Lead or Lead-Free PCB Assemblies: What is the difference?

When discussing your project with an electronics manufacturer, it is essential to address the topic of lead-free requirements. This pertains specifically to the metals used in the components and solder employed in the manufacturing process. Historically, leaded solder was the gold standard for printed circuit board (PCB) assembly, being easy to use while providing robust, reliable connections. In more modern times, however, the effects of lead on the environment and human health, as well as government regulations, have popularized the usage of lead-free solders and electronic components. While often interchangeable, lead and lead-free solder have some significant differences, and the best choice between the two will depend on the application.



Lead vs Lead-free Solder Materials

Solder is a metal alloy that is used to join electrical components to a PCB, creating a strong and conductive bond between the component leads and the copper pads on the board. Leaded solder is made of a mixture of tin and lead, usually in a ratio of either 63% tin and 37% lead, or 60% tin and 40% lead.

Lead-free solder is a mixture of tin and other metals such as silver, copper, bismuth, and indium. The most common lead-free solders have a high concentration of tin — upwards of 99.3% — with small quantities of copper and silver. The omission of lead from these solders results in a higher melting point, which necessitates additional heat during the manufacturing process.

The higher temperatures used to accomplish this can contribute to increased thermal stress on both the board and its components, potentially reducing the integrity and durability of the materials. Proper thermal management techniques are crucial to mitigate any adverse effects caused by the elevated temperatures associated with lead-free manufacturing.


Mechanical Performance and Reliability

Leaded solder is known for its notable tensile strength and elasticity, rendering it highly resilient against mechanical stress and vibrations. These characteristics make it particularly well-suited for applications that demand robust joint integrity. In comparison, lead-free solders tend to be more brittle, although different alloys have varying elasticities. However, in most scenarios, the disparity in strength between leaded and lead-free solder is insignificant and has minimal impact on performance.

Despite this, leaded solder remains the first choice in industries that demand high reliability, such as aerospace and military applications. It provides better solder connections thanks to its superior wetting, or ability to bond with pads and components. In addition to lesser wetting capabilities, the higher tin content in lead-free solders makes them more susceptible to certain manufacturing defects. For example, one defect is tin whiskers, which are microscopic filaments of tin that grow out from the surface of the solder and may cause short circuits or failures. Another common defect is the growth of tin dendrites, which are tree-like structures that grow along the surface of the board and can cause similar issues. In either case, proper manufacturing techniques and procedures are necessary to minimize defects and ensure proper solder connections.


Cost Considerations

Leaded solder tends to be less expensive than lead-free solder, as lead is cheaper and more abundant than the metals used in lead-free solder. The higher concentration of tin, and especially the introduction of metals such as copper and silver, leads to a higher price. This disparity in material costs can also impact the pricing of components, often resulting in lead-free parts being slightly more expensive than their leaded counterparts.


Health, Environment, and Regulations

Using leaded solder is not dangerous when handled properly. However, lead fumes and dust have the potential to contaminate surfaces and the environment through the air, soil, or water. Extreme exposure to lead through improper handling or disposal has linked to a myriad of health issues including heart disease, nerve disorders, and decreased kidney function.

As a result, many countries have enacted regulations that restrict or ban the use of lead in electronics. These regulations include the Restriction of Hazardous Substances (RoHS) directive in the European Union, which prohibits the use of lead and five other hazardous substances in electrical and electronic equipment. Even in the United States, where leaded solder is not prohibited, lead is regulated as a pollutant and may impact product disposal practices.

Proponents of lead-free solder claim it is safer for human health and the environment, as it does not contain toxic metals that can cause various health problems or environmental pollution.


Impact on the Manufacturing Process

Despite the distinctions between leaded and lead-free components and solder, the manufacturing process itself remains largely unaffected. Both leaded and lead-free PCB assemblies utilize the same pick and place machines, reflow ovens, and quality control measures. The primary differences are the tools used and the variation in reflow profiles required for each type of solder.

Working with both lead and lead-free PCB assemblies involves having entirely separate tool sets for either occasion to ensure no cross-contamination. Soldering irons, tweezers, stencils, and even work mats are switched out when changing between these projects. These practices ensure that there is no cross-contamination on lead-free assemblies.

A more intricate change to the manufacturing process involves adjusting the reflow profile when attaching surface mount components. Reflow profile refers to the temperature of the ovens that melt, or reflow, solder paste to attach surface mount components to the board. The higher melting point of lead-free solder naturally requires a different reflow profile than is used for leaded assemblies. Occasionally leaded assemblies will include lead-free components such as ball grid arrays that necessitate a mixed reflow profile or solder sphere replacement to match the leaded or lead-free process. In addition, if building with lead free solder it is important to verify any leaded components can handle the higher lead-free temperatures.  While it is possible to create reflow profiles to accommodate both types of parts, it is preferred to use only leaded parts on leaded assemblies.


Which to Choose?

Today, many applications utilize lead-free PCB assembly. Commercial companies may want to get ahead of health and environmental regulations that mandate lead-free components and solder. As per the RoHS directive, lead-free assembly is already a requirement for the sale of electronics in European Union countries. Despite this, leaded solder and components are the standard for mission critical or other high reliability applications where circuit board failure can endanger human life.


How MJS Can Help

At MJS Designs, we provide comprehensive solutions for both leaded and lead-free manufacturing requirements. We can help with any stage of the manufacturing process, whether you have a developed product, a potential idea, or need to make an existing design RoHS compliant. Contact MJS Designs today to learn more about how we can fulfill your PCB manufacturing needs here.