ENIG vs ENEPIG: A Comprehensive Comparison
Eminent PCB manufacturers like Rush PCB Inc. coat the exposed pads with a metal/organic coating, which they call the surface finish. The surface finish protects the copper pads from scratches and oxidation while boosting soldering in the reflow soldering ovens. Although there are many types of surface finishes, ENIG and ENEPIG are two versatile types. Manufacturers typically use them for rigid PCBs, flex boards, and rigid-flex PCBs to provide them with high reliability. ENIG and ENEPIG are better surface finishes in comparison to OSP, HASL, immersion tin, and others.
How to Select Surface Finish?
So far, manufacturers used the more commonly accepted surface finish like OSP or Organic soldering preservatives, HASL or hot air solder leveling, Immersion tin, Immersion gold, ENIG, and ENEPIG. Each of these surface finishes has its advantages and disadvantages, and it is necessary to choose the one most compatible with the application. Selecting a surface finish requires considering factors like cost, application environment, fine-pitch components, use of leaded or lead-free solder, frequency of operation, shelf life, resistance to drop and shock, volume and throughput, and thermal resistance.
With PCBs trending towards micro vias and finer lines and the disadvantages of HASL and OSP, such as flatness and flux elimination issues, becoming more prominent, surface finishes like ENIG were growing in demand. Furthermore, ENEPIG emerged as an improvement over black pads, the leading weakness of ENIG.
What is ENIG Surface Finish?
ENIG, or Electroless Nickel Immersion Gold, is also known in the electronics industry as chemical gold or immersion gold. This type of finish provides two metallic layers—gold and nickel—which the manufacturer deposits on the PCB pad surface one after the other. This surface finish is a selective one, meaning some select pads may have an ENIG surface finish, while other pads may have other types such as OSP, HAL, HASL, or Immersion tin. The manufacturer applies the ENIG surface finish in several steps:
Copper Activation
The PCB manufacturer starts by first activating the copper layer by cleaning it. This helps to get rid of any dust and oxidation residues on the surface. They also moisten the surface to remove trapped gas or air in perforations. The next step is micro-etching the surface with peroxide or sulfuric acid. Some manufacturers also employ a pre-dip catalyst to get rid of oxidation residues.
Electroless Nickel
The next step in the process is coating the activated copper surface with a layer of nickel. This nickel layer acts as a barrier or inhibitor, preventing the copper surface from reacting with any other element.
Immersion Gold
This is the last step in the process. The manufacturer immerses the PCB in a mixture that oxidizes the nickel surface, producing nickel ions and reducing gold from the mixture. The reduced gold forms a coating of the metal that protects the nickel surface. The thickness of the gold surface must match the specifications.
Advantages of ENIG Surface Finish
An ENIG coating offers many benefits, primarily:
● Flatness of the surface — especially useful for fine pitch components like BGA and other small shape factor components.
● Suitable for press-fit components — provides an excellent connection for electrical testing.
● Suitable for wire bonding and edge card connectors.
● Less expensive compared to ENEPIG.
Disadvantages of ENIG Surface Finish
An ENIG coating also has a few disadvantages:
● Black Pads — a result of unregulated nickel and gold plating process.
● Variable PCB coating thickness — a result of unregulated nickel and gold plating process.
● Low wetting — during soldering.
What is ENEPIG Surface Finish?
ENEPIG is the abbreviation for Electroless Nickel Electroless Palladium Immersion Gold. This type of metallic coating on the PCB pad surface has three layers—nickel, palladium, and gold—that the manufacturer deposits one after the other. Apart from protecting the copper surface from corrosion and oxidation, this type of surface finish is suitable for high-density SMT designs. The manufacturer starts by activating the copper surface, follows it up by depositing a layer of electroless nickel, then by a layer of electroless palladium, and finally, by a layer of immersion gold. The process is somewhat similar to what they follow in the ENIG process. The ENEPIG process is developed based on the ENIG technology with the addition of the palladium layer.
Adding a palladium coating improves the PCB surface protection. The palladium layer prevents the nickel from deteriorating and interacting with the gold coating. Chemical reduction during the electroless process forms thin layers of nickel and palladium. Finally, the gold layer protects the palladium from the elements.
Advantages of ENEPIG Surface Finish
● Reduces quality problems like black pads — due to the presence of palladium.
● Excellent solderability and high solder reflow phase.
● Offers highly dependable wire bonding capability.
● Supports highly dense vias.
● Satisfies wide standards of miniaturization.
● Suitable for thin PCBs.
Disadvantages of ENEPIG Surface Finish
● More expensive than ENIG.
● A thicker palladium layer can decrease the efficiency of SMT soldering.
● Higher wetting time.
Similarities Between ENIG and ENEPIG Surface Finishes
Although these are two different types of surface finishes, they have some common features as well. ENIG and ENEPIG surface finishes both make use of electroless nickel and immersion gold. Both offer a straight, flat surface for soldering, making it easier to solder small form factor components on the PCB surface. Moreover, being free of lead, both the surface finishes are green alternatives for PCB products.
Both ENIG and ENEPIG offer excellent electrical performance and thermal diffusion. Coating the PCB pad surface with either ENIG or ENEPIG requires following similar procedures. For instance, initiation for both processes begins with copper activation, followed by a coating of electroless nickel. The final layer for both surface finishes is immersion gold.
Differences Between ENIG and ENEPIG Surface Finishes
A major difference between the two surface finishes is the presence of the palladium layer in ENEPIG. This layer of palladium introduces high oxidation resistance, improving the electrical performance of the surface finish. However, the extra palladium layer increases the cost of an ENEPIG surface finish as compared to that of ENIG.
Moreover, the ENIG surface finish is not consistent for gold-wire bonding, caused by low solder-joint reliability. Additionally, an extra procedure is necessary to stop nickel corrosion in ENIG. For issues like these, the ENIG surface finish is most suited for low-end electronics.
Considerations for Selecting ENIG or ENEPIG
-
-
-
Application
-
-
Selecting the most optimum surface finish depends on the application and the use case. For instance, if the application requires the PCB to operate in a hot environment, it is preferable to use ENIG. This is because the ENIG surface finish can withstand high temperatures.
-
-
-
Cost
-
-
This is an important factor to consider. For instance, if you are looking for a low-cost surface finish, ENIG could satisfy your requirements. As ENEPIG has an additional layer of palladium, it adds to the cost. However, ENEPIG is less costly than a solid gold coating.
-
-
-
Flatness
-
-
Many conventional surface finishes are not adequately smooth and flat. This makes it difficult to mount very small SMT components with tiny form factors. The non-smooth surface is also problematic for fine-pitch components like BGAs. However, both ENIG and ENEPIG surface finishes are very smooth and form thin and uniform layers over the pad.
-
-
-
Wire Bonding
-
-
The best option for wire bonding is offered by ENEPIG, as the surface finish has a strong capability for wire bonding.
-
-
-
Touch Interface
-
-
ENEPIG can handle touch interfaces without any issues.
-
-
-
RoHS Compliance
-
-
Many conventional surface finishes contain hazardous substances and, hence, do not comply with the requirements of RoHS. Both ENIG and ENEPIG comply with RoHS requirements fully and are lead-free.
-
-
-
Shelf Life
-
-
The Shelf life of a printed circuit board is a crucial factor. Both ENIG and ENEPIG surface finishes have a great shelf life of up to a year.
Conclusion:
According to Rush PCB Inc., modern surface finishes of ENIG and ENEPIG are highly dependable. They are eminently suitable for high-quality printed circuit boards. As they are free of any hazardous substances like lead, the PCBs comply with RoHS and WEEE requirements. Additionally, both surface finishes offer a very flat and level surface for SMT mounting. However, ENEPIG can meet any extra specification for PCBs using various SMT packages. ENEPIG is frequently used in PCBs required by the military, aerospace, and medical industries.