Is HASL lead free?

HASL vs Lead-free HASL - PCB Surface Finishes

Posted: 05:20 PM December 06, 2022

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writer: NextPCB

Introduction to HASL Surface Finish

Hot air solder leveling (HASL) and lead-free HASL are prominent surface finishes in PCB manufacturing. Lead-Free HASL uses lead-free alloys, distinguishing it from conventional HASL, which uses a tin-lead solder. Selecting the right surface finish is crucial, both for cost-effectiveness and RoHS compliance. This guide will explore the differences, advantages, and disadvantages of HASL and Lead-free HASL.

What is PCB Surface Finish?

Surface Finish is metalizing the outer layer of a PCB to protect the copper circuitry from corrosion and ease soldering of components. Common types of surface finishes include HASL, ENIG, ImAg, OSP, and ImSn.

Hot Air Solder Leveling (HASL)

HASL is the least expensive PCB surface finish, widely used due to its economical application. The process involves dipping the board in molten solder and scraping off excess solder with a hot air knife. It's ideal for through-hole and larger surface mount technology designs but not suitable for smaller boards. Typically, HASL uses Tin-Lead solder, making it non-compliant with RoHS.

Pros of HASL

• Excellent solderability.
• Cost-effective and economical.
• Wide industry acceptance and experience.

Cons of HASL

• Not ideal for small SMT circuit boards.
• Not suitable for HDI products.
• Can cause bridging on circuit boards.

Lead-free HASL

Lead-free HASL employs lead-free solder alloys instead of Tin-Lead eutectic alloys used by conventional HASL. The process involves applying molten solder for wetting, followed by scraping off excess solder with air knives, and passing the board through a washer to remove solder flux residues. Though an environmental alternative, lead-free HASL is less commonly employed due to the availability of other innovative surface finishes like OSP and ENIG.

Pros of Lead-free HASL

• Relatively inexpensive.
• Excellent solderability.
• Suitable for large through-hole technology circuits.

Cons of Lead-free HASL

• Utilizes higher temperatures, between 260°C and 270°C.
• Not suitable for small circuit boards.
• Significant thickness difference between small and large pads.

Differences between HASL and Lead-Free HASL

HASL uses lead in its solder alloy, which is harmful to humans, while lead-free HASL does not, making it safer for both humans and the environment. HASL solders have lower melting points than lead-free HASL due to the lower melting temperature of lead. Furthermore, HASL finishes are brighter and shinier, whereas lead-free HASL finishes appear dimmer and duller. Finally, lead-tin finishes offer better mechanical strength compared to lead-free finishes.

Choosing the appropriate surface finish during PCB production is vital. Considering PCBs are found in nearly all electronic components, opting for a non-toxic material like lead-free HASL ensures better safety and compliance with the RoHS directive, which has been effective since 2006.

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Conclusion

Hot Air Surface Leveling (HASL) is a common surface finish in the PCB industry. This guide has outlined the differences between HASL and lead-free HASL, emphasizing their respective physical and mechanical strengths. While HASL offers cost-effective benefits, it is not RoHS compliant. Therefore, ensuring high safety upon application of these finishes is crucial.

Lead-Free HASL

There is a significant demand for replacing tin-lead solders with lead-free alternatives due to health and environmental concerns. Lead-free HASL remains a popular finish, providing a robust and durable solution for PCBs.

What is lead-free HASL PCB surface finish?

Lead-free HASL uses lead-free alloys instead of tin-lead alloys. The process involves applying molten solder for wetting and then scraping excess solder with air knives. The board is then washed to remove flux residues. There are two primary methods for applying lead-free HASL:

• Vertical process: Boards are dipped vertically into the solder bath and then lifted through hot air knives that ensure a sufficient thickness of solder, followed by a wash.
• Horizontal process: Boards are flooded with solder and then passed through hot air knives arranged horizontally. This method provides a more uniform thickness compared to the vertical process.

The lead level in lead-free solder is less than 0.1%. Common alloys include Castin (Sn 93-98%, Ag 1.5-3.5%, Cu 0.2-2%, Sb 0.2-2%) and SnCu 0.7.

Why is lead-free HASL used?

The HASL process has certain drawbacks such as damage from thermal variations, coplanarity issues due to pad thickness variations, and heat and fumes considered inappropriate for modern board shops. Lead-free HASL offers a thinner and more uniform coating and better coplanarity, making it more compatible with modern production environments.

Coating thickness

The coating thickness in lead-free HASL is defined by the volume of solder left on the pad after passing through hot air knives. Due to higher surface tension, lead-free solder forms a thinner and more uniform coating. The thickness is typically checked using XRF (X-ray fluorescence spectroscopy).

Whisker formation

Compressive stress can cause whiskers on tin-coated surfaces. According to JESD22A121 specifications, whiskers appear under severe conditions like high temperature and humidity. Lead-free HASL offers a corrosion-resistant finish that ensures long-term solderability.

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