Introduction

Printed circuit boards (PCBs) are essential components in almost all modern electronic devices. They provide the platform for mounting and interconnecting electronic components using conductive tracks and traces etched from copper sheets laminated onto a non-conductive substrate.

One of the most critical aspects of PCB fabrication is the surface finish – the coating applied to the bare copper traces to protect them from corrosion and improve solderability. The surface finish acts as the interface between the PCB trace and the solder joint that attaches electronic components. An optimal finish promotes excellent wetting during soldering and forms reliable solder joints for robust connections.

Over the years, a variety of metallic surface finishes have been developed and deployed in PCB fabrication. These include Hot Air Solder Leveling (HASL), Immersion Silver, Immersion Tin, Electroless Nickel Immersion Gold (ENIG) and many more. Each finish has its strengths and limitations depending on the application.

One of the more popular options in recent years is the Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) finish. As the name suggests, it is composed of three layers – a layer of electroless nickel covered by electroless palladium and finally immersion gold on the outer surface.

This article provides a comprehensive overview of ENEPIG – how it is processed, its characteristics and performance profile in different situations, comparisons with other finishes and more. We will also look at the pros and cons of ENEPIG and its applications where it shines.

What is ENEPIG?

ENEPIG refers to an advanced PCB surface finish composed of three metallic layers sequentially deposited on the copper traces. The layers are:

• Electroless Nickel (EN) – an alloy of 90% nickel and 10% phosphorus deposited using an auto-catalytic chemical process.
• Electroless Palladium (EP) – a pure palladium layer deposited using an electroless plating chemical process.
• Immersion Gold (IG) – a pure gold layer deposited by immersing the PCB in a solution of a gold salt.

The electroless nickel and palladium layers are deposited using auto-catalytic chemical reactions while the gold layer is deposited using a simpler immersion process. The three layers provide an excellent balance of solderability, corrosion resistance, and wire bondability.

Why use a multilayer finish?

A multilayer finish like ENEPIG provides synergistic benefits compared to single layer finishes:

• The electroless nickel provides excellent corrosion resistance to the copper traces. Nickel alloys have inherently higher corrosion resistance than copper.
• Palladium and gold outer layers provide excellent solderability and ductility. This improves shelf life and withstands thermal cycling stress better.
• The palladium acts as a diffusion barrier preventing the nickel and gold layers from interdiffusing or dissolving into each other. This maintains soldering performance.
• Overall, ENEPIG provides higher reliability and consistent performance over a long lifetime compared to single layer finishes.

Typical layer thicknesses

The ENEPIG layers have the following typical thicknesses:

• Electroless nickel – 4 to 8 micrometers
• Electroless palladium – 0.05 to 0.3 micrometers
• Immersion gold – 0.05 to 0.1 micrometers

The nickel layer is thickest as it provides the core corrosion protection while the palladium and gold layers are very thin, just enough to coat the nickel uniformly underneath.

Cost considerations

ENEPIG provides excellent reliability and performance but comes at a higher cost than simpler finishes like HASL or immersion tin:

• The electroless plating chemicals required are more expensive.
• More process steps are involved necessitating additional equipment.
• The palladium and gold metals used are inherently more costly.

However, the cost premium is justified in mission-critical applications where the improved solder joint reliability outweighs the higher processing costs.

Why use ENEPIG?

ENEPIG provides some unique benefits that make it an appealing choice in a variety of applications:

Excellent solderability

The ENEPIG finish promotes superb wetting when soldering components onto the PCB traces. The outer gold layer dissolves rapidly into the solder, ensuring very high quality solder joints.

This makes ENEPIG well-suited for advanced soldering processes like lead-free soldering which require excellent wetting.

Corrosion resistance

The thick electroless nickel coating provides maximum corrosion resistance to the underlying copper. This protects the PCB over long usage periods and during exposure to corrosive atmospheres.

ENEPIG is often preferred for aerospace applications where resistance to high-altitude atmospheric corrosion is desired.

Wire bondability

The gold outer layer also permits wire bonding – connecting silicon dies directly to the PCB using fine gold wires. This is used in advanced packaging like multi-chip modules (MCMs).

Lead-free solder compatibility

ENEPIG is fully compatible with lead-free solders which have become the norm following the ban on lead usage. Many finishes like HASL are incompatible. The gold interacts readily with lead-free solders to form reliable joints.

Shelf life

The inert gold outer layer allows ENEPIG finished boards to remain unsoldered for over 12 months without losing solderability. Other finishes like immersion tin degrade faster. This makes ENEPIG suitable where long shelf lives are needed.

Thermal cycling reliability

Testing has shown ENEPIG to withstand over 1000 cycles of thermal stress (-65 °C to 125 °C) with minimal degradation. This is crucial for automotive under-hood electronics and other applications requiring thermal cycling endurance.

Flexible & rigid-flex PCBs

ENEPIG works very well on flexible printed circuits (FPCs) and rigid-flex boards where the finish must withstand repeated flexing stresses. The nickel/palladium/gold stack remains highly ductile and resistant to cracking.

ENEPIG plating process

The ENEPIG layers are sequentially deposited on the PCB using precise auto-catalytic chemical processes. Here is an overview of the plating process steps:

Surface preparation

The copper traces must first be properly cleaned to remove all surface contamination like oils and oxides. Standard cleaning involves:

• Alkaline cleaning to remove organic residues
• Microetching to strip surface oxides
• Acid cleaning to eliminate surface contaminants

This prepares the traces for the optimal electroless plating of the nickel layer.

Electroless nickel plating

Electroless nickel plating involves submerging the PCB in a heated nickel plating bath containing nickel salts and a reducing agent like sodium hypophosphite.

The plating chemistry utilizes an auto-catalytic reaction where the reducing agent reacts with the nickel ions at the copper surface to deposit nickel metal. The deposited nickel acts as a catalyst to continue the reaction.

Key process parameters:

• Bath temperature – 85 to 90 deg C
• pH – 4.5 to 5.5
• Plating time – depends on desired thickness

The nickel forms a hard, corrosion resistant alloy coating on the copper traces.

Electroless palladium plating

This deposits a thin layer of pure palladium on top of the nickel using a similar auto-catalytic plating reaction. The PCB is immersed in a heated solution containing palladium salts and a reducing agent.

Key parameters:

• Bath temperature – 50 to 60 deg C
• pH – 5 to 7
• Plating time – to achieve 0.05 to 0.3 micron thickness

The palladium provides excellent solderability and inhibits nickel corrosion.

Immersion gold plating

The final gold layer is deposited by simply immersing the PCB in a room temperature solution containing a gold salt. The gold ions exchange with palladium atoms at the surface, thereby depositing a pure gold layer.

Post-treatment

The plated boards will undergo a final rinse and drying process before solder mask application and other downstream pcb fabrication steps.

ENEPIG properties

The electroless nickel, palladium and gold layers each impart distinct properties that together provide an optimized PCB finish:

Electroless nickel properties

• Hardness – 550 to 650 HV (50 to 60 Rockwell C)
• Tensile strength – 550 to 700 MPa
• Elongation – 2 to 12%
• Melting point – 1450 deg C
• Density – 7.9 g/cm3
• Resistivity – 15 to 25 microOhm-cm

Nickel-phosphorus alloys have excellent hardness, strength and temperature resistance. The moderate resistivity provides good conduction. Overall, nickel protects the copper traces from corrosion and mechanical wear.

Electroless palladium properties

• Purity – 99.95% or higher
• Density – 12 g/cm3
• Melting point – 1555 deg C
• Resistivity – 10.8 microOhm-cm

Palladium is a precious metal similar to platinum that has excellent corrosion resistance. It is very ductile and interacts readily with solders. The high purity electroless palladium layer improves solderability and shelf-life.

Immersion gold properties

• Purity – 99.9% or higher
• Density – 19.3 g/cm3
• Melting point – 1064 deg C
• Resistivity – 2.44 microOhm-cm

Gold is well known for its chemical inertness and conductivity. The thin immersion gold layer maintains the solderability of the finish while also allowing wire bonding.

Together, the ENEPIG stack provides the ideal engineering properties for a high-reliability PCB finish.

ENEPIG Finish vs Other Finishes

ENEPIG has both advantages and disadvantages compared to some other common metallic PCB finishes:

vs Immersion tin (IS)

Advantages

• Better shelf life – 12+ months vs 3-6 months for IS
• Withstands higher temperature soldering
• More resistant to thermal cycling stresses

Disadvantages

• More expensive process chemistry
• Slower plating rate than IS

vs Immersion silver (ImAg)

Advantages

• Better resistance to sulfur contamination and tarnishing
• More ductile – suits flex PCB applications

Disadvantages

• Slightly more expensive than ImAg

vs OSP (Organic Solderability Preservative)

Advantages

• Inorganic finish with longer shelf life
• Withstands multiple soldering heat cycles
• Lower long-term cost – saves reprocessing steps

Disadvantages

• Higher initial processing cost than OSP

vs HASL (Hot Air Solder Leveling)

Advantages

• Compatible with lead-free solders unlike leaded HASL
• Lower thermal stress on components during soldering
• Even finish thickness and consistency

Disadvantages

• Cannot match ultra-low cost of HASL
• Longer processing time

vs ENIG (Electroless Nickel Immersion Gold)

Advantages

• Added electroless palladium layer improves shelf life
• Better wire bondability
• Lower long term cost

Disadvantages

• Slightly more expensive than ENIG

Overall, while ENEPIG costs more than the simpler single layer finishes, it provides the best all-round performance that justifies the extra expense in many applications.

ENEPIG Process Control

To consistently achieve high quality ENEPIG coatings, careful process control is required:

Bath Chemistry

The plating bath chemical composition must be maintained optimally:

• Regular analysis of metal ion concentrations
• pH monitoring and adjustment
• Replenishment of reducing agents
• Filtration to remove suspended contaminants

This ensures deposition rates and material quality are within specifications.

Temperature Control

Precise temperature regulation is critical as deposition rates are highly temperature dependent:

• Heaters and chilled recirculation for nickel and palladium baths
• Thermocouples and control systems for rapid measurement and adjustment

Agitation

Vigorous solution agitation is needed to transport metal ions quickly and uniformly to the PCB surfaces. Agitation methods include:

• Air sparging
• Mechanical pumps
• Solution recirculation

Hull cells

Test hull cell panels can be periodically plated and analyzed to dial in optimal currents and operating windows. This also provides early warning of any process issues.

Thickness measurement

Plating thickness needs to be regularly verified using techniques like X-ray fluorescence (XRF). This monitors any drifts from specifications.

ENEPIG Troubleshooting

Like any chemical process, problems can occur with ENEPIG deposition. Some common issues and remedies include:

Rough, powdery deposits

Causes:

• Low bath temperature
• High pH
• Depleted reducing agent
• Contaminated solution

Fixes:

• Increase temperature to specified range
• Adjust pH to optimum
• Replenish reducing agent concentration
• Filter solution and replace if needed

Thin, incomplete coatings

Causes:

• Insufficient plating time
• Low metal ion concentrations
• Incomplete surface prep

Fixes:

• Increase plating duration
• Replenish metal ion levels in bath
• Ensure suitable alkaline/acid cleaning

Solderability issues

Causes:

• Insufficient gold thickness
• Surface contamination
• Palladium or gold bath problems

Fixes:

• Check immersion gold deposition rate
• Improve post-plating cleaning
• Analyze bath chemistry and rectify issues

Corrosion spots

Causes:

• Pinholes, inclusions or porosity in nickel layer
• Scratches from handling
• Localized bath chemistry imbalance

Fixes:

• Increase nickel thickness
• Improve component handling
• Ensure uniform solution agitation

ENEPIG Applications

Some of the key application areas where ENEPIG shines are:

Aerospace and Aviation Electronics

The excellent corrosion resistance satisfies stringent aerospace requirements. ENEPIG withstands the temperature extremes and jet fuel exposure encountered.

Automotive Under-the-Hood Electronics

Provides thermal cycling durability and temperature resistance for engine control units and other automotive subsystems.

High-Reliability Telecom/Networking Equipment

Ideal for enterprise grade routers, switches, base stations etc. where long service life and field reliability are critical.

High-End Consumer Electronics

Used to enhance the reliability of gaming devices, wearables, smart home hubs etc. where consumer satisfaction is crucial.

Medical Electronics

Strict FDA regulations demand maximum reliability for medical devices. ENEPIG offers very high MTBFs (Mean Time Between Failures)

Flexible and Rigid-Flex PCBs

ENEPIG withstands repeated bending without cracking. Well suited for flex PCBs in gadgets and other movable applications.

Lead-Free Soldering Processes

Fully compatible with lead-free solders. Provides excellent wetting during reflow soldering.

Wire-Bonded Components

The gold layer allows wire bonding of silicon dies directly onto the PCB traces.

ENEPIG Finish Process Summary

To recap, here is an overview of the key steps in the ENEPIG finish process:

1. Base copper is chemically cleaned and microetched
2. Electroless nickel plating onto copper using auto-catalytic process
3. Electroless palladium plating onto nickel also via auto-catalysis
4. Immersion gold – gold ions exchange with palladium
5. Post-treatment rinsing and drying
6. Strict process control of bath chemistry and operating conditions
7. Quality verification using hull cells, XRF thickness measurement etc.

Conclusion

The ENEPIG finish comprising electroless nickel, palladium and immersion gold layers provides an advanced surface finish option for PCB fabricators.

Compared to simpler single layer finishes, ENEPIG delivers:

• Maximum shelf life and solderability
• Excellent corrosion resistance
• Wire bondability
• Thermal cycling durability
• Compatibility with lead-free solders

These characteristics make ENEPIG well suited for mission-critical and high-reliability electronics applications despite the higher cost.

With meticulous process control and monitoring, ENEPIG can be implemented reliably in production to satisfy the most demanding customer requirements.

Frequently Asked Questions

What is the typical cost premium for using ENEPIG vs HASL?

The ENEPIG process requires more expensive chemistry and additional process steps, so it typically costs around 25-50% more than a leaded HASL finish. However, the far superior soldering performance and reliability justify the extra cost for critical applications.

What PCB pad geometry works best with ENEPIG?

ENEPIG can be used with a wide range of pad designs. It is not constrained by the need for “solder dams” like HASL. In general, pad geometries optimized for lead-free soldering work well. Lands with an oval shape help manage thermal stress.

Can ENEPIG be stripped and reworked?

Yes, the ENEPIG finish can be chemically stripped using solutions that dissolve the gold and nickel layers selectively. However, re-plating will never be quite as good as the original finish. Minimal rework should be the goal.

Does ENEPIG contain any lead?

No, ENEPIG is a completely lead-free finish. This makes it ideal for high-reliability electronics where contamination risks must be minimized. The electroless plating process does not use any lead at all.

Is ENEPIG RoHS compliant?

Yes, ENEPIG meets all Restriction of Hazardous Substances (RoHS) regulations globally since it does not