Hot Air Solder Leveling (HASL) Process for PCBs

HASL is a process where lead-free tin solder is applied to the surface of the board and then leveled with heated compressed air to create an oxidation-resistant copper coating on the surface.  It is one of the most common types of PCB surface finishes used in the industry today. It involves coating the exposed copper surfaces of the PCB with solder, either lead-based (63% tin, 37% lead, also referred to as 63/37) or lead-free (99.99% tin), to prepare the board for component assembly.

The Hot Air Solder Leveling (HASL) process is a crucial step in the manufacturing of Printed Circuit Boards (PCBs). It involves coating the exposed copper surfaces of the PCB with solder, either lead-based (63% tin, 37% lead, also referred to as 63/37) or lead-free (99.99% tin), to prepare the board for component assembly.

Here's an overview of the HASL process, broken down into its key steps:

1. PCB Panel Cleaning
The PCB panel undergoes a thorough cleaning process to remove any contaminants, oxidation, or debris, ensuring the surface is clean and ready for further processing.

2. Flux Application
A layer of flux is applied to the cleaned PCB to facilitate better solder adhesion. The flux also acts as a protective layer, preventing oxidation during the soldering stage.

3. Solder Bath Immersion
The PCB panel is then dipped into a molten solder bath, which ensures that all exposed copper surfaces are evenly coated with solder. The temperature of the solder bath depends on the type of solder being used:

Lead-Free Solder Bath: Approximately 260°C to 270°C (500°F to 518°F).
Leaded Solder Bath: Approximately 235°C to 245°C (455°F to 473°F).
Due to the higher temperature of lead-free solder, it is recommended to use substrates with higher thermal resistance, such as FR4-TG 170-180. This material can tolerate temperatures up to 170°C to 180°C (338°F to 356°F) without risk of damage. For FR4-TG 130 (rated up to 130°C or 266°F), the risk of thermal stress increases, especially during extended dwell times.

4. Hot Air Leveling
Once the PCB panel is removed from the solder bath, hot air knives are used to blow heated air across the surface. This removes any excess solder and ensures that a uniform, flat solder coating is applied to all pads. The air flow also clears any solder from the plated-through holes. The solder mask protects areas where solder should not be applied, ensuring precise coating.

5. Inspection and Quality Control
After soldering, both visual and automated inspections are conducted to check for consistency in the solder coating and to identify any defects, irregularities, or imperfections.

6. Cleaning and Final Processing
The PCB is cleaned to remove any residual flux or contaminants. This is typically followed by routing, where the large panel is cut into individual PCBs or arranged into arrays.

In summary, the HASL process is essential for preparing PCBs for component soldering, ensuring that the board has a reliable and even solder coating. By following these steps carefully, manufacturers ensure high-quality and functional PCBs ready for assembly. HASL is a very cost-effective surface finish compared to other types of finishes, and thus is considered a great choice for general purpose boards.