Hard Gold on PCBs
Hard gold is an electroplated metal that’s most often used for edge connectors (gold fingers) and keypads on PCBs. It is a good choice for areas that need high levels of wear resistance.
The non-noble metallic elements alloyed with the gold deposit in hard gold plating create a finer grain structure that’s more resistant to sliding wear than soft gold deposits. The finish is also brighter and more lustrous than soft gold plating.
Fine Grain Sizes
The microstructure and grain size of a piece of gold can be examined under an optical microscope. This is a process called metallography. In hard gold plating, the grain sizes are much smaller than soft gold. This gives hard gold a brighter finish that can be better for some applications where the surface will get brushed or touched.
Unlike pure gold, hard gold plating typically contains other elements like nickel and cobalt. These non-noble metals give hard gold its superior wear resistance and a high level of electrical conductivity. In fact, it is this combination of characteristics that makes hard gold the choice for applications that require sliding wear or make/break switching events. Hard gold is capable of withstanding a high number of cycles and can last up to twice as long as ENIG plating.
In addition to the aforementioned benefits, hard gold hard gold is also a great choice for applications that require a high degree of solderability. However, it should be noted that hard gold is not as easily bonded to copper as soft gold. This is due to the presence of non-noble metals, which can cause the substrate to oxidize and create other compounds. This is why many manufacturers use a barrier coat of 150 to 200 micro inches of nickel before using hard gold on their PCBs.
Low Coefficient of Friction
The small, smooth grain structure of hard gold plating leads to a lower coefficient of friction than soft gold. While this may seem like a minor detail, it can be crucial when comparing metals for an application. The low coefficient of friction helps reduce contact resistance and improve conductive paths between components. It also makes it easier to perform soldering and other types of electrical connections.
A major benefit of hard gold is its ability to withstand high temperatures without oxidizing. This means that it can be used in corrosive environments, such as those that include sulfur or chlorine gases. This is a significant advantage when compared to other metallics, which can react with these gases and deteriorate quickly.
While hard gold is a good choice for areas that require force friction, it is not ideal for soldering. It is typically plated over a barrier coat of nickel, making it unsuitable for visible interconnect applications. Soft gold, on the other hand, is ideal for these applications because it will stand up to abrasion and burnishing better than hard gold.
Often, it is possible to combine the benefits of hard and soft gold in order to get the best of both worlds. For example, a sulfamate nickel undercoat can be added to hard gold to give it additional ductility and strength while preserving its low coefficient of friction.
Longer Life Cycles
Depending on the thickness of the gold deposit, hard gold can power through more cycles than soft. This makes it ideal for projects that see regular sliding wear or on-off switching. For instance, most smart industrial machines feature this type of circuit board or are equipped with a keyboard that experiences many actuation events.
These longer life cycles in hard gold can be attributed to the unique electron sea properties of this metal. Unlike other metals that have a tight structure, gold atoms can slip by each other really easily. This is because of Hard Gold PCB Supplier the way they were formed when stars exploded and seeded this material across the universe.
This is also the reason why hard gold can be plated over a barrier coat of nickel. It can also be deposited directly onto PCBs, especially in areas that require high wear like edge connector fingers or keypads. However, it’s not recommended for solder-able surfaces because it has non-noble elements like cobalt and nickel that oxidize during soldering. This can lead to poor connections and weaker joints.
While hard gold offers some useful benefits, it can’t compete with soft and ENIG plating in terms of solderability and bonding. Therefore, it should be used in areas that require beautiful and cosmetic gold contacts, such as visible interconnect applications. Moreover, for high-temperature applications, hard gold will experience an increase in contact resistance due to the accelerated formation of oxides and other compounds.
Better Solderability
Hard Gold is a type of PCB plating that consists of high concentrations of gold electroplated over a nickel barrier. It’s a very durable finish and is used for heavy wear areas such as edge connector fingers or keyboards; it also has great application in projects that require repetitive contact and friction such as rubbing, plugging-unplugging and moving contacts. It’s also commonly known as Electroplated Nickel Gold (ENIG).
It isn’t very compatible with soldering and oxidation is common at elevated temperatures. It’s important to choose the proper underplate for this type of plated material in order to ensure optimal corrosion/oxidation protection, solderability and contact cycle life.
A nickel underplate that is free of organic codeposits such as sulfamate nickel is recommended to maintain maximum load-bearing and ductility. This type of underplate can withstand high-temperature cycling without a reduction in load bearing capability.
Because hard gold includes non-noble metals like nickel and cobalt, it isn’t suitable for any kind of bonding process; the metals oxidize at soldering temperatures and weaken the strength of the solder joint. If your project requires ultrasonic wire bonding or thermosonic bonding, then ENIG is a better choice.
