Lead-Free HASL

Contents

If you want to learn more, please visit our website 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.

 

Design for Manufacturing Handbook

10 Chapters - 40 Pages - 45 Minute Read

What's Inside:

• Annular rings: avoid drill breakouts
• Vias: optimize your design
• Trace width and space: follow the best practices
• Solder mask and silkscreen: get the must-knows

Download Now

 

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.

HASL (Hot Air Solder Leveling) and Lead-free HASL are both surface finish techniques used in the printed circuit board (PCB) manufacturing process. They are used to protect copper traces and pads on the PCB from oxidation, provide a solderable surface for components, and ensure reliable solder joints during assembly. The main difference between HASL and Lead-free HASL lies in the composition of the solder used in the process.

Composition:

· HASL (Hot Air Solder Leveling): Traditional HASL involves using a solder alloy that contains lead (Pb) as a major component. This composition has been used for many years, but due to environmental concerns and regulations, the use of lead in electronics manufacturing has been reduced.

For more multilayer pcb design tipsinformation, please contact us. We will provide professional answers.

· Lead-free HASL: As the name suggests, lead-free HASL uses a solder alloy that does not contain lead. These alloys typically consist of a mix of tin (Sn), silver (Ag), and sometimes other metals like copper (Cu). These alloys comply with RoHS (Restriction of Hazardous Substances) regulations, which restrict the use of certain hazardous materials in electronics.

Environmental Concerns:

· HASL: The lead-based solder used in traditional HASL poses environmental and health risks, as lead is a toxic substance. The disposal of waste containing lead is regulated and requires special handling to prevent environmental contamination.

· Lead-free HASL: Lead-free HASL addresses the environmental concerns associated with lead-based solder. By eliminating lead from the process, the resulting PCBs are more environmentally friendly and compliant with regulations.

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 , HASL has taken on a new approach by not utilizing lead in its surface finishes.

Solder Characteristics:

· HASL: Lead-based solder used in HASL tends to have a lower melting point than some lead-free alternatives. This can make the soldering process easier, but it also means that the resulting solder joints might be more susceptible to heat-related issues over time.

· Lead-free HASL: Lead-free solder alloys used in Lead-free HASL have higher melting points compared to traditional lead-based solders. This can make the soldering process slightly more challenging, and manufacturers need to ensure that proper temperature profiles are used during soldering to avoid issues like thermal stress.

Reliability and Performance:

· HASL: While lead-based solder joints might be easier to work with due to their lower melting point, they can be more prone to developing brittle connections and undergoing thermal cycling-related fatigue over time.

· Lead-free HASL: Lead-free solder joints are generally considered to be more reliable and less prone to thermal fatigue in the long term. However, they might require more precise control of soldering processes to ensure good joint formation.

Conclusion

In summary, the primary difference between HASL and Lead-free HASL lies in the composition of the solder used. Lead-free HASL addresses environmental concerns and regulations while offering improved long-term reliability, although it might require more careful process control during soldering.

Want more information on pcb surface finish comparison? Feel free to contact us.