Types of PCB surface treatment

Common PCB surface treatment processes including: HASL (hot air solder leveling), organic coating (OSP), Immersion gold, Immersion silver, Immersion tin and so on.

HASL（hot air solder leveling）

Hot air leveling, also known as hot air solder leveling, it is the process of coating the PCB surface with molten tin lead solder and heating compressed air leveling (blowing), so that it forms a coating layer that is both resistant to copper oxidation and can provide good solderability. The solder and copper form copper tin compounds at the joint of hot air conditioning, and its thickness is about 1 ~ 2mil.

The PCB is immersed in molten solder during hot air finishing, and the air knife blows the liquid solder flat before the solder sets, and can minimize the meniscus of the solder on the copper surface and prevent solder bridging.

Hot air is divided into vertical type and horizontal type two, it is generally considered that the horizontal type is better, mainly the horizontal hot air leveling coating is more uniform, can achieve automatic production. The general process is: micro-etching - preheating - coating flux - tin-spraying - cleaning.

OSP PCB

OSP is different from other surface treatment processes in that it acts as a barrier layer between copper and air. In simple words, OSP is a chemically grown organic skin film on a clean, bare copper surface. This film has anti-oxidation, heat shock, moisture resistance, to protect the copper surface in the normal environment no longer rust (oxidation or vulcanization, etc.); At the same time, it must be easily removed by flux quickly in the subsequent welding high temperature for welding.

The simple process and low cost of organic coating make it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which act as rust inhibitors, and the latest molecules are mainly benzoimidazole. In order to ensure that multiple reflow soldering can be performed, only one layer of organic coating on the copper surface is not enough, there must be many layers, which is why the chemical tank usually needs to add copper liquid. After coating the first layer, the coating layer adsorbs copper. Then a second layer of organic coating molecules binds to the copper until twenty or even hundreds of organic coating molecules converge on the copper surface.

The general process is: degreasing - micro-etching - pickling - pure water cleaning - organic coating - cleaning, the process control is relatively easy to show that the treatment process.

ENIG PCB

Immersion gold is 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 in the long-term use of PCB and achieve good electrical properties. In addition, it also has environmental tolerance that other surface treatment processes do not have.

The reason for nickel plating is because gold and copper will diffuse between each other, and the nickel layer can prevent the diffusion between them, if there is no nickel layer barrier, the gold will spread into the copper in a few hours. Another advantage of electroless nickel plating/gold leaching is the strength of nickel, only 5um thickness of nickel can control the Z direction expansion at high temperatures. In addition, electroless nickel plating can also prevent the dissolution of copper, which will benefit lead-free welding.

The general process is as follows: pickling cleaning - micro-etching - preleaching - activation - electroless nickel plating - chemical gold leaching. There are 6 chemical tanks in the process, involving nearly 100 kinds of chemicals, and the process is more complex.

Immersion Silver

The silver plating process is between OSP and ENIG plating, and the process is simple and fast. Immersion silver is not to put a thick armor on the PCB, even if exposed to heat, humidity and pollution, still provide good electrical properties and maintain good solderability, but will lose luster. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of immersion gold.

Silver plating is a displacement reaction, it is almost submicron level of pure silver coating. Sometimes the silver leaching process also contains some organic matter, mainly to prevent silver corrosion and eliminate silver migration problems, it is generally difficult to measure this thin layer of organic matter, analysis shows that the weight of the organism is less than 1%.

Immersion Tin

Since all solders are tin-based, the tin layer can be matched with any type of solder, and from this point of view, the tin-dipping process has great prospects for development. However, the previous PCB is prone to tin whisks after tinning process, tin whisks and tin migration in the welding process will bring reliability problems, so the use of tinning process is limited. After adding organic additives to the leaching solution, the tin layer structure has a granular structure, which overcomes the previous problems, and has good thermal stability and weldability.

Immersion Tin process can form flat copper-tin metal intermetallic compounds, which makes immersion tin have the same good solderability as hot air smoothing without the troublesome flatness problem of hot air smoothing. There is also no diffusion problem between electroless nickel plating/immersion gold. It's just that the tin can't be stored for too long.

Other surface treatment processes are less applied, among which electroplating nickel gold and electroless palladium are more applied.

ENIG Plating

Electroplated nickel gold is the originator of PCB surface treatment process, since the appearance of PCB it has appeared, slowly evolved other processes. Nickel gold plating is to first electroplate a layer of nickel on the PCB surface conductor and then electroplate a layer of gold, mainly to prevent the diffusion between gold and copper. There are two types of nickel gold plating: soft gold plating (pure gold, the surface of gold does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing cobalt and other elements, the surface looks brighter). Soft gold is mainly used for chip packaging gold wire. Hard gold is mainly used in non-welded electrical interconnections (such as gold fingers).

Under normal circumstances, welding will cause the electroplating to become brittle, which will shorten the service life, so avoid welding on the electroplating. However, electroless nickel plating/immersion gold rarely occurs it because the gold is very thin and uniform.

Pd Plating

The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce palladium ions to palladium on the catalyzed surface by a reducing agent (such as sodium dihydrogen hypophosphate), and the newly formed palladium can be called the catalyst to promote the reaction, so that palladium can be deposited with any thickness. The advantages of electroless palladium plating are good welding reliability, thermal stability and surface flatness. The disadvantage is that palladium is a relatively rare precious metal, so the cost increases.

At present, the surface technology of Seekpcb is complete to meet the needs of customers, and PCB surface treatment is crucial to improve reliability and shelf life. We will cover various types of surface treated printed circuits to provide boards to enhance good intermetallic bonding and shelf life of components.