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In the electronics industry, “gold” is often used on the surface of products. But do you know what hard gold, soft gold, electroplated gold, ENIG (Electroless Nickel Immersion Gold), and flash gold are in printed circuit boards? What are the differences between them, and what should be considered in their applications?
Workingbear wants to clarify that the information shared in this article is based solely on personal experience and knowledge. Since I haven’t worked directly in the PCB manufacturing industry, my understanding of hard gold, soft gold, and flash gold comes from years of interacting with PCB manufacturers and asking questions. If you notice any mistakes, please feel free to point them out.
Many colleagues in system assembly often get confused by terms like “hard gold,” “soft gold,” and “flash gold” when it comes to printed circuit boards. Some even believe that plated gold is always hard gold and ENIG (Electroless Nickel Immersion Gold) is always soft gold. However, that’s only partially true.
Below, Workingbear will try to explain the differences between hard gold, soft gold, and flash gold on PCB in a way that’s easier to understand.
In the industry, the distinction between “hard gold” and “soft gold” is related to the difference between “alloy” and “pure gold.” Pure gold is softer, while gold mixed with other metals (alloy) becomes harder and more resistant to wear. So, the purer the gold, the softer it tends to be.
Electroplated Nickel-Gold
“Plated gold” itself can be divided into hard gold and soft gold. Electroplated hard gold is essentially an alloy (a mix of Au and other metals), which makes it harder and more durable. This hardness is ideal for areas that need to withstand pressure or repeated friction. In the electronics industry, hard gold is typically used on the edge connectors of printed circuit boards (commonly known as “gold fingers,” like the one in the image at the top of the article) and for card slot contacts. Soft gold, on the other hand, is often used for wire bonding in COB (Chip On Board), for contact surfaces on mobile phone buttons, and more recently, it has been widely used on both sides of BGA substrates.
To understand the difference between hard and soft gold, it helps to know a bit about the electroplating process. Without going too deep into the pre-plating steps like acid cleaning, the basic goal of electroplating is to coat the copper surface of the PCB with gold. However, if gold is applied directly onto copper, a physical reaction known as electron migration (due to the potential difference) occurs. To prevent this, a layer of nickel is plated first as a barrier, and then the gold is plated on top of the nickel. Therefore, what we commonly refer to as “plated gold” is technically “electroplated nickel-gold.”
The difference between hard gold and soft gold lies in the composition of the gold layer. When plating gold, you can choose to use pure gold or an alloy. Since pure gold is softer, it is called “soft gold.” Pure gold bonds well with aluminum, so COB applications that require wire bonding will specifically ask for a thicker layer of pure gold.
If you choose to plate a gold-nickel alloy or a gold-cobalt alloy, the alloy will be harder than pure gold, and this is referred to as “hard gold.”
Here’s a simplified overview of the electroplating process for soft and hard gold:
Hard Gold: Acid cleaning → Nickel plating → Flash gold plating → Gold-nickel or gold-cobalt alloy plating
Chemical Gold or Immersion Gold (ENIG)
Today, the term “chemical gold” or call “immersion gold” usually refers to the ENIG (Electroless Nickel Immersion Gold) surface treatment method. One of its main advantages is that it doesn’t require the complex electroplating process to bond nickel and gold to the copper surface. Moreover, its surface is actually smoother than electroplated gold, which is especially important for the increasingly smaller electronic components and high flatness requirements of fine-pitch and 0201 and smaller chip components.
Since ENIG uses a chemical displacement process to form the gold layer, its maximum thickness generally can’t match that of electroplated gold, and the gold content decreases the closer you get to the base layer.
Because of the displacement process, the ENIG gold layer is considered “pure gold,” so it is often categorized as a type of “soft gold.” Some also use it for COB aluminum wire bonding, but it’s crucial to ensure the gold layer thickness is at least 3 to 5 micro-inches (μ”). Achieving a thickness over 5μ” with immersion gold is difficult, and too thin of a gold layer can impact aluminum wire adhesion. On the other hand, electroplated gold can easily reach a thickness of 15 micro-inches (μ”) or more, though the cost increases with the thickness of the gold layer.
[Video] Introduction to PCB production processes (PCB Production Process)
Flash Gold
The term “flash gold” comes from the English word “Flash,” meaning a quick plating process. Essentially, it’s the pre-gold plating step of hard gold electroplating. Referencing the “Electroplated Nickel-Gold” process mentioned earlier, flash gold uses a higher current and a more concentrated gold solution to form a thin, dense gold layer on the nickel surface. This layer serves as a foundation for the subsequent plating of gold-nickel or gold-cobalt alloys.
Some manufacturers realized that flash gold could produce a gold-plated PCB more quickly and at a lower cost, so they began selling these “flash gold” PCBs.
Because flash gold skips the final gold plating step, it’s much cheaper than true electroplated gold. However, since the gold layer is extremely thin, it often doesn’t fully cover the nickel underneath. As a result, these boards are more prone to oxidation over time, which can affect solderability.
When comparing surface treatment methods for PCBs, electroplated nickel-gold is generally more expensive than other options like ENIG or OSP(Organic Solderability Preservative). Given the high price of gold today, it’s less commonly used unless for special purposes, such as connector contact surfaces or sliding contact elements (e.g., gold fingers). However, among current surface treatment technologies, electroplated nickel-gold still offers unmatched durability in terms of friction resistance and oxidation prevention.
Because flash gold skips the final gold plating step, it’s much cheaper than true electroplated gold. However, since the gold layer is extremely thin, it often doesn’t fully cover the nickel underneath. As a result, these boards are more prone to oxidation over time, which can affect solderability.
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