How many kinds of PCB surface treatments are there？

Common Surface Treatment: OSP, HASL(low cost)  / Immersion Gold ,Flash Gold (more expensive).

Specially Surface Treatment: Carbon , Immersion Silver , Immersion Tin, ENIG + OSP , Golden Finger and etc.

HASL, hot air solder leveling

Hot air leveling, also known as hot air solder leveling, is a process of coating molten tin-lead solder on the surface of the PCB and leveling (flattening) with heated compressed air to form a layer that is resistant to copper oxidation and provides good solderability Sexual coating. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction, and its thickness is about 1 to 2 mils.

When the PCB is leveled with hot air, it must be immersed in the molten solder. The air knife blows the liquid solder before the solder solidifies, and can minimize the meniscus of the solder on the copper surface and prevent solder bridging.

There are two types of hot air leveling: vertical and horizontal. Generally, the horizontal type is considered to be better. The main reason is that the horizontal hot air leveling is more uniform and can realize automated production. The general process is: micro-etching --> preheating --> coating flux --> spraying tin --> cleaning.

OSP

OSP is different from other surface treatment processes in that: its role is to act as a barrier between copper and air; in simple terms, OSP is to chemically grow an organic film on the clean bare copper surface. This layer of film has anti-oxidation, heat shock resistance, and moisture resistance to protect the copper surface from rusting (oxidation or sulfidation, etc.) in a normal environment; at the same time, it must be easily assisted in the subsequent welding high temperature The flux is quickly removed to facilitate soldering.

The organic coating process is simple and low in cost, making it widely used in the industry. The early organic coating molecules are imidazole and benzotriazole, which play a role in preventing rust, and the newest molecules are mainly benzimidazole. In order to ensure that multiple reflow soldering can be performed, only one organic coating layer on the copper surface is not acceptable. There must be many layers. This is why copper liquid is usually added to the chemical bath. After coating the first layer, the coating layer adsorbs copper; then the organic coating molecules of the second layer are combined with copper, until twenty or even hundreds of organic coating molecules gather on the copper surface.

The general process is: degreasing-->micro-etching-->pickling-->pure water cleaning-->organic coating-->cleaning. The process control is easier than other processes.

Immersion gold

Electroless nickel/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 performance. In addition, it also has environmental tolerance that other surface treatment processes do not have.

The reason for nickel plating is that gold and copper will diffuse each other, and the nickel layer can prevent the diffusion between them. Without the barrier of the nickel layer, gold will diffuse into the copper within a few hours. Another benefit of electroless nickel/immersion gold is the strength of nickel, and only 5um thickness of nickel can control the expansion in the Z direction at high temperatures. In addition, electroless nickel/immersion gold can also prevent the dissolution of copper, which will benefit lead-free soldering.

The general process is: de-pickling cleaning-->micro-etching-->pre-dip-->activation-->electroless nickel plating-->chemical immersion gold; there are 6 chemical baths in the process, involving nearly a hundred kinds Chemicals, the process is more complicated.

Immersion Silver

The immersion silver process is between OSP and electroless nickel/immersion gold, and the process is simple and fast. Immersion silver is not a thick armor for PCB. Even if exposed to heat, humidity and pollution, it can still provide good electrical performance and maintain good solderability, but it will lose its luster. Because there is no nickel under the silver layer, immersion silver does not have the good physical strength of electroless nickel/immersion gold.

Immersion silver is a displacement reaction, it is almost submicron pure silver coating. Sometimes the immersion silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate the problem of silver migration. 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 based on tin, the tin layer can be matched with any type of solder. From this point of view, the immersion tin process is very promising. However, tin whiskers are prone to appear in the previous PCBs after the immersion tin process, and the tin whiskers and tin migration during the soldering process will cause reliability problems, thus restricting the use of the immersion tin process. Later, organic additives were added to the tin immersion solution to make the tin layer structure in a granular structure, which overcomes the previous problems and also has good thermal stability and solderability.

The immersion tin process can form a flat copper-tin intermetallic compound. This feature makes immersion tin have the same good solderability as hot air leveling without the headache flatness problem of hot air leveling; there is no electroless nickel plating/dipping Diffusion between gold and metal; it’s just that the immersion tin plate can’t be stored for too long.

Electroplated nickel gold

Electroplating nickel gold is the originator of PCB surface treatment process. It has appeared since PCB appeared, and other processes have slowly evolved. Nickel-gold electroplating is to first electroplate a layer of nickel on the PCB surface conductor and then electroplate a layer of gold. Nickel plating mainly prevents the diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains cobalt and other elements, and the surface looks brighter). Soft gold is mainly used for gold wire during chip packaging; hard gold is mainly used for electrical interconnection in non-soldering places (such as gold fingers).

Under normal circumstances, welding will cause the electroplated gold to become brittle, which will shorten the service life, so avoid welding on the electroplated gold; while the electroless nickel/immersion gold is very thin and consistent, and the brittleness rarely occurs.

Electroless Palladium

The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to reduce the palladium ions to palladium on the catalytic surface through a reducing agent (such as sodium dihydrogen hypophosphite). The new palladium can be called a catalyst to promote the reaction, so a palladium plating layer of any thickness can be obtained. The advantages of electroless palladium plating are good welding reliability, thermal stability, and surface smoothness. The disadvantage is that palladium is a relatively rare precious metal, so the cost will increase.

Selective chemical nickel/gold and OSP

Application: It can be used for PCBA that requires mechanical contact area and installs fine-pitch devices. In this application, OSP is used as a highly reliable solderable layer, and ENIG is used as a mechanical contact area, such as the keyboard of a mobile phone, which uses this surface treatment.

Cost: medium to high.

Compatibility with lead-free: compatible.

Storage period: 6 months.

Solderability (wettability): low.

Disadvantages:

The cost is relatively high.

ENIG is not suitable for use as the ground edge plating for channel installation, because the ENIG layer is very thin and not resistant to friction.

There is a risk of Gavani effect in OSP potions.

ipcb can do all kinds of PCB surface treatments, If you need PCB, please contact ipcb.