Hard Gold
Hard gold is a type of electroplated gold that includes non-noble metals like cobalt and nickel. This makes it harder than soft gold and more resistant to corrosion.
It also boasts a more precise grain structure that makes it more lustrous than ENIG. It’s capable of surviving more use cycles than soft gold, too.
Corrosion Resistance
Corrosion is a natural process that affects the binding energy of metals and makes them lose electrons to other materials in their surroundings. Different metals react differently in this environment, and it’s important to choose a plating that will be resistant to the specific conditions in which it will be used.
The corrosion resistance of hard gold is much higher than that of soft gold. This is due to its incredibly fine grain size, which runs from 20 to 30 nanometers, and its high hardness rating of between 130 and 200 HK25. These characteristics make it far more resistant to sliding wear than its soft counterpart, and can power through a number of durability cycles before failure.
Additionally, hard gold has a polished appearance that many customers find appealing. The luster that this finish produces is due to the refined structure of its grain, which gives it a dazzling look. This is another reason why it is favored in hard gold projects that require cosmetically appealing gold contacts, such as visible interconnect applications.
However, the fact that hard gold contains non-noble elements like nickel and cobalt makes it unsuitable for soldering. These materials oxidize at soldering temperatures, reducing the strength of the soldered joint. As a result, this finish is not suitable for ultrasonic wire bonding and thermosonic bonding, where ENIG is preferred.
Wear Resistance
A hard gold electroplating deposit is actually an alloy of gold and other metals. As such, it is much more durable than pure gold. Because of this, hard gold plating is perfect for applications requiring force and friction such as the contact edge on a chip carrier or aluminum wire on cob (chip on board).
The non-noble metallic elements alloyed with the gold in a hard gold deposit — typically cobalt, nickel or iron — alter its grain structure making it far more lustrous and resistant to sliding wear. Its typical grain size runs 20-30 nanometers and it has a hardness between 130 and 200 Knoop hardness. This is a significant improvement over the soft, ductile properties of ENIG and allows it to withstand contact loads of 50 grams or more.
In addition, the use of cobalt or other elements increases hard gold’s resistance to oxidation and prevents formation of harmful compounds at elevated temperatures. This is especially important for electrical connections that need to maintain a low level of contact resistance.
However, the presence of these non-noble elements can also make soldering with hard gold more difficult and less receptive to sensitive joining applications like ultrasonic bonding. Generally speaking, it is better to use ENIG in these instances. This is because soft gold retains the true noble characteristics of elemental gold and protects against acids, unlike hard gold.
Solderability
Hard gold plating is an electroplated gold deposit that has been alloyed with a non-noble metal element (typically cobalt, nickel or iron). The presence of these elements in the deposit changes the grain structure and makes it harder, thus more resistant to sliding wear. The Hard Gold PCB Supplier deposits also have a very small grain size that is much finer than soft gold and offers better resistance to on-off switching events.
The addition of these non-noble elements also reduces the corrosion resistance of the deposit as well as its ability to resist high temperature oxidation. This reduced resistance may be an issue for some applications. Additionally, the presence of these impurities makes it difficult to solder hard gold. This is why it is typically not used for buried vias or blind vias on the PCB.
A hard gold surface finish is most often applied to the edge connectors (gold fingers) of a PCB, though it can be applied throughout the circuit board. During the PCB fabrication process the nickel under layer is tape laminated to expose only the area where hard gold is required. The hard gold is then electroplated on top of the nickel. This process requires careful control since the thickness of the hard gold deposit is critical to proper performance. This process is also expensive as well as more time consuming than other PCB surface finishes.
Contact Resistance
Hard gold is more durable than soft due to the presence of metallic components such as nickel, iron and cobalt. This translates into finer grain structures that look more glossy and resist sliding wear.
This toughness also extends to its ability to withstand normal contact forces of 50 grams or more. The absence of insulating oxide and compound formation also allows hard gold to perform consistently over a longer lifecycle than soft.
These benefits are enhanced further with the addition of a barrier layer of nickel. The nickel underplate provides both mechanical support to the gold and acts as a diffusion barrier that inhibits copper oxide from forming underneath the plating. It also prevents tarnish film formation, improves the surface finish and helps to keep the gold deposits clean and shiny.
Unfortunately, the inclusion of non-noble metals in hard gold reduces its ability to withstand high temperature oxidation. This makes it unsuitable for solderability applications such as ultrasonic wire bonding or thermostatic bonding. It also impedes the effectiveness of copper corrosion inhibitors (CCI’s) applied under the plating. Consequently, hard gold is typically used in non-solderable areas like edge connector fingers and keypads. However, a nickel underplate can be applied to enhance the appearance of hard gold in solder-able areas. In these cases, the nickel underplate can be plated to a thickness of 30-50 micro inches.
