A Comprehensive Guide on ENIG PCB Surface Finish

Introduction

Printed circuit boards (PCBs) provide the foundation for mounting and interconnecting electronic components in products ranging from consumer electronics to high-end telecom and aerospace applications. The exposed copper pads and traces on PCBs readily oxidize when exposed to ambient environments, which severely impairs solderability and reduces product reliability over time. Therefore, metallic surface finishes are commonly applied on PCBs to protect the copper features from oxidation and enhance solderability.

Electroless nickel immersion gold (ENIG) is one of the most popular and versatile PCB surface finishes in use today. It provides excellent solderability along with unmatched corrosion and oxidation resistance. This article provides an in-depth overview of ENIG technology and its application for protecting PCBs against oxidation while maintaining reliable long-term solderability.

What is ENIG PCB Surface Finish?

ENIG refers to the deposition of a thin layer of immersion gold over electroless nickel plating on the copper pads and traces of a printed circuit board. It is a dual-layer metallic coating that leverages the properties of nickel and gold to protect the exposed copper surfaces on PCBs.

The key characteristics of ENIG finish are:

• An electroless nickel underlayer of around 3-6 μm thickness is deposited first on the PCB copper by means of an autocatalytic chemical reaction.
• This is followed by depositing a thin immersion gold top layer of 0.03-0.15 μm thickness atop the nickel layer through a galvanic displacement reaction.
• The intermediate nickel layer acts as a diffusion barrier to the copper and also provides a stable, solderable metallic foundation.
• The outer gold layer serves to prevent oxidation of the nickel layer and maintains excellent solderability over prolonged durations.
• The dual-layer ENIG finish thus resists oxidation and corrosion effectively to provide a long shelf-life before PCB population and soldering. The immersion gold layer also facilitates formation of highly reliable solder joints without any wetting issues.

This combination of corrosion resistance and outstanding solderability makes ENIG one of the most versatile and reliable surface finishes for PCBs across a wide array of applications.

Process Steps for Electroless Nickel Immersion Gold (ENIG) Plating

The ENIG plating process involves multiple steps carried out in sequence to produce the two metallic layer depositions that make up the ENIG finish:

Surface Preparation

This first phase involves thorough cleaning and etching of the PCB’s copper features to remove oils, oxides and surface films that can otherwise hinder plating adhesion:

• Alkaline Cleaning – This removes particulates, greases and organic contaminants using detergent solutions.
• Microetching – A quick dip in an etchant like sulfuric acid/hydrogen peroxide roughens the copper surface and removes a tiny top layer of copper, improving plating adhesion.
• Acid Dip – This step removes any residual oxides, surface films or etching solution residues that may be left from prior process steps.

Catalyzation

This step activates the PCB’s copper surfaces in preparation for the subsequent electroless nickel plating:

• A dilute aqueous solution containing palladium chloride complexes deposits a thin layer of palladium catalyst particles on the copper surfaces of pads/traces.
• The palladium seeds serve to initiate the autocatalytic deposition reaction used for electroless nickel plating.
• The catalyst solution is maintained around 30-50°C with a pH of 1-3 for optimal activation.

Electroless Nickel Plating

This stage involves the autocatalytic (no external power) electroless deposition of a nickel layer around 3-6 μm thickness on the PCB’s copper features:

• The nickel plating solution contains nickel sulfate as the metal source along with reducing agents like sodium hypophosphite.
• The plating reaction proceeds spontaneously at a deposition temperature between 75-95°C based on the bath formulation.
• A mildly acidic pH of 4.5-6.0 is maintained to obtain a suitable plating rate.
• The resulting nickel deposit provides an excellent solderable metallic coating while also acting as a diffusion barrier over the copper.

Immersion Gold Deposition

Following the nickel plating, immersion gold deposition produces the thin protective gold overlayer:

• The PCB panel is simply immersed in an acidic immersion gold plating solution containing a gold salt.
• The gold ions undergo galvanic exchange with nickel atoms to spontaneously deposit a thin gold layer around 0.03-0.15 μm thickness.
• No external electrical current is required unlike electrolytic gold plating methods.

After plating, thorough rinsing steps are performed to remove chemical residuals from the PCB surface. The board is now ready for population and soldering, with the ENIG finish protecting it against oxidation during production and enhancing solder joint reliability in the field.

Key Benefits of ENIG as a PCB Surface Finish

ENIG plating provides a number of important advantages that make it one of the most versatile PCB surface finishes in use today:

• Oxidation Resistance – The thin immersion gold layer effectively prevents oxidation of the underlying nickel layer. This enables excellent shelf-life with minimal loss of solderability over extended durations.
• Solderability – The immersion gold surface allows highly reliable and consistent solder joint formation across pad interfaces.
• Wire Bondability – The gold layer enables high-yield gold wire bonding of integrated circuits onto ENIG coated copper pads on the PCB.
• Wear Resistance – The hard gold coating strongly resists mechanical abrasion damage during PCB handling, connector mating cycles, card insertion etc.
• Corrosion Resistance – Gold provides unmatched protection against harsh industrial contaminants, polluted environments and humid/saline operating conditions.
• Conductivity – The gold surface maintains the highest electrical conductivity between joints without any corrosion induced conductivity degradation over time.
• Coplanarity – The thin uniform gold deposit maintains excellent pad coplanarity, which is essential for reliably assembling fine-pitch and leadless components.
• Lead-free Compatibility – ENIG finish is fully compatible with lead-free solder alloys, reflow profiles and assembly processes.
• RoHS Compliance – ENIG meets RoHS requirements for elimination of hazardous substances like lead, making it an eco-friendly surface finish.

In summary, ENIG finish provides a highly solderable, conductive and durable PCB surface while protecting against corrosion, abrasion or tarnishing over long-term use. This makes it a versatile finish suitable for a diverse range of demanding applications.

Limitations and Disadvantages of ENIG Surface Finish

While providing excellent properties, ENIG finish does have certain limitations and disadvantages:

• Relatively higher cost compared to immersion tin or organic solderability preservative (OSP) finishes since it utilizes expensive gold.
• The thickness of the immersion gold layer needs tight control between 0.03-0.15 μm, since excessively thick gold risks embrittlement while thin gold provides inadequate protection.
• Rework is challenging on ENIG boards since the hard gold deposit is difficult to penetrate by soldering irons for component removal. Complete pad metal stripping is required before resoldering.
• The interface between nickel and gold layers is susceptible to corrosion, especially when there is phosphorus contamination. This can result in brittle “black pad” defects.
• The electroless nickel bath composition and plating conditions require careful monitoring and control to minimize phosphorus co-deposition on the nickel surface.

However, these limitations of ENIG finish can be largely addressed through proper process control and thickness optimization during plating. Overall, the benefits of ENIG far outweigh its disadvantages for most applications.

RoHS Compliance for ENIG Surface Finish

RoHS stands for Restriction of Hazardous Substances, which prohibits the use of hazardous materials like lead, mercury, cadmium, hexavalent chromium etc. in electronic products sold in Europe.

ENIG plating is fully RoHS-compliant since the process does not utilize any restricted substances in the plating chemistry. The immersion gold bath contains gold salts like potassium gold cyanide. The electroless nickel plating formulation also does not introduce lead or other toxins.

By eliminating lead and other hazardous substances, ENIG meets RoHS requirements for eco-friendly PCB fabrication. This has contributed greatly to its widespread adoption across consumer, telecom, automotive, medical and defense electronics sectors.

Materials Used for ENIG Plating

The ENIG plating process primarily utilizes:

• Electroless nickel plating solution containing nickel sulfate as the nickel source and reducing agents like sodium hypophosphite to deposit the intermediate nickel layer.
• Immersion gold solution containing gold salts such as potassium gold cyanide which exchange with the nickel surface atoms to form the thin gold protective outer finish.

No hazardous, restricted or toxic materials are introduced in either of the plating steps. The gold layer thickness is tightly maintained in the 0.03 to 0.15 micron range to obtain optimal solderability and protection without embrittlement issues.

Comparison Between ENIG vs HASL Surface Finishes

ENIG and hot air solder leveling (HASL) are two alternative PCB surface finishes with significant differences:

• Process Complexity – ENIG involves multiple stages of chemical cleaning, etching, plating etc. whereas HASL only dips boards in molten solder followed by hot air knife leveling.
• Environmental – ENIG is lead-free and RoHS-compliant while HASL allows the use of leaded solders.
• Reparability – Reworking and removing components is easier on HASL PCBs versus ENIG boards where the gold layer is difficult to penetrate by soldering irons.
• Solderability – HASL suffers from solder oxidation over shelf life which can impair wetting. ENIG maintains consistent solderability over prolonged storage.
• Coplanarity – The waved HASL deposit provides relatively poor coplanarity while ENIG gives an excellent flat pad surface.
• Temperature/Corrosion Resistance – ENIG withstands much higher temperatures and corrosive environments versus HASL.
• Cost – HASL is a cheaper process while ENIG has higher material and processing costs due to the use of gold.

In essence, ENIG provides far superior solderability, bondability and environmental resistance than HASL finish, albeit at a higher cost.

Comparing ENIG and ENEPIG Surface Finishes

ENEPIG finish enhances ENIG by introducing an additional electroless palladium layer between the initial nickel plating and the final immersion gold layer. This provides several advantages compared to ENIG:

• The thin palladium coating acts as an effective diffusion barrier over the nickel, which eliminates the risk of black pad corrosion defects plaguing ENIG.
• ENEPIG allows for thicker immersion gold layers to be deposited since the palladium prevents interface corrosion between gold and nickel.
• The palladium strengthens bonding with the nickel underlayer and final gold coating, providing better adhesion.
• ENEPIG withstands repetitive and aggressive soldering excursions along with extreme thermal cycling without blistering or delaminating.
• Wire bonding pull strengths exhibit greater consistency versus ENIG due to the palladium adhesion layer.

However, ENEPIG has a higher cost due to the additional palladium plating step. Control of the tri-layer deposition thicknesses also adds complexity. But for maximum reliability, ENEPIG outperforms ENIG.

Soldering Issues with ENIG Surface Finish

While ENIG offers excellent and consistent solderability over prolonged storage durations before assembly, certain soldering defects can still occur:

Non-Wetting – The solder paste fails to properly wet and spread across the ENIG coated pads during reflow soldering. Potential root causes are:

• Thick immersion gold exceeding 0.15 μm. This prevents solder alloy interaction with the nickel layer.
• Contamination from finger oils, dust or chemicals during handling. This interferes with wetting.
• Corrosion of the nickel layer stemming from plating chemistry issues. This manifests as black pad defects which impair solderability.

Solder Cracking – Cracks within solder fillets and joints on ENIG finished pads. This loss of adhesion can arise due to:

• Excessive hardness of thick gold layers above 0.15 μm thickness which induces stresses.
• Black pad defects creating a weak intermetallic region between the solder and pad finish.
• High phosphorus co-deposition in nickel plating bath decreasing ductility.

Mitigation

• Tight control of ENIG layer thicknesses and bath chemistries during plating processes.
• Validation of thickness uniformity across PCBs using microsectioning and X-ray fluorescence measurements.
• Correct handling procedures to prevent ENIG surface contamination prior to soldering.
• Solderability testing using wetting balance tests to identify any non-wetting issues.