A Brief Description Of The Surface Treatment Process Of The Circuit Board PCB？

PCB surface treatment technology refers to the process of artificially forming a surface layer different from the mechanical, physical and chemical properties of the substrate on the PCB components and electrical connection points. Its purpose is to ensure good solderability or electrical performance of the PCB. Since copper tends to exist in the form of oxides in the air, which seriously affects the solderability and electrical performance of the PCB, surface treatment of the PCB is required.

1. Hot air leveling (spray tin HASL)

    Wave soldering is the best method of soldering where through-hole devices are predominant. The use of hot air leveling surface treatment technology is sufficient to meet the process requirements of wave soldering. Of course, for the occasions where the joint strength (especially the contact connection) is required to be high, the method of electroplating nickel/gold is mostly used. HASL is the main surface treatment technology used worldwide, but there are three main driving forces that drive the electronics industry to consider alternatives to HASL: cost, new process requirements and lead-free requirements.

2. Organic antioxidant (OSP)

    Organic Solderability Protective Layer is an organic coating used to prevent copper oxidation before soldering, that is, to protect the solderability of PCB pads from damage.

    After the surface of the PCB is treated with OSP, a thin layer of organic compounds is formed on the surface of the copper to protect the copper from being oxidized. The thickness of Benzotriazoles type OSP is generally 100 A°, while the thickness of Imidazoles type OSP is thicker, generally 400 A°. OSP film is transparent, its existence is not easy to be identified by naked eyes, and detection is difficult. During the assembly process (reflow soldering), the OSP is easily melted into the solder paste or the acidic Flux, and at the same time, the copper surface with strong activity is exposed, and finally a Sn/Cu intermetallic compound is formed between the component and the pad. Therefore, OSP has very good characteristics for treating the welding surface. OSP has no lead pollution problem, so it is environmentally friendly.

3. Immersion Gold (ENIG)

    Protection mechanism of ENIG: A thick layer of nickel-gold alloy with good electrical properties is wrapped on the copper surface and can protect the PCB for a long time. Unlike OSP, which only acts as a rust barrier, it can be useful and achieve good electrical performance during long-term use of the PCB. In addition, it also has the tolerance to the environment that other surface treatment processes do not have;

     The copper surface is chemically plated with Ni/Au. The deposition thickness of the inner layer Ni is generally 120-240μin (about 3-6μm), and the deposition thickness of the outer layer Au is relatively thin, generally 2-4μinch (0.05-0.1μm). Ni forms a barrier layer between the solder and copper. During soldering, the Au on the outside will quickly melt into the solder, and the solder and Ni will form a Ni/Sn intermetallic compound. The outer gold plating is to prevent Ni oxidation or passivation during storage, so the gold plating layer should be dense enough and the thickness should not be too thin.   

4. Chemical Immersion Silver

Between OSP and electroless nickel/immersion gold, the process is simpler and faster. It still provides good electrical properties and maintains good solderability when exposed to heat, humidity and pollution, but tarnishes. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel plating/immersion gold;

5. Electroplating nickel gold

     The conductor on the surface of the PCB is first electroplated with a layer of nickel and then a layer of gold is electroplated. Nickel plating is mainly to prevent the diffusion between gold and copper. Now there are two types of electroplated nickel gold: soft gold plating (pure gold, gold means that it does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains cobalt and other elements, and the surface looks brighter). Soft gold is mainly used for gold wires in chip packaging; hard gold is mainly used for electrical interconnections (such as gold fingers) in non-soldering places.

6. PCB mixed surface treatment technology

   Choose two or more surface treatment methods for surface treatment. Common forms are: immersion nickel gold + anti-oxidation, electroplating nickel gold + immersion nickel gold, electroplating nickel gold + hot air leveling, immersion nickel gold + hot air leveling .