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 a strong flux can be used to remove most copper oxides in subsequent assembly, the strong flux itself is not easy to remove, so the industry generally does not use strong flux. There are many PCB board surface treatment processes, the common is hot air smoothing, organic coating, electroless nickel plating/gold dipping, silver dipping and tin dipping these five processes, will be introduced one by one below.

Hot air smoothing

Hot air smoothing, also known as hot air solder smoothing (commonly known as spray tin), it is the process of coating molten tin (lead) solder on the surface of the PCB circuit board and heating compressed air to smooth (blow), so that it forms a layer of copper oxidation resistance, but also to provide good solderability of the coating layer. During hot air conditioning, a copper-tin metal intermetallic compound is formed at the joint of solder and copper. The PCB circuit board should be sunk in the molten solder for hot air finishing; The air knife blows out the liquid solder before it solidifies; The air knife minimizes the meniscus of solder on the copper surface and prevents solder bridging.

Organic solderability protectant (OSP)

OSP is a process for copper foil surface treatment of printed circuit boards (PCB circuit boards) that meets the requirements of the RoHS directive. OSP is short for Organic Solderability Preservatives, also known as organic solderability preservatives, also known as Preflux in English. Simply put, 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.); However, in the subsequent welding high temperature, this protective film must be easily removed by the flux quickly, so that the exposed clean copper surface can be immediately combined with the molten solder in a very short time to become a solid solder joint.

All plates plated with nickel gold

Nickel plating is the first layer of nickel plating on the surface conductor of the PCB circuit board and then a layer of gold plating, nickel plating is mainly to prevent the diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, gold surface does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing other elements such as cobalt, gold surface looks brighter). Soft gold is mainly used for chip packaging gold wire; Hard gold is mainly used in non-welded electrical interconnections.

Heavy gold

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

Sunk tin

Since all current solders are tin-based, the tin layer can be matched to any type of solder. The process of sinking tin can form flat copper-tin metal intermetallic compounds, which makes the sinking tin have the same good solderability as the hot air leveling without the headache flat problem of hot air leveling; The tin plate cannot be stored for too long, and the assembly must be carried out according to the order of the tin sinking.

silvering

Silver sinking process is between organic coating and electroless nickel/gold plating, the process is relatively simple and fast; Even when exposed to heat, humidity and pollution, silver is still able to maintain good weldability, but will lose its luster. Silver plating does not have the good physical strength of electroless nickel plating/gold plating because there is no nickel underneath the silver layer.

Chemical nickel palladium

Compared with the precipitation of gold, there is an extra layer of palladium between nickel and gold, and palladium can prevent the corrosion phenomenon caused by the replacement reaction and make full preparation for the precipitation of gold. Gold is closely coated with palladium, providing a good contact surface.

Electroplated hard gold

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

With more and more high user requirements, more and more stringent environmental requirements, more and more surface treatment processes, in the end, the choice of the kind of development prospects, more versatile surface treatment process, it seems a little dazzling, confusing. Where the PCB surface treatment process will go in the future, it is also impossible to predict accurately. In any case, meeting user requirements and protecting the environment must be done first!