Long-term gradual changes will lead to significant changes. With the increasing call for environmental protection, the surface treatment processes for PCBs will definitely undergo major changes in the future.
The Purpose of Surface Treatment
The most fundamental purpose of surface treatment is to ensure good solderability or electrical performance. Due to the tendency of copper in nature to exist in the form of oxides in the air, it is unlikely to remain as pure copper for long, hence the need for additional treatment of copper. Although strong flux can be used to remove most copper oxides during subsequent assembly, strong flux itself is difficult to remove; therefore, it is generally not used in the industry.
Five Common Surface Treatment Processes
There are many PCB surface treatment processes available today, the most common being hot air leveling, organic coating, chemical nickel/gold plating, immersion silver, and immersion tin. Below, each will be introduced in detail.
Also known as hot air solder leveling, it is a process where molten tin-lead solder is coated on the PCB surface and leveled with heated compressed air to form a coating that resists copper oxidation while providing good solderability. During hot air leveling, a copper-tin intermetallic compound forms at the joint between the solder and copper. The thickness of the solder protecting 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 it solidifies; the air knife minimizes the meniscus shape of the solder on the copper surface and prevents solder bridging. Hot air leveling can be divided into vertical and horizontal types, with horizontal being generally considered better, mainly due to its more uniform coating, allowing for automated production. The general process for hot air leveling is: micro-etching → preheating → applying flux → tin spraying → cleaning.
2. Organic Coating Process
Unlike other surface treatment processes, it acts as a barrier layer between copper and air; the organic coating process is simple and cost-effective, which allows it to be widely used in the industry. Early organic coating molecules were imidazole and benzotriazole for rust prevention, while the latest molecules mainly consist of 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 necessary. This is why copper solution is often added in the chemical tank. After applying the first layer, the coating layer adsorbs copper; then the second layer’s organic coating molecules bond with the copper, continuing up to twenty or even hundreds of layers of organic coating molecules accumulating on the copper surface, ensuring multiple reflow soldering. Tests 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 cleaning → pure water cleaning → 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 give the PCB a thick armor. Additionally, the chemical nickel/gold plating process is not merely a rust prevention barrier layer; it can provide useful and good electrical performance during the long-term use of the PCB. 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 for an extended period; it also has environmental resistance that other surface treatment processes do not possess. The reason for nickel plating is that gold and copper can diffuse into each other, and the nickel layer can prevent 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 the nickel; just a 5-micron thickness of nickel can limit expansion in the Z-direction at high temperatures. Additionally, chemical nickel/gold plating can prevent copper dissolution, which is beneficial for lead-free assembly. The general process for chemical nickel/gold plating is: acidic cleaning → micro-etching → pre-immersion → activation → chemical nickel plating → chemical gold immersion, mainly involving six chemical tanks and nearly 100 types of chemicals, making process control quite difficult.
4. Immersion Silver Process
Situated between organic coating and chemical nickel/gold plating, the process is relatively simple and quick; it is not as complex as chemical nickel/gold plating, nor does it provide a thick armor for the PCB, but it can still provide good electrical performance. Silver is the little brother of gold; even when exposed to heat, moisture, and pollution, silver can still maintain good solderability, although it may lose its luster. Immersion silver does not possess the good physical strength that chemical nickel/gold plating has because there is no nickel beneath the silver layer. Moreover, immersion silver has good storage properties; it can be assembled years after immersion without major issues. Immersion silver is a displacement reaction, resulting in almost sub-micron level pure silver coating. Sometimes, the immersion silver process also includes some organics, primarily to prevent silver corrosion and eliminate silver migration issues; this thin layer of organic material is generally difficult to measure, with analyses indicating that the organic weight is less than 1%.
Currently, all solders are tin-based, making the tin layer compatible with any type of solder. From this perspective, the immersion tin process has great development potential. However, previous PCBs experienced tin whiskers after immersion tin processing, leading to reliability issues during soldering due to whiskers and tin migration, thus limiting the adoption of immersion tin processes. Later, organic additives were added to the immersion tin solution, allowing the tin layer structure to form a granular structure, overcoming previous issues while also providing good thermal stability and solderability. The immersion tin process can form a flat copper-tin intermetallic compound, a characteristic that gives immersion tin similar good solderability to hot air leveling without the troublesome flatness issues associated with hot air leveling; immersion tin also does not have the diffusion issues between metals found in chemical nickel/gold plating—copper-tin intermetallic compounds can bond stably together. Immersion tin boards cannot be stored for too long, and assembly must follow the order of immersion tin processing.
6. Other Surface Treatment Processes
Other surface treatment processes are used less frequently; below we look at the more commonly applied nickel-gold electroplating and chemical palladium plating processes. Nickel-gold electroplating is the ancestor of PCB surface treatment processes; it has been around since PCBs emerged and has gradually evolved into other methods. It involves first plating a layer of nickel on the PCB surface and then a layer of gold; nickel plating primarily prevents diffusion between gold and copper. There are currently two types of nickel-gold electroplating: soft gold (pure gold, which does not appear shiny) and hard gold (smooth and hard surface, wear-resistant, containing cobalt and other elements, with a shinier gold surface). Soft gold is mainly used for bonding wires during chip packaging; hard gold is mainly used for electrical interconnections in non-soldering areas. Considering costs, the industry often employs selective electroplating through image transfer methods to reduce gold usage. Selective nickel-gold electroplating has been increasingly adopted in the industry, mainly due to the difficulties associated with process control in chemical nickel/gold plating. Normally, soldering will cause electroplated gold to become brittle, which shortens its lifespan; therefore, soldering on electroplated gold should be avoided; however, because the gold in chemical nickel/gold plating is very thin and consistent, brittleness is rarely observed. The process of chemical palladium plating is similar to that of chemical nickel plating. The main process involves reducing palladium ions on the catalytic surface using a reducing agent (such as sodium hypophosphite) to form palladium, with the newly formed palladium acting as a catalyst for the reaction, allowing for any desired thickness of palladium plating. The advantages of chemical palladium plating include good solder reliability, thermal stability, and surface flatness.
Choosing Surface Treatment Processes
The choice of surface treatment processes mainly depends on the type of components to be assembled ultimately; the surface treatment process will affect PCB production, assembly, and final use. Below, we will specifically introduce the common five surface treatment processes and their applicable scenarios.
Hot air leveling used to dominate PCB surface treatment processes. In the 1980s, over three-quarters of PCBs used hot air leveling processes, but the industry has been reducing the use of hot air leveling processes over the past decade, with an estimated current usage of about 25%-40% of PCBs. The hot air leveling process is quite dirty, smelly, and dangerous, thus it has never been a favored process, but it is excellent for larger components and wider lead spacing.
In high-density PCBs, the flatness of hot air leveling will affect subsequent assembly; therefore, HDI boards generally do not use hot air leveling processes. With technological advancements, hot air leveling processes suitable for smaller assembly spacings, such as QFP and BGA, have emerged, but practical applications remain limited. Currently, some factories are replacing hot air leveling processes with organic coating and chemical nickel/gold plating processes; technological advancements have also led some factories to adopt immersion tin and immersion silver processes. Additionally, the trend towards lead-free soldering in recent years has further restricted the use of hot air leveling. Although so-called lead-free hot air leveling has emerged, it may involve compatibility issues with equipment.
It is estimated that about 25%-30% of PCBs currently use organic coating processes, and this proportion has been steadily increasing (likely making organic coating the leading process, surpassing hot air leveling). Organic coating processes can be used in both low-technology and high-technology PCBs, such as single-sided TV PCBs and high-density chip packaging boards. For BGA applications, organic coating is also quite common. If a PCB does not have functional requirements for surface connectivity or storage period limitations, organic coating will be the ideal surface treatment process.
3. Chemical Nickel/Gold Plating
The chemical nickel/gold plating process differs from organic coating in that it is mainly used on boards with functional connectivity requirements and longer storage periods, such as in the button areas of mobile phones, edge connection areas of router casings, and electrical contact areas for flexible connections of chip processors. Due to the flatness issues with hot air leveling and the removal problems of organic coating flux, chemical nickel/gold plating saw widespread use in the 1990s; however, its application has decreased due to the emergence of black pads and brittle nickel-phosphorus alloys. Nonetheless, almost every high-tech PCB factory now has a chemical nickel/gold plating line.
Considering that removing copper-tin intermetallic compounds can make solder joints brittle, relatively brittle nickel-tin intermetallic compounds can lead to many issues. Therefore, portable electronic products (like mobile phones) almost exclusively use organic coatings, immersion silver, or immersion tin to form solder joints with copper-tin intermetallic compounds, while chemical nickel/gold plating is used for button areas, contact areas, and EMI shielding areas. It is estimated that about 10%-20% of PCBs currently use chemical nickel/gold plating processes.
Immersion silver is cheaper than chemical nickel/gold plating; if a PCB has functional connectivity requirements and needs to reduce costs, immersion silver is a good choice; coupled with its good flatness and contact properties, immersion silver should be selected. Immersion silver is widely used in communication products, automotive applications, and computer peripherals, and it is also applied in high-speed signal design. EMS recommends the use of immersion silver processes due to their ease of assembly and good inspectability. However, due to defects such as loss of luster and solder joint voids, the growth of immersion silver has been slow (but not declining). It is estimated that about 10%-15% of PCBs currently use immersion silver processes.
The introduction of immersion tin as a surface treatment process has occurred in the past decade, resulting from the demands of production automation. Immersion tin does not introduce any new elements during soldering, making it particularly suitable for communication backplanes. Beyond the storage period, tin will lose solderability, thus immersion tin requires good storage conditions. Additionally, the use of carcinogenic substances in the immersion tin process has led to restrictions on its use. It is estimated that about 5%-10% of PCBs currently use immersion tin processes.
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