What solder actually is
Solder is a metal alloy that melts at a relatively low temperature, wets the surfaces it touches, and then solidifies into a joint that is both mechanical and electrical. For electronics it is usually supplied as a wire with a core of flux, a chemical that cleans oxide from the metal surfaces as the joint forms. The alloy and the flux together determine how forgiving the material is in use.
The two broad families are leaded solder, based on tin and lead, and lead-free solder, usually based on tin with small amounts of other metals such as copper or silver. Everything else in the decision flows from the differences between them.
Leaded solder
The classic electronics alloy is a tin and lead mix. Common ratios are about 60 percent tin to 40 percent lead, and about 63 percent tin to 37 percent lead. The 63/37 mix is a eutectic, meaning it melts and solidifies at a single temperature rather than passing through a pasty range, and that temperature is often quoted as roughly 183 degrees Celsius. The 60/40 mix has a small pasty range around that figure.
Practical advantages are well known among hobbyists:
- A lower melting point, so the iron works with less thermal stress on parts and boards.
- Good flow and wetting, which makes joints easier to form and rework.
- Joints that tend to look shiny and smooth, which makes visual inspection easier.
The main disadvantage is the lead itself, which is a toxic metal. That drives the health and regulatory points below.
Lead-free solder
Lead-free alloys typically combine tin with copper, or tin with silver and copper. Several common variants exist, and the exact composition is shown on the packaging. These alloys generally melt at higher temperatures than leaded solder, often in the region of 217 degrees Celsius or above for common tin-silver-copper types, though values vary with composition.
Practical consequences:
- The iron usually needs to be set hotter, which can lift pads or stress sensitive parts if you hold the heat too long.
- The solder often wets less readily, so it can feel stickier and less forgiving to a beginner.
- Joints often look duller or grainier even when they are sound, so judging quality by shine alone can mislead you.
- Tips can wear faster at higher temperatures, and good tip care matters more.
Quality flux and a clean, well-tinned tip help considerably with all of these.
Health considerations in general terms
Lead is a recognised toxic substance, and the main exposure route when soldering with leaded wire is usually from hands: touching solder and then eating, drinking, smoking or touching your face. The visible fume from soldering is mostly from the flux rather than from the lead, because lead does not vaporise appreciably at normal soldering temperatures. That does not make the fume harmless. Flux fumes can irritate the eyes, airways and skin, and for some people can contribute to breathing problems.
Sensible habits apply whichever alloy you choose:
- Wash your hands after soldering and before eating or drinking.
- Do not eat or drink at the bench.
- Keep solder, offcuts and dross away from children and pets.
- Take care of fumes with extraction or good ventilation, which is part of any sound soldering technique.
- Collect scraps and cleaning waste and dispose of them as local rules require, rather than sweeping them into household rubbish without thought.
Lead-free does not mean risk-free. It avoids lead exposure, but the flux and the metal fume at higher temperatures still call for ventilation. If you are pregnant, planning a pregnancy, or soldering with or near children, take extra care and consider seeking medical or occupational health advice.
Regulation in general terms
Rules on lead in electronics differ by region and by product. Many places restrict lead in electrical and electronic equipment that is sold commercially, and manufacturers have largely moved to lead-free processes as a result. The most commonly cited example is the European restriction on hazardous substances, often known as RoHS. Whether those rules apply to a one-off hobby project, a repair, or something you sell is a separate question that depends on where you live, so check the guidance from your local authority or a relevant standards body if you intend to sell or distribute anything.
If you build for sale, products often need to comply with the relevant restrictions and labelling requirements. For personal projects the rules are usually less strict, but you should still check what applies to you instead of assuming.
Mixing alloys and rework
Mixing leaded and lead-free solder on the same joint is generally discouraged. The result can have an unpredictable melting range and properties, and a joint that looks fine may be weaker or more prone to cracking. When repairing a board made with lead-free solder, using lead-free wire is typically the safer match, and vice versa. If you do rework a joint, clean off the old solder thoroughly first.
Keep separate tips or at least clean tips thoroughly if you switch alloys, and label spools clearly so you do not grab the wrong one.
Which should a beginner choose?
There is no single right answer. Some points to weigh:
- Learning on leaded solder can be easier because it flows at lower temperatures, but handling discipline is essential, and you should consider whether you are comfortable having lead in your home.
- Lead-free is the more cautious choice for health and is what most modern boards use. It takes slightly more practice, and a temperature-controlled iron helps.
- If you will repair older equipment, you may meet either type.
- If your projects might ever be sold or shared, lead-free is the safer default.
Whatever you pick, buy solder from a reputable supplier, check the labelling for composition, and read any safety information supplied with it. Good technique matters more than alloy: a clean joint with adequate heat, the right flux and a steady hand is the aim for either type. The principles of reading a datasheet also help here, since component data often states maximum soldering temperatures and times that you should respect.
Soldering is a heat process with burn risks and fumes, so set up a suitable workspace, as covered in guidance on a home workshop setup, and follow the manufacturer’s directions for your iron, solder and flux.