Lead-Free HASL

Contents

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The electronics industry is increasingly moving away from tin-lead (Sn/Pb) solders due to growing health and environmental concerns. Among several alternatives, lead-free HASL has emerged as a favored finish, recognized for offering exceptional durability for printed circuit boards (PCBs).

What is lead-free HASL PCB surface finish?

Lead-free HASL refers to a surface finishing method that replaces traditional tin-lead (Sn-37Pb) alloys with lead-free mixtures. During the process, molten solder is initially applied to a cleaned and fluxed board, promoting wetting. Excess solder is removed using air knives, tools operating at temperatures above the solder’s melting point. Once solidified, the circuit board undergoes a washing process to eliminate any residual solder flux.

There are two main techniques for applying lead-free HASL:

• Vertical process: In this approach, a frame equipped with multiple boards is lowered into a solder bath for 2-3 seconds before lifting through air knives. These knives help clear through-holes and vias while ensuring the solder’s thickness is optimal for all necessary solderable areas. After cooling, the boards pass through a washer/dryer setup for residue removal, following which an operator inspects the finish quality.

• Horizontal process: The horizontal method involves flooding the board with solder through nozzles or rollers on both sides before passing it through hot air knives located above and below the panel.

Typically, vertical HASL lines are designed to yield high-quality finishes. Fully automated systems can manage tasks from fluxing to drying, whereas horizontal processes excel in uniform thickness across the coating.

Lead (Pb) content in lead-free solder stands at less than 0.1%. Common alloy compositions include Castin (Sn 93-98%, Ag 1.5-3.5%, Cu 0.2-2%, and Sb 0.2-2%) and SnCu 0.7.

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Why is lead-free HASL used?

Despite some complications associated with the HASL process, such as:

• Damage from thermal fluctuations
• Coating thickness variations leading to coplanarity issues

• Stencil gasketing challenges tied to variations in coating thickness and common mushroom cap profiles on smaller pads.

• Heat and fumes produced during the process, which are less suitable for modern manufacturing environments.

With lead-free HASL, the solder coating thickness is generally about half that of conventional tin-lead finishes, and it offers improved coplanarity. Modern HASL lines are built to maintain conditions conducive to contemporary manufacturing, thus mitigating issues related to heat and fumes.

Coating thickness

The solder coating profile primarily depends on the volume of solder remaining after passing through hot air knives, influenced by surface tension forces. Smaller pads often experience thicker coatings due to higher surface tension in lead-free solder, resulting in a generally thinner and more uniform coating compared to tin-lead alternatives.

XRF (X-ray fluorescence spectroscopy) is the standard method for assessing coating thickness.

Whisker formation

Whiskers, tiny filaments protruding from surfaces with tin finishes, occur under compressive stress conditions. As outlined in the JESD22A121 specification, such whiskers predominantly form on hot-dipped lead-free finishes in copper only under extreme conditions that include high temperatures and moisture levels.

With lead-free HASL, corrosion resistance is enhanced, ensuring long-lasting solderability.

HASL (Hot Air Solder Leveling) and Lead-free HASL are both techniques utilized as surface finishes in PCB manufacturing. They safeguard copper traces and pads from oxidation, support a solderable surface for components, and ensure dependable solder joints during assembly. The key distinction between HASL and Lead-free HASL lies within the solder's compositional make-up.

Composition:

· HASL (Hot Air Solder Leveling): Predominantly lead-based solder alloys have been traditionally implemented in HASL processes for years. However, environmental regulations are pushing the industry towards lead-free alternatives today.

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· Lead-free HASL: As its name indicates, lead-free HASL employs solder alloys without lead. Commonly, mixtures of tin (Sn), silver (Ag), and occasionally copper (Cu) are used. These comply with RoHS (Restriction of Hazardous Substances) directives banning certain hazardous substances from electronics.

Environmental Concerns:

· HASL: The traditional lead-based solder poses significant environmental health risks, making its proper disposal paramount to avoid contamination.

· Lead-free HASL: By negating lead, lead-free HASL minimizes environmental impacts and meets regulatory standards.

With serious emphasis on PCB production protocols to prevent exposing toxic materials, the advent of EU RoHS regulations has shifted HASL practices to eliminate lead from surface finishes.

Solder Characteristics:

· HASL: Lead-based solder often exhibits a lower melting point than some lead-free options, easing the soldering process but potentially making the joints susceptible to thermal-related issues long-term.

· Lead-free HASL: Lead-free uni-metal solder alloys display higher melting points requiring more care in temperature management during soldering for avoiding thermal stress complications.

Reliability and Performance:

· HASL: Lead-based joints may be simpler to manipulate, yet they have a heightened risk of brittleness and thermal cycling fatigue in time.

· Lead-free HASL: Conversely, lead-free solder joints tend to be more resilient, although they necessitate precise soldering management for optimal joint formation.

Conclusion

In conclusion, the principal variation between HASL and Lead-free HASL rests with solder composition. By addressing environmental issues, lead-free HASL strives to enhance long-term reliability while requiring meticulous control during the soldering process.

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