Introduction
ENIG (electroless nickel immersion gold) is a surface finish commonly used on printed circuit boards to protect exposed copper pads and improve solderability. It involves electroless plating of nickel followed by immersion plating of a thin gold layer.
This article provides a comprehensive overview of ENIG for PCBs covering:
- The composition and structure of the ENIG finish
- Pros and cons compared to other PCB finishes
- How ENIG is deposited through electroless plating
- Typical ENIG thickness and plating process steps
- Factors impacting solderability and shelf life
- ENIG removal and waste treatment
- Comparison to similar finishes like ENEPIG
- Guidelines for design, quality control, and processing
Read on for an in-depth look at everything PCB designers need to know about the popular ENIG surface finish.
ENIG Composition
ENIG is a surface coating composed of two metallic layers:
Nickel (Ni) Layer
- Intermetallic nickel-phosphorus (NiP) alloy
- Amorphous structure lacking grain boundaries
- Phosphorus content typically 7-11% by weight
- Deposited via electroless plating process
Immersion Gold (Au) Layer
- Pure gold layer
- Deposited through immersion plating displacement reaction
- Provides oxidation protection for nickel
- Visible gold color for cosmetic appearance and solderability
The combination offers a highly solderable and conductive surface resistant to corrosion and oxidation. The structure protects underlying copper from damage while enabling reliable solder joint formation.
Benefits of ENIG Finish
ENIG offers several benefits that make it a popular PCB surface finish:
Excellent Solderability
The immersion gold layer readily dissolves into molten solder, ensuring reliable wetting and solder joint formation.
Oxidation Resistance
The inert gold layer prevents surface oxidation of nickel or copper over long time periods. This preserves solderability on shelf or during assembly.
Copper Dissolution Protection
The nickel barrier layer prevents leeching of copper up into the solder joint which can cause voids or fractures.
Corrosion Resistance
The dense nickel deposit resists corrosion and environmental effects.
Wire Bonding Ability
The smooth nickel layer enables adhesion of gold or aluminum wire bonds.
Suitable for PTH/Blind/Buried Vias
The plating process allows coating inner surfaces of through holes, blind vias, and buried vias.
RoHS Compliance
ENIG contains no lead or other hazardous substances restricted under RoHS.
Thanks to these attributes, ENIG can support a wide range of PCB assembly applications from consumer products to military-aerospace.
Drawbacks of ENIG Finish
No coating is perfect, and ENIG does have some potential drawbacks to consider:
Higher Cost Compared to OSP or Immersion Tin
The nickel and gold metals required are more expensive than organic OSP or tin plating.
Difficult Stripping and Rework
The hard immersion gold layer is difficult to strip for rework and complicates depaneling.
Black Pad Risk
Insufficient nickel thickness risks gold diffusing down causing unreliable “black pad” intermetallic formation when soldering.
Thermal Fatigue Concerns
The dual-layer structure has distinct mechanical properties from solder which can lead to thermal fatigue fractures.
Waste Treatment Requirements
ENIG chemistry requires wastewater treatment for proper metal disposal.
Limited Thickness
Maximum total thickness around 6 microns constrains applications like wire bonding requiring thicker gold.
Despite these limitations, judicious quality control and design considerations allow mitigating the risks successfully in most applications.
ENIG vs Other Finishes
How does ENIG performance compare to alternate PCB finishes? Here is a brief comparison:
Vs Immersion Tin
- ENIG better shelf-life and oxidation resistance
- Immersion tin cheaper process and easier rework
Vs Immersion Silver
- ENIG avoids silver migration shorting risks
- Immersion silver lower cost and resistance
Vs OSP
- ENIG prevents copper dissolution into solder
- OSP much lower cost process
Vs Hard Gold Plating
- Thick hard gold allows wire bonding
- ENIG cheaper and avoids embrittlement issues
Vs Flash Gold
- Flash gold simplifies stripping but less oxidation protection
- ENIG avoids flash gold’s purple plague risk
The right choice depends on cost, assembly methods, rework needs, and performance requirements. ENIG provides the best all-around capabilities for many applications.
The ENIG Plating Process
The ENIG finish gets deposited through a two-step electroless plating process:
1. Electroless Nickel Plating
- PCBs immersed in nickel plating solution
- Solution contains nickel salt and reducing agent
- The reducing agent reacts, causing nickel to deposit globally
- Amorphous Ni-P alloy coating forms on all exposed metal
- Thickness typically 4-6 μm
2. Immersion Gold Plating
- PCBs immersed in solution containing gold salts
- The nickel surface acts as a reducing agent
- Gold ions exchange with nickel ions in an oxidation reaction
- Pure gold layer deposited thickness 0.05-0.10 μm
This process allows selective coating of just exposed nickel and copper surfaces without using electrical current. The controlled chemical reactions yield a uniform, conformal plating layer.
Typical ENIG Layer Thicknesses
Nickel Thickness
- 4-8 μm nickel is typical
- Thinner nickel risks gold diffusion causing black pad during soldering
- Thicker nickel is unnecessary cost without benefit
Gold Thickness
- 0.05-0.10 μm immersion gold is standard
- Thick gold not beneficial for soldering and impacts cost
- Too thin risks incomplete coverage and oxidation
Total Thickness
- Around 6 microns total is ideal (4 μm Ni and 0.1 μm Au)
- Thinner risks reliability issues
- Thicker wastes materials without improving function
With proper chemistry control and process tuning, ideal thicknesses can be maintained.
Design Rules for ENIG PCBs
To optimize results, designers should follow certain design rules when using ENIG:
- Minimum Annular Ring: 0.10mm or 4 mils
- Minimum Trace/Space: 0.15mm or 6 mils
- Minimum Hole Size: 0.20mm or 8 mils
- Solder Mask Over Bare Copper (SMOBC): Recommended for finer features
- Flux Compatibility: Ensurecompatibility with no-clean fluxes
- Mixed Finishes: Avoid combining ENIG with other incompatible finishes
Following datasheet recommendations for parameters like soldering temperature and dwell time also helps maximize joint quality and reliability.
Soldering with ENIG PCBs
The immersion gold layer in ENIG readily dissolves into molten solder during reflow or hand soldering operations. This helps form highly reliable solder joints.
Key tips for soldering include:
- Use solder alloys compatible with immersion gold
- Apply a “no-clean” solder flux
- Heat above the gold dissolving temperature of ~150°C
- Allow adequate dwell time above this temperature
- Cool gradually to avoid disturbance of wetted joints
- Visually inspect for successful wetting and meniscus formation
With proper soldering process, ENIG boards can achieve over 99% first pass solder joint yields.
Shelf Life and Solderability
The thin immersion gold deposit provides excellent oxidation protection. ENIG finished boards stored under controlled conditions exhibit long shelf lives exceeding 1 year before solderability degrades.
Key factors impacting shelf life include:
Handling Precautions
- Use clean gloves when handling bare boards
- Avoid mechanical abrasion or scratching
- Keep boards in ESD packaging before assembly
Storage Conditions
- Temperature <40°C
- Humidity <90% RH
- Avoid sulfur-containing pollution sources
Coating Durability
- Solder mask coating helps protect surface
- Spot probe locations to limit gold damage
With proper handling, storage, and coating strategies, maximum solderability retention can be achieved over extended durations.
ENIG Stripping and Rework
The refractory immersion gold layer makes ENIG more difficult to strip than common finishes like OSP or immersion tin. Two main approaches are used:
Mechanical Abrasion
- Disk sanding or wire brushing removes ENIG through mechanical polishing
- Labor intensive process only suitable for small rework quantities
- Can damage boards and components if not done carefully
Chemical Etching
- Two-step process first strips gold then nickel
- Typically uses cyanide mixtures hazardous to human health
- Generates metal-laden waste requiring treatment before disposal
- More practical for large-scale depaneling operations
For maximum yield, designs should minimize or avoid needs for stripping ENIG. But when essential, proper safety precautions are mandatory.
ENIG Waste Treatment
ENIG chemistry contains metals like nickel, gold, and phosphorus requiring specialized waste treatment:
- Wastewater gets collected in holding tanks
- Solids are filtered out through processes like flocking
- Metals are reclaimed through electrolytic recovery or ion exchange
- pH levels are neutralized before final disposal
- Sludge gets dewatered and sent for refining
Proper waste handling protects the environment and allows reclaiming and recycling valuable metals. Most PCB fabricators are equipped to treat ENIG waste streams responsibly.
ENEPIG vs ENIG
ENEPIG is an alternate finish combining electroless nickel, electroless palladium, and immersion gold. The thin palladium layer offers additional benefits:
Pros of ENEPIG vs ENIG
- Improved corrosion resistance
- Higher temperature soldering ability
- Reduced risk of black pad defect
- More process flexibility
Cons of ENEPIG
- Additional cost over ENIG
- Potential palladium contamination of solder
- More complex waste treatment
So while ENEPIG does offer some advantages, standard ENIG remains suitable for many applications where cost is a key factor.
Conclusion
This guide covered everything needed to understand ENIG as a PCB finish – from the plating process used to the nickel and gold layer structure formed. The combination of solderability, oxidation resistance, and reliability makes ENIG a versatile surface coating suitable for assemblies from consumer products to mission-critical aerospace applications.
With knowledge of ENIG’s benefits, limitations, processing, quality control, and design considerations, PCB designers can determine when it is the optimal finish choice. Although not the cheapest option, the excellent technical properties of electroless nickel immersion gold ensure it will continue seeing widespread use protecting bare copper and enabling robust solder joints across countless electronic systems.
Frequently Asked Questions
What is the minimum gold thickness for ENIG?
0.025 microns is the minimum, but 0.05 microns is recommended. Below 0.025 microns risks incomplete coverage and rapid oxidation. The maximum usable thickness is around 0.127 microns.
What nickel alloy is typically used in ENIG?
Electroless nickel phosphorous (NiP) with 7-11% phosphorous content is standard. This amorphous structure provides corrosion protection superior to crystalline nickel-boron alloys.
What is the typical roughness of ENIG coatings?
A surface roughness of 0.2 microns RMS or lower is typical for ENIG, providing a smooth finish helpful for soldering, aluminum wire bonding, and other microelectronics assembly processes.
Does ENIG prevent tin whiskers?
Yes, the immersion gold layer essentially eliminates the tin whisker risk associated with pure matte tin finishes by preventing diffusion of tin from solder through to the surface.
What are common defects seen in ENIG?
Pitting, gold embrittlement, black pad, lack of gold coverage, stains, and embedded contaminants are potential ENIG defects requiring process controls to avoid.
Conclusion
With its excellent solderability, oxidation resistance, shelf life, and longevity, ENIG has become a workhorse PCB finish for all types of electronic assemblies. By mastering the nuances of ENIG processing, quality assurance, and design integration, engineers can deploy it effectively across many applications to protect copper traces and deliver reliable solder joints over product lifetimes.
