Immersion Gold Flexible PCB: All You Need To Know

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Oakley Mae

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Immersion gold flexible PCB manufacturing has become a norm in the industry.

More than ever, the design of electronics operating in harsh environments has increased.

Therefore, manufacturers have been forced to coat flexible PCBs with different materials to protect and survive in different operation conditions. 

And that is why immersion gold comes in handy as the best surface finish solution.

Apart from limiting copper oxidation, immersion gold also improves electrical performance.

But what is immersion gold for flexible PCBs, and how is it different from other options? 

Today’s piece explores everything relating to immersion gold flexible PCB. Let’s get rolling. 

Table of Contents

What Is Immersion Gold Flexible PCB?

Flexible circuit boards are designed with conductive copper layers to transmit power and signals.

However, when exposed to oxygen, copper undergoes oxidation to form an oxide layer.

This oxide layer negatively affects flexible circuit board performance. 

Therefore, there is a need to cover the layer with a material that limits copper surface oxidation.

Also, there is a need to create a solderable PCB surface for components.

In this case, immersion gold or electroless nickel immersion gold is the perfect surface finish for flexible PCBs. 

Flexible PCB manufacturers prefer immersion gold to other alternatives since it offers a durable, lead-free finish.

The finish layer is always about 120-240 microns of nickel placed below 2 to 8 microns of gold. 

What Is Immersion Gold Rigid-flex PCB?

Female engineers working on circuit boards

(Female engineers working on circuit boards)

Rigid-flex PCBs are just a special class of flexible circuit boards combining flexible and rigid materials.

Therefore, regardless of the board’s number of layers, you must use a surface finish material to protect the conductive copper.

And just like with flexible circuit boards, there is no better option than immersion gold for use in rigid-flex boards

Immersion gold for rigid-flex PCBs also involves applying about 120-240 microns of nickel and 2-8 microns of gold.

As a result, you’ll be guaranteed that your rigid-flex PCB copper layer will be free from oxidation and provide a better solderable surface. 

How Does Immersion Gold Work?

Circuit board with Immersion gold surface finish

(Circuit board with Immersion gold surface finish)

Immersion gold features a layer of nickel plating clad with an immersion gold layer.

Manufacturers then employ displacement to create a gold layer over nickel.

The process will go on until the nickel covers all the generated gold. 

The aim is to prevent copper and nickel layers from oxidizing and limiting the PCB conduction properties.

Generally, immersion gold is highly durable and can last for many years. 

Immersion Gold Plating Process

First, clean the flexible PCB surface you want to plate to remove dirt and oil.

Therefore, you have the option of stripping or polishing the board.

However, most manufacturers use abrasive material, water, or solvents to clean the surface. 

The next step involves micro etching, removing the copper surface roughness.

This helps to improve the bonding of the plated nickel and copper layers. 

Next, you activate immersion gold by employing different parameters to regulate the plating process.

For example, you can determine the nickel layer thickness. You can then immerse the nickel-plated pad in acidic gold water.

As a result, gold will displace from the solution through the chemical displacement reaction. 

Finally, clean off any dirt on the pad surface. 

Properties of Electroless Nickel Immersion Gold Flexible PCB

A complete beautiful flexible PCB ready for project design

(A complete beautiful flexible PCB ready for project design)

Immersion gold is a top-level surface finish that has the following properties:

Excellent oxidation resistance: Immersion gold presents an excellent oxidation resistance feature thanks to the thick layer of gold that protects nickel and copper from oxidation.

Therefore, you’ll be guaranteed that your flexible board won’t experience any corrosion issues. 

High-temperature resistance: Immersion gold resists high temperatures, thus perfect for applications that require high temperatures.

Also, it has an excellent thermal diffusion property. 

Lead-free: We know how harmful lead is, especially to human health and the environment.

The good news is that immersion gold is lead-free and, thus, environment-friendly.

Therefore, the manufacturer and the end user won’t expose themselves to harmful lead. 

Excellent electrical performance: Regarding the electrical performance of a circuit board, everything, including design, component placement, and surface finish, matters.

Luckily gold itself is a good conductor of electricity, thus boasting high performance. 

Long shelf life: Gold and nickel are among the most long-lasting materials.

No wonder gold is used to preserve wealth not just by individuals but also by countries.

Therefore, immersion gold means your circuit board will have a longer shelf life than if you used a different finish. 

Strong chemical properties: Immersion gold has excellent chemical properties, including coplanarity, surface planarity, and wettability.

We can attribute this to the combination of nickel and gold. 

Advantages of Immersion Gold Flexible PCB

A flexible circuit board dismantled from a computer

(A flexible circuit board dismantled from a computer)

Immersion gold presents many benefits, including working in different conditions.

The other advantages include the following:

Flat surface: One benefit of immersion gold is that it can fit perfectly on flat surfaces. Most manufacturers apply them in lead-free applications that require a flat surface. 

Perfect for untreated surfaces: If you leave a PCB surface untreated, it’ll immediately oxidize.

Therefore, manufacturers apply immersion gold on such surfaces to eliminate copper layer oxidation. 

Perfect for PTH: Plated through-hole contains noticeable wires from the bottom and top, making it easier to stick into the circuit.

Therefore immersion gold is ideal for PTH. 

Easy to Weld: Immersion gold develops different crystal structures, making welding easy.

And since it has soft gold, it doesn’t present any welding effects. 

You can use them for different electronics: Immersion gold is the industry’s most popular surface finish solution.

Therefore, you can apply it to almost any product you could think about, including smartphones and motherboards. 

Doesn’t need electroplating: Immersion gold will offer corrosion resistance properties without electroplating.

As a result, this reduces the cost and time spent on electroplating.

Limitations of Immersion Gold Flexible PCB

Despite the various benefits above, immersion gold also presents different downsides, as we’ll see below: 

Weaker soldering joints: Immersion gold offers inadequate wetting, responsible for solder joint weakness.

As a result, your circuit board is likely to experience weaker connections. 

Black nickel risks: Sometimes, the nickel layer might experience oxidation leading to the formation of nickel ions.

As a result, the bigger gold atoms deposit develop a rough and loose arrangement. 

In that case, the gold layer won’t entirely cover the nickel layer.

Therefore, nickel ions get exposed to air and cause rusting.

And no matter what you do, your circuit board will experience circuit board failure. 

High price: Immersion gold finish is generally more costly than other options like HASL and OSP

FAQs

What’s direct immersion gold?

Direct immersion gold involves depositing gold directly on the flexible PCB conductive copper layer.

In short, it uses immersion and electroless deposition without nickel.

And research has shown that this method experiences superior coverage than the common immersion gold method.

Also, the copper in this method won’t co-deposit with gold. 

However, the solder spreading abilities reduces with a rough copper layer. 

Can immersion gold flexible PCB oxidize?

Generally, gold doesn’t oxidize.

However, gold is naturally porous, and immersion gold with low gold plating thickness can expose nickel to oxidation.

And if nickel also becomes porous, it’ll expose the copper layer to oxidation, forming a copper oxide layer.

Therefore, you must completely seal the nickel layer to avoid exposure to air. 

Final Remarks

Immersion gold finish is among the most popular flexible circuit board surface finish.

This option is easy to weld and works well for the untreated surface.

Moreover, it’s perfect for PTH, works for different electronics, and doesn’t need electroplating. 

However, complaints exist about weaker soldering joints, black nickel risks, and high prices.

All in all, immersion gold is necessary for flexible circuit boards, especially with boasting electrical conductivity and preventing oxidation.