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Introduction
Choosing the perfect surface finish in PCB manufacturing is critical to complete performance and long life. On this article we compare surface Hard Gold vs ENIG plating. Understanding the characteristics of the Hard Gold plating and ENIG plating and how they are each used will help you choose one that will fit in to your PCB.
What is Hard Gold Plating?
Hard Gold surface finish (Hard Electrolytic Gold) is a tough, hard wearing coating. Gold hardeners are an addition, over nickel. It’s also used on edge plating, and PCB contact points, where it is often used this to provide strong, wear resistant, connections. Other devices such as membrane switches are guided to last longer when put in for repeated use by gold contacts or “gold fingers”. Masking is followed by electroplating 24 carat gold onto the exposed nickel. This finish does a good job protecting PCB surfaces, including the reliable, long lasting connections.
What is ENIG Plating?
A quite popular PCB finish is Electroless nickel immersion gold (ENIG). It has two layers: nickel and gold. The first step is to cover with nickel the copper pads. The nickel layer also is a protection and permits soldering. And a layer of gold is applied on top resulting in a thin film. What keeps the nickel from oxidizing is the gold, which helps to present a smooth and flat surface. This is a great surface to solder components. RoHS compliant, ENIG is a commonly used durable material with high conductivity.
Hard Gold vs ENIG Differences
One of the most important decisions you make when it comes to your board in terms of durability is to use Hard Gold or ENIG in PCBs manufacturing. Each of Hard Gold and ENIG has its merits with different applications. In this case, here, we will break down the differences of these finishes in helping you choose the proper ones for your needs.
Process Principle
The chemical process used by ENIG is called nickel and gold plating, which is applied to the copper surface. It prevents copper diffusion and makes the thin gold durable by being the foundation. The gold layer makes solderability and resists oxidation. Unlike Hard Gold plating, which uses electroplating to create a thicker and harder gold layer directly on top of the copper, this material is comprised mainly of nickel. This produces a very hard void free welding with good wear resistance, suitable for use where parts will be subjected to intensive wear.
Process Sequence
Application sequence varies between ENIG and Hard Gold. After the soldermask is applied there is ENIG plating in that gold is chemically deposited across the exposed copper. The solder mask has to be after the Hard Gold plating, which needs to happen before as the process needs to be conductive. That’s because the difference in this means if Hard Gold needs an electric current to be plated, which the mask would block, it can only be plated on to open copper areas.
Soldering Performance
ENIG is well known for its smooth, ‘soft’ gold layer that offers great quality solder joints. Easy soldering across most PCB components is supported by its even surface on which it supports strong bonds. Although we can make the hardest gold and it is super durable, soldering becomes a bigger ordeal. The surface is hard, therefore can lead to soldering issues which makes it less suited to applications which require a high degree of solder reliability.
Wear Resistance
The thin, softer gold layer of the ENIG renders it with relatively low wear resistance. The mechanical contact is limited and that is something it is better suited for. As for Hard Gold, it is made of an extremely wear resistant, thicker, harder gold plating. It’s the perfect quality for connectors and other high contact areas which require extreme durability.
Oxidation Resistance
The thin gold layer of ENIG provides for a good level of oxidation resistance and therefore creates clean, functional contacts that outlast the PCB and work correctly over time. It’s not the strongest for long term exposure to harsh conditions, though. Even though Hard Gold plating produces a thicker gold layer, the gold layer provides superior oxidation resistance, earning it the reputation of a better choice for environments that need super durability.
Color
The ENIG plating is slightly more yellow in tone due to its chemical deposition of its thin gold layer. Unique electroplated crystal structure give Hard Gold plating an appearance which is more dark yellow. With this finish, PCBs look warm, golden. Sometimes the color distinction between a finished board and a PCB with a bare board finish can help you identify the finish on a PCB.
Cost
In ENIG, the thinner gold layer makes it a cheaper surface finish, so it’s more common in many standard PCBs. However, Hard Gold demands a more significant gold application, and this increases the cost. Because of this, hard gold is often designated for special, very high wear applications where the cost is warranted by the longevity.
Hardness
When analyzing hard gold vs soft gold plating, hardness is the defining factor. One nice thing about ENIG is that the relatively soft, very flexible gold layer is good for standard soldering tasks and use on standard PCB. Hard Gold is much harder than the metal, leading to added strength and resistance against high wear areas such as connectors. The hardness lends Hard Gold to where contact or handling is repeated.
Corrosion Resistance
ENIG has great corrosion resilience making it fit for long term use in moderate conditions. It keeps PCB components from ever being destroyed by gradual wear, protecting them over time. However, while Hard Gold is too hard to be bent, ENIG is less corrosion resistant but far more durable and useable.
Thickness
Hard Gold plating is much thicker than the gold layer in ENIG, which is on the order of 1-3 microinches. Hard Gold can be several tens of microinches thick and thus provide the strength needed on high wear areas. Therefore the Hard Gold’s great advantage in applications with mechanical contact is just this thickness difference.
Applications
Being extremely hard and relatively durable, Hard Gold is often found on connectors, gold fingers, or other areas which are frequently in contact mechanically. However, ENIG is more effective in those areas where you need to solder precisely smoothly and flatter to the less leaded components and ball grid arrays (BGAs). Every finish has its own application so knowing the uses can help you in choosing the proper one for your PCB.
Hard Gold Plating: Advantages and Disadvantages
We will then go into more detail about the advantages and disadvantages of how Hard Gold Plating is amazing for durability, high surface smoothness and lengthy working wear life. However, it also addresses challenges such as complex soldering, inadequate bonding for sensitive applications as well as higher costs compared to other plating processes.
Advantages
- Small Grain Size: The grain structure of the hard gold is very fine, typically 20 – 30 nanometers. As a result of this it has a very smooth and visually lustrous surface.
- Durability and Hardness: It can withstand high contact forces and wear due to a hardness rating of 130-200 HK25.
- Long Lifespan: Hard Gold plating can go through many cycles of use because it is great for areas of heavy friction or sliding.
Disadvantages
- Difficult to Solder: Soldering Hard Gold is difficult due to the metals comprising it, nickel, cobalt, and iron, compared to ENIG.
- Not Applicable to Sensitive Applications: This kind of plating is not so well suited for methods such as ultrasonic or thermostatic bonding.
- Higher Cost: Hard Gold plating requires more material and lots of material resulting in it costing the most of the finishes.
ENIG Plating: Advantages and Disadvantages
Let’s see the good and the bad of ENIG plating, which is easy to solder, good for joining application and better corrosion resistance. But it also reveals its lower durability, its lesser wear resistant quality and its susceptibility to scratching because of its softness.
Advantages
- Softer and Easier to Solder: Hard Gold is harder and less solder and bond friendly than ENIG plating.
- Better for Joining Applications: ENIG plating is the softest and purest of the finishes and consequently is ideal for sensitive joining methods such as ultrasonically or thermostatically bonding.
- Corrosion Resistance: Over time ENIG plating provides better corrosion resistance and therefore provides more reliability in harsh environments.
Disadvantages
- Lower Durability: Hard Gold is more durable than ENIG plating, which rates in the 20 – 100 HK25 hardness.
- Limited Wear Resistance: ENIG plating can only handle the contact forces of up to 35 grams and if you are experiencing high wear, it just isn’t going to last.
- More Susceptible to Scratching: ENIG plating is softer than Hard Gold, so that it is more likely to be scratched and damaged.
How to Choose the Right Plating Approach
The choice of the plating used for your PCB matters. How the board is mounted is then determined by this. Depending on the different applications, different properties such as wear resistance, solderability, and corrosion resistance, are required for them. A right finish will make your PCB last and do as they are meant to.
Contact Force Absorption and Resistance to Wear
If you experience high contact forces Hard Gold plating is a better choice. It is designed for repeated motion wear and tear. Hard Gold is used where parts are more likely to be subject to mechanical stress. It is used in smaller parts. So, these parts should be able to bear higher loads for longer time periods. Due to this, it is a better alternative to softer finishes such as ENIG.
Adhesion / Solderability
Because of its soft and pure gold layer, ENIG plating is much easier to solder than Hard Gold. For sensitive connections, like wire bonding, it’s perfect. ENIG quality makes it the best layer to be used on delicate components. Soldering with Hard Gold, especially on the non-precious metals side, can be difficult.
Appearance
The plating can have a shiny, lustrous finish which reads well in applications where appearance matters. This provides a polished and elegant looking to components. ENIG provides functionality, but in an equally unattractive way. If looks matter, Hard Gold is usually the wiser choice.
Bonding
Sensitive bonding applications such as ultrasonic or thermosonic bonding match well with ENIG plating. A secure and dependable bond is a result of its soft gold layer. Because the hardness of Hard Gold may cause difficulties with such delicate processing, we will not recommend it as your choice. ENIG is the better choice for intricate bonding.
Corrosion Resistance
Hard Gold will corrode while ENIG plating has superior corrosion. Its high purity prevents oxidation and extends the lifetime of your components. The added metals in Hard Gold can speed its corrosion. ENIG plating is the best option if you need long term corrosion resistance.
FAQs About Hard Gold vs ENIG
Is ENIG Hard Gold?
No, ENIG is not hard gold. It’s often called soft gold. The gold layer is thin and smooth. ENIG is easy to solder by this. It gives a high reliability level for solder joint. For most electronic components, ENIG is better than hard gold. The soft gold layer makes solderability easier. It is good for many soldering applications.
How Thick Is Hard Gold PCB?
The layer of hard gold on a PCB varies by thickness. Naturally, 3 to 50 microns (μin) belong to its classification. 32 μ in is used as the standard thickness. A thinner gold layer is usually chosen for prototypes. This reduces cost and just meets a set of basic requirements. It also needs to be slightly thicker gold for edge connectors. Besides this, these areas get plugged and unplugged obviously, more durably. The gold plating is usually up to 150 μin in some special application. When done right, it is only used for particular needs. The thickness will vary depending on how the PCB will be used.
How Thick Is Gold in ENIG Finish?
In an ENIG finish, the gold layer is typically between 0.05 and 0.23 µm. Usually the nickel layer beneath it is 2.5 to 5.0 µm thick. A chemical reaction forms this gold layer, moving gold towards nickel atoms before combining to create the gold layer. Gold flows through the PCB making a thin and even layer of gold. Gold thickness is a driver for solderability. It can cause a too porous layer if it is too thin. Also, if it’s too thick it affects the strength of the nickel layer. The final gold thickness also depends on process temperature.
Conclusion
In this PCBTok blog, we compared Hard Gold vs ENIG plating for PCB manufacturing. It bridged their process differences, soldering performance, wear resistance and corrosion resistance. ENIG is the best for solderability, and Hard Gold for high wear areas. The post also included tips on deciding which plating is right for your specific application.
