PCBs are a vital part of electronic devices. The advent of the circuit board has had an impact on the electronic industry. However, these boards have to survive different operational conditions. Hence, the need to coat them with various types of surface finishes. Immersion gold for PCB is an ideal solution for most surface finish problems.
These finishes help to minimize oxidation and as well as impact the electrical performance of circuit boards. There are different surface finishes for circuit boards. Immersion gold is a type of surface finish for PCBs. Therefore, we will concentrate on immersion gold surface finish in this article
Electroless Nickel Immersion Gold ENIG
There has been a huge development in the electronic Industry. The electroless nickel immersion gold has contributed to this development. Electroless nickel immersion gold, also known as immersion gold, is the commonest surface finish option. Most PCB manufacturers prefer this type of surface finish to other types.
ENIG is a lead-free alternative that provides a durable finish. It is a layered metallic surface finish featuring 2 to 8 micron Au over a nickel of 120 to 240 micron. The nickel serves as a hindrance for the copper. It also offers a surface to solder components. Electroless nickel immersion gold is a reliable and effective finish type for circuit boards. The gold layer shields the nickel plating during storage.
The use of nickel in this surface finish makes it unique. Nickel produces a smooth, flat surface. It is also ideal for a lead-free soldering process. Immersion gold prevents oxidation and enhances copper contacts’ solderability. ENIG features electroless nickel plating, clad with gold.
This surface finish uses quick immersion to apply the gold. The gold immersion process involves using a solution that contains gold salts. PCB manufacturers can apply immersion gold on circuit boards before or after solder mask. Overall immersion gold is when you apply Au before solder mask. Selective immersion gold involves applying Au after solder mask. The latter is more common since less gold is needed to clad the solder pads only.
Immersion gold plating solution incorporates the principle of displacement of the substrate’s surface skin. PCB manufacturers have incorporated immersion gold in PCB fabrication and gold wire bonding. However, immersion gold can’t take the place of hard gold PCB. This is because immersion gold thickness is limited. Immersion gold ENIG is a straightforward surface finishing process that requires no technical leads.
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Properties of Electroless Nickel immersion Gold ENIG
ENIG is a surface finish type that features great properties like;
Good oxidation resistance
ENIG features great oxidation resistance. The thin layers of gold present in this surface finish shields nickel from oxidation. Since it prevents oxidation, it ensures that a PCB resists any form of corrosion. Electroless nickel immersion gold resists oxidation to a great extent.
High temperature resistance
The ability of ENIG to resist high temperatures is another great property. ENIG finish is ideal for applications that demand high temperature. Electroless nickel immersion gold can withstand high temperatures. ENIG tolerates varying environmental conditions while maintaining the functionality of the PCB. Also, immersion gold features great thermal diffusion properties.
Lead free
ENIG is lead free. Frequent exposures to high levels of lead can cause several health complications for humans. Immersion gold is environmentally-friendly since it contains no lead. This means that manufacturers don’t expose themselves to harmful chemicals.
Great electrical performance
Every detail matters during the manufacturing process of PCBs. The component placement, design, and finishing touches are all important. As regards surface finishing, the preferred choice is ENIG. This surface finish type features great electrical performance. As a result of this, it has become so popular across the PCB industry. It provides excellent electrical performance in between electronic components on a circuit board.
Long shelf life
This is another great property of electroless nickel immersion gold. The finish type a PCB features will determine its shelf life. Unlike other surface finishes, ENIG features a long shelf life. This is one of the reasons manufacturers prefer it.
Strong chemical properties
Immersion gold ENIG features strong chemical properties. It has great wettability, coplanarity, and surface planarity. This is as a result of the combination of gold and nickel.
What is Immersion Gold PCB?
Immersion Gold PCB features immersion gold as its surface finish. Immersion gold is also known as Electroless Nickel Immersion Gold (ENIG). This immersion gold PCB features electroless nickel plating. This plating is then clad with a layer of immersion gold. Manufacturers use displacement to generate the gold layer on the nickel layer.
This process continues until nickel covers the generated gold layer. Immersion gold PCB features a thin layer of gold. This layer prevents the nickel from oxidizing. Immersion gold is one of the most common surface finish types in the PCB industry. This surface finish is very durable. Immersion gold has a long span, it can last for years. Due to this feature, it is very expensive.
Immersion gold PCBs are very ideal in high-speed applications. This is because of the great properties of immersion gold. This PCB type features the combination of gold and nickel as its finish. Immersion gold PCBs are one the best in the PCB industry. These circuit boards don’t oxidize even when stored for longer periods.
Immersion Gold Plating Process
The surface of the PCB the manufacturer wants to plate must be clean. It must be free of dirt or oils. Surface preparation may include polishing or stripping. Manufacturers use solvents, water, or abrasive materials to complete this process.
Micro etching is the next step for the immersion gold plating process. Micro-etching for PCB electroplating helps to remove the roughness of the surface of the copper plate. This will then help to enhance the binding force of the copper surfaces and the electroplated layers.
Activation of the immersion gold is the next step in this process. The major step is using parameters to plate the copper pad. You can regulate the thickness of the nickel layer. After that, immerse the nickel-plated pad in acidic gold water. Through a process called chemical displacement reaction, the gold displaces from the solution. The immersion gold plating process is complete after the dirt remaining on the pad surface is cleaned.
Advantages of Immersion Gold PCB
Immersion gold PCBs offer a lot of advantages. These PCBs can withstand various operational conditions. We will discuss their advantages below;
Flat surface
Immersion gold plating is common in the PCB manufacturing industry. It is becoming a preferred choice among PCB manufacturers. Immersion gold ENIG features a dual layer metallic coating that consists of nickel. The nickel in this surface finish serves as a surface on which designers solder electronic components.
Moreso, immersion gold provides an ideal solution to the latest industry demands. This includes the increase in complicated surface components. ENIG can fit well into flat surfaces. This makes it ideal for lead-free applications that demand flat surfaces.
Ideal for Untreated Surface
Untreated surfaces will immediately oxidize if left untreated. Therefore, PCB designers protect these surfaces to avoid oxidation. Immersion gold ENIG is ideal for untreated surfaces like membrane switches and contact points.
Ideal for PTH
PTH has wires or leads that appear from the sides or bottom. This enables a designer to stick them into a circuit. Another benefit of immersion gold plating is its suitability for PTH.
Easy to weld
Electroless nickel immersion gold creates different crystal structures. This makes it easy to weld. In addition, ENIG doesn’t have any welding defects. This is because it features soft gold.
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Suitable for a variety of electronic products
This is another benefit of immersion gold for PCB. Most electronic devices feature immersion gold PCBs. This is a major reason it is popular in the electronic industry. Electronic devices like motherboards and smartphones among others feature ENIG PCBs.
Doesn’t require electroplating
Electroless nickel immersion gold doesn’t need electroplating. Immersion gold plating is known for its corrosion-resistance property. This surface finish doesn’t need an external power source to deposit coatings on the base material.
Limitations of Immersion Gold for PCB
Weak soldering joints
This is one of the limitations of immersion gold for PCB. One of the causes of solder joint weakness is inadequate wetting. Improper wetting of solder joints can cause weak solder joints. This can further cause a weak connection within the board.
Risk of Black Nickel
If the nickel surface experiences an oxidation reaction, the nickel might change into nickel ions. The bigger gold atoms will deposit to create a loose and rough arrangement. This indicates that the gold layer might not cover the nickel layer properly. As a result, the nickel layer will have contact with the air. This will finally cause nickel rust under the thin layer of gold. Eventually, this will result in soldering failure.
Highly priced
Electroless nickel immersion gold is more expensive than other surface finish types. Immersion gold PCBs are very expensive. ENIG surface finish is more expensive than OSP and HASL.
What is Direct Immersion Gold?
The PCB manufacturing industry needs new concepts of final finishes. The Direct Immersion Gold (DIG) process has provided an alternative that is free from nickel. DIG combines immersion and electroless deposition. This approach results in a tight lattice that restricts copper diffusion to the surface.
Direct Immersion Gold is an approach used for plating gold on the surface of a PCB. It is becoming popular in the PCB industry. DIG involves plating gold directly on copper. Also, it is a surface finish type for PCBs. PCB manufacturers use solder to mount electronic components on circuit boards. Lead free solder is an alternative material to lead based material.
Sn/Ag/Cu solder materials are suitable in circuit boards due to their solder joint reliability. The major concern is that surface mounting reliability will diminish. This is because the high reflow temperature for Sn/Ag/Cu solder material is between the range of 240 to 260°C.
Direct Immersion Gold (DIG) is an effective surface finish for circuit boards. It can directly deposit a uniform gold layer on the copper surfaces. However, it is important for manufacturers to have a better understanding of how Direct Immersion Gold works. Some characteristics determine the success of this surface finish on circuit boards.
- Gold plating layer must feature good coverage
- Reduced corrosion of the copper surface
- Co-depositing of copper with gold doesn’t happen
DIG is partial immersion and electroless in deposition mode. DIG deposits can withstand the higher demands of PCB designers as regards wire bonding and soldering. By plating gold directly on copper, there are greater advantages.
Benefits of Direct Immersion Gold (DIG)
- Coating free of nickel
- Ideal for fine pitch layouts
- High solder joint reliability
- Low cost of plating as a result of few process steps
- Dense and uniform gold layer
- Ductile film for flex PCB applications
Frequently Asked Questions
How thick is immersion gold plating?
The thickness of the immersion gold layer should range between 0.05 to 0.23 µm. The electroless nickel layer must be within the range of 2.5 to 5.0 µm. A thicker immersion gold layer may lead to a black pad. Therefore, it is important you pay attention to the thickness of ENIG.
Can ENIG PCBs oxidize?
ENIG uses soft gold for its surface finish. Gold is a material known for its great properties. Gold doesn’t oxidize. However, by nature, immersion gold is porous. ENIG with very low gold plating thickness can cause nickel oxidation. If the nickel becomes porous, copper atoms can oxidize to copper oxide. To avoid this, the manufacturer must seal the nickel layer between the copper and gold layers.
Conclusion
Electroless nickel immersion gold is one of the most popular surface finishes. This finish type offers a lot of benefits. Hence, it is a great option for PCB manufacturers. In the PCB industry, there are several types of surface finishes. However, ENIG is mostly preferred by manufacturers.
The nickel layer serves as a barrier between the copper and the gold. Therefore, it is considered an environmentally-friendly surface finish option for PCB manufacturers. However, this surface finish type has its own limitations. This article has provided well-detailed facts about electroless nickel immersion gold.
