What is the Difference Between Leaded and Lead-Free Solder?

Understanding the difference between leaded and lead-free solder is crucial for making the right choice. It ensures your PCBA project meets various requirements, such as reliability and RoHS compliance. Below is a detailed comparison of the two types of soldering alloys.

What is Leaded Solder?

Leaded solder is solder made from a lead-tin alloy. The amounts of Sn and Pb can vary, giving rise to various ratios with varying properties. The most common ratios are 60:40 and 63:37.  The 63/37 ratio is a eutectic alloy, which means it solidifies instantly, creating highly reliable joints.

Although not common, this type of solder remains popular in specific applications and industries. You need it when circuit boards must meet high-quality standards or when repairing older PCBs.

What is Lead-Free Solder?

This type doesn’t contain lead. Instead, it uses various other metals to make tin alloys. The metals include copper, silver, bismuth, antimony, indium, gold, palladium, and zinc.

The solder is best for general electronics, such as consumer products. It pollutes the environment less and poses fewer health risks. It also meets regulatory standards across RoHS-compliant nations.

Difference Between Leaded and Lead-Free Solder

The main difference between lead solder and lead-free solder is their composition. The following are key variations and their impact on the assembly process and PCBA reliability.  

Composition Difference

One contains Pb, while the other doesn’t. This difference in leaded and lead-free solder composition is what gives rise to their varying characteristics. Lead-based type is commonly 60/40 or 63/37 Sn-Pb.

On the other hand, the Pb-free type is usually a silver/copper and tin alloy. It can also combine several other metals to achieve desired properties and other requirements.

Melting Point

Lead-tin alloy has a lower melting point of 183°C (361°F). The low melting temperature makes the alloy easier and safer to use, regardless of whether you’re assembling PCBs by hand or automatically.

The lead-free solder melting point is slightly higher. It typically liquefies at 217° Celsius (423° F) or higher, depending on the specific alloy (metal composition). The increased temperature brings assembly challenges and component safety risks.  

Joint Appearance

Pb-based solder joints have a shiny look. On the other hand, completed Pb-free joints have a dull appearance. This difference makes leaded joints easier to inspect and identify assembly errors.

As a result, Pb-solder joints offer higher visual and optical inspection accuracy than Pb-free types. The increased accuracy is crucial when examining high-reliability circuit boards.

Joint Reliability

Pb-based solder produces the most reliable joints, given its wettability and improved inspection visibility. The joints are robust and less likely to leave the factory with undetected defects.

Lead also prevents the formation of tin whiskers. These are tiny projections that grow from the soldered points. They cause short circuits, leading to potential board failures and risks to devices or systems.  

While modern lead-free alloys are very reliable, they can be more brittle and susceptible to failures from mechanical stress or thermal cycling compared to the more ductile tin-lead joints. Furthermore, lead-free tin surfaces have a higher propensity to grow ‘tin whiskers’.

Environmental Impact

One of the most significant drawbacks of Pb-based solders is their environmental impact. Lead is toxic, and its disposal requires greater care to prevent contamination. It’s one of the reasons for its ban on consumer electronics.

The Pb-free type is less toxic to the environment. However, fumes generated during soldering—regardless of alloy—can be harmful due to flux content and should not be inhaled. This attribute makes it a safer choice unless the PCBA has special reliability needs.  

Regulatory Requirements

The Restriction of Hazardous Substances (RoHS) regulations prohibit the use of lead in electronics manufacturing. Many countries uphold this ban, only accepting its presence in circuit boards that rarely come into contact with people.

Such circuit boards are popular in various applications. Popular uses include industrial electronic systems, some medical devices, automotive systems, military electronics, and the aerospace industry.

Cost Difference

Leaded and lead-free solder also differ in cost. The Pb-based type is less challenging to produce, making it less costly. The unleaded type costs more, primarily due to the use of expensive metals like silver and gold.

While the cost difference may not influence usage, it’s one of the variations that users should know. It affects the final PCBA price, depending on the type of metal alloy used.

Lead-Free Solder vs. Leaded Solder

Both leaded and lead-free solder types are in use today, with each offering advantages and disadvantages in PCB electronics. A brief look at the pros and cons should help you make the right choice.

Leaded Solder Advantages and Disadvantages

Advantages

• The alloy is easier to melt and work with
• It flows well, improving wettability and producing strong joints
• It reduces whisker formation, making circuit boards less susceptible to short circuits
• Lower Cost: The raw materials are less expensive, making it a more economical choice for legally exempt applications.

Disadvantages

• It poses health risks mainly from lead exposure during handling, in addition to the general hazards of soldering fumes.
• It poses a danger to the environment if users discard it inappropriately

Lead-Free Solder Advantages and Disadvantages

Advantages

• The alloy is less toxic to work with
• It is less harmful to the environment than the leaded type
• It complies with environmental protection requirements imposed on electronics

Disadvantages

• High-melting temperature that reduces usability and increases assembly costs
• The higher melting temperature requires more heat, which can damage PCB components
• It costs more than the leaded type, mainly if it contains expensive metals, such as silver

Is Lead-Free Solder Better?

It depends on various factors, such as application type and environmental considerations. The alloy is best suited for mounting circuit boards in regions with strict guidelines for electronic materials.

Lead-free alloys are also preferable in applications involving direct human contact, such as when assembling printed circuit boards for consumer products.

On the other hand, it isn’t a good choice when joint reliability is paramount. That includes circuit boards whose failure can be disastrous.

Such circuit boards operate mission-critical systems that require higher quality levels. They also often work in demanding conditions that subject the joints to certain extremes. The conditions include excess heat and vibrations.

When to Use Leaded Solder

• When manufacturing aerospace, military, industrial, automotive, medical, and other high-dependability circuit board assemblies
• If your PCB project involves mounting heat-sensitive components. The alloy melts at a lower temperature, allowing their safe assembly
• When fixing traditional leaded PCBs (the two alloys must not assemble parts on the same board)
• If you intend to use your PCBA in a region that doesn’t prohibit the use of lead  in electrical and electronic equipment

When to use Lead-Free Solder

• When assembling general electronics circuit boards, such as PCBs for consumer electronics
• If your region or target market observes Hazardous Substances Directive or RoHS regulation, such as European Union member countries
• If your company policies uphold strict environmental safety requirements, even if the law allows using lead

Conclusion

The difference between leaded and unleaded solder is crucial in the electronics industry. It influences the component mounting process, joint quality, RoHS compliance, and various other requirements.

At Venture, we offer both leaded soldering services and unleaded options to meet your exact requirements—whether you’re building high-reliability industrial systems or aiming for full RoHS compliance. Our team is here to ensure your PCBA assembly delivers on both safety and performance.