In modern PCB (Printed Circuit Board) fabrication, the choice of surface finish plays a crucial role in ensuring reliability, performance, and manufacturability of electronic assemblies. Among the available surface treatments, gold-based finishes, such as electroplated gold (hard and soft), ENIG (Electroless Nickel Immersion Gold), and flash gold, are particularly significant due to their exceptional conductivity, corrosion resistance, and solderability. Below are the gold finishes' distinctions, applications, and production methods to help engineers and designers make informed decisions.
Electroplated Nickel Gold (Electroplated Gold)
Electroplated gold, commonly called "nickel gold plating," is achieved by first electroplating a layer of nickel on the copper traces of a PCB, followed by a layer of gold. This nickel layer acts as a diffusion barrier, preventing direct interaction between copper and gold, which could otherwise result in undesirable intermetallic diffusion due to their differing electrochemical potentials.
Electroplated gold can be subdivided into hard gold and soft gold, depending on the final gold composition.
Hard Gold vs. Soft Gold: Key Differences
1. Hard Gold (Electroplated Gold Alloy):
Hard gold is formed by alloying gold with other metals such as nickel or cobalt, which increases its surface hardness and wear resistance. It is ideal for applications that require frequent mechanical contact, such as:
- Edge connectors
- Gold fingers
- Switch contacts
- Keypads
The increased hardness ensures a longer lifespan under repeated insertions and friction. The manufacturing process for hard gold involves:
Acid cleaning → Nickel electroplating → Flash gold (pre-plating) → Electroplating gold alloy (typically gold-nickel or gold-cobalt)
2. Soft Gold (Pure Gold Electroplating):
Soft gold, as the name suggests, is a layer of pure gold that lacks alloying elements, resulting in a softer finish. It is primarily used in:
- COB (Chip-On-Board) wire bonding (especially with aluminum wires)
- Contact points in flexible or delicate circuits
- High-frequency RF components
The absence of alloying materials makes it suitable for forming reliable bonds with aluminum wires. However, it is less resistant to abrasion and mechanical wear compared to hard gold. The soft gold plating process generally includes:
Acid cleaning → Nickel electroplating → Pure gold electroplating
At PCBYES, with more than 10 years in the field of PCB prototype and fabrication, we have developed advanced capabilities to deliver both hard and soft gold finishes with precise control over plating thickness and uniformity. Our engineers ensure your application requirements—whether durability or bondability—are matched with the right gold finish.
ENIG: Electroless Nickel Immersion Gold
ENIG (Electroless Nickel Immersion Gold) is a chemically-applied surface finish widely used in high-density PCBs due to its planarity and solderability. It involves:
- An electroless nickel layer chemically deposited on copper
- A thin layer of immersion gold that replaces some of the nickel atoms via chemical displacement
Key benefits of ENIG include:
- The flat surface is ideal for fine-pitch components like BGA and QFN
- Excellent oxidation resistance
- No need for electroplating equipment
However, ENIG gold layers are typically thinner than electroplated gold (3–5 micro-inches), and the gold content diminishes closer to the base layers due to the displacement nature of the process. It is still classified as pure gold, thus soft and wire-bondable, though manufacturers must ensure a minimum thickness to maintain reliable aluminum wire bonding.
One limitation of ENIG is that it generally cannot reach the high gold thickness levels of electroplated gold, which can easily exceed 15 micro-inches. However, ENIG remains cost-effective for mass production, especially in consumer electronics, mobile devices, and HDI PCBs.
Flash Gold (Pre-Plated Gold)
Flash gold, or "flash plating," refers to a thin layer of gold electroplated rapidly on top of the nickel layer, typically used as an intermediate step before plating a harder gold alloy. The process uses:
- High current
- High-concentration gold solution
This produces a thin but fine-grained gold layer that prepares the surface for subsequent alloy plating (e.g., gold-nickel or gold-cobalt). In some cases, PCBs are marketed with only this flash gold layer due to its reduced cost and quick turnaround time.
However, flash gold has critical limitations:
- Extremely thin gold layer (often <1 μ")
- Poor oxidation resistance over time
- Inferior solderability if stored for extended periods
Because flash gold lacks the final hard plating step, it is not suitable for critical applications requiring durability or long-term stability. It's important to verify the specifications before choosing a flash gold finish for production.
Application Considerations and Cost Trade-offs
The cost of electroplated nickel gold is generally higher than other surface finishes like ENIG, OSP (Organic Solderability Preservative), or HASL (Hot Air Solder Leveling). The high price of gold and the additional electroplating steps make it less favorable for high-volume applications unless performance requirements justify the investment.
Typical use cases where electroplated gold is essential include:
- Edge connectors and card-edge contacts
- Sliding contacts
- Wear-resistant switch pads
Meanwhile, ENIG is the preferred choice for:
- BGA pads
- High-density interconnects (HDI)
- Multilayer boards with flatness requirements
At PCBYES, we pride ourselves not only on delivering high-quality boards but also on acting as your reliable partner in determining the optimal surface finish for your PCB or PCBA project. Whether you're designing for cost-sensitive consumer electronics or high-end RF applications, our team ensures the perfect balance between function, longevity, and budget.
