HASL-Hot Air Solder Leveling Surface Finish
Hot Air Solder Leveling, or HASL, is a common surface finish in the PCB industry that typically involves solder with 63% tin and 37% lead. Due to its cost-effectiveness, it is often used for general-purpose PCB boards with larger components and wider-spaced wires. However, with the increasing focus on sustainable development, more and more electronics manufacturers require PCB factories to comply with RoHS standards, which require lead-free finishes. While HASL finish can be made lead-free, fewer people choose it due to its unpleasant, dirty, and difficult process.
Features of HASL Finish
Electronics manufacturers must choose the appropriate surface treatment for PCB manufacturing, which is widely used in industrial machines and children’s electronic applications. It is crucial to ensure that these electronic products provide maximum benefits. Therefore, understanding the characteristics of HASL PCB surface finish is essential.
- Board surface detection can be easily achieved by direct observation with the naked eye;
- HASL provides good wetting during component joining and soldering;
- Hot air solder leveling cannot level oxidized copper on the PCB;
- This finish is more accessible;
- It can prolong the service life of printed circuits;
- Materials used for this finish are inexpensive and readily available;
- It can be easily applied to the board without requiring specific equipment;
- Boards with HASL can operate at temperatures up to 265 degrees Celsius;
- HASL has a longer processing window, allowing for quick manufacturing schedules without requiring a large number of boards;
- It has good aesthetic parameters;
- HASL finish thickness can be made uniform by controlling the amount of solder present on the pad after hot knife and thermal tension;
- The hot air soldering process involves irregular surfaces, making it unsuitable for SMT assembly;
- The soldering process can lead to non-wetting or dewetting when the solder coating does not fully form the intermetallic layer;
- High-temperature processing can damage plated through holes, and plated hole tolerances can be difficult to handle;
- HASL is a great choice for wire binding;
- It is not suitable for fine-pitch components, and a leveler may be required to increase their flatness.
HASL vs Lead-Free HASL
- The HASL PCB finish typically consists of a lead-tin alloy solder, which can be harmful to the human body and environment. Lead-free HASL, on the other hand, does not contain lead as the solder alloy, making it more environmentally friendly and less harmful, and aligning with future trends.
- The melting point of lead is actually higher than that of tin, so the melting point of lead-free HASL is higher than that of HASL that uses lead as one of the raw materials.
- PCBAs treated with HASL surfaces may exhibit a bright finish, while lead-free HASL can provide a matte surface finish.
- The mechanical strength of PCBs treated with HASL and lead-free HASL can be similar.
- Lead-free HASL contains less than 0.5% lead, while HASL typically contains 37% lead.
- The addition of lead can make the handling of the tin wire easier during the soldering process, but using lead-free wire can also produce satisfactory results with proper handling techniques.
Tin and lead (37% of lead)
Tin (lead less than 0.5%)
Reflow soldering temperatures
210℃ to 245℃
240℃ to 270℃
Wave soldering temperatures
- HASL PCB Finish:
There are three steps involved in applying HASL finish on a PCB board: cleaning, fluxing, and spraying melted solder to achieve the required wetting. Hot air knives are then used to remove any excess solder, with the air knife modules adjusted to the melting temperature of the solder. Once the solder becomes solid, the boards are passed through a washer to remove any flux residue.
Some HASLs are configured at a ninety-degree angle to allow for complete immersion of the board in the solder bath, resulting in high-quality finishes. Alternatively, horizontal lines can be used, where the board is immersed in the solder through nozzles or rollers connected to the upper and lower parts of the panel. Through this horizontal process, a uniform thickness is achieved, unlike the vertical process where the impact of gravity can affect solder distribution.
HASL has good wettability and is preferred over other finishes because it can withstand solder material for a longer time without the use of any protective material. Finish quality can be measured through air pressure, board movement towards and away from the solder during dipping, and air knife angle.
Although effective for soldering components, the HASL surface is not compatible with fine-pitch elements due to its thickness. HASL deposition consists of a tin and lead eutectic mixture and is the best option for through holes and larger SMD components. However, it is not the best option for SMD components less than 0805 in size.
- Lead Free HASL Finish:
Lead-free Hot Air Solder Leveling is a type of surface finish that uses lead-free solder alloys instead of the conventional Tin-Lead eutectic alloys used by HASL. The application process involves spraying molten solder onto the board to achieve the required wetting, followed by the use of hot air knives without lead. Once the solder becomes solid, the circuit board is passed through a washer to remove any remaining flux residue, similar to the HASL process.
Lead-free HASL is considered an environmentally-friendly alternative to standard HASL since it does not contain lead. However, it is less commonly used for surface finishing compared to other options such as ENIG and OSP, which are more readily available.
HASL surface treatment has a long history and most PCBA manufacturers can provide this service, including FS Technology. Although it is a cheap alternative, it does not comply with RoHS regulations, so we should ensure the use of high-safety finishes in the manufacturing and actual use of electronic products.
Electroless nickel immersion gold (ENIG) for your next PCB project ENIG, or electroless nickel immersion gold, is a surface finishing process applied to the copper
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