Electroless nickel immersion gold (ENIG) for your next PCB project

ENIG, or electroless nickel immersion gold, is a surface finishing process applied to the copper pads on a PCB board to protect them from environmental conditions such as corrosion. This finish involves two layers: first, a nickel layer is applied to the copper layer, and then immersion gold is applied over the nickel layer. The nickel layer prevents oxidation and corrosion, while the immersion gold layer acts as a protective layer for the nickel layer and makes the surface easier to handle during soldering. The thickness of the nickel layer is typically between 4 and 7 microns, and the immersion gold layer has a thickness of about 0.05 to 0.23 microns.

As one of the leading PCBA manufacturers in China, we offer ENIG plating as a surface treatment option. While the ENIG plating process is somewhat complex, it is RoHS compliant and offers easy solderability, oxidation resistance, and a flat surface structure, making it one of the best surface finishes available, as recognized by engineers. In this article, FS Technology will provide some guidelines for using ENIG surface finish, which will be beneficial for your electrical projects.

Why use ENIG finish

Features of ENIG

Solderability: The resulting surface of ENIG is flat and uniform, making it easier to obtain accurate solder joints. The nickel layer on the board surface enhances conductivity, leading to better soldering results.
Corrosion Resistance: ENIG provides high corrosion resistance due to the nickel layer. Nickel is known for its oxidation and corrosion resistance, which protects the copper traces from environmental conditions.
Reliable Wire Bonding: ENIG is a good option for wire bonding due to its flat structure. The nickel layer is suitable for wire bonding but must be free from pitting to avoid any effect on the wire bonding process.
Shelf Life: ENIG has a longer shelf life compared to other finishes. It can operate for a longer duration without being affected by oxidation and corrosion, making it suitable for applications where long-term reliability is needed.
Cost-effective Finish: ENIG is less expensive than other finishes such as ENEPIG or immersion silver. Its application process is simple, making it a cost-effective option.
Difficulty Reworking: The board with ENIG finish is difficult to repair if it gets damaged.
Craft Puzzle: The difference in the thickness of the nickel and gold layers can affect the overall performance of the board. Pitting is another issue that can affect the board’s performance, caused by the nickel layer.

Enig Vs ENEPIG

Feature	ENIG	ENEPIG
Process	The electroless plating process is used to apply nickel on the board surface after it is immersed in the gold solution.	There are different processes used for ENEPIG applications, which typically involve a combination of electroless palladium, electroless nickel, and immersion gold plating.
Thickness	In some cases, the thickness of the nickel layer is about three to six microns, while the gold layer is about 0.05 to 0.1 microns thick.	For ENEPIG, the nickel layer typically has a thickness of 4 to 8 microns, the palladium layer thickness is 0.1 to 0.3 microns, and the gold layer thickness is about 0.025 to 0.05 microns.
Solderability	The use of a gold layer in the ENIG finish results in excellent solderability, but it can cause black pad corrosion as the nickel layer gets corroded.	The presence of gold and palladium layers enhances its solderability and minimizes the risk of black pads.
Wire Bonding	This finish is often utilized for wire bonding as it provides a flat surface, but the bonding effectiveness can be negatively impacted by pitting created by the nickel layer.	It is also an excellent option for wire bonding as the palladium layer helps to achieve more precise wire bonding than the nickel layer used in ENIG.
Cost	The simple plating process involved in ENIG and the fewer plating phases make it less expensive than ENEPIG.	The ENEPIG process is more complex, which makes it a more expensive finish compared to ENIG.
Corrosion Resistance	The nickel layer provides excellent corrosion resistance to the finish.	ENEPIG is also known for its high resistance to corrosion, thanks to the use of nickel, gold, and palladium.
Shelf Life	ENIG has a longer shelf life compared to other surface finishes, but it may degrade over time due to the formation of black pad defects.	Compared to ENIG, ENEPIG has a longer shelf life thanks to the presence of the palladium layer, which acts as a barrier against oxidation and corrosion.

ENIG Plating Needs Attention

Black pad of ENIG in PCB

The black pad is a common PCB problems that can occur during the ENIG finishing process on PCB boards. It results in the formation of a black layer between the nickel and gold layers, causing the gold layer to be eliminated from the board’s surface. The black pad defect is mainly caused by poor adhesion between the nickel and gold layers, which is often due to a thin nickel layer. The black layer is actually nickel oxide, produced by a reaction between nickel and environmental contaminants. This defect can significantly decrease the board’s reliability and lead to damage in some cases.

To prevent black pad defects, it is crucial to ensure a precise PCB manufacturing process. This involves controlling the thickness of the nickel layer, performing the finishing process in clean conditions, and using high-quality materials. Black pads are more likely to occur in BGA and chip scale packaging, which have a smaller pin pitch. Overheating during PCB assembly, such as during the soldering process, can also cause black pads to form. Black pads can cause electrical faults as well as mechanical faults such as solder joint cracking.

ENIG Quality Control and Testing

To ensure the ENIG finish meets the quality requirements of the board, various quality control tests are performed. These tests include:

X-ray inspection is a common technique used to detect the presence of black pads on a PCB board. This technique can also identify other anomalies in the nickel layer, which may indicate the presence of black pads.
XRF or X-Ray fluorescence: This test is used to check that the finish is in compliance with the board’s requirements. XRF is a non-destructive test that detects the composition of the surface layers by using X-rays.
SEM or Scanning Electron Microscopy: SEM testing is used to inspect the surface quality of the finish and to examine the morphology of the nickel and gold layers.
Solderability testing: These tests are performed to evaluate the soldering features of the board. The solderability testing includes the wetting balance test, the solder ball test, and the solder spread test.

Conclusion

The ENIG gold plating is a critical process for PCB manufacturing, offering features such as corrosion resistance, excellent solderability, and longer shelf life. It is widely used in various industries, including medical equipment, electronic devices, aviation, and telecommunications. Despite its benefits, ENIG also presents some challenges that can be addressed through ongoing research and development efforts.