Lead vs Lead-Free Solder Alloys — What's the Real Difference?

What’s the difference between lead and lead-free alloys for soldering and repairing printed circuit boards? It’s not just about which is harmful, and which is safe, although that also matters. I’ll explain the practical use of different soldering alloys in manufacturing, repair, and home soldering.

• Lead solder alloy is the classic choice. The most common alloy: Tin 63 and Lead or plumbum 37 with melting temperature — 183°C. It’s ideal for soldering and repairing all electronics where reliability is important. Aviation, military equipment, railway technology, and medical devices are almost always soldered with lead alloys, unless there are special requirements.
Why?
• Excellent wetting. Lead alloys can flow into the smallest holes in PCB with soldering flux of course. Lead alloy creates a strong joint if you properly preheat all surfaces. Video about soldering temperatures in description.
• High mechanical and vibration resistance because the lead alloy is more elastic. It withstands vibration tests and thermal shocks — sharp temperature changes in -40 +80°C (-40 +176°F). Most PCBs experience dynamic loads during use, drops, shocks, and shaking. Due to its elasticity, lead solder joints are less prone to self-destruction and cracking. Of course, assuming the production technology is followed.
• Lead alloy forgives mistakes better — if you overheat it during soldering, it usually does not have a major impact. Still, avoid overheating — will be poor quality.

After 2006, thanks to the R O H S directive, the mass market switched to lead-free alloys. The most common are SAC305 — Sn96.5Ag3Cu0.5, or similar lead-free compositions. Their main purpose is to be “eco-friendly.”
• Higher melting temperature: 217–221°C (422-429°F). The higher soldering temperature puts more and unnecessary thermal stress on components. Yes, modern electronic components can briefly tolerate such temperatures, but it's still thermal shock.
• Lead-free alloys have much worse elasticity and flowability than traditional lead solder alloys. PCBs using lead-free solder are more fragile and fail more often due to microcracks in lead-free joints. They also handle vibrations worse.
• You must properly set the cooling profile when using lead-free alloys — it directly affects the brittleness and reliability of solder joints. Link to the video about soldering profiles in description.
• Lead-free alloys perform worse under thermal cycling. That’s why video cards using lead-free solder alloy often require chip replacements or BGA reballing due to overheating.
• There are upsides: higher melting temperatures allow Lead-free alloys to withstand higher temperatures during operation without suffering melt damage. They also don’t contain lead, which accumulates on the planet as waste from old PCBs.
Did you know that NASA does not use lead-free alloys? And Canadian customs often reject PCBs soldered with lead alloy.

In manufacturing — No. It’s inconvenient to solder a PCB with different types of solder alloys. This makes no sense and violates quality control. But in repair — yes, if you know what you’re doing.
If the PCB was originally soldered with a lead alloy, then it is better to use a lead alloy for repair. But if you are replacing a lead-free BGA chip on a lead PCB, then when heated with hot air, the lead alloy on the PCB will melt before the lead-free solder balls on the BGA chip.
It’s ok. Lead alloys are forgiving. The main thing is to make sure that there are no heavy components on the bottom of the PCB that can fall when the solder melts.
The opposite situation when soldering lead BGA chip on a lead-free PCB. In this case, the lead alloy at the BGA chip will melt first and easily soldering to contact pads of the PCB.

The use of solder alloy containing bismuth for soldering PCB is not recommended. These alloys are very unreliable and sensitive to heat during future work of electronics. Bismuth solder is unreliable under vibration, mechanical stress and thermal shock. Using bismuth in printed circuit boards is a serious mistake.
Bismuth alloys can be useful when desoldering components — adding bismuth alloy can lower the melting point of lead-free joints, making easier to remove components.
After desoldering, be sure to clean the pads with copper braid. I have never use bismuth alloys for desoldering. I do fine with bottom preheating station and hot air.

In manufacturing, consistency and predictability are crucial. In practice, it doesn’t matter which alloys is used. It is more important that the soldering processes and profiles are set up correctly.
For manual soldering with lead-free alloys, higher-quality soldering iron tips and professional soldering stations are needed to avoid early iron tip degradation due to high temperatures.

Thank you for reading!