Forewrod

With the continuous improvement of human requirements for living environment, the environmental problems involved in pcb production are particularly prominent. The topics of lead and bromine are the hottest, and lead free and halogen-free will affect the developmentof PCB in many ways.

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

Introduction of Surface Treatment

There are many kinds of PCB surface treatment processes, such as hot air leveling, organic coating, electroless nickel plating/gold immersion, silver immersion and tin immersion. Each surface treatment is described below.

HALS/Hot Air Leveing (Lead free)

Hot air leveling, also known as spray tin, is a process that coats the surface of pcb with molten tin (lead or lead free) solder and heats compressed air to level it to form a coating that is resistant to both copper oxidation and good solderability. Hot air solders and copper form copper-tin intermetallic compounds at the junction. The pcb’s hot-air process usually sinks in molten solder; the wind knife flattens the liquid solder before the solder solidifies; and the wind knife minimizes and prevents bridging of the crescents of the solder on the copper surface

Organic Solderable Protector(OSP)

OSP is a process for surface treatment of PCB copper foil that meets the requirements of the RoHS instruction.The film is resistant to oxidation, heat, and humidity to protect the copper surface from continued rust (oxidation or vulcanization, etc.) in the normal environment. However, at subsequent welding temperatures, the film must be easily removed by flux so that the exposed clean copper surface can be immediately bonded to the molten solder in a very short time become firm solder joints.

Electro-less Nickel immersion gold(ENIG)

Sinking gold is a thick, well-electrical layer of nickel-gold alloy on the copper surface, which protects PCB’s for a long time; it also has environmental endurance that other surface treatment processes do not possess. In addition, the sinking gold can also prevent the dissolution of copper, which will be beneficial to lead-free assembly.

Immersion Tin（IT）

Since all solders are currently based on tin, the tin layer can match any type of solder. The tin-sinking process can form flat copper-tin intermetallic compounds, a property that allows tin-sinking to have the same good solderability as hot-air leveling without the headache of hot-air leveling flatness; the tin-sinking plate cannot be stored for too long and must be assembled in order of tin-sinking.

Immersion Ag/silver（IS）

The silver sinking process is between organic coating and electroless nickel/sinking gold, and the process is relatively simple and fast; even if exposed to hot, wet and contaminated environments, silver is still able to maintain good solderability but will lose its luster. Sinking silver does not have the good physical strength of electroless nickel/sinking gold because there is no nickel underneath the silver layer.

Electroless Nickel,Electroless Palladiuns and Immersion Gold(ENEPIG)

Chemical nickel palladium gold is a layer of palladium between nickel and gold, which can prevent the corrosion caused by the displacement reaction and prepare for the sinking gold. Gold is tightly covered in palladium, providing a good contact surface.