Electroless nickel electroless palladium immersion gold (ENEPIG) is another alternative for the surface coating of a PCB — it uses multiple layers to create a universal finish that can work with numerous applications, such as soldering or wire bonding with gold, aluminum and copper. ENEPIG is one of the more optimum choices for wire bonding as well as most PCB applications.
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The order of the layers over the substrate corresponds to the name of this surface finish. Over a copper base is an electroless nickel layer. Atop that layer goes an electroless palladium coating. The palladium prevents the nickel from passing through to the gold layer on top. The final coating, an immersion gold layer, keeps the palladium from interacting with environmental contaminants that could otherwise impede soldering.
ENEPIG printed circuit boards were popular many years ago due to their overall board support and are now rising once again thanks to a significant decrease in the cost of palladium. The design has been shown to successfully operate in conjunction with lead-free and eutectic solder alloys as well as most assembly processes.
ENEPIG is often referred to as a “universal” finish because it can be used for almost any PCB and in all assembly processes. Palladium completely dissolves during soldering so there is no oxide present in the nickel surface. This means your application has less probability of errors, and you can safely use it with varying solder applications.
Compared to other finishes, ENEPIG printed circuit boards will have a greater solder joint strength and are more likely to meet your industry requirements for overall PCB life and durability.
The advantages and disadvantages of ENEPIG are:
Advantages: The gold surface layer protects the PCB from tarnish and corrosion. The use of multiple layers inhibits corrosion from metal diffusion. Additionally, the composition of this finish provides a lead-free surface with a high solder strength while offering a more cost-effective option compared to electroless gold or electrolytic nickel gold. Lower contact resistance – The electrical resistance of this finish is uniform due to its production process and creates a situation where amperage is easier to predict and manage. Other advantages include:
And perhaps the best news of all is that the ENEPIG process tends to provide significant savings over electrolytic nickel gold and electroless nickel or electroless gold.
Disadvantage: Due to the inclusion of palladium and gold, both precious metals, this surface finish may not be as cost-effective as options that do not use these materials. Additionally, the application of the multiple layers must follow careful procedures for success and good solderability.
Choosing the surface finish is an essential step in the design of your printed circuit boards.
The PCB surface finish helps protect copper circuitry from corrosion. It also provides a solderable surface for your components. There are a number of factors to consider, including:
Below are 5 types of PCB surface finishes, along with their benefits and drawbacks.
“Hot Air Solder Leveling” is the least expensive type of PCB surface finish.
It is widely available and very economical. The board is dipped in molten solder and then leveled off with a hot air knife. If you’re using through-hole or larger SMT components, HASL can work well. However, if your board will have SMT components smaller than 0805 or SOIC, it is not ideal.
The surface is not completely level, so this can cause issues with small components. The solder used is typically Tin-Lead. That means that it isn’t RoHS compliant. And if reducing the amount of lead you use is important, you may want to choose Lead-Free HASL instead.
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Lead-Free HASL is similar to standard HASL, but with an obvious difference… It doesn’t use Tin-Lead solder.
Instead, Tin-Copper, Tin-Nickel or Tin-Copper-Nickel Germanium may be used. This makes Lead-Free HASL an economical and RoHS compliant choice. But like standard HASL it is not ideal for smaller components.
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For boards with smaller components, immersion coatings can be a better choice. They are slightly more expensive but more suitable for this purpose.
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With all immersion coatings, a chemical process is used.
A flat layer of metal is deposited on the copper traces. The flatness of the coating makes it ideal for small components. Tin is the least expensive type of immersion coating. Although it is an economical choice, it comes with some drawbacks.
The main drawback is that after the tin is deposited onto the copper it begins to tarnish. That means that if you want to avoid lower quality solder joints, you need to do your soldering within 30 days.
If you are expecting a high volume of production this may not be an issue. And if you are using large batches of boards quickly, you can avoid tarnishing. However, if your production volume isn’t high, it may be better to choose a coating like immersion silver.
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Immersion silver does not react with copper the way that tin does. However, it does tarnish when exposed to air.
That means it needs to be stored in anti-tarnish packaging.
When stored in proper packaging it will still be solderable for 6-12 months. But once the PCB is removed from its packaging, it will need to go through solder reflow within a day. A higher shelf life can be achieved with gold plating.
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Electro gold flash plating consists of a thin layer of gold over electroless or electrolytic nickel.
This type of plating is hard and durable. It also has a long shelf life, lasting for years. However, its durability and shelf life make it more expensive than any of the finishes mentioned above.
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Remember that when choosing the finish for your PCBs, you need to consider component types and production volume. You’ll also need to consider requirements for durability, environmental impact, and cost. By taking all these factors into consideration, you can be sure to make the right choice.
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