When you build a PCB, it is essential that you administer a PCB surface finish. What’s that? A surface finish is there to protect the foundation, that is, the copper surface. It basically allows the copper to do its job without getting damaged in the process.
There are several different surface finishes you can apply. They all come with different qualities, and likewise, different setbacks. Here are some of the most popular ones:
- Hot Air Solder Leveling — HASL
- Immersion Tin — ISn
- Immersion Silver — IAg
- Organic Solderability Preservative — OSP
- Electroless Nickel Immersion Gold — ENIG
According to research carried out in 2016, roughly half of all surface finishes were ENIG surface finishes. Today, some estimate that PCB manufacturers apply ENIG 80% of the time. Why’s that? Let’s take a look!
The Importance of Surface Finish
A surface finish is crucial in the process of designing a highly-functioning PCB. Essentially, the surface finish is a coating (it can be organic or metallic) that you apply to the copper of a circuit board. The finish has two purposes. Firstly, it protects the copper. Secondly, it provides you with a solderable surface so that you can add electronic components with ease.
When you have a PCB, its performance value depends on the conductivity of its copper pads. The entire electronic connection between different components relies on it. But, copper is not a passive substance. In fact, when you expose it to humidity, it will oxidize.
Obviously, you want to avoid oxidized copper at all costs. First off, it will reduce the performance level of your PCB. Additionally, it stops soldering from doing a proper job, making components unstable. Overall, your PCB will become unreliable and ultimately unusable.
A surface finish is there to stop that oxidation from happening. That’s its primary function. However, a surface finish also helps you with soldering components onto the PCB. The finish is more agreeable to the soldering process, making it easier to achieve stability of any added components. In other words, the finish acts as a connectivity layer between a device and the board.
At the end of the day, a surface finish is something that can make or break your PCB entirely.
What Is ENIG Finish?
Now, as you can expect, there are different types of finishes. They differ in the materials you use and the way you apply them. For a long time, HASL was the go-to finish (and the only one at first) for all manufacturers.
However, as time went by and the technology advanced, other finishes were available. Also, production standards such as RoHS became a thing, and there were lead-free requirements (HASL was initially a leaded surface).
Today, the ENIG finish is by far the most popular one, surpassing HASL as its main competitor. As the name suggests, the ENIG finish uses nickel and gold to create a double-layer metallic coating. This double-layer design makes sure the ENIG finish satisfies both of its purposes. The nickel layer is essentially a protective barrier to the copper circuitry that’s underneath it.
After that, you have the gold layer, which protects the nickel during storage, increasing the PCB’s shelf life. The gold layer is also a great surface for soldering, fulfilling the second purpose of a surface finish we talked about.
Perhaps the main advantage of the ENIG surface finish is its durability. The gold and nickel combination makes up for long shelf life — the ENIG finish lasts over a year, with only the HASL finish being able to compete in that regard. Other finishes fall short. For instance, the ISn finish lasts about six months.
What Makes It So Popular?
Apart from the long shelf life we’ve mentioned above, there are two other crucial reasons why the ENIG finish is the most common one nowadays. First of all, when it came to be, the ENIG finish was completely adherent to the RoHS and WEEE regulations. The HASL finish, on the other hand, wasn’t, and it needed to adapt. That’s why you have lead-free HASL today, which provides a different approach just so that it would be RoHS-friendly.
Another big reason for its popularity is the high yield-rate. As you know, technology never stagnates, and new developments are always behind the corner. The same goes for the ENIG finish. In recent years, new developments in the ENIG finish have come up.
What are these developments? They have improved the quality of production by minimizing the corrosion that occurs when depositing gold. They’ve also reduced the level of availability of hydride ion, which is what makes nickel corrode. Furthermore, they’ve reduced the amount of gold concentration. All of this results in a whopping 50–80 percent reduction of the initial investment.
The Plating Process
As we’ve mentioned earlier, the ENIG finish is a dual-layered finish. It adds a layer of 120–240μ inches of electroless nickel. Then, we cover up that layer with 2–5μ inches of immersion gold.
The EN in ENIG goes on first. It’s an auto-catalytic process during which we deposit nickel on the palladium-catalyzed surface. During this process, it is necessary that we use a reducing agent. Without it, the concentration, pH levels, and temperature would be off, and we wouldn’t be able to achieve consistency in the coating.
The IG in ENIG goes next. We adhere the gold to the nickel via a molecular exchange. As we’ve said, the gold protects the nickel during storage, making it intact until we begin soldering components. If the gold is too thick or too thin, it can affect the solderability of nickel. That is why we always make sure the gold thickness is in compliance with extremely precise tolerance levels.
Pros and Cons of ENIG Finish
There are several advantages to the ENIG finish, which is why it’s the most popular method out there. But, as with everything in life, it doesn’t come without its drawbacks.
Let’s begin with its solderability. The immersion finish of the gold layer means that the ENIG has an extremely flat surface. Therefore, the soldering process is much easier than it is with, for example, an HASL finish. That makes the ENIG finish perfect for PCBs with dense topography. If you want to make your PCB using BGA, smaller, or fine pitch components, you can’t go wrong with ENIG.
What’s more, the combination of nickel and gold makes for a very strong surface. Consequently, it’s resistant to high temperatures. This means that you can use aluminum wire for bonding. Finally, as we’ve already explained, the ENIG finish is completely lead-free, and it comes with a shelf life over a year.
One of the main drawbacks should have been evident the moment we mentioned the word “gold.” It is on the expensive side. However, the good thing is, its price has come down recently due to the development of gold processing.
Another drawback is what we call “the black pad syndrome.” This syndrome is especially applicable to BGA. Black pad (sometimes referred to as black nickel) represents corroded nickel. It’s a buildup of phosphorus between the two ENIG layers that deteriorates the quality of nickel.
Basically, the gold starts corroding the nickel. Then, you get (previously non-existent) areas in the joint in which phosphorus starts gathering. The phosphorus immensely affects the solderability. However, the biggest problem with the black pad syndrome is that we cannot know it’s there until the later stages, i.e., when it’s too late. Sometimes, issues don’t occur until the testing phase.
Thankfully, in recent years, the industry has come up with a solution to this. In short, the black pad occurs when the gold layer is too thick, but the IPC-4552 standard now controls the thickness. With it, the black pad syndrome is fast becoming a thing of the past.
There’s another con that we can’t get rid of (yet). It comes in the shape of rework-ability or lack thereof. The ENIG finish is really not agreeable to making adjustments. Basically, once you’ve done it, it is done. It’s also a complicated process, so if you’re sitting in your garage trying things out for the first time, you’re probably not going to pull it off perfectly.
Finally, the ENIG finish can sometimes (although seldom) occur in signal loss for signal integrity applications.
A surface finish is essential to the way any PCB will function, as it protects the copper and provides a solderable surface. There are different types of these finishes, but, by far, the most popular one is the ENIG finish. Mixing nickel and gold, this finish provides your PCB with a strong surface that’s flat and great for adding components.
The nickel makes sure that the copper remains highly functional, while the gold adds an extra layer of protection and comes with a high solderability. It’s a complicated finish to apply to your PCB, and you can easily make unremovable mistakes. However, with over ten years of experience, we at MKTPCB will make sure the surface finish is impeccable.
If you wish to hear more from us, and you need help choosing a finish for your PCB, feel free to contact us! We’ll provide you with further information, and you can also request a completely free quote for your PCB fabrication and PCBA!