package), or BGA manufacturers. (We will discuss the black pad issue in more detail in a future topic.)

Therefore, ENIG boards are very suitable for products requiring electrical contact (e.g., spring-loaded contacts, button contacts). Additionally, ENIG is suitable for small batch products, reducing the scrap rate of unused PCBs.


Related Posts:

Leave a Reply

Your email address will not be published. Required fields are marked *