In the circuit board manufacturing process, it may be necessary to apply PCB surface finish to achieve corrosion protection, enhanced wetting properties, and more reliable soldering. However, there are several different types of finish methods, including HASL, ENIG, OSP, and Immersion Silver. To provide you with a better understanding of this process, this TechSparks article will offer a comprehensive overview of the ENIG finish.
What is ENIG Finishing
ENIG in PCB stands for Electroless Nickel Immersion Gold, which refers to covering the copper pads on the circuit board with a layer of metal to protect them from environmental conditions such as corrosion. Despite its higher cost, this treatment method is often considered worthwhile due to its RoHS compatibility. It consists of two metal layers – the first layer being nickel and the second layer being gold. Nickel serves as a barrier layer for copper and material for soldering components, while gold safeguards nickel during storage and offers lower contact resistance. It’s important to note that the thickness of nickel varies from 4 to 7 micrometers, while the thickness of gold is approximately 0.05 to 0.23 micrometers. The processing temperature for ENIG plating is around 80℃ Celsius.
How can you differentiate an ENIG PCB?
- Golden Appearance: Due to the uniform nickel plating followed by a thin layer of gold immersion on the pads, the solder pads often exhibit a golden or gold-yellow color.
- Smooth Texture: The treated solder pad surface appears smooth without the solder “whiskers” that can be seen in processes like HASL.
Characteristics of ENIG Plating
- Due to the different crystal structures created by gold electroplating and immersion gold processes, the ENIG surface is more solder-friendly compared to gold electroplated surfaces, avoiding slow soldering speeds and solder joint failures.
- With excellent wettability and stable chemical characteristics, ENIG circuit board offers a longer shelf life (1 year). Additionally, it provides a flatter surface, making it an optimal choice for ball grid array.
- Having good wettability, surface flatness, and coplanarity, along with the robust chemical properties of immersion gold, contribute to its extended shelf life, approximately one year.
- During PCB design, engineers often consider tolerance compensation, ensuring that ENIG does not affect pad spacing, making the application process easier.
- A metallic intermetallic compound forms between nickel and gold, offering good solderability through reaction with tin. This compound is Ni3Sn4.
- On the finished board, the pads are composed of nickel-gold and are embedded within the copper layer rather than being on the surface. This could potentially cause interference with signal transmission or hinder tight-spacing pads, leading to short circuits.
- High cost remains a primary deterrent for engineers to utilize this method.
How to Apply ENIG Surface Finish
When to use?
As previously mentioned, there are various PCB surface treatments available, but in certain scenarios, ENIG surface treatment proves to be the optimal choice. Firstly, for applications prone to oxidation-reduction reactions, ENIG treatment is evidently more favorable as it prevents such issues by isolating from the air. Secondly, when PCB trace widths and spacing are limited, TechSparks recommends using ENIG to minimize short circuits between exposed copper layers and offer anti-creep properties. Additionally, if wire bonding is required during the assembly process, ENIG must be used for your circuit board.
How to use?
Now, let’s delve into the process of implementing ENIG on a printed circuit board:
- Materials: This includes electroless nickel solution, gold solution, cleaning agents, activators, etc.
- Cleaning: The prepared cleaning agent is applied to wipe the circuit board, ensuring the removal of surface grease, dust, and contaminants. After confirming cleanliness, rinsing is performed to eliminate cleaning residues, followed by a drying process.
- Activation: This step aims to enhance the reactivity of the metal surface for better interaction with subsequent chemical treatments. The circuit board is immersed in a prepared activator, which can be an acidic solution or specific chemicals. Activation time and conditions are controlled based on specific process specifications.
- Chemical Nickel Deposition: The board is immersed in an electroless nickel solution, where the reducing agent and nickel ions undergo a chemical reaction to form a uniform nickel layer, providing protective benefits.
- Gold Deposition: Subsequently, the circuit board with the nickel layer is immersed in a bath containing gold solution. Similar to the previous step, a gold layer is formed on the surface, providing a reliable and durable surface for soldering and wire bonding.
- Finishing: After the ENIG plating process, thorough cleaning and drying are conducted again to prevent excess chemicals from causing damage. Quality inspection may be performed if conditions allow, ensuring the quality and performance of the solder pads.
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