Long-term slow changes will lead to significant transformations. With the increasing calls for environmental protection, the surface treatment processes for PCBs will undoubtedly undergo tremendous changes in the future.
The Purpose of Surface Treatment
The most fundamental purpose of surface treatment is to ensure good solderability or electrical performance. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as pure copper for long. Therefore, additional treatments for copper are necessary. Although strong fluxes can be used to remove most copper oxides during subsequent assembly, strong fluxes themselves are difficult to remove, which is why they are generally not adopted in the industry.
Five Common Surface Treatment Processes
Currently, there are many PCB surface treatment processes, the most common being Hot Air Solder Leveling, Organic Coating, Chemical Nickel/Gold Plating, Silver Plating, and Tin Plating. Below, each will be introduced one by one.
1. Hot Air Solder Leveling
Also known as Hot Air Solder Leveling, this process involves coating the PCB surface with molten tin-lead solder and leveling it using heated compressed air, forming a layer that is resistant to copper oxidation and provides good solderability. During hot air leveling, intermetallic compounds of copper and tin form at the joint. The solder thickness on the copper surface is approximately 1-2 mils. During hot air leveling, the PCB is immersed in molten solder; the air knife levels the liquid solder before solidification; the air knife minimizes the convex shape of the solder on the copper surface and prevents solder bridging. Hot air leveling can be categorized into vertical and horizontal types, with horizontal generally considered better due to its more uniform plating layer, allowing for automated production. The typical process flow for hot air leveling is: Micro-etching → Preheating → Flux Application → Tin Spraying → Cleaning.
2. Organic Coating Process
Unlike other surface treatment processes, it acts as a barrier between copper and air; the organic coating process is simple and cost-effective, which allows for its widespread use in the industry. Early organic coatings used imidazole and benzotriazole as rust inhibitors, while the latest molecules primarily use benzotriazole, which chemically bonds nitrogen functional groups to the copper on the PCB. During subsequent soldering, having only a single layer of organic coating on the copper surface is insufficient; multiple layers are necessary. This is why copper liquid is usually added to the chemical bath. After applying the first layer, the coating layer adsorbs copper; then the second layer of organic coating molecules bonds with the copper, continuing until twenty or even hundreds of layers of organic coating molecules accumulate on the copper surface, ensuring multiple reflow soldering processes. Tests show that the latest organic coating processes can maintain good performance over multiple lead-free soldering processes. The typical process flow for organic coating is: Degreasing → Micro-etching → Acid Cleaning → Pure Water Rinse → Organic Coating → Cleaning, with process control being relatively easier compared to other surface treatment processes.
3. Chemical Nickel/Gold Plating
The chemical nickel/gold plating process is not as simple as organic coating; it seems to provide a thick armor for the PCB. Additionally, unlike organic coatings that serve as rust barrier layers, chemical nickel/gold plating can be useful during long-term use of the PCB and achieve good electrical performance. Therefore, chemical nickel/gold plating involves wrapping a thick layer of electrically conductive nickel-gold alloy around the copper surface, which can protect the PCB over the long term; it also possesses environmental tolerance that other surface treatment processes do not have. The reason for nickel plating is to prevent the diffusion between gold and copper; the nickel layer can stop the diffusion between gold and copper; without the nickel layer, gold would diffuse into copper within hours. Another benefit of chemical nickel/gold plating is the strength of nickel; even a thickness of just 5 microns of nickel can limit expansion in the Z-direction at high temperatures. Furthermore, chemical nickel/gold plating can prevent the dissolution of copper, which is beneficial for lead-free assembly. The typical process flow for chemical nickel/gold plating is: Acid Cleaning → Micro-etching → Pre-dipping → Activation → Chemical Nickel Plating → Chemical Gold Dipping, involving six chemical baths and nearly 100 types of chemicals, making process control relatively difficult.
4. Silver Plating Process
Situated between organic coating and chemical nickel/gold plating, this process is relatively simple and fast; it is not as complex as chemical nickel/gold plating nor does it provide a thick armor for the PCB, yet it still offers good electrical performance. Silver, being the little brother of gold, can maintain good solderability even when exposed to heat, humidity, and pollution, although it may lose its luster. Silver plating does not possess the good physical strength that chemical nickel/gold plating has because there is no nickel beneath the silver layer. Additionally, silver plating has good storage properties; it can be assembled after being silver-plated for several years without significant issues. Silver plating is a displacement reaction, resulting in nearly sub-micron levels of pure silver coating. Sometimes, the silver plating process also includes some organic materials, mainly to prevent silver corrosion and eliminate silver migration issues; this thin layer of organic material is generally difficult to measure, with analyses showing that the weight of the organic material is less than 1%.
Since all solder is currently tin-based, the tin layer can match any type of solder. From this perspective, tin plating has great development potential. However, previous PCBs have experienced tin whiskers after undergoing tin plating, which can lead to reliability issues during soldering due to whiskers and tin migration, thus limiting the adoption of tin plating. Later, organic additives were added to the tin plating solution, allowing the tin layer structure to adopt a granular structure, overcoming previous issues while also providing good thermal stability and solderability. The tin plating process can form flat copper-tin intermetallic compounds, a characteristic that gives tin plating good solderability similar to hot air leveling without the troublesome flatness issues of hot air leveling; tin plating also avoids the diffusion problems of chemical nickel/gold plating—copper-tin intermetallic compounds can bond stably together. Tin-plated boards should not be stored for too long; assembly must follow the order of tin plating.
6. Other Surface Treatment Processes
Other surface treatment processes are less commonly applied; below we will look at the relatively more common electroplated nickel-gold and chemical palladium plating processes. Electroplated nickel-gold is the ancestor of PCB surface treatment processes, appearing alongside the advent of PCBs and gradually evolving into other methods. In this process, a layer of nickel is first plated onto the PCB surface, followed by a layer of gold; nickel plating mainly prevents diffusion between gold and copper. Currently, there are two types of electroplated nickel-gold: soft gold (pure gold, which does not appear shiny) and hard gold (smooth and hard surface, wear-resistant, containing other elements like cobalt, and appears shinier). Soft gold is mainly used for bonding wires in chip packaging; hard gold is mainly used for electrical interconnection in non-soldering areas. Considering costs, the industry often employs 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 complex process control of chemical nickel/gold plating. Normally, soldering can make electroplated gold brittle, shortening its lifespan, so soldering on electroplated gold should be avoided; however, due to the thin and consistent gold layer in chemical nickel/gold plating, brittleness rarely occurs. The process of chemical palladium plating is similar to that of chemical nickel plating. The main process involves using a reducing agent (such as sodium hypophosphite) to reduce palladium ions on a catalytic surface into palladium, with the newly formed palladium acting as a catalyst for the reaction, allowing for any thickness of palladium plating. The advantages of chemical palladium plating include good soldering reliability, thermal stability, and surface flatness.
Selection of Surface Treatment Processes
The selection of surface treatment processes mainly depends on the type of components that will be assembled at the end; the surface treatment process will affect the PCB’s production, assembly, and final use. Below, we will specifically introduce the common applications of the five surface treatment processes.
1. Hot Air Solder Leveling
Hot air solder leveling once dominated PCB surface treatment processes. In the 1980s, over three-quarters of PCBs used hot air solder leveling; however, the industry has been reducing its use over the past decade, with an estimated 25%-40% of PCBs currently using this process. The hot air leveling process is relatively dirty, unpleasant, and hazardous, thus never a popular choice, but it is excellent for larger components and wider pitch wires.
In high-density PCBs, the flatness of hot air leveling will affect subsequent assembly; thus, HDI boards generally do not use hot air leveling. With technological advancements, hot air leveling processes suitable for smaller assembly pitches like QFP and BGA have emerged, but practical applications are rare. Currently, some factories are using organic coating and chemical nickel/gold plating processes to replace hot air leveling; technological developments have also led some factories to adopt tin plating and silver plating. Additionally, the trend towards lead-free processes has further limited the use of hot air leveling. Although there are now so-called lead-free hot air leveling processes, this involves compatibility issues with equipment.
It is estimated that about 25%-30% of PCBs currently use organic coating processes, a proportion that has been rising (likely making organic coating the leading process now, surpassing hot air leveling).
The organic coating process can be used for both low-technology and high-technology PCBs, such as single-sided TV PCBs and high-density chip packaging boards. Organic coating is also widely used for BGA applications. If a PCB does not have functional connectivity requirements or storage period limitations, organic coating will be the ideal surface treatment process.
3. Chemical Nickel/Gold Plating
Unlike organic coating, the chemical nickel/gold plating process is mainly used for boards with functional connectivity requirements and longer storage periods, such as the edge connection areas of mobile phone keypads, router housings, and the electrical contact areas for elastic connections of chip processors. Due to the flatness issues of hot air leveling and the removal of organic coating flux, chemical nickel/gold plating was widely used in the 1990s; however, its use has decreased due to the emergence of black pads and brittle nickel-phosphorus alloys. Nevertheless, almost every high-tech PCB factory now has chemical nickel/gold plating lines.
Considering that solder joints may become brittle when removing copper-tin intermetallic compounds, many issues arise at the relatively brittle nickel-tin intermetallic compounds. Therefore, portable electronic products (such as mobile phones) almost exclusively use organic coating, silver plating, or tin plating to form copper-tin intermetallic compound solder joints, while chemical nickel/gold plating is used for key areas, contact areas, and EMI shielding areas. It is estimated that about 10%-20% of PCBs currently use chemical nickel/gold plating processes.
Silver plating is cheaper than chemical nickel/gold plating; if a PCB has functional connectivity requirements and needs to reduce costs, silver plating is a good choice; along with its good flatness and contact properties, silver plating should be chosen. Silver plating is widely applied in communication products, automotive, and computer peripherals, and is also used in high-speed signal designs. EMS recommends using silver plating because it is easy to assemble and has good inspectability. However, due to defects such as loss of luster and solder voids, its growth has been slow (but not declining). It is estimated that about 10%-15% of PCBs currently use silver plating processes.
This introduced surface treatment process has emerged in the past decade as a result of the demands for production automation. Tin plating does not introduce any new elements at the soldering points, making it particularly suitable for communication backplanes. Beyond the board’s storage period, tin will lose its solderability, thus requiring good storage conditions for tin plating. Additionally, the use of carcinogenic substances in the tin plating process has led to restrictions on its use. It is estimated that about 5%-10% of PCBs currently use tin plating processes.
Source: Internet
