Is HASL lead free?

HASL vs Lead-free HASL - PCB Surface Finishes - NEXTPCB

Posted:05:20 PM December 06, 2022

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Introduction to HASL Surface Finish

Hot air solder leveling and lead-free HASL (Hot Air Solder Leveling) are the two prominent surface finishes in the PCB Manufacturing and Assembly Process. However, there is a noticeable difference; Lead-Free HASL does not use tin and lead solder.

Yet, when choosing the right surface finish for your circuit boards, it is vital to choose a cost-effective and RoHS-compliant surface finish. Suppose you want to use HASL as your surface finish choice and need help deciding which surface finish to use. In that case, this guide examines the pros and cons differences between HASL and lead-free HASL.

What is PCB Surface Finish?

Surface Finish is metalizing the outer layer of a PCB being soldered. The function of PCB surface finish is to offer protection against corrosion on the copper circuitry. Also, it fosters easy soldering of circuit board components.

There are varied kinds of surface finishes employed in the PCB Industry. They are: Hot Air Solder Leveling (HASL), Electroless Nickel/Immersion Gold (ENIG), Immersion Silver (ImAg), Organic Solderability Preservative (OSP), and Immersion Tin (ImSn)

Hot Air Solder Leveling (HASL)

Hot air solder leveling (HASL) is the least expensive PCB surface finish. Many PCB Manufacturers commonly use this surface finish as it is economical and widely available. When applying Hot Air Surface Leveling on circuit boards, the board is dipped in molten solder and then scrapped off with a hot air knife.

PCB Manufacturers that employ through-hole technology or larger surface mount technology designs, hot air surface leveling excels in these designs. For smaller boards, it is not ideal. Also, The solder used is typically Tin-Lead solder, making it RoHS non-compliant.

Pros of HASL

• This surface finish offers excellent solderability.
• It is inexpensive and economical in its application.
• It is a popular surface finish with an exceptional industry experience than other surface finishes.

Cons of HASL

• Not suited for small SMT circuit boards.
• It is not ideal for HDI products.
• It can cause bridging on circuit boards.

Lead-free HASL

Lead-free hot air solder leveling is a type of surface finish that employs lead-free solder alloys. Instead of the conventional Tin-Lead eutectic alloys used by HASL, Lead-free HASL employs other solder alloys that do not contain lead.

When applying Lead-free HASL on the circuit board, molten solder is first used on the circuit board to achieve wetting. Then, air knives (usually set at a temperature above the lead-free solder melting point) are used to scrap off excess solder.

After the solder has solidified, the circuit board is passed through the washer, where solder flux residues are removed.

Despite being considered an environmental alternative to the standard HASL, lead-free HASL is least employed in surface finishes. The reason is that other innovative surface finishes are available in the PCB market. They include OSP and ENIG.

Pros of Lead-free HASL

• It is relatively inexpensive
• Lead-free HASL offers excellent solderability
• This kind of surface finish applies to large through-hole technology circuits

Cons of Lead-free HASL

• Lead-free HASL utilizes higher temperatures between 260℃ and 270℃
• It is not suited for small circuit boards
• Lead-free HASL offers a huge difference in thickness between small and large pads

Differences between HASL and Lead-free HASL

Many Customers understand the HASL process. Still, they need to learn the difference between HASL and Lead-free HASL. However, the differences between them are:

• HASL uses lead as part of its solder alloy, which is considered harmful to humans. However, Lead-free HASL does not use lead as its solder alloy, making it safe for humans and the environment
• HASL solders have lower melting points than lead-free HASL because Lead melts at lower temperatures than other solder metals.
• Lead Tin solders (HASL) appear brighter and shiny on the circuit board when applied as surface finishes. In contrast, lead-free HASL appears dim and dull on the circuit board.
• Lead-tin surface finishes offer improved mechanical strength than lead-free surface finishes.

Choosing the proper surface finishing during PCB production is essential. This careful consideration is because PCBs are found in every electronic component worldwide, ranging from colossal industrial engines to children's toys.

Hence, PCB production is taken seriously to avoid exposing toxic materials such as lead to children and adults, which could adversely affect their health. Hence, since EU RoHS's inception in 2006, HASL has taken on a new approach by not utilizing lead in its surface finishes.

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Conclusion

Hot Air Surface Leveling (HASL) application in surface finishing is a common trend in the PCB industry. This guide has provided differences between HASL and lead-free HASL as they provide respective physical and mechanical strengths in their application.

Also, while HASL offers cost-effective benefits to PCB surface finishes, it is not RoHS compliant. As a result, should ensure high safety upon application of these finishes.

Lead-Free HASL

There is a huge demand for the replacement of tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry because of health and environmental concerns. Lead-free HASL is a popular finish that provides the most durable finish for PCBs.

What is lead-free HASL PCB surface finish?

Lead-free HASL is a type of surface finish that uses lead-free alloys instead of tin-lead (Sn-37Pb) eutectic alloys. Initially, molten solder is applied on a board (after cleaning and fluxing) to achieve wetting and excess solder is scraped with the help of air knives (a tool that is set at a temperature above the melting point of the solder). After the solder solidifies, the circuit board is passed through the washer where the solder flux residues are removed.

There are two methods of applying lead-free HASL:

• Vertical process: Vertical process involves a panel of boards mounted on a frame that moves vertically downward into the solder bath, holds for a certain time (usually 2-3 seconds), and then lifts the panel through the operating air knives. These knives are adjusted to clear through-holes and vias while leaving a sufficient thickness of solder on all areas where a solderable finish is essential. After the solder cools and solidifies, the panel passes through the washer/dryer to remove flux residues. The operator then inspects the quality of the finish.

• Horizontal process: In the horizontal process, the board is flooded with solder applied through nozzles or between rollers on the top and bottom of the panel before being moved between hot air knives placed above and below the panel.

Usually, HASL lines are of vertical design and deliver a high-quality finish. Fully automated vertical lines can deliver rack-to-rack performance including fluxing, preheating, washing, and drying. The finish in the horizontal process is more uniform in thickness compared to the vertical process.

The lead (Pb) level in lead-free solder is less than 0.1%. Castin (a composition of Sn 93­ 98%, Ag 1.5-3.5%, Cu 0.2-2%, and Sb 0.2-2%) alloy and SnCu 0.7 are widely used in this process.

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Why is lead-free HASL used?

There are several problems associated with the HASL process as given below:

• Damage due to additional thermal deviations
• Coplanarity issues because of pad to pad variation in coating thickness

• Stencil gasketing issue related to the difference in coating thickness and mushroom cap profile (of the coating) found on small pads.

• Heat and fumes associated with the process were considered out of place in modern board shops.

The difference in the solder coating thickness of a lead-free HASL finish is usually about half that of the tin-lead HASL finish. The coplanarity is also good in this finish. The modern HASL line is designed to provide the operating conditions in line with other equipment in modern board shops to avoid issues with heat and fumes.

Coating thickness

The solder coating profile is defined by the volume of solder left on the pad after passing the board through the hot air knife and surface tension forces. Due to surface tension, the coating has a tendency to be thicker on smaller pads.

Because of the higher surface tension of lead-free solder, the coating formed in the HASL process is both thinner and uniform compared to tin-lead solder formed in similar conditions.

The coating thickness can be checked by XRF (X-ray fluorescence spectroscopy) technique.

Whisker formation

Compressive stress is the driving force for whiskers (a thin filament protruding from a board surface having tin as a final finish). According to the JESD22A121 specification, whiskers appear on the hot-dipped lead-free finish on copper only in areas where compressive stress is intentionally introduced and under the most severe conditions such as the combination of elevated temperature (60°C) and high humidity (87%RH).

Lead-free HASL provides a corrosion-resistant finish that can assure solderability over long periods.

Contact us to discuss your requirements of pcb surface finish comparison. Our experienced sales team can help you identify the options that best suit your needs.