As one of the core components of modern electronic equipment, the surface finish of printed circuit board (PCB) not only affects the appearance quality, but more importantly, it is directly related to electrical performance, reliability and production efficiency. Therefore, a reasonable choice of PCB surface treatment is essential to ensure product performance and reduce costs.

This paper will discuss the importance of PCB surface finish, common treatment methods and their application scenarios in detail, aiming to provide a scientific basis for engineers and designers to make decisions.

What is surface finish

Surface finish refers to the smoothness of the surface of the object, which is an important index to measure the micro-geometric shape characteristics of the surface of the material. In the manufacture of printed circuit board (PCB), the surface finish not only affects the appearance quality of the PCB, but more importantly, is directly related to the reliability of the electrical connection, moisture resistance and corrosion resistance, and thermal management performance. Good surface finish can improve the welding quality and reduce the occurrence of problems such as false welding and missing welding. At the same time, it can effectively prevent the erosion of moisture and harmful gases, and extend the service life of the circuit board. In addition, for electronic devices with high power density, excellent surface treatment can also increase the heat transfer efficiency and help the PCB better heat dissipation. Therefore, in the PCB design and manufacturing process, it is crucial to choose the right surface finish treatment method.

The importance of PCB surface finish

1. Ensure reliability of electrical connections

The copper foil circuit and pad on the PCB need to maintain good electrical conductivity to ensure the smooth transmission of current. Proper surface treatment can improve the welding quality and reduce the occurrence of problems such as virtual welding and missing welding, thus improving the overall reliability of the circuit board.

2. Enhance moisture-proof and anti-corrosion ability

When working in a humid or corrosive environment, the untreated PCB surface is easy to oxidize and rust, resulting in reduced electrical performance or even failure. Through surface treatment, a protective film can be formed on the surface of the circuit board to effectively prevent the erosion of water and harmful gases.

3. Improve thermal management

For electronic devices with high power density, effective heat dissipation measures are crucial. A good surface treatment can increase the heat transfer efficiency, help the PCB better dissipate heat, and extend the service life.

4. Improve appearance quality

Although the appearance is not the main factor in determining the performance of the PCB, for some consumer electronics products, beautiful and generous circuit boards can also attract the attention of consumers and increase the market competitiveness of the product.

Common PCB surface treatment methods

1. Hot Air Solder Leveling (HASL)

Process flow
Preheat: Heat the PCB to a certain temperature to soften the solder paste.
Solder dipping: The PCB is immersed in the melted solder, so that the surface of the solder pad is evenly covered with a layer of tin.
Air cooling: Use hot air to blow the PCB surface to remove excess solder and form a smooth surface.
Merits and demerits
Advantages: low cost, wide application range, good weldability and heat resistance.
Disadvantages: May produce slight uneven, not suitable for fine pitch components welding.

2. Organic Solderability Preservative (OSP)

Process flow
Clean: Remove oil and oxide from PCB surface.
Coating: Spray a thin layer of organic flux on the surface of the pad.
Drying: The organic flux is cured by heating to form a protective film.
Merits and demerits
Advantages: Environmentally friendly, lead-free, suitable for welding of fine-pitch components, does not affect signal transmission.
Disadvantages: poor heat resistance, long-term exposure to high temperature environment may fail.

3. Electroless Nickel Immersion Gold (ENIG)

Process flow
Chemical nickel: A layer of nickel is deposited on the surface of the pad by a chemical reaction.
Electroplating: A thin layer of gold is deposited on top of the nickel layer to increase oxidation resistance and weldability.
Merits and demerits
Advantages: high surface smoothness, suitable for BGA and other high-density package components welding, with good oxidation resistance and corrosion resistance.

Disadvantages: high cost, complex process, there is a risk of gold migration.

4. Immersion Silver

Process flow
Clean: Remove contaminants from the PCB surface.
Silver deposition: A layer of silver is deposited on the surface of the pad by a chemical reaction.
Protection: Coat the surface of the silver layer with a layer of antioxidants to prevent oxidation of the silver layer.
Merits and demerits
Advantages: moderate cost, good weldability and oxidation resistance, suitable for the welding of fine-pitch components.
Disadvantages: there may be discoloration after a long time of storage, affecting the appearance quality.

5. Immersion Tin

Process flow
Clean: Remove oil and oxide from PCB surface.
Tin deposition: A layer of tin is deposited on the surface of the pad by chemical reaction.
Protection: Apply a layer of antioxidants to the surface of the tin layer to prevent oxidation of the tin layer.
Merits and demerits
Advantages: Low cost, good weldability and oxidation resistance, suitable for the welding of fine-pitch components.
Disadvantages: After long-term storage, there may be problems with the growth of tin, which affects the stability of the circuit.

How to choose the right PCB surface finish

1. Select based on the application environment

Indoor dry environment: Lower cost OSP or tin can be selected.
Outdoor humid environment: Strong corrosion resistance ENIG or silver is recommended.
High temperature environment: It is recommended to use HASL or ENIG with good heat resistance.

2. Consider the component type

Common SMD components: OSP or sunk tin can meet the requirements.
High-density packaging components such as BGA: ENIG or silver sinking is recommended to ensure welding quality and reliability.

3. Comprehensive consideration of cost and performance

Low cost requirements: OSP or sunk tin is preferred.
High performance requirements: Choose ENIG or silver, which, despite the higher cost, provides better electrical performance and reliability.

4. Focus on production process compatibility

When selecting a surface treatment method, it is also necessary to consider compatibility with existing production lines to avoid reducing production efficiency or increasing costs due to the introduction of new processes.

conclusion

In summary, choosing the right PCB surface finish is a comprehensive technical decision that needs to be weighed against a number of factors such as the specific application environment, component type, cost budget, and production process. Through in-depth understanding of the characteristics and scope of application of various surface treatment methods, combined with actual needs to develop the best solution, not only can effectively improve the electrical performance and reliability of PCB, but also optimize the production cost, enhance the market competitiveness of products.
I hope this article can be a powerful assistant in your PCB design and manufacturing process to help your product development more smoothly and efficiently. If you have any questions or need further assistance, please feel free to contact us.