Detailed Explanation of PCB Hot Air Solder Leveling（HASL）

1. What is PCB Hot Air Solder Leveling (HASL)?

Hot Air Solder Leveling (HASL) is a surface finish process for PCBs. It involves applying a layer of tin-lead alloy to the copper surface of the PCB, which helps protect the pads from oxidation and maintains solderability. The solder used in HASL typically consists of 63% tin and 37% lead, which melts and bonds with solder paste during the SMT soldering process. Before the advent of lead-free processes, HASL was dominant in the electronics manufacturing industry. While its usage has declined, HASL remains widely used in sectors such as medical, military, and aerospace.

2. Detailed Steps of the HASL Process

Before performing HASL surface finish, the PCB needs to be cleaned, usually through a micro-etching process to clean the copper surface. After cleaning, the PCB is preheated and coated with flux. The flux reduces surface tension between the PCB and solder, allowing the solder to better cover the PCB. Once flux is applied, the PCB is immersed in molten tin/lead solution, which covers the exposed copper pads and forms a tin-lead layer. After forming the tin-lead layer, high-pressure hot air knives are used to remove excess solder from the PCB surface and holes, ensuring uniform solder deposition, and leaving a thin, even protective layer on the pads.

Key parameters of the HASL process include solder temperature, dip time, air knife pressure, air knife angle, air knife distance, and PCB rise speed:

• Solder temperature: The melting point of Sn63Pb37 solder is 183°C. For optimal intermetallic compound formation, solder temperature is typically controlled between 230°C and 250°C.
• Dip time: Typically between 2-4 seconds; prolonged exposure can lead to excessive intermetallic growth.
• Air knife pressure and flow rate: Higher pressure results in thinner solder layers. Generally, air knife pressure is controlled between 0.3-0.5 MPa.
• Air knife angle: Improper angles can cause uneven coating thickness on either side of the PCB. Most air knives are angled downwards by 4°.
• Air knife distance: Usually set between 0.95-1.25 cm.
• PCB rise speed: This is adjusted based on the specific situation. A slower rise increases hot air finish time, thinning the solder layer. Conversely, a faster rise increases the solder layer thickness.

3. Comparison of HASL with Other PCB Surface finish Processes

Advantages of HASL:

1. Excellent solderability: The uniform solder layer formed by the HASL process ensures superior solderability. This even layer melts and flows more easily during soldering, creating reliable joints and improving the reliability of component connections. HASL is particularly well-suited for traditional through-hole components and some surface-mount devices.

2. Cost-effectiveness: Compared to other surface finish processes such as Electroless Nickel Immersion Gold (ENIG) or electroplated gold, HASL is more cost-effective. This lower cost makes HASL a practical choice for mass production, especially in cost-sensitive projects. Manufacturers can achieve reliable soldering and electrical performance without significantly increasing costs.

3. Wide application: HASL is suitable for most general-purpose PCB applications, covering sectors like consumer electronics and industrial equipment. Its mature process is easy to control and implement, meeting various design needs. Though not ideal for high-density or high-precision applications, HASL remains a preferred choice for many traditional applications.

Disadvantages of HASL:

1. Uneven thickness: A major drawback of HASL is the potential for uneven solder thickness, which is particularly problematic for high-density PCBs where fine-pitch components require precise solder thickness to avoid defects like bridging or opens, negatively affecting overall PCB performance.

2. Thermal stress: The high temperatures involved in HASL can cause thermal stress on the PCB substrate, leading to board warping, especially with thinner PCBs. This warping can result in assembly issues or degraded electrical performance. Care must be taken when working with heat-sensitive materials or thin substrates.

3. Environmental concerns: Traditional HASL uses lead-based solder, which poses potential risks to the environment and human health. Despite the introduction of lead-free HASL, lead-based processes are still used in many regions. With increasing environmental regulations, manufacturers must consider HASL's environmental impact and may need to seek alternatives to meet regulatory requirements.

4. Leaded vs. Lead-Free HASL: How to Choose?

In PCB manufacturing, HASL is a common surface finish process. However, with increasing environmental regulations, manufacturers are faced with the challenge of choosing between leaded and lead-free HASL. Below is a comparison of the two processes to help you make informed decisions during design and manufacturing.

Leaded HASL: Maintaining the Traditional Process

• Composition and Benefits: Leaded HASL uses tin-lead alloys, typically a 63/37 tin-lead mix. This alloy is known for its excellent solderability and relatively low melting point, making the soldering process smoother and more reliable. Additionally, leaded HASL is a mature process with proven performance and stability, giving manufacturers confidence in its application.
• Environmental Challenges: Lead is a substance with potential environmental and health hazards. As environmental regulations become stricter, many countries have placed restrictions on the use of lead, leading to the gradual phasing out of leaded HASL in some regions and applications.

Lead-Free HASL: Leading the Environmental Trend

• Composition and Benefits: Lead-free HASL uses tin-silver-copper alloys, which comply with environmental regulations such as RoHS and significantly reduce environmental impact. For companies focused on environmental responsibility, lead-free HASL is an ideal choice.

Process Challenges: While offering clear environmental benefits, lead-free HASL also presents challenges. The higher melting point of lead-free solder requires higher soldering temperatures, which may increase thermal stress on the PCB, especially for thinner substrates. Additionally, manufacturers may need to adjust equipment and processes to accommodate lead-free materials.

When choosing between leaded and lead-free HASL, consider the following factors:

• Environmental Regulations: If your products are destined for regions with strict lead restrictions, lead-free HASL is necessary.
• Application: For industrial applications where environmental concerns are less pressing, the maturity and cost benefits of leaded HASL may be more appealing.
• Manufacturing Capability: Assess whether your equipment can accommodate lead-free processes and, if necessary, make technical adjustments.
• Cost Considerations: While lead-free materials are more expensive, products compliant with environmental regulations may offer broader market opportunities in the long term.

Conclusion

Leaded and lead-free HASL each have their own advantages and disadvantages, and the choice depends on your specific needs and market environment. As technology continues to advance and environmental awareness grows, lead-free HASL is becoming the mainstream choice. However, for certain applications, leaded HASL remains a viable option. By understanding the characteristics of both processes, you can better select the appropriate surface finish solution for your project.