What is Lead Solder on PCB? A Complete Guide

PCB manufacturers can use lead or lead-free solder to mount components. The two options have application-related advantages and disadvantages. This article focuses on the use of lead solder on PCB assemblies and why you may want to choose it for your project.

What is Lead Solder?

Lead solder is a metal alloy formed by lead (Pb) and tin (Sn). The two metals produce a material with various properties, such as a low melting point and excellent wettability.

Lead-free solder doesn’t contain the Pb. Instead, it uses various other metals, such as copper, silver, antimony, and bismuth. The soldering alloys have varying characteristics that suit different uses.

Lead Solder on PCB

PCB manufacturers use solders to attach components to printed circuit boards, producing long-lasting but reparable joints. The available types of solders are leaded or lead-free. The manufacturers can use either option.

Lead solder composition varies widely depending on the desired characteristics. The most common is 60%-70% Sn-Pb content. This ratio produces better results in different applications.

Tin resists corrosion and has a shiny appearance. It also weighs less than Pb and melts at a much lower temperature. Combining it with Pb introduces these qualities to produce a high-quality alloy.

For example, the solder has a clean, shiny finish. This property makes its joints easier to inspect either manually or with the help of AOI equipment.

The solder is available in wire form for small-scale, manual assembly. You can also obtain it as a paste for large-scale, automated circuit board mounting.

Is Lead Solder Banned in Electronics?

Yes, but only in some regions. These are countries that follow the Restriction of Hazardous Substances (RoHS) directives. These restrictions prohibit its use in electrical and electronic equipment.

Countries that impose the ban include the European Union members, China, and some parts of the United States of America.

Using lead in solder is banned in several other regions, such as the UAE and EAEU (check this link for a more comprehensive list).

That said, the restrictions mainly affect consumer electronics. It remains usable in some PCBs, such as military, aerospace, medical, and automotive types. These boards have high-reliability needs, requiring sturdy joints.

Other applications that are exempt from the regulation include industrial systems and fixed installations. Hobbyists can also use the solder to assemble personal circuit boards.

What are the Benefits of Lead Solder on PCB?

Lead-based solder offers several benefits when used to assemble circuit boards. These explain its continued use and exemption from RoHS restrictions. The following are its key advantages.

Low Melting Point

It has a low melting point of between 180°C -190°C. Compare that with the 217°C-220°C range of the Pb-free types. Its thermal conductivity is also comparatively low at 50 W/m·K, which is 13 points lower than non-leaded types.

The low melting temperature and conductivity make mounting circuit boards less challenging. They also make it safer since the temperature remains low enough to assemble heat-sensitive components without damage.

Good Wetting Properties

It has good flow characteristics. This quality enhances wetting, making components attach to pads more firmly. In other words, you get better results than when using the Pb-free type.

Robust and Safer Joints

It creates joints that excellently resist damage. For instance, they withstand excessive vibrations better than lead-free joints. Printed circuit boards for tough applications need the higher mechanical strength of Pb-based solder.

Additionally, introducing Pb reduces the chances of tin whiskers. These are tiny protrusions that grow from tin. They can short board circuits, leading to unforeseen board failures or safety risks in application devices or systems.

Reduced Cost

It costs less than other types, given the availability of lead. Other solder types contain costly metals, such as copper and silver. The low price range makes it easy to obtain and use, especially when dealing with higher production volumes.

PCB Lead-Based Solder Disadvantages

The material has a few downsides that make it less favorable. They include health and safety concerns, usage restrictions, and the inability of some users to discard it safely. Let’s examine each drawback and its impact.

Safety Risks

Pb is toxic when inhaled into the human body. Hobbyists working with lead solder must ensure they do so in well-ventilated rooms. They should also wear protective gear, such as masks and gloves.

The measures minimize breathing flux fumes and hand-contamination risks. We also recommend using a fume extractor when soldering your board. This equipment ensures you do not inhale toxic substances.

RoHS Restrictions

RoHS regulations restrict its presence in most consumer electronics. Therefore, you cannot use it if you live in a compliant region – not unless the board is a special application type. That also applies to suppliers sending circuit boards to such markets.

In other areas, the material remains available to use. Electronic enthusiasts can also buy the product for personal use. It’s usually readily available and cheaper than the Pb-free type.

Disposal Concerns

It has a higher environmental impact than the Pb-free type. Again, this usually occurs if you wrongly discard the product, circuit board, or electronic product.

Avoid mixing it with other waste or discarding it on open grounds. That can cause land and water contamination. Instead, ensure you take Pb-containing boards to designated collection points for proper disposal or recovery.

When to Use Lead Solder on PCB Electronics

The solder performs excellently in PCB assembly despite its environmental concerns. This benefit makes it invaluable where PCB reliability is paramount. Popular applications include those listed below.

High Reliability Boards

We use it to assemble critical circuit boards. Examples include medical, military, and aerospace types. It produces sturdy solder joints, improving their dependability in demanding conditions. The joints also last longer in the harsh conditions.

Delicate Components

The material’s low melting point makes it an excellent choice when assembling delicate parts. These are devices that assembly temperatures could damage. It allows manufacturers to mount such parts safely.

Non-RoHS Regions

Users in areas where RoHS directives do not apply can choose the solder to assemble their circuit boards. They can also use it if the boards are critical or meant for harsh environments.

However, that also depends on the specific region. That’s because some countries have stricter exemptions than others. Ensure you understand your area’s restrictions before deciding which solder to use.

Legacy Electronics Repair

Older circuit boards that manufacturers assembled with the Pb-based solder are still in use today. These are boards manufacturers are no longer producing. They include parts of older computers, aircraft, and industrial equipment.

Conclusion

Using lead-based solder to populate printed circuit boards offers several advantages. They include robust joints and the ability to mount heat-sensitive components. It mainly assembles high-reliability and tough-condition circuit boards, given its better flow and wettability qualities compared to lead-free types.