HASL

HASL stands for hot air solder leveling is a type of finish used on printed circuit boards (PCBs). The PCB is typically dipped into a bath of molten solder so that all exposed copper surfaces are covered by solder. Excess solder is removed by passing the PCB between hot air knives.

The hot air leveling process is relatively simple, mainly including: plate laying (gold-plated plug protection tape) - hot air leveling pretreatment - hot air leveling - cleaning after hot air leveling - inspection. Although the hot air leveling process is simple, there are many process conditions to be mastered if you want to produce a good and qualified PCB by hot air leveling, such as solder temperature, air knife air temperature, air knife pressure, immersion welding time, lifting speed, etc. These conditions all have set values, but they should be changed according to the external conditions of the PCB and the requirements of the processing sheet, such as: board thickness, single side, double side and multi-layer boards with different board length. The conditions they adopt are different. Only when they are familiar with various process parameters, adjust the machine according to different types and requirements of PCB, patiently, carefully and reasonably, can the qualified PCB be leveled by hot air.

- Advantages:

Excellent wetting during component soldering

Avoids copper corrosion

1) After hot air leveling, the composition of solder coating remains unchanged, so that the solder coating has good consistency and good weldability. The composition (lead tin ratio) of lead tin alloy coating changed with the change of bath composition.
(2) Neither infrared hot-melt process nor hot oil hot-melt process can protect the side edge of the conductor 100%. The solder coating with hot air leveling can completely cover the side edge of wire, avoid corrosion and breakage on printed circuit board, prolong the storage and use time of printed circuit board, and improve the reliability of electronic products of the whole machine. Hot air leveling is now widely used in SMT process.
(3) By adjusting the air knife angle, PCB rising speed and other process parameters, the coating thickness can be controlled to obtain the required solder coating thickness, which is more convenient and flexible than hot-melt.
(4) The printed circuit board produced by pattern electroplating and etching is easy to bridge due to the lead tin alloy on the wire during wave soldering. Similarly, the flow of lead tin alloy causes the solder mask to wrinkle and warpage. The PCB produced by hot air leveling process has no solder, which eliminates solder bridging, film wrinkling and falling off.

- Disadvantages:

Low planarity on vertical levelers may make this surface finish unsuitable for use with fine pitch components. Improved planarity can be achieved using a horizontal leveler.

High thermal stress during process may introduce defects into PCB.

1) Copper contamination of solder bath. In hot air leveling, the printed circuit board should be immersed in the solder bath for several seconds, resulting in the dissolution of copper. When the concentration of copper is more than 0.29%, the fluidity of solder becomes poor, the solder layer is semi wetted, and the solder-ability of PCB 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 for sale to replace lead tin alloy for production.
(3) The production cost is high. The price of an imported hot air leveler is about 300000 US dollars, so its production cost is higher than that of hot melt technology.
(4) The thermal shock of hot air leveling is large, and it is easy to deform and warp the printed circuit board. High thermal stress.