May 21, 2018
When you are building a printed circuit board (PCB), every detail matters. All the finishing touches, wire placements and design of the board come together to make a PCB that meets every required need. However, it is what you put on the surface of the board that is critical to the assembly of the PCB.
The surface finish is what prevents copper oxidation that can lower the quality of the board. When you cannot maintain high-quality levels of soldering, it affects the performance of the electrical on the board. Plus today’s standards of PCB are to be small and lightweight as possible all while meeting the highest performance standards. To achieve this, new technology is continuously evolving to meet this demand.
Although not entirely new, the process of Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) has made its way back into the spotlight when it comes to the finish of a PCB. This method was first introduced roughly ten years ago and has since begun to take back its popularity. Before, the price of palladium was too high for everyone to be able to afford the finish. However, since the recent drop in the price of palladium, ENEPIG is more affordable and considered the best option for wire bonding.
When the time comes to finish off your PCB, consider looking at the ENEPIG method. Let’s look at what this is, the advantages and disadvantages, as well as when this process is most suitable.
What is ENEPIG?
When talking about ENEPIG, you are referring to a process of metal plating on a PCB. You are joining one metal to a conductive surface to finish the piece off. Quite commonly metal plating is associated with jewelry, but it is an essential process for building PCBs. Often you would see gold and silver plating. But in recent years, ENEPIG plating is making a comeback.
ENEPIG is known as the universal surface finish among PCB assembly. That is because an ENEPIG finish works almost any PCB finish. Instead of using a typical gold plating process, with ENEPIG plating you use palladium (similar look to platinum).
The process of ENEPIG involves four metal layers: copper, nickel, palladium and gold. First, you activate the copper layer by using a displacement reaction to cause the layer to act as a catalyst. You then add the nickel layer to the catalytic copper which protects the copper from negatively interacting with other metals.
Next is the layer of palladium for an additional layer of protection. It helps protect the nickel layer from corroding and affecting the gold layer. By using an electroless reaction, there is a chemical oxidation-reduction that occurs which forms a thin layer of the nickel and palladium. You then have the immersion gold layer which offers superior protection and preserves the palladium. The PCB is completely immersed in covering the whole board.
ENIG vs ENEPIG
What differentiates ENEPIG from ENIG (Electroless Nickel Immersion Gold) is the palladium layer. With ENIG, there is no palladium layer between the gold and nickel layers. The finish still offers a good amount of electric performance from the oxidation resistance, as well as being temperature resistant.
ENIG is not a reliable for gold wire bonding and quite often reduces the reliability of the solder joints. It is also not suitable for touch contact parts.
Advantages of ENEPIG
One of the main benefits of ENEPIG plating is the cost. Since the price of gold is more expensive than palladium, the cost of the process is more affordable compared to hard/soft gold plating alternatives. Compare that to 10 years ago when the method was first introduced, now more and more businesses and individuals can afford an ENEPIG plating finish.
Another advantage is the thinness of the finish. Because of the interaction between the nickel and palladium from the electroless reaction, the layer is between .05 and .1 microns which is ideal for many PCBs.
What makes ENEPIG quite appealing as a finish is how it not only protects the board, but it also makes other elements simpler to work with. It not only makes soldering easier and more reliable, but it also reduces the amount of soldering needed altogether. The plating also won’t tarnish which means the shelf life of your PCB increased exponentially.
Disadvantages of ENEPIG
Although the ENEPIG finish provides superior quality compared to other forms, there are disadvantages to this method. Even though this method is more affordable in recent years, it is still one of the more expensive methods of finishes. Gold and palladium are precious metals which means they are relatively expensive to use.
There are still issues when it comes to reliability with an ENEPIG finish if a board shop does not maintain their chemistry. The integrity of the lead and tin bonding of a PCB is compromised, mainly due to the use of copper and palladium. The ENEPIG coating is also prone to fractures if not correctly used, since the layers on top of the nickel plating can be quite brittle. Also, if the palladium layer is too thick, the performance and reliability of the solder are reduced. This means that you need a trusted source for ENEPIG as choosing correct chemistry and maintaining the chemistry is key for quality ENEPIG finish.
ENEPIG Use Cases
PCB that have multiple layers will significantly benefit from an ENEPIG finish. Not only that, if you have a PCB with differing packing technologies that need wire bonding, ENEPIG can meet all of those additional requirements. Touch contact also works well with ENEPIG and would be cheaper than hard gold.
Because of the superior quality finish compared to other methods, ENEPIG is used in areas like the military, medical industries and even aerospace. Basically, if you need wirebonding, or have push contact requirements, you would use ENEPIG.