For many years, solder was made from a mix of lead and tin, but recent concerns about exposure to lead are changing that. Lead-free solder is safer and more eco-friendly, making it a material of choice for many PCB assembly services (including our team at My EMS Solutions).
(Blaz Erzetic/pexels)
What Is Lead Solder?
Lead solder—also called SnPb solder—is a metal alloy with tin (Sn) and lead (Pb). This type was preferred because it cools slowly, reducing joint cracks. It wets joints well, ensuring a good electrical connection. It also has a defined melting point, making it easier to do automated PCB assembly. The lower melting point also means less chance of damaging electronics.
However, lead has severe health and environmental risks. The CDC warns about issues like memory loss and reduced fertility. The EPA has laws against using lead solder due to these concerns.
What Is Lead-Free Solder?
Lead-free solder is a mix of metals like bismuth, antimony, copper, nickel, silver, zinc, and tin. The most common blend is tin and copper. It has a higher melting point and creates stronger joints. Due to lead’s toxicity, it’s now the standard for consumer goods.
Lead-free solder is essential for everyday electronics. It’s not ideal for critical applications like aerospace, but for most electronics, it’s perfect. Lead-free joints can lack surface tension, but most electronics don’t need to handle high stress.
Comparing Lead vs. Lead-Free Solder
Here’s how the proprieties of lead vs. lead-free solder differ from each other:
Melting Point
Lead-free solders have a higher melting point than lead-based alloys. Lead-based solder, like Pb-37Sn, melts at 183°C (361°F). In contrast, lead-free solders melt at higher temperatures, like Sn-3.5Ag at 221°C (429.8°F).
Here’s a quick look at some melting points:
- Lead-based Pb-37Sn: 183°C (361.4°F)
- Lead-free Sn-3.5Ag: 221°C (429.8°F)
- Lead-free Sn-58Bi: 139°C (282.2°F)
- Lead-free Sn-0.7Cu: 227°C (440.6°F)
- Lead-free Sn-8.8Zn: 198°C (388.4°F)
- Lead-free Sn-3.4Ag-4.8Bi: 205°C (401°F)
- Lead-free Sn-3.8Ag-0.7Cu: 217°C (422.6°F)
- Lead-free Sn-8Zn-3Bi: 197°C (386.6°F)
Higher melting points mean more challenging soldering processes. Lead-based solder flows better and heats up quickly, making it easier to use. However, the great thing about high melting points is that they can withstand high temperatures, making them very durable when things heat up.
Physical Outlook
You can often tell the difference by just looking. Lead solder is shiny, while lead-free solder appears a little more dull and rough. The appearance of the soldered area is a giveaway.
Lead solder is also heavier—about 8.5G/m2 compared to 3.5G/m2 for lead-free. Another difference is the flux. Lead solder uses lead flux, while lead-free solder uses a different mix—lead-free flux.
Performance Difference
In PCB soldering, lead-based and lead-free solder have several differences. Melting point is one, but another is wettability—the way solder bonds with components when melted. In addition, lead-based solder is smoother, easier to apply, and cools solid without defects.
That said, lead-free solder is not prone to spread out during reflowing like lead solder is. The result is a stronger bond that does not carry the risks of rapid stencil wear. Lead soldering often leads to voids or fillet lifting.
Tin Whiskers
Tin whiskers are tiny, thread-like structures that grow from pure tin surfaces. They create electrically conductive paths and can cause short circuits. We don’t know why or how they form.
Pure tin surfaces often produce many whiskers. To reduce this, tin is mixed with other elements like lead. Lead-free solders are more prone to tin whiskers, so they need treatment to improve reliability.
Cost
In terms of cost, lead is cheaper than tin or silver. Adding lead to solder reduces overall expenses.
Material prices can change based on market conditions, location, and the amount needed. Lead-free solders are more expensive than lead-based ones, but the big benefit, of course, is that they don’t introduce the health and environmental risks that come with lead solders. This is why many companies are shifting to lead free PCB manufacturing despite the higher cost.
Longevity
Lead-based solders form a thinner intermetallic layer. This layer can be less brittle and resist cracks.
Lead-free solders, meanwhile, form a thicker intermetallic layer. This layer can be more brittle and expand more. In some situations, this can lead to cracking, but expert solderers can often minimize these risks.
At My EMS Solutions, our team is expertly trained in the key aspects of lead-free soldering, including evaluation of solder paste, wave solder flux, components, and equipment. We know how to optimize design and soldering processes to create reliable products with well-managed production costs.
Environmental Impact
Lead is a toxic metal that bioaccumulates in nature. It can be released into the air and water throughout its life cycle. When soldering, lead-based solders release dust and fumes that are highly toxic to humans. Lead can worsen asthma, irritate eyes, and is known to be carcinogenic.
Always wear a mask, avoid eating or drinking, and follow safety instructions when soldering with lead alloys.
The European Union introduced the WEEE and RoHS directives in the early 2000s to ban the use of lead and other toxic elements in electronics manufacturing. Products sold in the EU must comply with RoHS. Many countries have banned lead solder, even for hobbyist projects.
Conclusion
Because lead-based solders are easier to work with and cheaper, hobbyists might prefer them for small projects. However, despite the benefits of lead-based solders, their toxic nature makes them hard to justify. Lead-free solders are the sensible choice for commercial use. They’re safer for health and the environment.
An EMS company that prioritizes lead free PCB assembly ensures compliance with health and environmental standards. Specialized assembly procedures, thorough evaluations, and adherence to WEEE and RoHS directives are essential. Emphasizing lead-free solutions helps reduce environmental impact and improves the overall safety and reliability of electronic products.
Video
Infographic
Solder has traditionally been made from a lead and tin mixture, but lead-free solder is now preferred for PCB assembly services due to lead exposure concerns. Learn about the differences in properties between lead and lead-free solder in this infographic.
