How to Choose the Right Solder Alloy for Your PCB?

Let’s set aside our beliefs and preconceptions about solder alloys for electronics for a moment. Let’s try to delve deeper, even if you’re firmly convinced about something. Before we tackle the question posed in the title, let's first answer some important questions:

• At what temperatures will the electronic device operate?
• What stresses and loads must an electronic device withstand?
• How professionally are the assembly processes for the electronic printed circuit board (PCB)?
• How will the electronic device be protected?
• How professionally is the PCB designed, and who evaluates this desigh?
• What stages of testing should the electronic device undergo?
• Ask yourself: how important is environmental friendliness?

If you find it challenging to answer even one of these questions honestly, you won’t be able to conclusively prove which alloy is better or worse for your situation (in fact, there could be even more questions). This is critical because the choice of solders, solder pastes, and solder fluxes is based on the future design concept of the device and its operational requirements. Let's put aside any misconceptions and start asking ourselves questions and answering them right away, weighing pros and cons.

The only rule: create a device with maximum longevity.

Let’s begin. Naturally, I’ll help you answer some of these questions.

Lead-free alloys withstand very high temperatures (from 120°C and up), making them preferable for soldering LEDs in powerful lighting fixtures. For motherboards, graphics cards, and even in space, due to their higher melting and degradation points by 20-40°C depending on the alloy.

Lead-based alloys are better for extremely low temperatures, though modern lead-free alloys, like Sn96.4%, Ag1.1%, Cu0.7%, and (Bi + Ni)1.8%, can also endure very low temperatures and serious vibrations.

If the electronics operate between -40°C and +85°C, both types of alloys will handle it well. There are numerous high-quality leaded and lead-free alloys, each excelling in certain characteristics while falling short in others. There’s no golden mean. Remove silver from any alloy, and the results can change drastically. Knowing in advance which alloy will withstand extreme temperatures is impossible without thorough testing.

It’s known that lead-based alloys are quite flexible, but that doesn’t mean equally flexible lead-free options don’t exist. Vibration tests, pressure chambers, and thermal shocks are relevant here. If these details are unimportant or you lack the inclination to explore them, you might simply rate lead as a “+” and lead-free as a “-”. But that doesn’t mean you’ve honestly answered the question.

This question is very important because the way the assembly stages are planned impacts the majority of initial failures. The materials, assembly techniques, soldering, and cleaning – all are crucial. If anything is overlooked or missed, the device will “say good bye” to you soon after leaving the manufactory. Assembly techniques differ slightly depending on the alloy used, though the differences are minimal and still need consideration.

To prevent temperature and humidity effects on the PCB and components, it must be protected. This is achieved with lacquers, sealants, and specially designed casings with effective insulation, but there are some nuances:

• Before applying any protective coating to the board, it’s essential to test material compatibility to avoid oxidation.
• The boards must be thoroughly cleaned before coating, not in makeshift conditions but with industrial techniques, to ensure predictable quality results.

Depending on the operating environment, the electronic device might face various types of moisture, such as saltwater, freshwater, acids, or vapors. Each of these environments interacts differently with leaded and lead-free alloys.

• A. There are always mistakes in development.
• B. If no mistakes are found – see point A.
/joke/

How many developer-versus-technologist debates have ended in frustration, with each side adamant about being right? Sound familiar?

Developers are primarily responsible for this. Laziness and pride can obstruct communication with production teams and technologists, which often leads to a host of issues during electronics assembly. These range from board layout errors to minor issues, like missing fiducial markers on the board, that can end up costing money when something needs to be redone. A wise developer consults everyone who might offer a different perspective.

Naturally, the use of leaded or lead-free solder directly affects melting temperatures and components exposed to those temperatures. The different properties of immersion finishes and solders should be calculated before the board is manufactured.

I hope you haven't forgotten about testing!

Vibration stands, electrical breakdown stands, dust chambers, heat chambers, thermal shocks chambers, pressure chambers... During any of these tests, the electronic device may fail due to the wrong solder, and there is no guarantee that lead solder will withstand more than lead-free solder. First, prove your assumptions with tests. You can find test reports in IPC standards.

Whether you want to or not, customers often have requirements or beliefs about the environmental impact of products, especially in the large-scale production of thousands or millions of PCBs.

If you assemble PCBs for other companies, you follow their requirements. However, if you produce your own PCBs, you’re responsible for considering their future disposal. If you ignore this, your users will handle disposal however they see fit, and your tens of thousands of products may end up in a landfill. If you acknowledge this, you can minimize environmental harm, as there are materials and technologies available, which, in some cases, are even cheaper than leaded options.

Thank you for reading!