Hard Gold for PCB Edge Connector Fingers

Hard gold is typically used in contact points and PCB edge connector fingers known as “gold fingers.” Gold’s high level of hardness can help withstand repeated insertion and removal in applications like membrane switches.

This type of gold offers long lifecycles based on its thickness. However, it does not solder well and can become oxidized in high heat applications.

Corrosion Resistance

Gold has a high resistance to corrosion and tarnishing. It is also a good conductor of electricity and is well-suited to applications in harsh environments where copper oxidizes faster or cannot provide the desired contact. Because it is a noble metal, however, gold has less tolerance for environmental conditions such as humidity or chemical vapors that can attack other base metals and corrode their coatings.

In order to protect against tarnishing and improve the durability of hard gold, we typically plate it over a layer of nickel. The nickel underplate serves to support the contact load of the gold deposit and to help it resist cracking. In addition, the nickel underplate can improve the appearance of the plated surface.

Hard electrolytic gold has a higher hardness than soft gold and includes smaller grain size particles, which contributes to its excellent wear resistance. It is also capable of preventing sliding wear. It is also far more lustrous than ENIG, and can withstand many more cycles of contact force before failing.

However, the presence of non-noble elements such as cobalt and nickel in hard gold electrodeposits can make it difficult to solder. Therefore, this type of finish is not recommended for applications requiring ultrasonic wire bonding or surface mount assembly. We suggest that you opt for ENIG for these sensitive joining applications.

Electrical Conductivity

Gold is a metal that conducts electricity quite well. This is because of the sea of delocalized electrons it carries, and the fact that its temperature coefficient is lower than many other metals.

This is why you see gold in a lot of electrical contacts, like connectors. But the ionization potential of gold is higher than copper, which makes it more difficult for electrons to get into the metallic structure. Luckily, this does not mean that contact resistance is high.

In general, plated gold is very thin, which is good hard gold for conducting the electrical signal. The flatness of gold also helps to create a more precise contact surface. This means that the edges of traces on a PCB are more precisely aligned than with other metals. This may lead to fewer rejects during quality control.

Hard electrolytic gold is often used in areas that are subject to a lot of wear, such as the edge connector fingers on a printed circuit board. It is deposited over a barrier coat of nickel to prevent oxidation. However, it is not recommended to use on areas that will be soldered, as nickel and cobalt oxidize at soldering temperatures. This is why ENIG (electroless nickel immersion gold) is preferred by some producers for these applications. ENIG has a much more refined grain structure and can withstand many more durability cycles than hard gold plating.

Wear Resistance

In addition to its shiny finish, hard gold has an advantage over soft gold in terms of wear resistance. Hard gold has a finer grain structure than soft gold, making it more resistant to sliding wear on contact surfaces. This is because it inhibits crystal growth in the gold deposit, creating a more refined microstructure and reducing sliding friction between metal contact points.

The hardness of the deposit also contributes to its wear resistance, as the higher hardness of the deposit makes it more difficult to scratch or mar the surface. This makes it a more durable alternative to soft gold plating, especially in high-contact applications like PCBs.

Additionally, hard gold has excellent conductive properties and can tolerate high temperatures, which make it ideal for use in electronic devices like cell phones and laptop computers. It is also common in telecommunications systems and automated machines that require a reliable and stable connection to other components.

However, it is important to keep in mind that the non-noble elements in hard gold electrodeposited coatings like nickel and cobalt can oxidize at soldering temperatures. Therefore, hard gold is not a good choice for projects that require soldering, ultrasonic wire bonding or thermosonic bonding.

Solderability

Hard gold provides a tough surface finish that resists wear and tear better than Hard Gold PCB Supplier other finishes. This finish is commonly used to create gold/edge fingers on PCBs or for edge connectors. However, it has poor solderability and is costly.

This is due to the alloys of non-noble metal elements in hard gold electroplating. These alloys usually include cobalt, nickel or iron and alter the grain structure of the deposit. Moreover, these elements are not suitable for soldering at high temperatures since they oxidize and reduce the effectiveness of the solder. Consequently, hard gold is not recommended for sensitive joining applications such as thermosonic or ultrasonic wire bonding.

Nevertheless, hard gold can be used for surface-mount devices (SMD). This process involves spreading solder paste on the components and inserting them on the PCB automatically using a SMT machine. The resulting PCB is then reflow-baked in an oven at a certain temperature to complete the SMT assembly.

Despite its limited solderability, hard gold is highly functional since it preserves the solderability of the underlay. This is because the underlying base metal needs to be free of oxides and in a solderable condition. Hence, the gold serves as a Saran Wrap to protect it. However, the thickness of the gold layer determines its ability to maintain this functionality. A thicker layer will provide better solderability.