This article is based solely on my personal experience and understanding regarding hard gold, soft gold, and flash gold plating on circuit boards.
The following represents only my personal opinions and experiences. If any errors are found, please feel free to leave comments for correction.
Many friends working in downstream assembly plants remain unclear about the distinction between “hard gold” and “soft gold” on circuit boards.
Some still believe that electroplated gold is always hard gold, while chemical gold is always soft gold. In reality, this classification is only partially correct.
Electroplated Nickel-Gold
Electroplated gold itself can be categorized into hard gold and soft gold.
Electroplated hard gold is essentially an alloy plating (consisting of Au and other metals), resulting in greater hardness.
It is suitable for areas subject to stress and friction. In the electronics industry, it is commonly used as the contact points on the edges of circuit boards (commonly known as “gold fingers,” as shown in the first image);
while soft gold is typically used for bonding aluminum wires on COB (Chip On Board) or as contact surfaces for mobile phone buttons.
Recently, it has been extensively applied to both sides of BGA carrier boards.
To understand the origins of hard and soft gold, it helps to briefly grasp the electroplating process.
Setting aside the earlier acid pickling step, the fundamental purpose of electroplating is to deposit “gold” onto the copper layer of a circuit board. However, “gold” cannot react directly with copper.
Therefore, a layer of ‘nickel’ must first be electroplated, followed by gold plating onto the nickel layer. Thus, what we commonly refer to as “gold plating” should technically be called “Nickel-Gold Electroplating.”
Additionally, if an alloy such as gold-nickel or gold-cobalt is chosen for plating, it is termed “hard gold” because alloys are harder than pure gold.
Electroplating Processes for Soft Gold and Hard Gold:
Soft Gold: Acid pickling → Nickel plating → Pure gold plating
Hard Gold: Acid pickling → Nickel plating → Pre-gold plating → Gold-nickel or gold-cobalt alloy plating
Electroless Nickel Immersion Gold
Today, “chemical gold” primarily refers to this ENIG (Electroless Nickel Immersion Gold) surface treatment method.
Its advantage lies in depositing nickel and gold onto copper substrates without requiring electroplating processes.
Moreover, its surface finish is smoother than electroplated gold, which is particularly crucial for increasingly miniaturized electronic components and parts demanding high surface flatness.
Since ENIG employs a chemical displacement method to form the gold layer, its maximum thickness cannot match that of electroplated gold. Furthermore, the gold content decreases progressively toward the substrate.
Because the ENIG gold layer is pure gold, it is often classified as “soft gold.” It is also used as a surface treatment for COB aluminum wire bonding, but strict requirements mandate a gold layer thickness of at least 3 to 5 microinches (μ“).
Achieving a chemical gold layer exceeding 5μ” is generally difficult, and too thin a layer compromises aluminum wire adhesion.
Conventional electroplated gold, however, can easily achieve thicknesses exceeding 15 microinches (μ”). The cost increases proportionally with the thickness of the gold layer.
Flash Gold
The term “Flash Gold” originates from “Flash,” meaning rapid gold plating. It essentially serves as a “pre-gold plating” step for hard gold electroplating.
Similar to the nickel-gold plating process, it employs higher currents and a more gold-rich solution to deposit a denser yet thinner gold layer onto the nickel surface. This facilitates subsequent gold-nickel or gold-cobalt alloy plating.
Some manufacturers recognized that this method could produce gold-plated PCBs at lower cost and faster turnaround times, leading to the sale of these “flash gold” PCBs.
Since “flash gold” omits the subsequent gold plating process, its cost is significantly lower than genuine gold plating.
However, due to the extremely thin gold layer, it fails to adequately cover the underlying nickel layer. This makes the gold layer more susceptible to oxidation during storage, which can adversely affect solderability.