Several PCB surface finish methods

The current surface finish methods are the following: 

1. Hot air solderleveling (HASL) 

The process of applying molten tin-lead solder to the PCB surface and leveling(blowing flat) with heated compressed air forms a coating that is resistant to copper oxidation and provides good solderability. Hot air leveling flat soldering copper at the junction of copper and tin metal compounds, the thickness of about 1 ~ 2mil.

2. Organic antioxidant(OSP) 

Ona clean bare copper surface, a layer of the organic film grows chemically. This layer of film with anti-oxidation, thermal shock resistance, moisture resistance, to protect the copper surface in the normal environment no longer continue to rust (oxidation or sulfide, etc.); at the same time must be in the subsequent high-temperature welding, can be easily assisted quick removal of flux for welding. 

3. Chemical nickelplating gold 

Wrap a thick layer of copper on the surface, good electrical properties of the nickel-gold alloy, and long-term protection of the PCB. Unlike OSP, which is only used as a rust barrier, it can be useful during long-term PCB use and achieve good electrical performance. In addition, it also has other surface treatment processes that do not have the patience of the environment. 

4. Chemical silver 

Between the OSP and electroless nickel/gold immersion, the process is relatively simple and fast. Exposure to heat, humidity and contaminated environments still provides good electrical properties and good solderability but is tarnished. Because there is no nickel below the silver layer, silver does not have all the good physical strength of electroless nickel/gold immersion. 

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5. Nickel plating 

Conductors in the PCB surface after plating a layer of nickel plating on a layer of gold, nickel plating is mainly to prevent the diffusion between gold and copper. Now there are two types of nickel plating: soft gold (gold, gold does not look bright) and hard gold plated (smooth and hard surface, wear, containing cobalt and other elements, the surface looks shinier). Soft gold is mainly used in the chip package hit the gold line; hard gold is mainly used in the non-welding of electrical interconnection (such as gold fingers). 

6. PCB mixed surface finish technology 

Choose two or more surface treatment methods for surface finish, the common forms are: Nickel gold + anti-oxidation, nickel-plated gold + nickel-plated gold, nickel +gold plated hot air leveling, nickel-plated gold + hot air leveling. 

Hotair leveling (lead-free / lead) is the most common and cheapest solution for all surface finishes, but please note the EU RoHS directive. 

RoHS: RoHS is a mandatory standard set by the EU legislation, its full name is"Restriction of Hazardous Substances in the use of electrical and electronic equipment directive" (Restriction of Hazardous Substances). The standard has been formally implemented on July 1, 2006, mainly for the standardization of electrical and electronic products, materials, and process standards to make 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 polybrominated diphenyl ethers in motor and electronic products, and to stipulate that the content of lead should not exceed 0.1%.