What Type of Gold Coating is Right For Your Product?

The purpose of a product or item will dictate what type of gold coating it requires. A circuit board, for example, may benefit from hard gold over soft to prevent wear.

However, hard electrolytic gold is expensive and does not offer good solderability. Thus, it’s not applied to through-hole vias in PCBs.

Corrosion Resistance

Gold’s natural corrosion resistance is an important consideration for items that may be exposed to corrosive environments like acid and chlorine gases. Copper, nickel and other metals often corrode in these types of conditions, but gold resists corrosion well. However, if an item is likely to encounter abrasion, softer gold may be more appropriate as it is less prone to scratches and dents than hard gold.

The addition of non-noble elements such as cobalt and nickel to hard gold deposits increases the deposit’s hardness and wear resistance. These alloying elements also change the deposit’s grain structure, giving it a finer overall appearance that is especially attractive in applications where sliding contact occurs regularly.

Because these alloying materials are hard gold not as noble as pure gold, they can cause issues in soldering and other applications where the deposits need to withstand high temperatures. These materials oxidize at soldering temperatures, weakening the strength of the resulting solder joint.

Soft gold is a more suitable option for medical applications that require biocompatibility and a noble coating that will not react with the body’s biochemistry. The pureness of soft gold also enables it to produce a pore-free film when plated to the required thickness. This helps reduce the risk of biofilm formation in critical areas such as catheters and stents.

Resistance to Wear

Gold is one of the most durable metals, able to withstand high levels of wear. This durability makes it a great choice for products that need to be handled and moved frequently, such as PCBs used in computers or handheld electronic devices. It is also resistant to corrosion, rust and tarnish.

Hard gold plating has a higher hardness than soft gold, making it more resistant to wear and abrasion. This increased hardness comes from non-noble metallic elements that are alloyed with the gold deposit — most commonly cobalt, nickel or iron. These elements alter the grain structure of the deposit, resulting in a finer and more lustrous deposit with a hardness that is around 130 to 200 HK25 (Knoop).

Another factor that contributes to the high level of wear resistance found in hard gold is the lack of oxides or compounds that can form on its surface. These compounds can cause the conductivity of the gold to diminish over time, so avoiding them is essential for long-term contact resistance performance.

A common use for hard gold is in areas that receive a lot of friction, like the edge connector fingers on a PCB or a keypad. Because of its high level of wear resistance, hard gold offers longer lifecycles than other coatings and can power through many actuation cycles.

Aesthetics

Gold is well known for its beauty and value as a monetary store of wealth. But it also brings many desirable, functional qualities to the products we use in our daily lives. When designing products that will be plated with gold, it is important to consider the specific design and end-use of the item as well as the type of Gold used. The choice of Gold plating can have major effects on everything from the ability to withstand corrosive environments and resistance to wear to the ease of soldering and bonding.

The dazzling look of hard gold is a result of its smaller, more refined grain structure. The appearance of hard gold is brighter than soft gold and it stands up much better to burnishing. This makes it a good choice for applications that require cosmetically acceptable gold contacts such as visible interconnect applications.

Due to the presence of non-noble metals like Hard Gold PCB Supplier nickel and cobalt, hard gold deposits are challenging to solder. At soldering temperatures, these metals oxidize and weaken the integrity of the solder joint. Because of this, hard gold is not recommended for any joining application except ultrasonic and thermosonic bonding where the thickness and purity of ENIG are ideal. Nevertheless, this finish is often used for cosmetically appealing gold cladding and coatings.

End-Use

Gold plating is a highly customizable option, with the type of plating dictating everything from the item’s reaction to high temperatures to its bonding characteristics. In addition, the item’s end-use can also impact what type of plating is required. For example, items that will be inserted and removed repeatedly may benefit from hard gold plating due to the fact that it resists wear better than soft.

With its smaller grain structure and harder finish, hard gold plating offers an appealing aesthetic. This makes it suitable for projects that require visually appealing gold contacts like visible interconnect applications. However, it’s important to remember that the metal underneath the gold coating is valuable as well, offering functional benefits such as superior soldering and bonding.

In addition, hard gold can also provide more dependable contact resistance than other metals. Other metals typically interact with oxygen to create insulating films that prevent current flow. However, hard gold does not react with oxygen to form these insulating layers and requires far smaller forces to operate effectively.

Hard gold plating is typically applied on printed circuit boards as gold contact points and PCB edge connectors known as “gold fingers.” These are used in applications such as membrane switches, which see repeated insertion and removal cycles. Often, these devices must be tested to ensure that the gold contact surfaces are operating correctly. This testing can include the flying probe test or universal grid test, as well as automated optical inspection (AOI).