How to Distinguish Hard Gold and Soft Gold in PCB

During the PCB manufacturing process, prolonged exposure of circuit boards to the air can lead to copper oxidation, affecting solderability. To address this issue, a common practice is to apply a layer of gold on the PCB surface as a protective coating. However, the concepts of hard gold and soft gold are often confused when selecting the type of gold layer. This article delves into the details of hard gold and soft gold in PCB surface finish, comparing the two to help make a more informed choice considering factors such as purpose and cost.

Gold in PCB

Table of Contents

Hard Gold vs. Soft Gold

When discussing hard gold and soft gold, some people may associate hard gold with electroplated gold and soft gold with chemical gold. However, this is not entirely accurate, as electroplated gold and chemical gold refer to the techniques of depositing gold on the PCB surface, while the terms hard gold and soft gold primarily denote the hardness of gold, differing in composition.

Hard Gold, also known as gold fingers or gold connectors, is a hard and wear-resistant metal alloy protective layer formed on the PCB surface through the electroplating process. This alloy consists mainly of gold, nickel, and palladium mixed in specific proportions to achieve higher hardness. The typical thickness of hard gold is around 30μm or 50μm. It finds widespread use in applications requiring high insertion and extraction cycles or under harsh environmental conditions, such as game card connectors. Although hard gold provides excellent reliability, its relatively thick gold layer makes melting and diffusion more challenging during soldering, reducing soldering reliability.

Hard Gold in PCB

In contrast to the alloy, pure gold has lower hardness. When pure gold is applied to the surface of copper, it is referred to as soft gold. The deposition of soft gold can be achieved through the same electroplating process as hard gold or by chemical means. The typical thickness of soft gold ranges from 20μm to 50μm. Due to its thinner gold layer, soft gold may exhibit lower wear resistance and increased sensitivity to oxidation. It is commonly used for wire bonding on COB, key contact surfaces, and both sides of BGA substrates.

Soft Gold in PCB

Apart from gold hardness, a clear visual distinction between the two can be made. The surface of hard gold appears smoother and shinier, while the surface of soft gold has a slightly dull appearance.

Electroplated Gold and Chemical Gold

Electroplated gold refers to the process of depositing gold on a PCB surface through electrolytic deposition, applicable to both hard gold and soft gold. Since gold cannot react directly with copper foil, a layer of nickel is plated on the copper surface first, and then gold is plated on the nickel. Therefore, electroplating gold is also known as “electroless nickel gold.” Specifically, the circuit board is immersed in a gold solution containing gold salt, where gold exists in ionic form. When an external electric current is applied, with the circuit board serving as the cathode, gold ions in the gold solution are reduced and deposited on the board’s surface. Precise control of electroplating time and other parameters ensures the desired thickness of the gold layer, typically within the range of 10μm to 100μm, ensuring selective plating only in the designated areas for the necessary gold layer protection.

Chemical gold, also known as immersion gold or ENIG (Electroless Nickel Immersion Gold), is a technology where a gold layer is formed on the PCB surface through chemical reactions instead of electric current. In this process, a layer of electroless nickel is first deposited on the PCB surface, followed by the deposition of a gold layer. This process occurs automatically without the need for external electric current.

Cost-Reduction Measures

It’s no secret that gold is a costly metal, and its quantity significantly impacts the cost of PCB. However, in some projects, the use of gold is unavoidable. In such cases, flash gold emerges as an alternative solution for its low cost and quick turnover. Flash gold is a pre-plating gold process that uses larger currents and high-concentration gold solution baths. Through this process, a thin but dense gold layer forms on the surface of the nickel layer. This gold layer serves as an excellent foundation for subsequent gold-nickel or gold-cobalt alloy plating, improving plating efficiency and quality. However, due to the relatively thin gold layer, issues may arise with complete coverage of all nickel layers, potentially leading to oxidation and soldering problems, especially over extended storage periods.

Additionally, although gold thickness is a crucial consideration, when designing hard gold or soft gold circuit boards, the designer must also consider the impact of copper area on the entire body plating. Full-body gold plating, combined with the high cost of gold, can significantly increase the manufacturing cost of the PCB. External ground layers, in particular, may double the price of the PCB. Therefore, designers pursuing hard gold or soft gold characteristics must ensure the minimization of outer layer copper to maintain cost-effectiveness.

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