Why Hard Gold Is Better Than Soft Gold
Hard gold is preferable to projects that require regular sliding wear or make/break switching events as it offers longer lifecycles based on thickness. According to laboratory testing it has better corrosion resistance than soft gold.
Hard gold contains alloying elements (like nickel or cobalt) which increase its deposit hardness, but they also interfere with wire bonding.
Resistance to Corrosion
Gold is an excellent material for use in situations with a lot of moisture or corrosive environments. Unlike copper or nickel, gold does not corrode under normal conditions. However, it can corrode under certain conditions, mainly when it comes into contact with sulfur dioxide and chlorine gases. This can be a problem for electronic components, since these metals can easily clog up the electrical conductors. This is why hard gold plating is a popular choice in electronics for circuit boards.
Using a hard gold coating prevents this from occurring and keeps the metal looking bright. This also helps the plated item to stay abrasion resistant. A hard gold coating can be tested for abrasion resistance by using energy dispersive x-rays (EDX).
Another way to improve the abrasion resistance of hard gold is to add an underlayer. This can include nickel, cobalt, or tin. However, this reduces the solderability and wire bonding ability of the finish, so it is not ideal for items that will be handled frequently or that will be placed in a harsh environment.
The hardness of gold plating is important because it allows the plate to withstand high amounts of force without scratching or breaking. This is especially important for electronic parts that will be used in mobile devices or automated machines, where the plated part will be subjected to a lot of friction.
Resistance to Wear
Hard gold plating resists wear and corrosion better than nickel or copper finishes. This is particularly important in humid hard gold or highly corrosive environments. These conditions cause metals to oxidize and create hard oxides that block current flow. Gold is not as susceptible to these conditions, and its ability to displace oxidation makes it an ideal metal for use in electrical devices.
Depending on the size and nature of the components being plated, hard electrolytic gold deposits can last longer than other coatings. This is largely due to its ability to displace oxides and keep the surface smooth, which helps to reduce friction between contacts. This also means that the normal contact force required to operate a component will not damage or wear the surface.
The addition of non-noble elements in hard gold electrodeposits decreases its corrosion resistance. This can also result in oxidation and the formation of other compounds when exposed to high temperatures.
Because of this, hard gold is most often used in high-wear areas like edge connector fingers and keypads. It is not, however, used in solder-able areas of circuit boards. A soft gold finish with a higher purity is a more suitable option for these applications. The gold finish most commonly plated at Valence is ENIG (Electrolytic Nickel – Gold), which has a Knoop hardness of 130 to 200 and offers many of the same benefits as hard gold with solderability that approaches that of pure gold.
Appearance
Hard gold looks brighter than soft gold plating and has a finer, more refined grain structure. This makes it better for areas where contact forces will be absorbed, such as the edge connector finger on a PCB or a keypad.
It also shows good corrosion and tarnish resistance and preserves its rich yellow colour well. In contrast, soft gold tends to show discolouration over time and may develop dark spots and scratches.
The difference between hard and soft gold is the non-noble metal elements alloyed into the electroplated deposit – typically cobalt, nickel or iron. These harden the deposits, resulting in greater resistance to sliding wear. They also reduce the deposit thickness, which helps with avoiding thermal shock or damage.
Another advantage of hard gold is its solderability. Due to the presence of nickel, iron and cobalt in the hard deposit, however, it is not suitable for ultrasonic wire bonding or thermosonic bonding processes. The deposit can also oxidize during soldering, which reduces the strength of the joints and shortens the life of the gold plate.
In comparison, ENIG is an alternative to hard gold for PCBs where the components require delicate connections. ENIG is softer than hard gold, but it still resists sliding wear and can withstand a maximum of 35 grams of contact force without showing signs of fatigue.
Solderability
Hard electrolytic gold is typically applied to keypads and edge connector fingers on PCB. Generally, it is applied over a barrier coat of nickel to prevent oxidation and extend wear life. The minimum plating thickness that IPC considers to be solderable is 17.8 micro inches. The Hard Gold PCB Supplier problem with applying too much hard gold to solder-able surfaces is that it will dissolve into the tin/lead solder, creating intermetallic compounds that can make brittle joints.
The way to avoid this is to apply the gold plate over a bright electrolytic nickel deposit free of organic codeposits. This will prevent the dissolution and prevent premature dewetting of the solder joint.
In addition to the corrosion resistance, hard gold is also resistant to sliding wear because it has a finer grain structure than soft gold plating. When non-noble elements are added to the gold, such as cobalt and nickel, it changes the grain structure of the electroplated deposit, making it harder. The result is a more durable deposit that can resist many sliding wear cycles before it fails, and the additional thicknesses of hard gold plating can power through even more wear cycles than soft. This is why hard gold is preferable to projects that use repetitive contact switching, such as keyboards. A lubricant or an underplate of nickel can help with this.
