1. With the continuous improvement of human requirements for the living environment, the environmental issues associated with current PCB production processes have become increasingly prominent. Currently, concerns about lead and bromine are particularly prevalent; lead-free and halogen-free materials will significantly impact PCB development. Although changes in PCB surface treatment processes may appear minor at present, gradual and sustained adjustments can lead to substantial transformations. With the growing emphasis on environmental protection, PCB surface treatment processes are expected to undergo significant changes in the future.
2. How can we “advance with the times” to address PCB surface technology?
3. Purpose of Surface Treatment
The primary purpose of surface treatment is to ensure good solderability and electrical properties. Since natural copper tends to form oxides when exposed to air, it is unlikely to remain in its original state for long, necessitating additional treatments. Although strong fluxes can remove most copper oxides during assembly, these fluxes themselves are difficult to remove, leading the industry to generally avoid their use.
4. Five Common Surface Treatment Processes
There are several PCB surface treatment processes, including hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver, and immersion tin. Each of these processes will be introduced in detail below.
5. Hot Air Leveling
Also known as hot-air solder leveling, this process involves coating the PCB surface with molten tin-lead solder and then flattening it using heated compressed air to form a coating that resists copper oxidation and ensures good solderability. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the interface. The solder layer protecting the copper surface is typically about 1-2 mils thick. In this process, the PCB is immersed in molten solder; the air knife blows the liquid solder before it solidifies, minimizing the meniscus of the solder on the copper surface and preventing solder bridging. Hot air leveling can be performed in two orientations: vertical and horizontal. Generally, the horizontal type is preferred due to its more uniform application and suitability for automated production.
6. The typical hot air leveling process includes: micro-etching, preheating, coating with flux, tin spraying, and cleaning.
2. Organic Coating Process
Unlike other surface treatment processes, the organic coating process acts as a barrier between copper and air. It is simple and cost-effective, making it widely used in the industry. Early organic coating molecules included imidazole and benzotriazole, which played a role in rust prevention. The latest molecules are primarily benzimidazole, which chemically bonds nitrogen functional groups to the PCB. During soldering, a single organic coating layer is insufficient; multiple layers are necessary. Hence, copper liquid is often added to the chemical tank. After the first layer is coated, it adsorbs copper; then, the second layer’s organic coating molecules combine with the copper. This process continues until twenty or even hundreds of organic coating molecules are gathered on the copper surface, ensuring multiple cycles of flow welding. Tests show that the latest organic coating processes maintain good performance during multiple lead-free soldering cycles. The general flow of the organic coating process is: degreasing-micro-etching-pickling-pure water cleaning-organic coating-cleaning. Process control is simpler than in other surface treatments.
3. Electroless Nickel Plating / Immersion Gold Chemical Nickel Plating / Immersion Gold Process
This process is more complex than organic coating. Electroless nickel/immersion gold applies a robust layer to the PCB. Unlike organic coatings, which act as anti-rust barriers, electroless nickel/immersion gold provides long-term protection and superior electrical performance. This method involves wrapping a durable nickel-gold alloy around the copper surface, offering extended protection and environmental benefits not found in other treatments. Nickel plating is necessary because gold and copper can diffuse into each other, and the nickel layer prevents this diffusion. Additionally, electroless nickel/immersion gold prevents copper dissolution, benefiting lead-free assembly. The general process for electroless nickel plating/gold immersion is: acid cleaning-micro-etching-pre-dipping-activation-electroless nickel plating-chemical immersion gold. This involves six chemical tanks and nearly 100 chemicals, making process control more challenging.
2. How can we “advance with the times” to address PCB surface technology?
3. Purpose of Surface Treatment
The primary purpose of surface treatment is to ensure good solderability and electrical properties. Since natural copper tends to form oxides when exposed to air, it is unlikely to remain in its original state for long, necessitating additional treatments. Although strong fluxes can remove most copper oxides during assembly, these fluxes themselves are difficult to remove, leading the industry to generally avoid their use.
4. Five Common Surface Treatment Processes
There are several PCB surface treatment processes, including hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver, and immersion tin. Each of these processes will be introduced in detail below.
5. Hot Air Leveling
Also known as hot-air solder leveling, this process involves coating the PCB surface with molten tin-lead solder and then flattening it using heated compressed air to form a coating that resists copper oxidation and ensures good solderability. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the interface. The solder layer protecting the copper surface is typically about 1-2 mils thick. In this process, the PCB is immersed in molten solder; the air knife blows the liquid solder before it solidifies, minimizing the meniscus of the solder on the copper surface and preventing solder bridging. Hot air leveling can be performed in two orientations: vertical and horizontal. Generally, the horizontal type is preferred due to its more uniform application and suitability for automated production.
6. The typical hot air leveling process includes: micro-etching, preheating, coating with flux, tin spraying, and cleaning.
2. Organic Coating Process
Unlike other surface treatment processes, the organic coating process acts as a barrier between copper and air. It is simple and cost-effective, making it widely used in the industry. Early organic coating molecules included imidazole and benzotriazole, which played a role in rust prevention. The latest molecules are primarily benzimidazole, which chemically bonds nitrogen functional groups to the PCB. During soldering, a single organic coating layer is insufficient; multiple layers are necessary. Hence, copper liquid is often added to the chemical tank. After the first layer is coated, it adsorbs copper; then, the second layer’s organic coating molecules combine with the copper. This process continues until twenty or even hundreds of organic coating molecules are gathered on the copper surface, ensuring multiple cycles of flow welding. Tests show that the latest organic coating processes maintain good performance during multiple lead-free soldering cycles. The general flow of the organic coating process is: degreasing-micro-etching-pickling-pure water cleaning-organic coating-cleaning. Process control is simpler than in other surface treatments.
3. Electroless Nickel Plating / Immersion Gold Chemical Nickel Plating / Immersion Gold Process
This process is more complex than organic coating. Electroless nickel/immersion gold applies a robust layer to the PCB. Unlike organic coatings, which act as anti-rust barriers, electroless nickel/immersion gold provides long-term protection and superior electrical performance. This method involves wrapping a durable nickel-gold alloy around the copper surface, offering extended protection and environmental benefits not found in other treatments. Nickel plating is necessary because gold and copper can diffuse into each other, and the nickel layer prevents this diffusion. Additionally, electroless nickel/immersion gold prevents copper dissolution, benefiting lead-free assembly. The general process for electroless nickel plating/gold immersion is: acid cleaning-micro-etching-pre-dipping-activation-electroless nickel plating-chemical immersion gold. This involves six chemical tanks and nearly 100 chemicals, making process control more challenging.
