Understanding the Differences Between Immersion Gold and Gold Plating

When it comes to applying a thin layer of gold onto a substrate material like copper or nickel for protective and aesthetic purposes, two common methods are immersion gold and gold electroplating. While both result in a gold surface finish, there are important differences between these processes that impact the properties and applications of the final product.

What is Immersion Gold?

Immersion gold, also known as electroless gold or chemical gold, is a process that deposits a thin layer of gold onto a substrate through a chemical exchange reaction, without the use of external electrical current. The substrate, typically copper or nickel, is immersed in a heated solution containing a gold salt. The more active metal substrate displaces the gold ions out of solution, causing the gold to deposit onto the surface.

Key characteristics of immersion gold:
– Thin deposits, typically 0.05-0.2 μm
– Deposits only on exposed copper or nickel, will not build up or plate onto itself
– Pore-free, uniform coverage even on irregular surfaces
– Fast deposition rate
– Limited to small thicknesses
– Acts as a diffusion barrier to prevent copper migration

What is Gold Electroplating?

Gold electroplating is an electrolytic process that uses an external electrical current to reduce gold ions in solution and deposit them onto a conductive substrate. The substrate is connected as the cathode and immersed in a gold plating bath along with an inert anode. When current is applied, gold ions are reduced at the cathode and the goldplate builds up on the substrate surface.

Key characteristics of gold electroplating:
– Thicker deposits possible, from 0.5 μm to several microns
– Deposits onto any conductive surface
– Can build up thicker layers
– Slower deposition rate than immersion gold
– More control over thickness and deposition rate by adjusting current
– Bright, smooth, and ductile gold deposits possible with proper chemistry

Comparing Properties of Immersion Gold vs. Gold Plating

While both immersion gold and electroplated gold provide a gold surface finish, the different deposition methods result in coatings with distinct properties. Understanding these differences is crucial for selecting the appropriate process for a given application.

Property	Immersion Gold	Gold Electroplating
Thickness	0.05-0.2 μm	0.5-5+ μm
Purity	99.9%+ pure gold	Wide range, 90-99.9%+ possible
Porosity	Pore-free, uniform coverage	May have pores if not optimized
Adhesion	Excellent on copper & nickel	Very good, depends on pretreatment
Diffusion Barrier	Excellent barrier for Cu	Good barrier at higher thicknesses
Wire Bondability	Excellent	Very good
Solderability	Limited by thin coating	Excellent, thickness helps
Wear Resistance	Limited by thin coating	Very good, improves with thickness
Corrosion Resistance	Excellent	Excellent
Cost	Lower cost for thin coatings	Higher cost, more gold used

Thickness and Coverage

One of the most significant differences between immersion gold and gold electroplating is the range of thicknesses that can be achieved. Immersion gold is a self-limiting process that only allows for very thin gold layers, typically 0.05-0.2 μm. Once the substrate surface is covered with a thin gold layer, the exchange reaction stops.

In contrast, gold electroplating allows for a much wider range of thicknesses, from around 0.5 μm up to several microns or more. The thickness can be controlled by adjusting the plating time and current density. Thicker gold layers provide better wear and corrosion resistance.

While immersion gold provides excellent, uniform coverage even on irregular surfaces due to its solution-based process, electroplated gold may have some pores or defects if the plating process is not carefully optimized. However, electroplating can deposit onto any conductive surface and build up to greater thicknesses.

Purity and Composition

Immersion gold baths typically deposit high purity gold, 99.9% or greater, as a thin, pore-free layer. The purity is inherent to the displacement reaction process.

Electroplated gold can have a wider range of purity and composition, depending on the plating bath chemistry. Purity levels from 90% to 99.9%+ are possible. Lower purity electroplated gold often contains cobalt or nickel as alloying elements to improve hardness and wear resistance. Special bath chemistries can produce bright, ductile, high-purity electroplated gold deposits for applications like semiconductor packaging and connectors.

Diffusion Barrier Properties

Immersion gold is an excellent diffusion barrier to prevent the migration of copper atoms to the surface. Even at thicknesses as low as 0.05 μm, immersion gold can effectively prevent copper diffusion, making it an ideal choice for printed circuit boards and semiconductor packaging.

Electroplated gold can also act as a diffusion barrier, but its effectiveness improves with increasing thickness. Thinner electroplated gold layers may still allow some copper migration over time, while thicker layers provide a more robust barrier.

Solderability and Wire Bonding

Immersion gold and electroplated gold both offer excellent solderability and wire bonding performance. The thin, pore-free nature of immersion gold allows for easy soldering and thermosonic or ultrasonic wire bonding, making it popular for printed circuit boards and semiconductor packaging.

Electroplated gold also provides very good solderability and wire bonding, especially for thicker deposits. The increased thickness can provide a more robust surface for soldering and improve the reliability of wire bonds.

Wear and Corrosion Resistance

The thin nature of immersion gold limits its wear and abrasion resistance compared to thicker electroplated gold coatings. However, immersion gold still provides excellent corrosion resistance due to its high purity and pore-free structure.

Electroplated gold offers better wear resistance, especially at higher thicknesses. The ability to deposit thicker layers makes electroplated gold a better choice for applications that demand durability, such as connectors and switch contacts. The corrosion resistance of electroplated gold is also excellent and can be further enhanced by increasing the thickness.

Applications for Immersion Gold and Gold Electroplating

Immersion Gold Applications

• Printed circuit boards (PCBs): Immersion gold is widely used as a surface finish for PCB pads and contacts, providing excellent solderability, wire bonding, and shelf life.
• Semiconductor packaging: Thin immersion gold layers act as a diffusion barrier and provide a bondable surface for wire bonding in semiconductor packages.
• Connectors: Immersion gold can be used on connector contacts for improved durability and corrosion resistance, especially when mated with gold-plated pins.

Gold Electroplating Applications

• Connectors: Thicker electroplated gold is ideal for connector contacts that require durability and low contact resistance, such as USB, HDMI, and audio connectors.
• Switch contacts: Electroplated gold provides wear resistance and low contact resistance for switch contacts in electronic devices.
• Semiconductor packaging: Thicker electroplated gold layers are used in semiconductor packages for improved wire bonding reliability and protection of underlying metals.
• Jewelry and decorative items: Electroplated gold is widely used for decorative purposes, providing a lustrous, durable gold finish on various substrates.

Choosing Between Immersion Gold and Gold Electroplating

When deciding between immersion gold and gold electroplating for a specific application, consider the following factors:

1. Thickness requirements: If a thin gold layer (0.05-0.2 μm) is sufficient, immersion gold may be the most cost-effective option. For applications requiring thicker gold layers, electroplating is necessary.

2. Substrate material: Immersion gold is limited to copper and nickel substrates, while electroplating can deposit onto any conductive surface.

3. Coverage and surface uniformity: Immersion gold provides excellent, pore-free coverage even on irregular surfaces. Electroplated gold may have some pores or defects if not optimized properly.

4. Wear and corrosion resistance: Thicker electroplated gold offers better wear and abrasion resistance compared to thin immersion gold. Both provide excellent corrosion resistance.

5. Cost considerations: Immersion gold is generally more cost-effective for thin coatings due to its fast deposition rate and lower gold usage. Electroplating becomes more economical for thicker deposits or larger surface areas.

6. Production volume and throughput: Immersion gold has a faster deposition rate and can be more easily scaled for high-volume production. Electroplating may be more suitable for smaller batch sizes or applications requiring precise control over thickness.

FAQ

1. Can immersion gold be applied over electroplated gold?

No, immersion gold will not deposit onto an existing gold surface, as there is no chemical displacement reaction between the two. If additional gold thickness is required, electroplating should be used to build up the layer.

2. How does the shelf life of immersion gold compare to electroplated gold?

Both immersion gold and electroplated gold offer excellent shelf life, providing a stable, non-reactive surface that resists oxidation and contamination. However, the thicker layers achievable with electroplating may provide a slightly longer shelf life due to the increased gold thickness acting as a barrier.

3. Can immersion gold be used for wire bonding?

Yes, immersion gold is an excellent choice for wire bonding in semiconductor packaging and other electronics applications. The thin, pore-free gold layer provides a clean, bondable surface that is compatible with thermosonic and ultrasonic bonding processes.

4. Is it possible to selectively deposit immersion gold or electroplated gold?

Yes, both immersion gold and electroplated gold can be selectively deposited using appropriate masking techniques. For immersion gold, areas where deposition is not desired can be masked off with a resist or covered with a sacrificial metal layer that is later etched away. Electroplating allows for selective deposition using photoresist masks or by controlling the current distribution on the substrate.

5. Can immersion gold be used as a final finish for connectors?

Immersion gold can be used as a final finish for connector contacts, providing good durability and corrosion resistance. However, for applications that require higher wear resistance or thicker gold layers, such as high-cycle connectors or contacts mated with gold-plated pins, electroplated gold may be a better choice.

Conclusion

Immersion gold and gold electroplating are two valuable surface finishing processes that deposit thin gold layers onto substrates for improved protection, performance, and aesthetics. While both methods result in a gold surface, they differ in their deposition mechanisms, achievable thicknesses, and resulting properties.

Immersion gold is a fast, cost-effective process that deposits thin, pore-free gold layers onto copper and nickel substrates through a chemical displacement reaction. It provides excellent coverage, acts as a diffusion barrier, and offers good solderability and wire bonding performance. However, its thin nature limits its wear resistance and applications.

Gold electroplating, on the other hand, uses an external current to deposit gold onto any conductive surface, allowing for a wider range of thicknesses and compositions. Thicker electroplated gold layers offer improved wear resistance, durability, and corrosion protection, making them ideal for demanding applications like connectors and switch contacts.

When selecting between immersion gold and gold electroplating, consider factors such as thickness requirements, substrate compatibility, coverage and uniformity needs, wear and corrosion resistance, cost, and production volume. By understanding the unique characteristics and capabilities of each process, engineers and designers can make informed decisions when specifying gold surface finishes for their applications.