PCB surface treatment technology refers to the process of artificially forming a surface layer with different 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.
At present, the common surface treatment methods are as follows:
1. Hot Air Leveling
The process of coating molten tin-lead solder on the surface of the PCB and leveling (blowing) with heated compressed air to form a coating that resists copper oxidation and provides good solderability. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction with a thickness of about 1-2 mils.
2. Organic Antioxidant (OSP)
On the clean bare copper surface, a layer of organic film is chemically grown. This film has anti-oxidation, thermal shock, moisture resistance, and is used to protect the copper surface from rusting (oxidation or vulcanization, etc.) in a normal environment. At the same time, it must be easily assisted in the subsequent high temperature of welding. Flux is quickly removed for soldering.
3. Electroless Nickel Gold
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.
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 will lose its luster. 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 plated with a layer of nickel and then a layer of gold is plated. 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 other elements such as cobalt, 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 .
Hot air leveling (lead-free/leaded) is the most common and cheapest of all surface treatments, but be aware of EU RoHS regulations.
RoHS: RoHS is a mandatory standard formulated by EU legislation. Its full name is “Restriction of Hazardous Substances”. The standard has been officially implemented since July 1, 2006, mainly used to regulate the material and process standards of electrical and electronic products, making it more conducive to human health and environmental protection. The purpose of this standard is to eliminate 6 substances of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers in electrical and electronic products, and focus on stipulating that the content of lead cannot exceed 0.1%.
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