Bare copper of PCB is easily oxidized by moisture when exposed to air, and the resulting oxide layer of copper has a great impact on the soldering, which may lead to cold soldering and even cause the pads unable to be soldered. So it is essential to implement the surface finishes that can ensure good solderability and electrical properties of the printed circuit boards. For rigid boards, PCBWay offers surface finishes including Hot Air Leveling (HASL), Organic Solderability Preservatives (OSP), Electroless Nickel Immersion Gold (ENIG), Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), Hard Gold Plating, Immersion Tin, Immersion Silver and also plain copper. Among them, the most commonly used finishes are the first three. Today we’ll mainly introduce HASL process to you.
HASL, short for Hot Air Solder Leveling, is a process of coating the PCB surface with molten solder and leveling it with heated compressed air to form a coating layer that can be resistant to copper oxidation and provides good solderability. Hot Air Leveling process needs to go through several steps, including pre-treatment, flux coating, tin spraying, hot air leveling, cooling, and post-cleaning.
Before applying solder, the board needs to be pretreated. Pre-treatment steps include micro-etching, water washing, and drying.
After drying, the board is first immersed in the flux. Flux can moisten the copper surface, increase the free radicals on the copper surface, and promote the solderability between tin and copper. Due to the acidic nature of the flux, operators need to wear protective gear such as protective gloves and overalls to avoid direct contact with the flux. Next, the board is going to be affixed by the clamps of the HASL machine and then immersed in the molten solder bath for a few seconds. After the board is taken out of the solder bath, the excess solder on the surface will be blown off under the action of hot air, and then the remaining solder is cooled and cured to form a thin even coating on the surface.
The cooled board needs to be cleaned again with hot water. Because the temperature of the circuit board has not been completely cooled down at the moment, using cold water may cause bow and twist or water mist on the board. After the cleaning step, the board surface is removed from all the oil, flux, and remaining tin powder. Finally, the board is dried to complete the entire HASL process. After HASL process, the first sample of the board needs to proceed with quality control. After making sure that the board is without problems such as rough tin surface, insufficient or excessive solder, the board is then ready to enter the next process.
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