Demystifying Soldering Alloys

What is Soldering?

Soldering is crucial in the realm of electronics as it allows one to create reliable connections between components on a circuit board or wires. Whether you are soldering in a professional setting or are doing a DIY project, understanding the right type of solder for the job is essential. In this blog, we will cover the different types of alloy combinations used in solder, what they are used in, and why it is that we use these metals.

Soldering involves using a heated soldering iron to melt a metal alloy (typically containing tin and lead) to join two metal surfaces. The molten solder bonds with the metals, forming a strong connection. However, solder can be composed of various metals in different proportions. These alloys have unique properties that make them suitable for specific applications.

Types of Solder

Lead-Based

Of all these lead-containing alloys, the most common are Sn-60 Pb-40, Sn-63 Pb-37, and Sn-62 Pb-36 Ag-2. The most affordable option, Sn-60 Pb-40 solder, can be tricky for beginners due to its plastic region where it can form weak joints if moved. Sn-63 Pb-37 solder is slightly more expensive but avoids this issue, making it a popular general-purpose choice due to its ease of use and reliable performance. Finally, Sn-62 Pb-36 Ag-2 solder with silver isn’t necessarily better for electronics but is the go-to choice for soldering silver wires to prevent the solder from dissolving the silver over time.

Lead-Free

Of all these lead-free alloys, the most common are SN100C, Sn-99.3 Cu-0.7, K100LD, and Sn-95 Ag-5. Sn100C is a popular choice for its ease of use but is expensive. For those looking for a similar option with potentially improved performance, K100LD is a good option. For a slightly cheaper option than those mentioned previously, Sn95 Ag-5 offers similar properties but without nickel/germanium doping. Finally, Sn-95 Ag-5 offers performance close to lead-based solders, but its large plastic region makes it challenging for beginners and its high silver content makes it the most expensive option. Lead-free solders are typically more expensive than their lead-based counterparts so it’s important to consider this factor when buying.

DION: Lead-Based vs. Lead-Free (Note: I completely removed the section ‘Isn’t Lead Dangerous’ and combined it into one section.)

As described above, solders typically come in two types: lead-based and lead-free. Lead-based solder has been widely used in electronics, plumbing, and metalwork due to its low melting melting point, strong electrical conductivity, and smooth reliable solder joints. For these reasons, lead-based solder is typically recognized as the more reliable and capable soldering option when compared to its lead-free counterpart. But if that’s the case why would anyone choose lead-free solder? 

The dangers and risks of lead has become increasingly more prevalent in recent years. Lead has known chronic health effects such as reproductive, digestive, memory, muscle and joint problems if preventative measures are not taken. There are also significant environmental and disposal concerns given that lead does not break down easily and accumulates in soil and water. This can cause long-term contamination where lead enters the food chain, affecting wildlife and humans. For these reasons, government mandates, specifically the EU’s Restriciton of Hazardous Substances (RoHS) directive have strongly pushed for lead-free solder, especially in applications where there is a higher chance of encountering the human body or environment. 

Although, lead-free solder is better for you and the environment, it may not always be suitable for your project. Lead-free solder has a higher melting point on average which can stress components and require adjustment to manufacturing processes. It is also more brittle than lead-based solder, leading to potential reliability issues such as tin whiskers, which can cause short circuits over time. Lead-free solder is also typically more difficult to work with and is especially harder for beginners to achieve a smooth, reliable and repeatable solder joint.

Understanding RoHS Compliance

As mentioned above, RoHS (Restriction of Hazardous Substances Directive) ensures products are free from hazardous materials like lead and mercury, and is crucial for many industries, including consumer electronics, medical devices, and telecommunications equipment. Different industries mandate RoHS compliance to meet safety and environmental standards. Distributors and manufacturers declare RoHS compliance for their products, so be sure to check these declarations when sourcing components. Additionally, placing a purchase order for PCB assembly often incurs an added charge for RoHS compliance, covering the use of compliant materials and processes. RoHS compliance is crucial, so always verify compliance and account for any additional costs in your manufacturing process.

Tin Whiskers in Aerospace Environments

One issue involved with lead-free solders is the formation of tin whiskers. These are crystalline structures of tin that grow on surfaces where tin was used as solder. These whiskers are electrically conductive and can grow to be several millimeters long. From their growth, they can cause accidental connections between components, thus leading to electrical shorts. There is not one universally accepted explanation for their formation: it is believed that they grow in response to external pressure. In high-pressure applications, like aerospace, tin whiskers have caused failures, including complete on-orbit commercial satellite breakdowns. However, the addition of lead into the solder can prevent the formation of these whiskers, thus reducing the chances of unpredictable failures. Thus, in applications of high stress like aerospace, adding lead to your solder could be the difference between success and failure.

Conclusion

Soldering is a fundamental process in electronics, allowing us to create reliable connections between components. When selecting a solder alloy to use, consider factors such as cost, ease of use, quality, and safety. Lead-based alloys strike a balance between affordability and performance while lead-free alloys offer health benefits at a higher price. No matter the use case, understanding the nuances of solder alloys ensures electronic assembly is perfect for your needs.

References/Sources

https://www.kester.com/Portals/0/Documents/Knowledge%20Base/Alloy%20Temperature%20Chart.pdf

https://neurochrome.com/pages/choosing-solder

https://nepp.nasa.gov/whisker/background

https://www.eptac.com/blog/leaded-vs-lead-free-solder-which-is-better

https://www.wevolver.com/article/lead-free-solder-vs-lead-solder

Tin Whiskers Image: Rocknrollsuicide, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons