Electrical solder is a metal alloy made primarily of tin, combined with either lead or a mix of other metals like silver and copper. The exact composition depends on whether you’re using traditional leaded solder or the newer lead-free formulas that have become standard in commercial electronics manufacturing.
Traditional Leaded Solder
For decades, the most common electrical solder was a simple two-metal alloy: 60% tin and 40% lead, often written as 60/40. An even more popular version is 63/37 (63% tin, 37% lead), which is called a “eutectic” alloy. That means it transitions directly from solid to liquid at a single temperature, around 361°F, rather than passing through a pasty, semi-solid stage. This clean melting behavior makes 63/37 solder easier to work with and produces more reliable joints.
Leaded solder flows smoothly, wets metal surfaces well, and creates shiny, strong connections. It’s forgiving for beginners and still widely used by hobbyists, repair technicians, and in certain industries like aerospace and military electronics where reliability standards are extremely high. You can still buy it at most hardware and electronics stores.
Lead-Free Solder
Regulations in the European Union and other regions now restrict lead in consumer electronics, pushing most manufacturers toward lead-free alternatives. The most widely used lead-free formula is called SAC305: 96.5% tin, 3.0% silver, and 0.5% copper. Other common variations use 3 to 4% silver and 0.5 to 0.7% copper, with tin making up 95% or more of the alloy.
Some lead-free solders swap in different metals entirely. You’ll find alloys based on tin and bismuth, tin and antimony (95% tin, 5% antimony), or tin and zinc (91% tin, 9% zinc). Each combination offers slightly different melting points and mechanical properties, but tin is always the dominant ingredient.
Lead-free solder generally melts at a higher temperature than leaded solder, typically requiring about 420°F or more compared to roughly 360°F for leaded types. It can also be slightly less forgiving to work with. The joints tend to look duller and more matte compared to the bright, shiny appearance of leaded solder joints, but this is purely cosmetic and doesn’t indicate a bad connection.
What the Flux Core Does
If you’ve ever looked at a cross-section of solder wire, you’ll notice it’s not solid metal all the way through. Most electrical solder is sold as a hollow tube with a core of flux running down the center. This flux is a chemical cleaning agent that melts and flows out ahead of the solder, removing the thin layer of oxidation on metal surfaces so the molten alloy can bond directly to clean metal.
Electrical solder uses rosin-based flux, which is derived from pine tree resin. When heated, rosin flux breaks down into compounds including abietic acid (the main active ingredient), along with trace byproducts like acetone, formaldehyde, and methanol. These byproducts are released as fumes during soldering, which is why ventilation matters even when using lead-free solder. The rosin residue left behind after soldering is mild and non-corrosive, making it safe to leave on circuit boards.
This is a key distinction from plumbing solder, which uses acid-based flux. Acid flux is more aggressive at cleaning heavily oxidized copper pipes, but it will corrode delicate electronic components and wiring over time. Never use plumbing solder for electrical work.
How Well Solder Conducts Electricity
Solder is a decent electrical conductor, but it’s far less conductive than the copper wires and circuit board traces it connects. Conductivity is often measured as a percentage of pure copper’s conductivity (called IACS). The classic 63/37 tin-lead solder conducts at about 11.5% of copper’s level. Lead-free alternatives perform in a similar range: a tin-silver alloy (96.5% tin, 3.5% silver) reaches about 16% of copper, while a tin-antimony alloy comes in around 11.9%.
In practice, this doesn’t matter much. Solder joints are tiny, and their job is to make a physical and electrical bond between components, not to carry current over long distances. The small amount of resistance in a solder joint is negligible in virtually all circuits.
Safety With Leaded Solder
Lead melts at 621°F, and lead fumes begin releasing at around 900°F. Since soldering irons typically operate between 600°F and 750°F, you’re generally below the threshold for significant lead fume generation. The bigger risk comes from the flux fumes (which are irritating to your lungs regardless of solder type) and from lead residue on your hands. Lead dust and oxide can transfer from the solder to your skin and then to your mouth if you eat or touch your face without washing up.
If you’re soldering with leaded solder, the practical precautions are straightforward: work in a ventilated area or use a small fume extractor, wash your hands thoroughly after handling solder, and avoid eating or drinking at your workbench. Store any solder scraps and dross (the crusty waste that forms on molten solder) in a sealed container.
Which Type to Use
For hobby electronics, Arduino projects, and general circuit board repair, 63/37 leaded solder with rosin core is the easiest to work with. It flows well at lower temperatures, produces clean joints, and is inexpensive. If you prefer to avoid lead exposure entirely, or you’re working on a product that needs to meet regulatory standards, SAC305 lead-free solder is the industry default. It costs a bit more and requires a slightly higher iron temperature, but modern formulations are much improved over early lead-free solders.
Solder wire comes in various diameters too. Thinner wire (around 0.031 inches) gives you better control for small electronics work, while thicker wire (0.050 inches or more) is better for larger connections and heavier gauge wiring. Whichever alloy you choose, make sure it’s labeled for electronics use with rosin core flux, not acid core.