Hard Gold

Hard gold is a surface finish used to create the edge connector fingers and keypads on PCBs. It offers tough resistance to wear and tear, but has poor solderability.

It’s important to consider how your product will be used when choosing between soft and hard gold plating. The type of use will determine whether the finish can withstand the wear and tear it’ll experience.

Corrosion Resistance

The corrosive-resistant properties of gold are augmented by its pure nature. When plated with hard gold, the metal is much more resistant to acids. This makes it hard gold a great choice for items that will be exposed to corrosive environments. It also offers excellent tarnish resistance.

However, it’s important to consider the item’s end-use. A dental piece that will be placed in the mouth has different corrosion needs than a chip that will be inserted and removed regularly. Those requirements can have an impact on the type of gold that is best to use.

Depending on the application, it may be necessary to add an underplate before plating with hard gold. A Nickel underplate can help protect against copper diffusion into the gold deposit. It can also reduce the occurrence of pore corrosion and creep corrosion.

Typically, hard electrolytic gold is applied over a barrier layer of sulfamate nickel. It’s often used on edge connector fingers and keypads on PCBs. It’s an excellent choice for wire bonding and offers a high level of corrosion/tarnish resistance. It’s also highly solderable. It meets the requirements of the new MIL-DTL-45204D specification that supersedes the old MIL-G-45204C. The new specifications include standards for hardness and purity vs. hardness, as well as copper etch and tape adhesion tests. It’s also available in a variety of thicknesses to meet your specific needs.

Abrasion Resistance

As a precious metal, gold has naturally low durability and is prone to wear and tear. However, metallurgists have long sought to harden it so that coins and jewellery are less likely to break and become brittle.

To achieve high abrasion resistance, hard gold is typically coated with a nickel underlayer. This serves to bear the contact load, reducing stress on the hard gold deposit and improving overall abrasion resistance. The addition of this underlayer also allows hard gold to be soldered without difficulty, which is particularly important for high-current, sensitive joining applications like thermosonic wire bonding.

A hard gold coating is also ideal for use in applications that Hard Gold PCB Supplier require a combination of corrosion resistance and a consistently low level of electrical contact resistance. The ohm-per-square readings of hard gold are lower than those of soft gold, making it ideal for applications that require a high level of stability and consistency of signal transmission.

When deciding whether to choose hard or soft gold, it is critical to consider the item’s end-use and environment. Items that will be subject to corrosive environments are more likely to benefit from hard gold, while items that will be handled frequently or used in the human body may need soft gold. This is especially true if they will be exposed to acid, as acid can cause erosion of soft gold.

Wear Resistance

Hard gold plating is more resistant to sliding wear than soft gold and offers longer lifecycles based on thickness of the deposit. This makes it ideal for applications that require repeated on/off switching events. Hard gold deposits also have a finer grain size than soft, which reduces frictional forces on contacts and enables the plating to withstand high contact force assertions.

To further improve durability, hard gold plating can be coated with a load-bearing underlayer of nickel. This reduces the likelihood of contact resistance due to oxidation, especially when exposed to extreme temperatures.

It is important to note that the presence of non-precious metal elements in hard gold electrodeposited deposits, such as cobalt and nickel can make soldering more difficult than on soft or ENIG plated surfaces. The co-deposited metals can oxidize at soldering temperatures, causing poor bonding and reducing the integrity of the plated contact area.

Although the addition of these metals to hard gold increases its corrosion resistance and wear resistance, they reduce the overall purity of the deposit. This can cause problems for some sensitive joining applications such as wire bonding or thermostatic bonding. In these cases, only soft or ENIG should be used. In addition, the higher metal impurity levels found in hard gold plating can compromise UL 94 flammability standards. However, a barrier layer of nickel can eliminate this problem.

Solderability

Hard gold surface finish is a layer of gold with hardeners added for increased durability plated over a barrier coat of nickel through electroplating. It is typically used on high-wear areas such as the gold fingers on edge connectors or keypads since it can withstand sliding wear better than soft gold. Due to its high cost and poor solderability, manufacturers rarely use a full hard gold surface finish on the PCB.

The non-noble elements in hard gold deposits (cobalt, nickel, iron) oxidize at soldering temperatures which makes it difficult to solder. This also reduces the life of the gold coating.

Soft Gold, on the other hand, has fewer metal impurities that improves corrosion resistance and high temperature oxidation resistance but at the expense of decreased solderability. The grain structure of soft gold plating is more refined allowing it to resist sliding wear but not as well as hard gold.

The other option is immersion gold which provides the best of both worlds. Immersion gold offers improved abrasion resistance and high-temperature oxidation resistance like hard gold but has better solderability than soft gold. The reason for this is that the softer gold particles have larger grains. In addition, they are deposited on the entire surface of the copper layers instead of only on the contact pads on the PCB.