What Is ENEPIG and How Does It Compare to ENIG in PCB Finishing?

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) is a type of surface plating applied on a Printed Circuit Board to protect it from environmental factors during storage and operation. ENEPIG is prepared by depositing electroless Nickel (Ni 3-5 μm), followed by electroless Palladium (Pd 0.05-0.1μm), and an immersion Gold layer (Au 0.03-0.05 μm). It is good for solder joint strength, gold wire bonding, aluminium wire bonding, and provides low contact resistance. It can easily be deposited on almost any PCB which is why it is also called‘Universal Finishing’.

Its compatibility with advanced HDI (High-Density Interconnect) designs empowers the creation of sleeker, more compact electronics without compromising on functionality. Today, we'll unravel the intricate nuances between two remarkable contenders ENIG (Electroless Nickel Immersion Gold) and ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold). So, strap in as we embark on this enlightening journey!

How ENPIG is different from ENIG PCB Finishing:

ENIG (Electroless Nickel Gold Plating) is a 2-layer metallic coating of 2-8um of gold on 120-240 um of nickel. On the other hand, ENEPIG plating has an additional layer of Palladium when compared to ENIG plating which consists of just Nickel and Gold. ENEPIG (Electroless Nickel-Palladium-Gold Plating) is a 3-layer metal coating consisting of nickel, palladium, and gold. This additional layer helps avoid what is known as the ‘Black Pad’syndrome which is the result of corrosion caused by the Gold to the Nickel layer. In addition, Recent studies have shown that ENEPIG-coated PCBs exhibit up to 30% higher reliability in extreme temperature fluctuations compared to ENIG.

ENIG PCB Plating:

ENIG, known for its widespread adoption, has long been a hallmark of PCB manufacturing. It offers a reliable foundation with corrosion resistance and excellent solderability. However, the ever-evolving landscape of electronics demands more than just reliability; it yearns for innovation that doesn't compromise the quality of high-speed circuits.

However, ENIG surface finishes are only ever used in low-end consumer products due to their quality issues. For starters, ENIG surface finishes don’t provide reliable gold wire bonding results. This is because the interaction between tin and gold produces problematic intermetallics, resulting in reduced solder joint reliability.

ENEPIG PCB Plating:

ENEPIG is a revolutionary surface finish that elevates the game to astonishing heights. ENEPIG is a type of metal plating for PCBs. Developed around a decade ago, this kind of surface finish has gained popularity in recent years for its comparatively low cost to other types of gold plating for PCBs. The marriage of nickel and palladium brings forth superior chemical stability, protecting your PCBs against the harshest environments. ENEPIG's enchanting attributes include exceptional wire bonding capabilities and a smoother, more uniform surface, setting the stage for high-frequency applications and miniaturized designs.

Nicknamed the “universal surface finish” for its ability to be deposited on just about any PCB assembly, ENEPIG finishes are primarily used to support soldering, gold and aluminium wire bonding. Overall, In the PCB prototype process, ENEPIG and ENIG belong to the surface treatment process. The difference is that ENIG is done before the solder mask while ENEPIG is done after the solder mask.

What is ENEPIG Plating Consist of?

● Electroless nickel – Corrosion-resistant layer

● Electroless palladium – Barrier layer prevents nickel diffusion

● Immersion gold – The outermost layer provides solderability

The term ‘electroless’ refers to autocatalytic deposition without using electrical current. The metals deposit through a chemical reduction reaction. This tri-metal finish provides excellent solderability while also resisting corrosion and oxidation. It is an alternative to conventional finishes like electrolytic nickel/gold and immersion tin.

ENEPIG PCBs Fabrication Process Steps:

ENEPIG involves four layers of metal deposited on a PCB surface. These are copper, nickel, palladium,  and gold. The process of creating an ENEPIG surface finish includes the following steps:

1) Copper Activation: This process is accomplished using a displacement reaction, which makes the copper layer act as a catalytic surface.

2) Electroless Nickel: Nickel acts as a barrier layer, preventing copper from interacting with the other metals. The layer is deposited on the catalytic copper surface using an oxidation-reduction reaction. The result is a layer that is between 3.0 to 5.0 microns thick.

3) Electroless Palladium: The layer of palladium is what differentiates ENEPIG from ENEG surface finish technology, acting as another barrier layer. Palladium prevents the nickel layer from corroding and diffusing into the gold layer. This palladium is deposited on a layer with a thickness of 0.05 to 0.1 microns, depending on the application.

4) Immersion Gold: Gold is the final layer added to ENEPIG surfaces, lending the benefits of low contact resistance, protection from friction, and resistance to oxidation. Gold also preserves palladium’s solderability. Palladium on the PCB dissolves and releases electrons that reduce the gold atoms surrounding it. The gold ions then attach to the surface of the PCB, replacing some palladium ions and resulting in a relatively thin outer layer, having a thickness of  0.03 and 0.05 micrometres.

Advantages/Disadvantages of ENEPIG Finish:

1) Avoid the risk of black pad: An extra layer prevents the diffusion and migration of nickel, avoiding the corrosion of electroless nickel-phosphorus alloys. To effectively inhibit the oxidation of the nickel surface layer and prevent the welding process of the occurrence of black pad.

2)  Long-term reliability of the coating: The ENEPIG coating has high stability, and there are no obvious defects generated by SEM after aging treatment, indicating its long-term reliability.

3) Lead-free soldering: The ENEPIG coating can form solder joints with high soldering strength with lead-free solder (Sn-Ag-Cu) to achieve the same soldering strength as leaded (tin/lead) solder. It can withstand multiple reflow cycles.

4) High reliability of welding and wire bonding: ENEPIG surface treatment can obtain good welding bonding and wire bonding performance.

5) Low cost: Using Palladium allows for the reduction in the thickness of the Gold layer, which eventually reduces the cost.

Limitation of ENEPIG Surface Finish:

It is more prone to fractures due to the brittleness of the tin-palladium layers formed on top of the nickel plating.

Why Plate PCBs Using ENEPIG?

Modern PCBs continue to advance, resulting in smaller, lighter electronics that maintain maximum functionality and speed. To help improve functionality while also maintaining structural integrity, plating PCBs with a metallic finish has become a priority. Of all the plating options available, ENEPIG provides the greatest number of benefits while keeping processing and material costs relatively low. It also presents more benefits than other finishing options, like Tin, silver, ENEG and ENIG plating, including excellent solderability, durability and anti-corrosive properties.

These benefits are not only important in consumer electronics but also in a variety of other industries. ENEPIG finishes help maintain the high functionality and reliability of PCBs, making it an ideal finish for applications where reliability is a high priority. Just a few examples include the automotive, aerospace, military & defence and medical industries.

Process Challenges Associated with ENEPIG:

● Uniformity: Electroless deposition depends on local chemical conditions. Careful monitoring and tank agitation is required.

● Palladium Activation: Insufficient activation can cause non-uniform nickel deposition and gold embrittlement.

● Bath Maintenance: Regular analysis and replenishment of electroless baths is critical.

● Solder Mask Adhesion: Compatibility between solder mask and ENEPIG chemistry must be ensured.

● Via Filling: Deposits thin coating only. For thicker coatings, additional electroless copper buildup may be required.

Properties of ENEPIG:

ENEPIG has slowly grown in popularity as a plating option, with a recent surge in popularity resulting from reduced prices in raw palladium. The method is also popular due to its profound benefits, including the following:

Low Contact Resistance: Low contact resistance is essential in PCBs to prevent overheating, energy losses and poor grounding. ENEPIG finishes can guarantee this low resistance because of the controlled deposition of nickel, palladium and gold.

Protective Barriers: Both the nickel and palladium in an ENEPIG finish act as barrier layers, preventing the copper from being affected by the tin in solder. These barriers improve solderability immensely. They also prevent copper from mixing with gold and affecting conductivity.

Unlimited Shelf Life: Unlike many other plating options like silver, copper and aluminium, palladium and gold do not tarnish or oxidize. These are progressive conditions that result due to exposure to air and humidity. Both conditions result in reduced solderability and overall reduced quality of a finish, and both worsen over time.

Uses of ENEPIG Surface Finishing:

ENEPIG Surface Finishing is ideal for Printed Circuits Boards that support various package types including THT (through-hole technology), SMT, BGA, wire bonding, etc. The key applications where ENEPIG finish provides benefits are:

● Lead-free Soldering: Makes it less dangerous to use.

● Automotive Electronics: Halogen-free and withstand high temperatures.

● Avionics and Aerospace: Provide High reliability required for extreme conditions.

● Medical Electronics: Biocompatible finishing

● High-Speed Digital Circuits: Gold provides low contact resistance stability.

Conclusion:

Overall we can conclude that ENPIG is a better surface plating technique having excellent solderability, contact reliability, compatibility with lead-free solders, allows wire bonding, and provides stable low contact resistance. On the other hand, it also has some disadvantages, it requires precise process control, Palladium is expensive, And multiple plating steps increase cycle time. But overall it is a better technology which provides an optimistic price to performance ratio.