answer：

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the production of PCB are particularly prominent. 

Lead and bromine are the hottest topics, and lead-free and halogenation will affect the development of PCB in many ways. 

The most basic purpose of surface treatment is to ensure good weldability or electrical performance. 

Because the copper in nature tends to exist in the form of oxide in the air and is unlikely to remain as raw copper for a long time, other treatment of copper is needed. 

Although in the subsequent assembly, strong flux can be used to remove most of the copper oxides, but the strong flux itself is not easy to remove, so the industry generally does not use strong flux. 

There are now many PCB surface treatment processes, which will be described one by one below. 

Hot air leveling (tin spray). 

Hot air leveling, also known as hot air solder leveling (commonly known as tin spray), is a process of coating molten tin (lead) solder on the surface of PCB and heating compressed air to flatten it to form a layer that is resistant to copper oxidation. 

It can also provide a good weldable coating. 

Hot air conditioning solder and copper form copper-tin intermetallic compounds at the junction. 

PCB hot air conditioning is usually sunk in molten solder; the air knife blows out the liquid solder before the solder solidifies; the air knife minimizes the meniscus of the solder on the copper surface and prevents the solder from being bridged. 

Organic solderable protective agent (OSP). 

OSP is a process for surface treatment of (PCB) copper foil on printed circuit board (PCB), which meets the requirements of RoHS instruction. 

OSP is the abbreviation of Organic Solderability Preservatives, which is translated into organic welding film, also known as copper protector, also known as Preflux in English. 

To put it simply, OSP is the chemical growth of an organic skin film on a clean, bare copper surface. 

This film has anti-oxidation, heat shock and moisture resistance, which is used to protect the surface of copper from rust (oxidation or vulcanization, etc.) in normal environment. 

However, at subsequent welding temperatures, the protective film must be easily removed by the flux so that the exposed clean copper surface can be immediately combined with the molten solder to form a strong solder joint in a very short period of time. 

Full plate nickel plating. 

The whole plate nickel plating is to plating a layer of nickel and then a layer of gold on the surface conductor of PCB. The main purpose of nickel plating is to prevent the diffusion between gold and copper. 

There are now two types of nickel-plated gold: soft gold (pure gold, the gold surface does not look bright) and hard gold (smooth and hard, wear-resistant, containing cobalt and other elements, the gold surface looks brighter). 

Soft metal is mainly used for gold wire in chip packaging, and hard metal is mainly used for electrical interconnection at non-welded places. 

Sinking gold. 

Precipitated gold is coated with a thick, good electrical nickel-gold alloy on the copper surface, which can protect PCB; for a long time. In addition, it also has tolerance to the environment that other surface treatment processes do not have. 

In addition, sinking gold can also prevent the dissolution of copper, which will be beneficial to lead-free assembly. 

Shen tin. 

Since all solder is currently based on tin, the tin layer can match any type of solder. 

The tin deposition process can form a flat copper-tin intermetallic compound, which makes the tin deposit as weldable as hot air leveling without the headache of hot air leveling. 

Tin plates should not be stored for too long and must be assembled in accordance with the sequence of sunken tin. 

Precipitate silver. 

Silver deposition process is between organic coating and electroless nickel / gold plating, the process is relatively simple and fast; even if exposed to hot, wet and polluted environment, silver can still maintain good weldability, but will lose luster. 

Precipitated silver does not have the good physical strength of electroless nickel plating / gold deposition because there is no nickel under the silver layer. 

Chemical nickel palladium gold. 

Compared with gold precipitation, chemical Ni-PD gold has an additional layer of PD between nickel and gold. Palladium can prevent the corrosion caused by replacement reaction and make full preparation for gold precipitation. 

Gold is tightly covered with palladium, providing a good contact surface. 

Electroplating hard gold. 

In order to improve the wear resistance of the product, increase the number of plug and pull and plating hard gold.