Why Hard Gold Is Ideal For PCB Applications

Hard gold has a precise grain structure and a dazzling appearance. It’s ideal for PCB applications that require visually acceptable gold contacts.

However, because hard gold deposits contain non-noble metals like nickel and cobalt, soldering them is difficult. Additionally, they have lower resistance than ENIG plating. This is due to a lack of insulating oxide or compound formation.

Corrosion Resistance

Corrosion resistance is an important characteristic to look for when selecting a metal. Different metals will corrode at different rates depending on their environment or if they are exposed to certain elements. The best materials for corrosion resistance have a high tolerance to a wide range of environmental conditions and will maintain their structural integrity when they are exposed to corrosive chemicals or acids.

Gold oxidizes slower than other metals and has a low affinity for oxygen. This makes it a durable material that will not easily erode in harsh environments like saltwater or chlorinated solvents. It is also more resistant to acid attack than copper or nickel.

Williams Plating has extensive experience in plating both soft and hard gold. When selecting the right material for a project, product teams must consider a variety of factors including the item’s exposure to corrosive environments, its resistance to wear, the need for adequate contact force, and its temperature requirements.

Hard electrolytic gold is typically applied to the edges of connector fingers and keypads on PCBs for durability and solderability. It consists of a layer of gold over a barrier coat of nickel. The nickel provides mechanical support for the gold and helps obstruct pores that can lead to pinhole corrosion. In addition, a thicker layer of nickel can provide better corrosion resistance than a thinner layer of gold.

Wear Resistance

Unlike copper, gold does not oxidize in humid or highly corrosive environments. This characteristic makes it the preferred metal for electrical contacts. It is also resistant to sulfur and hard gold chlorine gas, which often attack other metals and impede the flow of electricity.

Additionally, hard gold offers superior wear resistance compared to soft gold. This is attributed to the absence of oxide and compound formation. This means that plated surfaces can be powered through more use cycles than those plated with other metals. This makes hard gold the ideal choice for projects that require regular sliding wear or make/break switching events, like keyboards that undergo many actuation forces.

The high level of wear resistance of hard gold can be attributed to its higher Knoop hardness. Unlike soft gold, hard gold deposits have a Knoop hardness between 130 and 200. The hardness of a gold deposit can be measured using energy dispersive x-ray (EDX), which allows for the identification of different metals within a gold plating and can help determine the wear pattern of the coating.

When compared to soft gold, hard gold has fewer metal impurities which improves corrosion resistance while optimizing the deposit for solderability and high temperature oxidation. In fact, the highest performance hard gold alloy currently available from Mintek has a peak hardness of HV 170 in the cold worked condition and can be heat treated to achieve HV 274 after ageing.

Electrical Conductivity

Gold is one of the most electrically conductive metals, second only to silver and copper. It doesn’t oxidize or form insulating compounds when exposed to high temperatures and humidity, making it an excellent choice for connecting components in harsh environments where corrosion is a risk.

The hardness of a gold deposit is a key consideration when specifying a PCB surface finish. Hard gold plating, or electroplated nickel-gold (ENIG), is made when non-noble elements like cobalt, nickel and iron are alloyed with the gold electrodeposit to increase its durability and lustre. This alters the grain structure of the deposit, making it harder and more resistant to sliding wear. Unlike soft gold, which has a coarser, more amorphous grain structure, hard gold deposits have a much finer grain size.

Hard gold is often chosen for applications that require regular sliding wear or make/break switching events because it can withstand many cycles. Its ability to withstand these repeated actuations is proportional to its thickness, and thicker hard gold plating offers a longer lifecycle.

However, if your project requires soldering, hard gold isn’t an ideal choice. The added non-noble elements in hard gold plating oxidize at the high temperatures of soldering, increasing contact resistance. This makes it more difficult to bond your soldering process with the gold. This is why ENIG is preferred for projects that require soldering.

Durability

Hard gold is a durable finish that’s best used for applications that will be handled frequently or for items whose chips are inserted and removed several times. It also stands up to sliding wear better than soft gold because its finer grain structure is less prone to burnishing or scratching.

The durability of hard gold is achieved through the addition of non-noble metallic elements – typically cobalt, nickel and iron – to the gold deposit during plating. These alloyed metals alter the crystalline structure of the deposit to make it much harder. The result is a hard lustrous finish that’s Hard Gold PCB Supplier more resistant to sliding wear and offers a higher lifespan based on the thickness of the deposit.

This finish is commonly applied to edge connectors (gold fingers) and keypads during PCB Fabrication. It’s also used for aluminium or gold wiring on COB (Chip On Board).

Because of its high resistance to corrosion and wear, hard gold enables components to withstand multiple cycles of actuation without sustaining any damage. However, the presence of these alloyed metals makes it a less suitable choice for soldering applications since they tend to oxidize at high temperatures and degrade the integrity of the solder joint. For this reason, hard electrolytic gold is often plated over a nickel barrier coating and only applied to areas that will not be subjected to soldering operations.