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Lead vs. Lead-Free Solder: Which is Better?

lead-free-solder-kit

Table of Contents

The soldering process involves heating the solder alloy to its melting point, which allows it to flow and create a bond between the component leads and the PCB pads. This process is typically performed using techniques such as wave soldering or hand soldering, depending on the scale and requirements of the manufacturing process. We are dedicated to tell you something about the solder alloy used in PCB solder paste, because there are some interesting stuffs you need to know!

Lead Solder

Lead solder, also known as lead-based solder, is a type of solder alloy that contains a significant amount of lead (Pb) as one of its main components.

The Origins and Qualities of Lead (Pb)
The Origins and Qualities of Lead (Pb)

Composition

Lead solder typically consists of a mixture of tin (Sn) and lead (Pb), with various compositions depending on the specific application. Common lead solder alloys include Sn63Pb37 (63% tin, 37% lead) and Sn60Pb40 (60% tin, 40% lead). The addition of lead provides desirable properties such as low melting point, good wetting characteristics, and ease of use during the soldering process.

Environmental Concerns

The use of lead-based solder has raised environmental concerns due to the potential hazards associated with lead. Lead is considered a toxic substance and poses risks to human health and the environment. The disposal and recycling of electronic waste containing lead solder can also contribute to environmental pollution. As a result, regulatory measures such as the RoHS directive have been implemented to restrict the use of hazardous substances, including lead, in electronic products.

Transition to Lead-Free Solder

The PCB manufacturing industry has undergone a significant transition from lead solder to lead-free solder in response to environmental regulations and concerns. Lead-free solder alloys, such as SAC (Sn-Ag-Cu) and other compositions, have been developed as alternatives to lead solder. These lead-free alloys aim to provide similar or improved performance while eliminating the use of hazardous materials.

Lead-Free Solder

Lead-free solder is a solder alloy that does not contain lead (Pb) or contains it in trace amounts within the legal limits set by regulations such as RoHS (Restriction of Hazardous Substances). It has gained significant importance in PCB (Printed Circuit Board) manufacturing due to environmental concerns associated with the use of lead-based solder.

Lead-Free Solder Products
Lead-Free Solder Products

Common Lead-Free Solder Alloys

Some commonly used lead-free solder alloys:

Tin-Silver-Copper (Sn-Ag-Cu or SAC): SAC alloys are among the most widely used lead-free solder alloys. They typically consist of tin (Sn), silver (Ag), and copper (Cu). The most common SAC alloy compositions include SAC305 (Sn-3.0Ag-0.5Cu) and SAC387 (Sn-3.8Ag-0.7Cu). SAC alloys offer good thermal and mechanical properties, excellent wetting characteristics, and reliable solder joint formation.

Tin-Copper (Sn-Cu): Sn-Cu alloys, also known as stannous copper alloys, are another popular choice for lead-free soldering. Common compositions include Sn-0.7Cu and Sn-0.5Cu. These alloys offer lower costs compared to SAC alloys while providing acceptable mechanical and thermal properties. Sn-Cu alloys are compatible with a wide range of applications, including consumer electronics and automotive electronics.

Tin-Bismuth (Sn-Bi): Sn-Bi alloys are used in specific applications where low melting temperature is desired. The most common composition is eutectic Sn-58Bi, which has a low melting point of approximately 138°C. Sn-Bi alloys are suitable for soldering temperature-sensitive components and PCBs with heat-sensitive substrates.

Tin-Zinc (Sn-Zn): Sn-Zn alloys are utilized in certain applications where lower soldering temperatures are required. These alloys have a lower melting point compared to other lead-free solder alloys. Sn-Zn alloys can be a suitable option for soldering heat-sensitive components and reducing thermal stress on PCBs.

Tin-Silver (Sn-Ag): Sn-Ag alloys, without the addition of copper, have been used in some applications. However, the addition of copper in SAC alloys has become more common due to its beneficial effects on mechanical and thermal properties.

Lead-free solder is compatible with modern electronic components and PCB materials. It can be used in surface-mount technology (SMT) processes, including reflow soldering, and through-hole assembly techniques. Lead-free solder alloys have been optimized to address compatibility challenges, ensuring successful soldering and reliable connections.

RoHS Directive and its Impact on PCB Manufacturing

The RoHS (Restriction of Hazardous Substances) directive is an EU regulation that restricts the use of certain hazardous substances in electrical and electronic equipment (EEE). It prohibits or limits the use of substances such as lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBBs), and polybrominated diphenyl ethers (PBDEs) in EEE placed on the EU market.

It has driven the transition from lead solder to lead-free solder in the industry, as lead is one of the restricted substances. PCB manufacturers need to ensure that their products comply with RoHS requirements to access the EU market. Compliance involves using lead-free solder and ensuring that other restricted substances are within the allowed limits.

Table: A Comparison of Lead and Lead-Free Solder

Parameters
Lead solder
Lead-free Solder
Units
Electrical Conductivity
11.5
15.6
IACS
Thermal Conductivity
50
73
(W / m)
Melting Point
183
218
°C
Resistivity
15
11
M ohm-cm
Surface Tension
481
548
mN/m
Fatigue Life
3
1
 
TCE
23.9
21.4
micrometers/M/°C
Density
8.5
7.44
g/cm(3)
Shear Strength
23
27
MPa

Differences Between Lead and Lead-Free Solder

There are some key differences between lead and lead-free solder. These differences impact their properties, uses, behaviors, and features. Here are some important differences

Health and Safety

Lead is a toxic substance that poses health risks to workers involved in soldering processes. Lead-free solder eliminates these health hazards, providing a safer working environment for PCB manufacturing professionals.

Melting Point

The melting point of the solder is the main problem in constructing a PCB because it alters the PCB surface, statistics, and components. Lead and lead-free solder have specific melting points. Lead solder melts at around 183 °C (361 °F). Lead-free solder has no set melting factor and melts at 217 °C (423 °F) or higher.

Rusting

Rusting is a big problem for revealed circuit boards (PCBs) because it reduces their provider life and might cause technical troubles together with quick circuits. This is because lead-free solder has a lower corrosion resistance while the layer of softness thins, as well as a stronger corrosion coating on its surface. As a result, lead-free solder is a higher preference for PCBs that must operate under wet conditions.

Wettability

When comparing the wettability of lead and lead-free solder, several major variations emerge. Wettability describes how well a solder extends and clings to the surfaces of the components and PCB during the soldering process. Leaded solder generally has better wettability because of its composition. This allows it to melt and flow smoothly, creating strong and reliable joints with good coverage. However, lead-free solder may have less wettability at times. Despite this, lead-free solder remains a better choice for a variety of reasons.

Cost

The transition to lead-free solder in PCB manufacturing has introduced some cost implications. Here are a few factors to consider:

  1. Solder Material Cost: Lead-free solder alloys often have higher material costs compared to lead-based solder due to the use of alternative metals like silver and copper. However, the price difference has decreased over time as lead-free solder has become more prevalent and economies of scale have been achieved.
  2. Process Adjustments: The transition to lead-free solder may require process adjustments, such as optimizing reflow profiles, reworking techniques, and potential changes to equipment and soldering materials. These adjustments may involve some initial costs for equipment upgrades or process optimization.
  3. Rework and Repair: Lead-free soldering may have different characteristics compared to lead solder, requiring additional care during rework and repair processes. This could result in increased labor costs or the need for specialized equipment or training.
  4. Long-Term Reliability: The improved reliability of lead-free solder can potentially lead to long-term cost savings by reducing the need for repairs and replacements due to solder joint failures.

The cost impact of the transition to lead-free solder in PCB manufacturing varies depending on factors such as the scale of production, specific requirements, and the availability of lead-free solder materials in the market. While there may be some initial cost adjustments, the long-term benefits of compliance with environmental regulations and access to global markets outweigh the potential cost implications.

Which One is Better to Use?

There is no option to choose between lead and lead-free solder because lead solder is restricted from RoHs due to its toxic nature. Despite all the advantages of lead-based solders, the toxic nature of the heavy metal makes it difficult to use. If there is an option to choose, then you can easily choose the suitable solder for your circuit board by noticing your PCB requirements in both environmental and electrical aspects. According to RoHs, lead-free solder is the best option to use in PCBs.

Disadvantages of lead solder

Health Hazards: Lead is poisonous and can have major health consequences. Even small levels of lead can cause lead poisoning. It can cause symptoms:

  • Headaches
  • Stomach pain
  • Skin problems
  • Inhalation
  • Neurotoxicity
  • Kidney Damage
  • Developmental Issues in Children

Environmental Harm: Lead solder can be dangerous to the environment. When gadgets containing lead solder are not properly gotten rid of, lead may escape into the land and water, harming them and hurting wildlife and people.

Professional Safety: Applying leaded solder may raise dangers at work. Workers who handle lead solder without enough safeguards risk swallowing lead fumes or getting lead atoms through their skin, which leads to serious health risks.

Regulatory Restrictions: Many countries restrict the use of lead in electronics. Using leaded solder can make it more difficult to stick with these regulations, possibly resulting in legal complications and charges.

Benefits of Lead-Free Solder

Ideal for circuit soldering: Compared to lead solder, lead-free alloys are more powerful for electric-printed circuit board applications. Lead-free solder doesn’t spread out an awful lot throughout reflowing, which results in stronger connections and reduces issues like rapid stencil wear. Lead soldering is more prone to problems such as voids or fillet lifting. Overall, the use of lead-free solder yields higher effects for PCBs used in electronics.

Best for the environment: Lead-free solder is a satisfactory option for the surroundings as it gets rid of the risks related to toxic lead. Its non-poisonous composition method has fewer fitness hazards for employees and the general public, and it guarantees safer disposal of electronics. This reduces the chance of soil and water contamination. Additionally, lead-free solder simplifies the recycling technique, making it more efficient and cost-effective, thereby minimizing electronic waste in landfills.

Different-Types-of-Solder
Different Types of Solder

Effect on Different Types of Soldering

The choice between leaded and lead-free solder paste has notable implications for the quality of the final product in PCB manufacturing. While controlled soldering parameters can mitigate some effects, proper design layout and component selection play crucial roles in overcoming quality control issues. In wave soldering or through-hole soldering, the decision to use lead-free products is determined by specific design parameters and the demands of the final assembled PCBs.

Yet, the higher temperatures associated with lead-free soldering emphasize the importance of meticulous board layout and component selection to prevent metal decomposition. And protects sensitive electronic components, particularly during reflow processes.

Conclusion

The selection between lead and lead-free solder extensively affects the quality, safety, and environmental footprint of PCB manufacturing. While lead solder has higher wettability and a lower price, its poisonous nature poses extreme health and environmental dangers, leading to strict regulatory restrictions.

Lead-free solder, regardless of its higher melting factor and value, affords a safer alternative for both human health and the environment. It creates more potent, more durable joints and complies with RoHS guidelines, making it the favored alternative for cutting-edge digital packages. By considering the necessities of your PCB, inclusive of both environmental and electric components, lead-free solder emerges because of the advanced choice, ensuring a more secure and sustainable method of electronics manufacturing.

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About the Author
Jeffrey Lee
I'm Jeffrey, an experienced electronics engineer. As a seasoned content creator, I bring a deep passion for PCB-related topics, allowing me to communicate complex concepts with clarity and precision, making them accessible to a wide audience.
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