Advanced PCB Surface Treatment - ENEPIG

Technology is always evolving, and as electronic applications continue to grow in depth, they are used in more complex environments. To ensure that these electronic products can function properly, we have to carry out some necessary means or measures, such as PCB surface finish. We currently have a variety of surface treatment options available, each of which produces different results, although they are all designed to avoid copper layers in contact with environmental moisture and air will produce oxides, resulting in the attenuation of copper during the soldering process. The ENEPIG we will discuss in this article is one of them, its full name is Electroless Nickel Electroless Palladium Impregnated Gold and it has many advantages and complies with RoHS and WEEE directives making it often the preferred choice for different applications.

WHAT IS ENEPIG?

We often use the term “universal surface treatment” to refer to ENEPIG, because it can be applied to all circuit boards. Compared to other surface finishes, the ENEPIG plating process is more mature, having first appeared 15 years ago. Later, due to the sharp drop in the price of palladium, ENEPIG became a more cost-effective surface finish method and became famous as an electronic plating solution.

ENEPIG is commonly used to support soldering and bonding of gold and aluminum wires to protect PCB performance from environmental factors during storage and operation. Electroless plating deposits several layers of metal on the circuit board surface: nickel plating (Ni 3-5 μm), palladium plating (Pd 0.05-0.1 μm), and an immersion gold layer (Au 0.03-0.05 μm). It is particularly suitable for high-end applications that require high performance, such as aerospace and military electronics. Due to its superior performance and reliability, ENEPIG is often used as an alternative to traditional surface treatments like HASL (Hot Air Solder Leveling) and OSP (Organic Solderability Preservative). However, it is also more expensive than other finishes and requires specialized equipment and processes to apply.

How ENEPIG Applied

Pre-treatment

Cleaning the PCB is an essential step in the PCBA production process, and it has been a long-standing concern at FS Technology. Any contaminants, whether from the environment or from handling by operators, can interfere with the adhesion of the plating layers, leading to issues with solderability and electrical performance. Therefore, it is crucial to ensure that the PCB is thoroughly cleaned before proceeding with the ENEPIG plating process to avoid these common PCB problems.

Advanced Reading: How to clean PCB?

Copper Activation

Copper activation is a crucial step in the electroless plating of metals onto printed circuit boards (PCBs). During this process, the copper layer to be protected is selectively activated to determine the deposition pattern during the subsequent electroless nickel plating step. This is achieved through a displacement reaction that requires the use of a chemical solution containing a reducing agent, such as palladium chloride.

Electroless Nickel

Electroless nickel plating is a coating process that deposits a layer of nickel onto a substrate without the need for electricity, and as such, is also referred to as autocatalytic nickel plating. Nickel acts as a barrier layer on the PCB, protecting copper from interacting with other metal materials, particularly gold, that are used for ENEPIG finish. This layer is deposited on the catalytic copper surface using redox reactions, resulting in a layer thickness of between 3 and 5 microns. In comparison to traditional electroplating methods, electroless nickel plating offers several advantages, including uniform thickness, better adhesion, and improved corrosion and wear resistance.

Electroless Palladium

The second metal layer deposited on the surface of the board is palladium, which offers a more stable surface for the final gold layer and improves the bonding of the gold wires. ENEG and ENEPIG are two surface treatments that are similar, with the essential difference being the presence of the palladium layer. Palladium serves to safeguard the nickel layer from corrosion and diffusion into the gold layer, as well as act as an anti-oxidation and anti-corrosion layer. Similar to nickel, the layer is applied through a chemical reaction using a redox reaction, which prompts the nickel surface to react with the palladium and form a thin layer. The thickness of the palladium layer typically ranges from 0.05 to 0.1 microns. There are numerous electroless palladium plating processes available, with Altarea TPD-23 being a suitable option for SMT PCBA applications.

Immersion Gold

The final step in the ENEPIG PCB finish process involves adding a gold layer. Once the electro-cleaning and drying steps are complete, the PCB can be assembled. The gold layer serves several functions, including reducing contact resistance, preventing friction, providing oxidation resistance, and protecting the palladium’s solderability.

Immersion gold plating is used for the gold layer and involves a displacement reaction. During this process, the covered plate is completely immersed, causing the palladium layer on the surface of the plate to dissolve, releasing electrons and reducing the surrounding gold atoms. Gold ions attach to the surface of the plate, replacing the palladium ions and forming a relatively thin outer layer that is only 0.03 to 0.05 microns thick, making it thinner than any other type of gold plating solution.

Advantages of ENEPIG Finish

High Durable Finish

The ENEPIG surface finish comprises multiple layers that offer superior corrosion resistance, safeguarding the copper lines and pads on the PCB against both corrosion and the presence of black nickel. Furthermore, its smooth and non-porous surface impedes the deposition of corrosive components on the board, preventing issues like warping during PCB assembly.

High Wire Bond Pull Strengths

ENEPIG plating shows good compatibility with both aluminum and gold wires in PCBs, resulting in greater resistance to pulling forces. Extensive testing has demonstrated that ENEPIG finishes can effectively protect aluminum and gold wires from high stresses over prolonged periods of time, thus highlighting their excellent bondability properties.

High-Quality Solderability

When it comes to electronics manufacturing, ENEPIG surface finishes are a popular choice for their ability to provide consistent and even layer thickness. This is crucial for ensuring high-quality soldering, which is essential for electronic components to function properly. During the PCB assembly process, PCBA boards need to be compatible with various alloys for lead-free reflow soldering. Thankfully, ENEPIG finishes can handle multiple reflow soldering cycles without any damage. This is important because the palladium coating dissolves completely after soldering, leaving behind a nickel surface that is free from oxide. The nickel-tin configuration that remains is perfect for creating strong and dependable solder joints.

Less Contact Resistance

When designing printed circuits, one important consideration for engineers is the contact resistance value, which measures the resistance to the flow of current at the interface of the circuit. As PCB engineers, we aim for lower contact resistance values, especially for applications that involve high-frequency or high-speed signal transmission. Fortunately, the ENEPIG electroplating process is an excellent solution for this requirement. When using ENEPIG, the solder joints of the circuit board components have low contact resistance, ensuring optimal performance of electronic equipment. With ENEPIG surface treatment, a uniform three-layer metal composition of nickel, gold, and silver is formed on the solder joint, providing excellent electrical conductivity. Furthermore, ENEPIG surface treatment is less demanding on the metallization quality of electronic components, and even in cases where the surface quality is poor, good contact resistance performance can still be achieved.

Protection Barriers

One way to enhance the solderability of a PCB is to use an ENEPIG finish, which includes nickel and palladium barrier layers. These barrier layers protect the copper from being affected by tin during soldering and prevent any unwanted reactions that could negatively impact the conductivity of the PCB. In other words, the nickel and palladium act as a protective shield to preserve the quality of the PCB’s components and ensure proper functionality. By using ENEPIG finish, engineers can reduce the risk of any unwanted mixing or degradation of materials, resulting in a high-quality and reliable PCB.

ENEPIG vs ENIG

FS Technology offers various surface treatment methods, including ENIG, ENEPIG, and immersion silver. To assist you in selecting the appropriate treatment, we have provided some helpful tips in the table. If you require further assistance, please do not hesitate to contact us directly for a quotation. Our team of professional engineers will promptly address your needs and provide effective recommendations.

Note: This table offers a general comparison between ENEPIG and ENIG. Actual performance may vary based on specific applications and requirements.

Property	ENEPIG	ENIG
Full name	Electroless nickel, immersion palladium, immersion gold	Electroless nickel, immersion gold
Substrates compatibility	High compatibility with aluminum and gold wires	Compatible with various substrates
Corrosion resistance	High	Moderate
Solderability	High-quality	Good
Surface finish	Matte	Shiny
Thickness	2-6 μm (nickel), 0.05-0.2 μm (palladium), 0.03-0.1 μm (gold)	2-6 μm (nickel), 0.05-0.2 μm (gold)
Contact resistance	Low	Low
Wire bonding	Excellent	Good
Cost	Higher than ENIG	Lower than ENEPIG
Shelf Life	6-12 months	3-6 months
Handling / Contact	Minimal impact	Sensitive to handling/contact
SMT Land Surface Planarity	Better	Acceptable
Multiple Soldering Cycles	Can withstand multiple cycles	Limited to a few cycles
No Clean Flux Usage	Limited usage	Widely used
Electrical Test Probing	Good	Poor
Corrosion Risk After Assembly	Low	Moderate
Contact Surface Application	Recommended for gold wire bonding	Recommended for copper wire bonding