Long-term gradual changes will lead to significant changes. With the increasing calls for environmental protection, the surface treatment processes for PCBs are sure to undergo major changes in the future. Currently, there are many PCB surface treatment processes, the most common being Hot Air Leveling, Organic Coating, Electroless Nickel/Immersion Gold, Immersion Silver, and Immersion Tin. Below, each will be introduced one by one.
1. Hot Air Leveling
Also known as Hot Air Solder Leveling, this process involves coating the PCB surface with molten tin-lead solder and leveling it with heated compressed air, forming a layer that is both resistant to copper oxidation and provides good solderability. During hot air leveling, intermetallic compounds of copper and tin form at the junction. The solder thickness protecting the copper surface is approximately 1-2 mils. When performing hot air leveling, the PCB is immersed in molten solder; the air knife levels the liquid solder before it solidifies; the air knife minimizes the meniscus of solder on the copper surface and prevents solder bridging. Hot air leveling can be vertical or horizontal, with horizontal being generally considered better because it provides a more uniform coating and allows for automated production. The general process of hot air leveling includes: Micro-etching → Preheating → Coating with Flux → Tin Spraying → Cleaning.
2. Organic Coating
Unlike other surface treatment processes, it acts as a barrier layer between copper and air; the organic coating process is simple and cost-effective, making it widely used in the industry. Early organic coating molecules served as rust prevention agents, such as imidazole and benzotriazole, while the latest molecules are primarily benzotriazole, which chemically bonds nitrogen functional groups to the copper on the PCB. During subsequent soldering processes, having only a single layer of organic coating on the copper surface is insufficient; multiple layers are required. This is why copper solution is often added to the chemical bath. After the first layer is coated, the coating layer adsorbs copper; the second layer of organic coating molecules then bonds to the copper, continuing until twenty or even hundreds of layers of organic coating molecules accumulate on the copper surface, ensuring multiple reflow soldering can occur. Experiments show that the latest organic coating processes maintain good performance during multiple lead-free soldering processes. The general process for organic coating is: Degreasing → Micro-etching → Acid Washing → Pure Water Cleaning → Organic Coating → Cleaning, with process control being relatively easier compared to other surface treatment processes.
3. Electroless Nickel/Immersion Gold
The Electroless Nickel/Immersion Gold process is not as simple as organic coating; it seems to dress the PCB in a thick armor. Additionally, unlike organic coating, which serves as a rust barrier layer, this process is useful for long-term use of PCBs and achieves good electrical performance. Therefore, Electroless Nickel/Immersion Gold involves wrapping a thick layer of electrically conductive nickel-gold alloy around the copper surface, which can protect the PCB for a long time; it also possesses environmental resistance that other surface treatment processes lack. The nickel layer is necessary because gold and copper can diffuse into each other, and the nickel layer prevents this diffusion; without it, gold would diffuse into copper within hours. Another benefit of Electroless Nickel/Immersion Gold is the strength of nickel; even a nickel thickness of just 5 microns can limit expansion in the Z-direction at high temperatures. Additionally, this process can prevent copper dissolution, which is beneficial for lead-free assembly. The general process for Electroless Nickel/Immersion Gold is: Acid Cleaning → Micro-etching → Pre-dipping → Activation → Electroless Nickel Plating → Immersion Gold, involving six chemical baths and nearly 100 types of chemicals, making process control quite challenging.
4. Immersion Silver
Positioned between organic coating and Electroless Nickel/Immersion Gold, this process is relatively simple and quick; unlike Electroless Nickel/Immersion Gold, it does not provide a thick armor for the PCB, yet it still offers good electrical performance. Silver, being a sibling to gold, can maintain good solderability even when exposed to heat, moisture, and contamination, although it may lose its luster. Immersion silver does not possess the good physical strength of Electroless Nickel/Immersion Gold because there is no nickel beneath the silver layer. However, immersion silver has good storage properties, and assembling after several years of immersion silver treatment typically poses no significant issues. Immersion silver is a displacement reaction that results in almost sub-micron pure silver coating. Sometimes, the immersion silver process includes some organic substances, mainly to prevent silver corrosion and eliminate silver migration issues; generally, it is difficult to measure this thin layer of organic material, with analyses showing that the weight of the organic matter is less than 1%.
5. Immersion Tin
Since all current solders are tin-based, the tin layer can match any type of solder. From this perspective, the immersion tin process has great development potential. However, earlier PCBs exhibited tin whiskers after undergoing immersion tin treatment, leading to reliability issues due to tin whiskers and tin migration during soldering, which limited the adoption of immersion tin processes. Later, organic additives were introduced into the immersion tin solution to create a granular structure in the tin layer, overcoming previous issues while also providing good thermal stability and solderability. The immersion tin process can form flat copper-tin intermetallic compounds, a feature that gives immersion tin good solderability similar to hot air leveling without the troublesome flatness issues of hot air leveling; immersion tin also avoids the diffusion problems of intermetallics in Electroless Nickel/Immersion Gold—copper-tin intermetallics can bond securely. Immersion tin boards should not be stored for too long, and assembly must follow the sequence of immersion tin treatment.
6. Other Surface Treatment Processes
Other surface treatment processes are less commonly applied. Below, we will look at the relatively more common processes such as Nickel-Gold Plating and Electroless Palladium processes. Nickel-Gold Plating is the pioneer of PCB surface treatment processes; it has existed since the advent of PCBs and has gradually evolved into other methods. It involves first plating a layer of nickel on the PCB surface conductor and then plating a layer of gold; the nickel plating primarily prevents diffusion between gold and copper. Current Nickel-Gold Plating processes can be divided into two categories: Soft Gold (pure gold, which appears dull) and Hard Gold (smooth and hard surface, wear-resistant, containing cobalt and other elements, with a brighter gold surface). Soft gold is mainly used for wire bonding in chip packaging; hard gold is primarily used for electrical interconnections in non-soldering areas. Considering costs, the industry often uses image transfer methods for selective plating to reduce gold usage. Currently, the use of selective gold plating in the industry continues to increase, mainly due to the difficult process control of Electroless Nickel/Immersion Gold. Normally, soldering can cause plated gold to become brittle, reducing its lifespan, so soldering on plated gold should be avoided; however, Electroless Nickel/Immersion Gold tends to have a thin and consistent gold layer, making brittleness less likely. The Electroless Palladium process is similar to the Electroless Nickel process. The main process involves using a reducing agent (such as sodium hypophosphite) to reduce palladium ions on the catalytic surface to palladium, with the newly formed palladium acting as a catalyst to drive the reaction, allowing for any thickness of palladium plating. The advantages of Electroless Palladium include good solder reliability, thermal stability, and surface flatness.
Selection of Surface Treatment Processes
The selection of surface treatment processes mainly depends on the type of components to be assembled. The surface treatment process will affect PCB production, assembly, and final use. Below, we will specifically introduce the common usage scenarios for the five surface treatment processes.
1. Hot Air Leveling
Hot air leveling once dominated PCB surface treatment processes. In the 1980s, over three-quarters of PCBs used hot air leveling; however, the industry has been reducing its use over the past decade, estimating that currently about 25%-40% of PCBs use hot air leveling. The hot air leveling process is relatively dirty, unpleasant, and hazardous, making it never a favored process, but it is excellent for larger components and wider spacing wires.In high-density PCBs, the flatness of hot air leveling will affect subsequent assembly; therefore, HDI boards generally do not adopt hot air leveling. With advancements in technology, there are now hot air leveling processes suitable for smaller assembly spacings like QFP and BGA, but actual applications are rare. Currently, some factories use organic coating and Electroless Nickel/Immersion Gold processes to replace hot air leveling; technological advancements have also led some factories to adopt immersion tin and immersion silver processes. Coupled with the trend towards lead-free processes in recent years, the use of hot air leveling has been further restricted. Although so-called lead-free hot air leveling has emerged, it raises compatibility issues with equipment.
2. Organic Coating
It is estimated that currently about 25%-30% of PCBs use organic coating processes, and this proportion has been rising (likely organic coating has now surpassed hot air leveling to become the most used). Organic coating processes can be used on both low-tech and high-tech PCBs, such as single-sided TV PCBs and high-density chip packaging boards. For BGA, organic coating is also widely applied. If the PCB does not have functional connectivity requirements or storage period limitations, organic coating is the ideal surface treatment process.
3. Electroless Nickel/Immersion Gold
The Electroless Nickel/Immersion Gold process differs from organic coating; it is mainly used on boards that have functional connectivity requirements and longer storage periods, such as the button areas of mobile phones, edge connection areas of router housings, and electrically contact areas of chip processors. Due to the flatness issues of hot air leveling and the cleaning problems of organic coating flux, the use of Electroless Nickel/Immersion Gold became widespread in the 1990s; however, its application has decreased due to the emergence of black pad and brittle nickel-phosphorus alloys. Nevertheless, almost every high-tech PCB manufacturer now has Electroless Nickel/Immersion Gold lines.Considering that removing copper-tin intermetallic compounds can make solder joints brittle, many issues arise at relatively brittle nickel-tin intermetallics. Therefore, portable electronic products (such as mobile phones) almost exclusively use organic coating, immersion silver, or immersion tin to form copper-tin intermetallic solder joints, while employing Electroless Nickel/Immersion Gold for button areas, contact areas, and EMI shielding areas. It is estimated that currently about 10%-20% of PCBs use the Electroless Nickel/Immersion Gold process.
4. Immersion Silver
Immersion silver is cheaper than Electroless Nickel/Immersion Gold; if the PCB has functional connectivity requirements and needs to reduce costs, immersion silver is a good choice. With immersion silver’s good flatness and contact properties, it should be chosen. Immersion silver is widely applied in communication products, automotive, and computer peripherals, and is also used in high-speed signal designs. EMS recommends using immersion silver due to its ease of assembly and good inspectability. However, immersion silver has slow growth due to defects such as loss of luster and solder joint voids (but no decline). It is estimated that currently about 10%-15% of PCBs use immersion silver processes.
5. Immersion Tin
The introduction of immersion tin as a surface treatment process is a recent development within the last decade, resulting from the demand for production automation. Immersion tin does not introduce any new elements at solder joints, making it particularly suitable for communication backplanes. Beyond the board’s storage period, tin will lose solderability, so immersion tin requires good storage conditions. Additionally, the use of carcinogenic substances in immersion tin processes has led to restrictions. It is estimated that currently about 5%-10% of PCBs use immersion tin processes.Source: Internet