If you are involved in the PCB – “printed circuit board” industry, you must already understand that these boards have copper finished surfaces. In case if they are left unprotected, the copper will start oxidizing and deteriorating, making your circuit board unusable.
The surface finish helps form an interface between the PCB and the component. This finish has two significant functions – the protection of exposed copper-based circuitry and providing a solderable surface during the assembling of the components to the PCB.
What is Hot Air Solder Leveling?
Hot Air Solder Leveling or HASL was once hailed as a tried, tested, and true way to produce consistent and quality PCB assembly results. That said, the ever-evolving and increasing circuit board complexity and density of components have pushed the capabilities of all horizontal soldering systems to their maximum limits.
HASL is probably the most common type of printed circuit board surface finish used within the industry in the modern era. The Hot Air Solder Leveling finish composition is in the proportions of 37 percent lead and 63 percent tin. However, you can also use HASL for a lead-free finish; all it takes is a minor tweak to the entire process.
HASL Process
This type of PCB surface finish is applicable by first dipping your circuit board into a pot of molten lead/tin alloy after the application of the solder mask. The next step is the removal of excess solder via Hot Air Leveler (HAL) using hot-air knives that results in leaving behind a thinnest possible layer.
The job of this remaining thin solder layer is to protect underneath traces from corrosion. The finish also facilitates the activity of soldering your components to the circuit board during the printed circuit board assembling process via pre-tinning the pads on your board.
Hot Air Solder Leveling is one of the most cost-effective PCB surface finishes compared to other finishes types available in the market. Therefore, it is a recommended and preferred choice of surface finish for general purpose PCBs.
Although considered a separate type of surface finish in its own right, we can discuss a little about Lead-Free HASL. It is identical to HASL in both usage and appearance; however, the solder in LF-HASL contains a mix of 0.6 percent copper and 99.3 percent tin.
This alloy has a higher melting point for lead-free soldering in comparison to leaded soldering. This raises the need for a slight modification to the reflow soldering operation in your printed circuit board assembly solution.
Lead-Free HASL is a viable replacement for the traditional lead sourced soldering process but only used when you require a RoHS compliant circuit board. You must also remember that you will need a laminate material with a high temperature to apply this type of finish. Otherwise, the rest of the soldering process is similar to HASL.
In the past years, Hot Air Solder Leveling was the most in-demand and popular PCB surface finish. It was due to two main qualities, i.e., robust solution, and low cost.
However, the recent evolution and fundamental changes to the printed circuit board industry introduced more complex and advanced surface mount technology (SMT), which exposed the shortcomings of HASL.
You cannot use HASL with fine pitch SMT circuit assembly because of the incompatibility between the uneven surfaces and fine pitch components. Although a lead-free HASL solution is not available in the market, there are several other options that you can use to achieve highly reliable products.
Pros & Cons of HASL
There are certain advantages and disadvantages to using HASL, and they are:
Advantages
- Widely available
- Low cost
- Excellent shelf-life
- Easily re-worked
Disadvantages
- It produces uneven surfaces
- Not an ideal solution for fine pitch components
- Poor wetting
- Not ideal for PTH or Plated Through-Hole
- Thermal shock
Still Worth it
HASL still stays relevant, and one of the lesser-known and unintended benefits of HASL is the exposure of your PCB to higher temperatures of up to 265⁰C. This will help identify potential delamination problems before you attach any expensive components to the board.
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