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The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxides, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.
The surface treatment process of the PCB includes: anti-oxidation, tin spray, lead-free tin spray, immersion gold, immersion tin, immersion silver, hard gold plating, full board gold plating, gold fingers, nickel palladium gold OSP, etc.
Introduction to common surface treatment methods of circuit boards:
1, chemical immersion silver
Between OSP and electroless nickel/immersion gold, the process is simpler and faster. When exposed to heat, humidity and pollution, it can still provide good electrical performance and maintain good solderability, but it will lose its luster. Because there is no nickel under the silver layer, immersion silver does not have the good physical strength of electroless nickel/immersion gold.
2, electroplating nickel gold
The conductor on the surface of the circuit board is electroplated with a layer of nickel and then electroplated with a layer of gold. The main purpose of nickel plating is to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, gold indicates 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 wire in chip packaging; hard gold is mainly used for electrical interconnection (such as gold fingers) in non-welded areas.
3, circuit board mixed surface treatment technology
Choose two or more surface treatment methods for surface treatment. The 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 .
Among all surface treatment methods, hot air leveling (lead-free/leaded) is the most common and cheapest treatment method, but please pay attention to the EU's RoHS regulations.
4, hot air leveling
The process of coating molten tin-lead solder on the surface of the circuit board and flattening (flattening) with heated compressed air to form a coating layer that is resistant to copper oxidation and provides good solderability. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction, and its thickness is about 1 to 2 mils.
5, organic anti-oxidation (OSP)
On the clean bare copper surface, a layer of organic film is grown chemically. This layer of film has anti-oxidation, thermal shock resistance, and moisture resistance to protect the copper surface from rusting (oxidation or sulfidation, etc.) in a normal environment; at the same time, it must be easily assisted in the subsequent welding high temperature The flux is quickly removed to facilitate soldering.
6, chemically immersed nickel gold
Wrap a thick layer of nickel-gold alloy with good electrical properties on the copper surface and can protect the PCB for a long time. Unlike OSP, which is only used as an anti-rust barrier layer, it can be useful and achieve good electrical performance during long-term use of circuit boards. In addition, it also has environmental tolerance that other surface treatment processes do not have.
