Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

Most PCB use copper as a conductor, but this metal tends to oxidize quickly when exposed to air. The oxidized copper is challenging to form good solder joints, necessitating surface finish after PCB manufacturing. With technological advancements, surface finishs have become diverse, and among them, ENEPIG is often overlooked due to its higher cost.

enepig plating

Table of Contents

What is ENEPIG?

The full name of ENEPIG is Electroless Nickel Electroless Palladium Immersion Gold. When you first recognize this word, it may confuse it with ENIG, but in reality, ENEPIG is an upgraded version of ENIG. 

ENEPIG’s structure includes gold on the top layer, palladium in the middle layer, and nickel on the bottom layer, which play different roles. Gold, as an excellent conductor, provides good electrical conductivity and remains chemically stable. Nickel, a metal with excellent physical and chemical properties, enhances soldering and increases joint strength. However, nickel is more prone to oxidation than copper. In the ENIG structure, which includes only two metal layers, black pad issues often arise. Additionally, nickel causes significant signal loss, making ENIG unsuitable for high-speed PCB. To address these concerns, ENEPIG introduces a palladium layer between the gold and nickel layers as a protective barrier against excessive corrosion of the nickel layer.

Further Reading: Causes and Solutions of ENIG Black Pad Defect

enepig structure

ENEPIG PCB finish follows the IPC-4556 standard. According to IPC, the nickel layer’s thickness ranges from 3μm to 6μm, the palladium layer’s thickness is typically between 0.05μm and 0.3μm, and the gold layer’s thickness is typically between 0.03μm and 0.1μm. For cost-effective projects the thickness of the gold layer should be at the lower limit.

ENEPIG Process Flow

Cleaning

Before electroplating, need to clean PCB to remove surface oils, dirt, and oxides to ensure uniform deposition of nickel, palladium, and gold layers. According to the composition, cleaners include alkaline and acidic ones, but for the ENEPIG process it is recommended to use an acidic solution, which can activate the copper surface while removing grease.

Microetching

Microetching involves creating tiny grooves and structures on the copper surface using sodium persulfate and sulfuric acid. These structures provide additional activation sites for the activator. However, Excessive etching may lead to PCB failure, so it’s advisable to maintain sulfuric acid concentration between 3% and 5%, solution temperature between 25°C and 30°C, and immersion time under 3 minutes.

Activation

This step addresses the challenge of chemically nickel-plating the copper surface directly. Specialized activation solutions, mainly composed of metal ions, are used. Common activator types include hydrochloric acid salt-type and sulfuric acid salt-type. Sulfuric acid salt-type activators are superior for overcoming infiltration effects.

The Pd (palladium) concentration in the activation solution should be maintained between 0.001% and 0.01% (10 to 100 ppm). Operational conditions include a temperature of 20°C to 30°C, with activation times generally between 1 and 5 minutes, depending on the presence of solder mask on the PCB. The activation solution should be promptly replaced when the copper ion concentration reaches 50 to 100 mg/L.

Chemical Nickel Plating

Chemical nickel plating involves a mixture of key components, including NiSO4, sodium hypophosphite (NaH2PO2), complexing agents, accelerators, and stabilizers. These components play a crucial role in preventing unnecessary reactions between copper and other metals used in the electroplating process. During the electroplating process, nickel layers are deposited on catalyzed copper surfaces through redox reactions. However, controlling the pH of the solution can be challenging and may result in defects.

In redox reactions, the generation of hydrogen ions can rapidly decrease the solution’s pH. To ensure a smooth electroplating process, it is essential to maintain the pH around 4.6. If the pH drops below 4.5, adjusting with ammonia is recommended, and if it exceeds 4.7, adding AR-grade sulfuric acid can help.

Moreover, the electroplating process is accompanied by the escape of hydrogen ions. If not promptly addressed, this phenomenon can lead to hydrogen ions being covered by the nickel layer, hidden on the PCB surface. When exposed to high temperatures, the escape of ions may occur again, eventually causing blistering. To prevent this situation, it is advisable to maintain a lower temperature during the process, slowing down the reaction rate and suppressing the rapid generation of hydrogen ions.

Chemical Palladium Plating

Chemical palladium serves as a protective layer to prevent black pad formation. Key components of the solution include PdCl2, sodium hypophosphite, ammonium chloride, and stabilizers like ammonium hydroxide. Recommended concentrations are PdCl2 at 1-5g/L, sodium hypophosphite at 10-15g/L, ammonium chloride at 40-50g/L, and ammonium hydroxide at 100-200g/L.

Chemical Gold Plating

Chemical gold plating deposits metal directly on the chemical matrix through a displacement reaction. The mechanism involves the significant difference in electrode potentials between nickel or palladium and gold, allowing nickel or palladium to displace gold ions in the plating solution. The porous nature of the gold layer enables continued displacement beneath the gold layer until the newly formed gold layer completely covers the nickel or palladium layer.

Characteristics of ENEPIG Surface Finish

  • ENEPIG’s metal layer combination allows multiple reflow soldering processes without damage.
  • Suitable for lead-free soldering, ensuring compliance with RoHS standards.
  • ENEPIG provides a longer lifespan compared to other surface finish methods.
  • Palladium layer helps avoid black pad issues and offers good signal performance.
  • ENEPIG exhibits high corrosion resistance.
  • Chemical nickel layer acts as a barrier to prevent copper migration.
  • Enhances wire bonding performance.
  • ENEPIG is a high-cost treatment method and may not be suitable for low-profit projects.
  • The palladium layer is brittle, making ENEPIG prone to cracking.
  • Additional steps increase manufacturing time and complexity.

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