A PCB surface finish is a coating you apply on a bare board and components to prevent the copper from oxidizing and create a solderable surface. However, some finishes are more popular and adaptable than others. As you evaluate your options, you will likely factor in durability, cost, and the materials found in the finish, like tin-lead, lead-free, gold finishes, and silver. Besides, you may need to consider the surface finish compatibility with the subsequent termination process – like wire bonding.
This article takes a deeper look into the ENIG surface finish. You will learn the ENIG process, how it compares with other finishing methods, and why it is taking the PCB industry by storm. But before that, let’s briefly discuss the importance of a surface finish.
What is the Purpose of Surface Finish on PCB?
The surface finish you use in your board will significantly affect its quality and performance. As mentioned in the introduction, surface finishes prevent the board’s copper layer from oxidizing, which would otherwise reduce its solderability. Applying an appropriate surface finish protects the PCB from oxidizing before the assembling process is done. This further ensures you solder your components seamlessly. Since your circuit’s electrical connections depend on proper copper transmission capacity, preventing the copper from oxidizing is crucial.
Choosing a suitable finish for your application is influenced by different factors, like the finishing process, the circuit design, and the quality of the end product. Remember, not every surface finish is ideal for every board. Here are the primary features to consider when selecting your surface finish.
Solderability
Dodging soldering problems is important in making a circuit with a high-performance. Smooth PCB surfaces create connections that work appropriately in their applications. Therefore, consider whether the finish can solder directly to the PCB, like in Immersion Tin, or it is a layered method, like ENIG.
Processing Time
The processing period for a surface finish may be long or short – finishes such as HASL have extended processing times. The time a finish takes to dry depends on the complexity of the assembly. Some finishes, such as OSP, have short temperature cycles and cannot endure several soldering rounds. After a few cycles, it may fade and expose the copper layers to oxidizing agents.
ENIG is a complex finishing process with a longer processing time, making it suitable for circuit board production lines that require few boards in a short time.
Reliability
How will the selected finish withstand the working environment? If your board must meet certain reliability requirements, you need a surface finish that accommodates them. While the finish cost is just one of the factors highlighted here, you need to consider the cost of failure in case your board doesn’t work as specified and fails to prevent the copper layers from oxidation.
The IPC has highlighted some standards under its TN-650 Test Method Manual, describing various ways of testing circuit boards for reliability and quality. These techniques include dimensional verification, chemical resistance, copper foil flexibility, and the quantity of signal loss.
Corrosion
Some PCB finishing methods, such as silver, are more prone to creep corrosion than ENIG. Creep corrosion takes place in any finish used, but it is commonly associated with immersion silver. As the breakdown of metal parts occurs on the board surface, it can interact with adjacent features and cause short-circuiting.
Creep corrosion is prevalent in lead-free surface finishes and humid and sulfur-rich environments. Therefore, you need to consider the type of settings your boards will be working under when choosing the right surface finish for your application. The working environment plays a significant role in influencing your PCB’s lifespan.
The Superior of All Finishes – ENIG Finish
ENIG is an abbreviation for Electroless Nickel and Immersion Gold. It is basically the initial application of nickel and then gold on the copper foil using chemical electroplating. Nickel and gold are preferred because of their unique benefits like excellent dispensability, solderability, wiring bond capacity, soldering capacity, and high compatibility with all types of flux materials.
Unlike other finishing methods, ENIG meets various assembly requirements like solderability, linking, wiring, and thermal dissipation. Furthermore, ENIG PCB surfaces and Surface Mount Devices (SMD) are smooth and ideal for fine applications.
Chemical nickel plating is basically a Ni-P alloy with 7-9% phosphorus. The PH value and stabilizer of the solution influence the phosphorus content in the plating layer. Therefore, it is essential to set the PH at 5.1. You should have a nickel layer thickness of more than 3μm.
The second step of immersing gold on top of the nickel layer is a replacement process. Basically, when you coat nickel with copper, the precipitation of gold ceases. But due to the multiple apertures on the gold layer, the nickel underneath will dissolve. The gold will continue to precipitate on nickel at a progressively low speed till it stops.
The immersion gold layer should have a thickness of between 0.03μm-0.1μm. You need nickel to protect the copper foil from oxidation and dialysis. Therefore, it should have a low thickness to avoid breaking down, regarding its fragility and insecurity.
The ENIG process is easy as it doesn’t require technical leads. However, a black pad may be formed during the soldering process, causing reliability problems. The cause of the black pad is the replacement reaction of the process and the partial dissolving of nickel; hence, easily worn out by the gold solution. This forms a metal compound between nickel and gold layer, which changes color when the PCB is stored for a long time.
You should regulate your phosphorus content to minimize nickel later corrosion. Additionally, you should control the amount of gold in your solution and change the additive and some working restrictions to reduce the formation of black pads. Currently, immersion gold solution has cyanide. It is advisable to use fewer cyanide quantities because it is a harmful substance that significantly affects the ecosystem.
The latest R&D efforts on ENIG have led to the development of a new nickel coating that regulates the stability of the mixture and reduces the nickel layer’s fragility. The PH of this new solution is neutral with less gold content, minimizing costs and corrosion.
Why ENIG PCB Finish is Taking the PCB Industry by Storm
Easy Process Structure
One of the primary benefits of ENIG plating is its natural application process. Unlike other surface finish processes, ENIG is easy to apply. Its highly solderable nature makes it easy to structure ENIG plating on a board.
Flat Surface
As mentioned earlier, ENIG contains two metallic coats of nickel and gold. These layers are an excellent solution to increasing complex surface finishes and lead-free requirements, specifying flat surfaces. Besides, the capacity to flawlessly fit into flat surfaces is another benefit of ENIG.
Good Oxidation Resistance
Most electronic manufacturers require finishes that are not easily oxidized. The ENIG finish is more resistant to oxidation than other surface finishes, like HASL and ENEPIG. The ENIG plating comprises an electroless plate and immersion gold layer that further protects the nickel from oxidation. This creates minimal chances of oxidation, ensuring that the finished board is corrosion-resistant and durable.
Excellent Electrical Performance
During PCB fabrication, each aspect is essential. The placement of wires, finishing touches, and the PCB’s overall design determine the proper functioning of the board. Regarding surface finishes, ENIG finishes are becoming favorites because of their good electrical performance. The high electrical performance ensures a good flow of current and signals between the board components.
High-Temperature Resistance
PCBs, especially those applied in military, aerospace, and medical fields, are exposed to severe environmental conditions, like high temperatures, dust, and humidity. Such PCBs require the right surface finish to protect them against these conditions. The ENIG finish effectively tolerates high temperatures without affecting the PCB performance. An ENIG surface coat is temperature-resistant and displays good thermal diffusion properties, necessary for PCBs operating in harsh environmental conditions.
Long Lifespan
The type of surface finish you use determines the shelf life of your PCB. Your board will last from six, twelve, or more months based on its surface finish. ENIG is well known for providing a longer service life than other finishes, like OSP and immersion tin.
Applicable even in Untreated Surfaces
As a PCB manufacturer, you must prioritize protecting the untreated board surfaces, which oxidize quickly when left untreated. ENIG’s suitability across untreated parts like contact joints and membrane switches makes it suitable for treating untreated surfaces.
Lead-Free
Constant exposure to high-lead levels can cause various health problems, eventually leading to death. Since ENIG does not contain any lead compound, technicians are safeguarded from the health complications caused by lead.
Perfect for Plated-Through Hole (PTH) Technique
PTH components contain wires projecting from the bottom or sides, enabling designers to attach them to breadboards. Unlike other surface finishing methods, like HASL and OPS, ENIG is ideal for applications that require the PTH technique.
ENIG does not Require Electroplating
ENIG is a corrosive-resistant surface finish consisting of nickel and gold layers. Unlike electroless nickel, ENIG plating does not require an additional power source to form coatings on the copper layer.
Conclusion
Are you looking for the best surface finish for your next PCB project? MKTPCB is a leading PCB manufacturing and assembling company, ready to guide you throughout the PCB manufacturing process. We are a reputable and reliable partner that prioritizes quality over quantity. Contact us today to learn more about various surface finishing methods, including ENIG.
