Hard Gold PCBs
Hard gold is a gold coating applied over a nickel underplate. It’s commonly used in PCB edge connector fingers and keypads.
Hard gold has more corrosion resistance and wear resistance than soft gold. It also withstands more durability cycles. However, hard gold does not perform well in soldering or wire bonding applications.
Durability
Gold is soft enough to be scratched by other metals, so it’s alloyed with a mixture of silver and copper, as well as traces of zinc to increase its durability. This also makes it more tarnish-resistant and durable against corrosion. Regardless, the material is still relatively delicate and should be used for applications where contact forces are minimal or where the item will remain in a protected environment such as in a jewelry box or dental work.
Hard gold plating, on the other hand, is a tougher metal due to the non-noble elements alloyed with it – typically cobalt, nickel or iron – that alter the deposit’s grain structure. The result is a smoother, brighter finish that can withstand more wear and tear than soft gold. In addition, it can withstand many durability cycles before showing signs of failure and can be improved even further with the use of lubricants or an underplate layer of nickel.
The impurities that make up hard gold deposits also reduce its overall corrosion resistance as they oxidize at soldering temperatures. For this reason, the material is typically not recommended for sensitive bonding techniques like thermostatic joining or ultrasonic wire bonding. However, hard electrolytic gold PCB plating is often utilized for edge connector fingers and keypads where a combination of wear resistance, corrosion resistance and high-temperature resistance are required.
Corrosion Resistance
Hard gold is a non-reactive metal that resists corrosion in many different environments. It is especially useful in hard gold moist and corrosive conditions where copper, nickel or other materials may be susceptible to sulfur or chlorine gas attack.
Due to its precise grain structure and dazzling appearance, hard gold plating is ideal for applications that require visually acceptable gold contacts, including visible interconnect applications. Its ability to withstand sliding wear and contact switching events makes it highly suitable for PCB edge connector fingers or keypads.
However, it is not recommended for areas that require soldering. This is because the alloying elements that are present in hard electrolytic gold deposits (cobalt, nickel and iron) cause oxidation at soldering temperatures, making it difficult to bond with other components or metals. Therefore, if you require soldering, soft gold or ENIG is more appropriate for your application.
The use of a nickel underplate prior to hard gold plating provides a diffusion barrier that prevents the solid-state metallic diffusion between copper base metals and the gold deposit, extending the life of the finish. The contrasting grain structures of the 2 gold layers also result in reduced general deposit porosity, which may improve corrosion resistance and increase durability.
A duplex gold finish that is applied over a hard gold deposit can provide an excellent solution for both corrosion resistance and solderability. This type of PCB gold plating is able to achieve high corrosion resistance, and can tolerate many durability cycles before wear-through.
High Temperature Resistance
The ability of gold to withstand high temperatures makes it ideal for electronics. Many metals can react with oxygen to create hard oxides that are electrically insulating. To displace these oxides and provide a path for current, normal force is required; this can lead to contact resistance problems. Hard gold is less reactive than these other metals, which means it requires much smaller forces to work properly.
Hard gold is also highly durable and can withstand many durability cycles. This is particularly important in applications that require repeated insertion and make/break switching events, such as PCBs. In some cases, a hard gold coating can last ten times longer than conventional copper plated finishes. A nickel underlayer is often added to hard gold plating to help support the load and reduce cracking.
Another benefit of hard gold is its resistance to corrosive environments. For example, jewellery pieces often need to withstand prolonged exposure to perspiration and certain corrosive chemicals, such as artificial sweat, dilute lactic acid, and dilute sulphuric acid. This type of corrosion can be problematic for other Hard Gold PCB Supplier metals, but not for gold. This is because gold does not oxidize under these conditions, and its pure noble characteristics protect it from chemical attack. Its resistance to corrosive environments is also enhanced by the fact that it is not subject to abrasion from these acids.
Solderability
Hard gold PCBs can withstand many cycles of sliding wear and make/break switching, but it’s important to select the proper plating thickness for your application. The most common thickness for functional hard gold plating is 0.000015 inch, which can withstand up to 30 million cycles and is often referred to as “Flash Gold.”
For the best results when soldering a hard gold-plated part, the underplate should be high purity nickel free of organic codeposits, such as sulfamate nickel. This helps bear the contact load of a hard gold deposit and prevents dewetting of the joint surface, which can cause the gold to crack.
Another important aspect to consider when choosing the right gold plating method is its bonding ability. Because hard gold deposits are alloyed with non-noble metals, like cobalt, iron and nickel, they tend to oxidize at soldering temperatures, which can compromise the integrity of the bond.
For this reason, a hard gold PCB should only be used for critical applications that require sensitive joining processes, such as ultrasonic wire bonding and thermostatic bonding. For most other applications, a soft gold, such as Type III, is the better choice.
