PCB Electronic Manufacturing Services

ENIG (Electroless Nickel Immersion Gold) Surface Finish

JHYPCB is a leading professional PCB manufacturer and assembly service provider in China. With over 10 years of experience, we specialize in providing rapid PCB prototyping, small-batch PCB production, and complete PCB assembly services. Our manufacturing capabilities include multilayer, HDI, Flex-Rigid, and metal-based PCBs using cutting-edge equipment and processes.

Surface finishing is a critical step in PCB fabrication. It involves coating the outermost layer of copper traces on a printed circuit board with a thin layer of metal or chemical compound. This applied finish serves important functions like protecting the copper from oxidation and corrosion, ensuring good solderability, and enhancing bonding and connectivity. Several options exist for PCB surface finishing, each with its own set of benefits and tradeoffs. Some of the most common finishes include:

HASL (Hot Air Solder Leveling) – Applies a solder coating for excellent solderability.
Immersion silver – Coats traces with a thin layer of silver for good conductivity.
OSP (Organic Solderability Preservative) – An organic coating to protect copper and provide temporary solderability.
ENIG (Electroless Nickel Immersion Gold) – Electrochemically deposits nickel and gold for corrosion resistance and solderability.

ENIG or Electroless Nickel Immersion Gold has emerged as one of the most popular surface finish options, especially for consumer electronics applications. In this guide, we provide a comprehensive overview of ENIG technology, including its benefits, applications, and a comparison to other PCB finishes.

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What is ENIG Surface Finishing?

ENIG stands for Electroless Nickel Immersion Gold. It is a PCB surface finish that involves electrochemically depositing two metallic layers onto the copper traces – a layer of nickel followed by a thin layer of gold.

The ENIG process involves two main steps:

Electroless Nickel Plating
This deposits a layer of nickel a few microns thick over the exposed copper on the PCB through an auto-catalytic chemical reaction. No external power source is required for this reaction. The nickel provides a protective barrier layer over the copper that prevents oxidation and corrosion.

Immersion Gold Plating
This deposits a thin layer of gold (typically 0.05 to 0.1 microns) over the nickel by immersing the PCB in a gold solution. The gold provides excellent conductivity and solderability. It also protects the nickel from oxidizing over time.

The combination of nickel and gold in ENIG provides several important benefits. The nickel gives the PCB surfaces excellent wear and corrosion resistance. The immersion gold outer layer enables efficient soldering and bonding. The thickness uniformity of the ENIG deposit also allows for consistent assembly and performance.

ENIG Plating Process Explained

The ENIG PCB plating process involves several steps to prepare the board surface and deposit the nickel and gold layers.

Step 1: Pretreatment: The bare copper traces are cleaned and micro-etched to remove oxidation and condition the surface. Solutions of sulfuric acid and other chemicals are used.

Step 2: Activation: A palladium catalyst is deposited on the copper to initiate the nickel deposition reaction. This can be done by immersion or colloidal seeding.

Step 3: Electroless Nickel Plating: The PCB is immersed in the nickel plating solution which deposits nickel 2-5 microns thick over the copper through an auto-catalytic reaction. The solution contains a nickel salt, reducing agent, and stabilizers/wetting agents. It is maintained at a temperature of 80-90°C and pH of 4-6.

Step 4: Rinsing: The board is thoroughly rinsed to remove excess chemicals after nickel plating.

Step 5: Immersion Gold Plating: The PCB is dipped in a solution containing a gold salt, typically gold potassium cyanide. This rapidly deposits a thin layer of gold around 0.05-0.1 microns on the nickel surface.

Step 6: Rinsing: Final rinse to remove excess gold solution. Drying completes the ENIG process.

Step 7: Post-treatments like hot air leveling can be done to ensure uniform thickness. The quality of the ENIG deposit is inspected under microscopes. With the right process controls, ENIG provides an excellent and consistent PCB finish.

Benefits and Advantages of ENIG

ENIG offers several important benefits that make it a popular choice for PCB finishing:

Excellent Solderability and Conductivity
The immersion gold outer layer enables reliable and efficient soldering. The gold metallurgy results in superb wetting and bonding with solder. This allows for consistent solder joint formation and interconnections.

Corrosion Resistance
The electroless nickel layer provides an effective barrier against oxidation and corrosion. This protects the underlying copper traces from environmental damage over time.

Surface Planarity and Uniformity
The dual ENIG layers are deposited with a high degree of thickness uniformity across the PCB surface. This ensures boards with excellent planarity and minimal finish thickness variation.

Smooth Surface Finish
ENIG deposits conform closely to the base copper topography. The resulting smooth surface is ideal for further PCB assembly and component placements.

Long Shelf Life
Properly processed ENIG finishes maintain solderability and oxidative stability for over 12 months of shelf life. The gold layer preserves performance over extended periods.

Suitable for Fine Features
The thin and conformal ENIG deposit makes it suitable for PCBs with fine line widths, small components, and high interconnect densities.

ENIG Applications and Use Cases

ENIG is widely used for finishing PCBs across many industries and applications including:

Consumer Electronics
ENIG is commonly used for coating PCBs in mobile phones, laptops, tablets, wearables, and other consumer electronics where miniaturization and reliability are critical.

High-Speed Digital Circuits
For high-frequency boards in computing, telecom infrastructure, etc. ENIG provides stable signal integrity and connectivity.

Automotive Electronics
ENIG’s resistance to harsh environments makes it suitable for automotive electronics which require thermal cycling durability.

Aerospace and Aviation
ENIG is used for aerospace PCBs which demand the utmost reliability in extreme conditions.

Medical Equipment
High-quality demands for medical device PCBs are met by ENIG’s excellent solder joint integrity and conductivity.

Harsh Environments
The corrosion resistance of ENIG is advantageous for boards used in humid, high-temperature, or corrosive environments.

High-Reliability Applications
Mission-critical defense, instrumentation, etc. applications use ENIG for maximizing PCB reliability over long service lives.

The uniformity, solderability, and longevity of ENIG finish make it a versatile choice suitable for prototyping through high-volume production across industries.

ENIG vs Other Surface Finishes

ENIG provides a unique set of advantages but has some limitations as well. Here is how it compares to other popular PCB finishes:

vs OSP (Organic Solderability Preservative):

OSP is a low-cost finish but has a short shelf life. ENIG maintains solderability for over 12 months.
OSP cannot withstand high temperatures or humidity. ENIG is far more durable.

vs Immersion Silver:

Immersion silver also provides excellent solderability. But it tarnishes over time affecting appearance.
ENIG’s gold outer layer is more resistant to tarnishment or oxidation.

vs HASL (Hot Air Solder Leveling):

HASL provides good shelf life and solderability. But the solder deposit is thicker and less uniform.
ENIG allows for more consistent assembly and a smooth surface.

vs Hard Gold Plating:

Hard gold is deposited using electroplating so can produce thicker layers. But ENIG offers better overall cost.
Hard gold may have worse thermal fatigue and adhesion performance.

So in summary, while no finish is ideal for every application, ENIG provides an optimal balance of solderability, shelf life, durability, cost, and process compatibility. This makes it one of the most versatile and widely used surface finishes for PCBs.

ENIG PCBs from JHYPCB

With over 10 years of experience, JHYPCB has extensive capabilities for manufacturing high-quality ENIG boards.

We utilize advanced PCB processes and equipment to deposit ENIG finishes that conform tightly to industry standards such as IPC-4552. Our production lines enable us to efficiently scale from prototyping small quantities to bulk orders of over 10,000 boards.

JHYPCB maintains stringent process controls to ensure the reliability and consistency of the ENIG plating. Parameters like solution concentrations, temperature, pH, and current density are closely monitored with statistical control methods.

We have expertise across a range of ENIG boards including:

High-density interconnects (HDI)
Multi-layer boards
Flex and rigid-flex circuits
High-frequency RF/microwave boards
Metal-core PCBs

Our quality assurance procedures include tests like solderability, porosity, coating thickness, and adhesion. We also test for surface cleanliness, corrosion resistance, and environmental durability.

JHYPCB is ISO 9001 and IATF 16949 certified ensuring we meet the highest standards for quality management. Contact us today at sales@pcbjhy.com to discuss your needs for rapid prototyping or volume production of ENIG PCBs. Our technical support team can provide guidance on design, fabricability, and technology considerations.