PCB Technical - PCB surface treatment technology process

With the continuous improvement of human requirements for the living environment, the environmental problems involved in the current PCB production process appear to be particularly prominent. The topics related to lead and bromine are the most popular. Lead-free and halogen-free will affect the development of PCB in many ways.

Although at present, the changes in the surface treatment process of PCB are not very big, it seems to be a relatively remote thing, but it should be noted that long-term slow changes will lead to huge changes. With the increasing demand for environmental protection, the surface treatment process of PCB will definitely undergo tremendous changes in the future.

The most basic purpose of surface treatment is to ensure good solderability or electrical properties. Since natural copper tends to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper. Although in the subsequent assembly, strong flux can be used to remove most of the copper oxides, the strong flux itself is not easy to remove, so the industry generally does not use strong flux.

There are many PCB surface treatment processes, the common ones are hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver and immersion tin, which will be introduced one by one below.

1. Hot air leveling (spray tin)

Hot air leveling is also known as hot air solder leveling (commonly known as tin spraying). It is a process of coating molten tin (lead) solder on the surface of the PCB and flattening (blowing) it with heated compressed air to form a layer that is resistant to copper oxidation. It can also provide a coating layer with good solderability. During hot air leveling, the solder and copper form a copper-tin intermetallic compound at the joint. When the PCB is leveled with hot air, it must be submerged in the molten solder; the air knife blows the liquid solder before the solder solidifies; the air knife can minimize the meniscus of the solder on the copper surface and prevent the solder from bridging.

2. Organic Solderability Preservative (OSP)

OSP is a process for surface treatment of printed circuit board (PCB) copper foil that meets the requirements of the RoHS directive. OSP is the abbreviation of Organic Solderability Preservatives, which is translated as Organic Solderability Preservatives in Chinese, also known as Copper Protector, or Preflux in English. Simply put, OSP is to chemically grow a layer of organic film on the clean bare copper surface.

This layer of film has anti-oxidation, thermal shock resistance, and moisture resistance to protect the copper surface from rusting (oxidation or sulfidation, etc.) in a normal environment; but in the subsequent welding high temperature, this kind of protective film must be very It is easy to be quickly removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder into a strong solder joint in a very short time.

3. The whole plate is plated with nickel and gold

The nickel-gold plating of the board is to plate a layer of nickel and then a layer of gold on the surface of the PCB. The nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold now: 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 gold surface looks brighter). Soft gold is mainly used for gold wire during chip packaging; hard gold is mainly used for electrical interconnection in non-welded areas.

4. Immersion gold

Immersion gold is a thick layer of nickel-gold alloy with good electrical properties on the copper surface, which can protect the PCB for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, immersion gold can also prevent the dissolution of copper, which will benefit lead-free assembly.

5. Shen Xi

Since all current solders are based on tin, the tin layer can be matched with any type of solder. The tin sinking process can form a flat copper-tin intermetallic compound. This feature makes the tin sinking have the same good solderability as the hot air leveling without the headache flatness problem of the hot air leveling; the tin plate can not be stored for too long, The assembly must be carried out according to the order of sinking tin.

6. Immersion Silver

Immersion silver process is between organic coating and electroless nickel/immersion gold. The process is relatively simple and fast; even if exposed to heat, humidity and pollution, silver can still maintain good solderability, but will lose its luster . Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer.

7. Chemical nickel palladium gold

Compared with immersion gold, chemical nickel-palladium-gold has an extra layer of palladium between nickel and gold. Palladium can prevent corrosion caused by substitution reaction and make full preparations for immersion gold. Gold is tightly covered on palladium, providing a good contact surface.

8. Plating hard gold

In order to improve the wear resistance of the product, increase the number of insertion and removal and electroplating of hard gold.

With the increasing requirements of users, stricter environmental requirements, and more and more surface treatment processes, the choice of surface treatment process with development prospects and more versatility seems to be a bit dazzling and confusing at present. . Where the PCB surface treatment process will go in the future cannot be accurately predicted now. In any case, meeting user requirements and protecting the environment must be done first!