New options for Chemical Nickel-Gold surface finish

Introduction to Nickel-Gold Finish

Nickel-gold (Ni-Au) plating is a popular surface finish used in electronics manufacturing to protect copper circuitry and components from oxidation and corrosion while providing excellent solderability. The standard nickel-gold finish consists of an electroless nickel layer (typically 3-6 μm thick) covered by an thin immersion gold layer (usually 0.05-0.2 μm).

This combination offers several advantages:

• Nickel provides a strong diffusion barrier to prevent copper migration
• Gold protects the nickel from oxidation and ensures good solderability
• Electroless nickel is a conformal coating that covers complex geometries evenly
• Immersion gold is a cost-effective, simple process compared to electroplated gold

However, the electronics industry continuously seeks to improve this crucial PCB surface finish. Challenges include:

• Minimizing cost while maintaining performance and reliability
• Adapting the finish to leadfree and high-temperature soldering
• Preventing “black pad” nickel corrosion and brittle solder joint failures
• Optimizing thickness and composition of the nickel and gold layers
• Developing more environmentally friendly nickel and gold plating chemistries

In this article, we will explore some of the latest developments and options available for enhancing the standard nickel-Gold PCB surface finish.

Recent Advancements in Nickel Plating

High Phosphorus Electroless Nickel

One way to improve the performance of electroless nickel is to increase its phosphorus content. Standard mid-phosphorus electroless nickel contains 7-9% phosphorus. High phosphorus electroless nickel has >10% phosphorus content, with some newer baths approaching 12%.

The benefits of high phosphorus electroless nickel include:

• Increased hardness and wear resistance
• Better corrosion protection, especially in acidic environments
• Higher melting point and thermal stability
• Smoother, brighter, more uniform deposits
• Potentially slower inter-diffusion with solder

Some PCB fabricators have transitioned to high phosphorus electroless nickel as their standard offering for nickel-gold finishes. The main tradeoff is slightly lower plating efficiency and deposition rate compared to mid-phosphorus baths.

Ductile Electroless Nickel

Another development is ductile electroless nickel, which contains additives that refine the grain structure and reduce the internal stress of the nickel layer. This results in a more ductile and less brittle coating.

Benefits of ductile electroless nickel include:

• Reduced risk of brittle fracture, even with thicker nickel layers
• Better resistance to thermal cycle stresses
• Improved mechanical shock and drop test performance
• Easier rework and component replacement

Ductile electroless nickel can be particularly advantageous for assembled PCBs that experience harsh mechanical and thermal stresses in applications such as automotive, aerospace, and military electronics.

Alternatives to Electroless Nickel

While electroless nickel remains the dominant nickel plating process for PCBs, there is growing interest in alternative nickel deposition methods, driven by environmental and cost considerations.

One option is electroplated nickel. Modern electroplated nickel baths can produce deposits with properties approaching those of electroless nickel. Advantages include faster plating rates, easier bath maintenance, and avoiding use of hazardous hypophosphite.

However, electroplated nickel is not an exact drop-in replacement for electroless nickel. Challenges include achieving even coverage on high aspect ratio through-holes and preventing nickel corrosion in corrosive environments under the gold layer.

Another nickel alternative being researched is electroless nickel-boron (Ni-B). Ni-B has a nanocrystalline or amorphous structure that provides very high hardness, wear resistance, and corrosion protection. However, Ni-B baths are even more complex and expensive than electroless Ni-P baths.

Advances in Gold Plating

Palladium-Gold and Palladium-Silver-Gold

To enhance the performance and cost-effectiveness of nickel-gold finishes, some PCB fabricators offer palladium-gold or palladium-silver-gold as an upgrade option.

Palladium (Pd) and palladium-silver (Pd-Ag) are applied as ultra-thin (10-30 nm) immersion layers between the nickel and gold. The benefits include:

• Palladium acts as a barrier to prevent nickel diffusion and corrosion
• Allows use of thinner gold layer (30-50 nm flash gold) while maintaining protection
• Prevents formation of weak gold-tin intermetallic compounds during soldering
• Reduces overall precious metal cost compared to thicker gold deposits

The main drawback of palladium is its tendency to absorb hydrogen, which can lead to embrittlement. However, the very thin palladium layer and short exposure time minimize this risk.

Palladium can be applied by either immersion or electroless deposition. Electroless palladium requires more complex and expensive baths but offers faster deposition rates and better deposit properties.

Gold Alloys and Nanostructured Gold

Another way to potentially improve nickel-gold finish performance is to use gold alloys or nanostructured gold deposits rather than pure gold.

Gold alloys being researched for electronics finishing include:

• Gold-cobalt (Au-Co): Harder, more wear-resistant, and more thermally stable than pure Au
• Gold-nickel (Au-Ni): Very hard and wear-resistant, with good corrosion protection
• Gold-palladium-nickel (Au-Pd-Ni): Combines barrier properties of Pd with hardness of Ni

These alloys can be deposited by immersion plating, but co-electroplating allows more control over composition and properties.

Nanostructured gold coatings have grain sizes <100 nm and can be pure gold or alloyed. The extremely fine grain structure provides higher strength, hardness, and wear resistance compared to conventional gold deposits. Nanostructured gold-nickel is a promising option, with hardness approaching that of electroless nickel.

However, gold alloy and nanostructured gold deposits are still largely in the research stage. More work is needed to scale up these processes for commercial PCB production.

Autocatalytic Gold and Gold-Nickel Plating

Most immersion gold baths are displacement reactions that are self-limiting and produce relatively thin deposits (50-200 nm). Thicker gold coatings are typically achieved by electroplating.

However, there is growing interest in electroless or autocatalytic gold plating processes for nickel-gold finishes. These baths use a reducing agent to continuously deposit gold without an external current source, enabling thicker, more uniform deposits than immersion gold.

One promising option is electroless gold-nickel plating. In this process, a thin layer of electroless nickel is first deposited onto the nickel substrate, followed by an autocatalytic gold-nickel alloy layer.

The benefits of electroless gold-nickel include:

• Thicker gold deposits (up to 1 μm) for enhanced durability and wire bonding
• More uniform coverage and better throw into deep vias and cavities
• Harder, more wear-resistant gold-nickel alloy coating
• Self-healing gold deposit that can withstand multiple reflow cycles
• Stable, easy-to-maintain bath compared to immersion gold

Electroless gold-nickel is a strong candidate to replace electroplated gold in the future, offering similar performance with simpler processing and lower cost.

Optimizing Nickel-Gold Thickness and Composition

Beyond exploring new types of nickel and gold deposits, PCB fabricators are also fine-tuning the thickness and composition of conventional nickel-gold finishes to balance cost and performance.

Thin and Ultra-Thin Nickel-Gold

One trend is toward thinner nickel-gold layers to reduce cost and improve signal integrity. Many PCB shops now offer thin nickel-gold with <3 μm Ni and <0.1 μm Au as a standard option, with ultra-thin nickel-gold (<1 μm Ni and <0.05 μm Au) available for high-frequency applications.

The benefits of thin and ultra-thin nickel-gold include:

• Lower material cost, especially gold consumption
• Reduced risk of nickel corrosion and solder embrittlement
• Better high-speed signal performance due to thinner dielectric layer
• Easier conformance to fine pitch geometries and small vias

However, thin nickel-gold may not provide sufficient durability and solderability for some applications. It is mainly suited for consumer electronics with short product lifetimes and mild operating environments.

Thick Nickel-Gold for Harsh Environments

At the other extreme, some applications demand extra-thick nickel-gold finishes for maximum protection against corrosion, wear, and extreme temperatures. Examples include automotive under-hood electronics, downhole drilling tools, and aerospace systems.

Thick nickel-gold typically has >7 μm electroless nickel and >0.2 μm hard gold (often electroplated). The thicker nickel provides a more robust diffusion barrier, while the thicker gold withstands multiple soldering and rework cycles.

However, thick nickel-gold poses challenges such as:

• Higher material and processing costs
• Risk of nickel corrosion if the gold is too thin or porous
• Embrittlement of solder joints due to excessive intermetallic formation
• Decreased signal integrity and difficulty plating small features

Therefore, careful engineering is needed to optimize the nickel-gold thickness for each specific application and reliability requirement.

Alternative Nickel-to-Gold Thickness Ratios

The standard nickel-to-gold thickness ratio for ENIG is around 30:1 to 50:1, such as 5 μm Ni to 0.1 μm Au. However, some researchers are exploring alternative ratios to enhance solder joint reliability.

One study found that a 10:1 ratio (e.g., 2.5 μm Ni to 0.25 μm Au) resulted in stronger, more ductile solder joints compared to the standard 50:1 ratio. The thicker gold layer helped absorb stresses and prevent solder joint fracture.

Another approach is to use a graduated nickel-gold ratio, with a thinner gold layer over a thicker nickel layer near the copper interface, and a thicker gold layer over a thinner nickel layer near the surface. This provides a balance of diffusion barrier protection and solder joint ductility.

However, these alternative nickel-gold ratios are not yet widely used in the PCB industry. More research is needed to validate their reliability and manufacturability.

Comparison of Nickel-Gold Finish Options

To summarize, here is a table comparing some of the key nickel-gold finish options discussed in this article:

Finish	Ni Type	Ni Thickness	Au Type	Au Thickness	Features & Applications
Standard ENIG	Mid-phos EN	3-6 μm	Immersion Au	0.05-0.2 μm	General purpose, widely used
High-phos ENIG	High-phos EN	3-6 μm	Immersion Au	0.05-0.2 μm	Harder Ni, better corrosion resistance
ENEPIG	Mid-phos EN	3-6 μm	Pd+Au or Pd-Ag+Au	0.05-0.1 μm	Thinner Au, Pd barrier, wire bonding
Ductile ENIG	Ductile EN	3-6 μm	Immersion Au	0.05-0.2 μm	Less brittle Ni, harsh environments
Ultra-thin ENIG	Mid-phos EN	0.5-1.5 μm	Immersion Au	0.03-0.06 μm	Low cost, consumer electronics
Thick ENIG	Mid-phos EN	>7 μm	Immersion or plated Au	>0.2 μm	Durable, automotive & aerospace
Electroplated Ni/Au	Plated Ni	3-7 μm	Plated Au	0.1-1 μm	Alternative to EN, thicker Au
Electroless Ni-Au	EN	3-6 μm	Autocatalytic Au-Ni	0.2-1.0 μm	Easier than plating, self-healing Au
Nanostructured Ni/Au	EN	1-5 μm	Nano Au or Au alloy	0.05-0.5 μm	Hard, wear-resistant, under development

This is not an exhaustive list, and the specific nickel-gold finish properties and applications can vary depending on the PCB fabricator’s process. Designers should work closely with their fabricator to select the optimal nickel-gold finish for their project requirements and budget.

Environmental and Safety Considerations

An important factor driving nickel-gold finish development is the need for more environmentally friendly and safer plating processes. Traditional electroless nickel and immersion gold baths contain hazardous substances such as:

• Nickel salts, which are carcinogenic and cause allergic reactions
• Hypophosphites, which are toxic and produce phosphine gas
• Cyanide salts, which are highly toxic
• Thiourea and other sulfur compounds, which are carcinogenic
• Lead and cadmium, which are toxic heavy metals

Stricter regulations and growing environmental awareness have spurred the development of “green” nickel and gold plating chemistries that minimize or eliminate these hazardous substances. Examples include:

• Nickel baths using amino acids or other organic complexing agents instead of ammonia
• Phosphorus-free electroless nickel baths based on hydrazine or dimethylamine borane
• Cyanide-free immersion gold baths using sulfite, thiosulfate, or ascorbic acid
• Lead and cadmium-free electroless nickel and immersion gold solutions

These green plating processes can provide similar deposit properties to conventional baths while improving worker safety and reducing environmental impact. However, they often require more precise control and may have higher operating costs.

As the electronics industry continues to prioritize sustainability, it is likely that green nickel and gold plating will become more widely adopted for PCB surface finishing.

FAQ

What is the most common nickel-gold surface finish for PCBs?

The most common nickel-gold finish is ENIG (electroless nickel immersion gold), which consists of 3-6 μm of electroless nickel followed by 0.05-0.2 μm of immersion gold. This provides a good balance of cost, performance, and ease of manufacturing for most PCB applications.

What are the advantages of ENEPIG over ENIG?

ENEPIG (electroless nickel electroless palladium immersion gold) has several advantages over standard ENIG:

• The palladium layer acts as a barrier to prevent nickel corrosion and solder joint embrittlement
• A thinner gold layer can be used, reducing cost while maintaining protection
• ENEPIG is more suitable for gold wire bonding due to the harder palladium surface
• ENEPIG has better thermal stability and solder joint reliability

However, ENEPIG is more complex and expensive to produce than ENIG, so it is typically used for high-reliability applications.

What is black pad and how can it be prevented?

Black pad is a defect that can occur with nickel-gold finishes, where the nickel surface appears black or gray instead of bright silver. It is caused by excessive corrosion of the nickel layer, usually due to the gold layer being too thin or porous.

Black pad can lead to poor solder joint strength and reliability. To prevent it, PCB fabricators should:

• Ensure sufficient gold thickness (>0.05 μm) and uniform coverage
• Optimize the electroless nickel bath chemistry and operating parameters
• Minimize the time between nickel and gold plating
• Use a palladium layer (ENEPIG) or thicker gold (>0.2 μm) for more protection

Black pad can also be mitigated by using a more corrosion-resistant nickel layer, such as high phosphorus or ductile electroless nickel.

How do I choose the right nickel-gold thickness for my PCB?

The optimal nickel-gold thickness depends on the specific application requirements for cost, reliability, and performance. Here are some general guidelines:

• For low-cost consumer electronics: thin ENIG (<3 μm Ni, <0.1 μm Au)
• For general purpose industrial electronics: standard ENIG (3-6 μm Ni, 0.05-0.2 μm Au