The development of PCB technology has continued until now, and many PCB surface treatment processes have been produced. The common processes are hot air leveling, OSP, electroless nickel plating,immersion Tin/Immersion Au
Hot air leveling
热风整平又名热风焊料整平HASL （Hot Air Solder Leveled），它是在PCB表面涂覆熔融锡铅焊料并用加热压缩空气整（吹）平的工艺，使其形成一层既抗铜氧化，又可提供良好的可焊性的涂覆层。可焊性好，热风整平时焊料和铜在结合处形成铜锡金属间化合物。
Hot Air leveling, also known as hot air Solder leveling HASL (hot air solder), is a process of coating the PCB surface with molten tin lead solder and leveling with heated compressed air, so that it forms a coating layer that is resistant to copper oxidation and can provide good weldability. Good weldability, hot air is usually solder and copper at the joint of the formation of copper tin gold compounds.
OSP（Organic Solderability Preservative）
Organic coating process OSP is different from other surface treatment processes. It acts as a barrier between copper and air to prevent the surface oxidation of the brazing pad. Organic coating is simple and inexpensive, which makes it widely used in the industry.
化学镍金（ENIG）化学镀Ni和浸镀金（ENIG）具有良好的可焊性。Ni作为隔离层和可焊的镀层，要求厚度≥3um；Au是Ni的保护层 ，如果太多的Au溶解到焊点里（无论是Sn－Pb还是Sn－Ag－Cu）都将引起“金脆”。所以一定要限定Au层的厚度，用于焊接的Au层厚度≤1?m （ENIG ：0．05～0．3?m).
Chemical nickel gold (ENIG) electroless Ni plating and electroplating (ENIG) have good weldability. Ni as isolation layer and weldable coating, required thickness ≥3um; Au is the protective layer of Ni, and too much Au dissolved into the solder joint (whether Sn-Pb or Sn-Ag-Cu) will cause “gold brittle”. Therefore, the thickness of Au layer must be limited. The thickness of Au layer used for welding is less than 1? m (ENIG: 0.05 ~ 0.3? m).
Immersion Tin PCB is prone to tin whiskers after immersion process, tin whiskers and tin migration in the welding process will bring reliability problems. After adding organic additives into the leaching solution, the tin layer structure is granular, which overcomes the previous problem, but also has good thermal stability and weldability
isadvantages: The biggest weakness of tin leaching is its short life, especially when stored in high temperature and humidity, Cu/Sn intermetallic compounds will continue to grow until they lose solderability. Therefore, the tin plate can not be stored for too long.