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Hot air leveling, also known as hot air solder leveling (commonly known as tin spraying), is a process of coating molten tin (lead) solder on the surface of PCB and leveling (blowing) with heated compressed air to form a coating layer that not only resists copper oxidation, but also provides good solderability. Hot air leveling solder and copper form copper tin intermetallic compound at the joint. PCB shall sink in molten solder during hot air leveling;The air knife blows the liquid solder before the solder solidifies;The wind knife can minimize the meniscus of solder on the copper surface and prevent solder bridging.

The working principle is to use hot air to remove the excess solder on the surface of the printed circuit board and in the hole, and the residual solder is evenly covered on the solder pad, unimpeded solder lines and surface packaging points. It is one of the methods of printed circuit board surface treatment.

The hot air leveling process is relatively simple, mainly including: plate laying (pasting gold-plated plug protective tape) – hot air leveling pre-treatment – hot air leveling – cleaning after hot air leveling – inspection.Although the hot air leveling process is simple, there are still many process conditions to be mastered if you want to level an excellent and qualified printed circuit board with hot air, such as solder temperature, air knife airflow temperature, air knife pressure, immersion welding time, lifting speed, etc.

Advantages:

(1) After hot air leveling, the composition of solder coating remains unchanged, so that the solder coating has good consistency and weldability.The composition (lead tin ratio) of the lead tin alloy coating changed with the change of the composition in the plating solution.

(2) Neither infrared hot-melt process nor hot oil hot-melt process can 100% protect the side edge of the conductor.The solder coating leveled by hot air can completely cover the side edge of the conductor, avoid corrosion and wire breakage on the printed board, prolong the storage and service time of the printed board, and improve the reliability of the electronic products of the whole machine.Hot air leveling is now widely used in SMT process.

(3) By adjusting the process parameters such as air knife angle and rising speed of printed board, the coating thickness can be controlled to obtain the required solder coating thickness, which is more convenient and flexible than hot melting.

(4) When the printed board produced by pattern electroplating and etching is wave soldering, it is easy to bridge because there is lead tin alloy on the conductor.Similarly, the flow of lead tin alloy wrinkles and warps the solder resist film.The printed board produced by hot air leveling process has no solder, that is, the phenomena of solder bridging, wrinkle and falling off of barrier film are eliminated.

Disadvantages:

(1) Copper pollution to solder trough.In hot air leveling, the printed board should be immersed in the solder bath for several seconds, resulting in the dissolution of copper.When the concentration of copper reaches more than 0.29%, the fluidity of solder becomes poor, the coated solder layer is semi wetted, and the solderability of printed board decreases.

(2) Lead is a heavy metal element in solder coating, which is harmful to human body and pollutes the environment.Now some lead-free solders have been produced and sold to replace lead tin alloy for production.

(3) High production cost.A good imported hot air leveling machine costs about US $300000, so its production cost is higher than that of hot melt process.

(4) The thermal impact of hot air leveling is large, which is easy to deform and warp the printed board.High thermal stress.