Gold plating is a process where gold plates are placed on other metals on PCBs to help with their conductivity. Gold plating is the most common way of depositing gold onto a PCB and is a relatively straightforward technique.
It works by creating a gold plating solution composed of an extremely large number of gold ions. Gold ions are gold atoms that are missing at least one electron. The gold solution gets applied to another surface with positively charged atoms – called the cathode – causing the negatively charged atoms to chemically bond with them.
This process creates a remarkably thin layer of gold – less than 15 millionths of an inch thick – on the surface that is being plated with gold.
Why Is Gold Used on PCBs?
Gold is used on PCBs over other metals for the combination of the following reasons:
- Conducts Electricity Efficiently – its high conductivity is essential for PCB applications
- Does Not Corrode – silver and copper also have high conductivity, but they corrode easily, which affects efficiency over time. Gold does not corrode, and can also protect metals underneath from corrosion caused by oxidation
- High Heat Resistance – tolerating high temperatures reduces fire hazards
- Reusable – it’s simple to melt and reclaim gold
- Flexible – it’s easily molded to fit different applications, and thin layers work just as efficiently
The Difference Between Immersion Gold and Gold Plating
Although they share some similarities, immersion gold and gold plating for PCBs are different in subtle and meaningful ways.
Beyond merely the differing colors, the two methods utilize differing chemical processes. Gold plating uses electron displacement to bind the gold atoms to the metal’s surface, whereas immersion gold works by submerging the board in a bath of gold salts.
The result of these competing processes is that they form differing crystal structures. The most significant outcome of this difference is that gold plating is much harder, whereas immersion gold is softer. The difference in hardness and softness is significant for durability and soldering ability.
These differing processes and qualities mean that gold plating and gold immersion may be preferable for different parts of a PCB, such as the gold fingers or spots which require soldering. It is important to have a working understanding of both processes, their advantages and drawbacks, and where they each should be used.
Why Use Gold Plating on PCBs?
The main reason to use gold plating for PCBs is that gold plating is harder than immersion gold. Hard gold is especially useful for parts that are regularly in contact with other electronic components. For instance, gold plating is more commonly used on a printed circuit board’s gold fingers, which connect the PCB with another circuit board.
Because some gold fingers are inserted and removed from another circuit board many times over a PCB lifespan, they must be able to withstand the abrasion that comes from repeated or imperfect insertion. Gold-plated gold fingers are less likely than gold fingers made from immersion gold to get damaged in this process.
Gold plating may also be preferable because it is simpler. If someone does not have the required equipment to use gold immersion for their circuit board, they may use gold plating instead. Additionally, if one only needs to plate a small area of a PCB, they may opt to brush just that area with the gold plating solution rather than submerge the entire PCB.
One drawback to gold plating is that it is more expensive simply because it’s a thicker layer of gold. Of course, if it’s too thick, you won’t be able to solder the PCB. Hard gold’s suitability for touch and sliding contacts may be worth the price for many manufacturers, though.
Why Use Immersion Gold with PCBs?
There are many advantages to immersion gold. Because it is softer, many people find immersion gold easier to solder than gold plating. This added ease means you are less likely to receive complaints from customers regarding the quality of the welding if you use immersion gold, and it can also save time for whoever is soldering the PCB.
For example, immersion gold is often preferable for electroless nickel immersion gold finish (ENIG). The purpose is to create a protective barrier for the nickel to avoid oxidation. But upon soldering, the gold dissolves into the solder. What’s left behind is a solid connection between the nickel and the solder.
Because immersion gold applies a thin layer of gold through the displacement of the surface atoms rather than merely bonding on top of them (as gold plating does) immersion gold creates a flatter surface where it has bonded. This feature can help if the PCB needs to be as flat as possible, and may help when further work is being done to the circuit board.
Because of its softness, it is easier to control stress on immersion gold. This softness is part of what helps with the soldering process and means that immersion gold is not as brittle, and is less likely to snap under pressure, which helps increase the longevity of immersion gold.
Importantly, the process of immersion gold provides the person producing the PCBs with more control and creates a more consistent product. This consistency means that immersion gold is more likely to give you the exact coverage you want, helping with general quality control issues and helping those working with the PCBs know what to expect.