Soldering
How soldering works and how to do it well: solder and flux, irons and tools, through-hole and surface-mount joints, reflow, wires, rework, quality and safety, with worked numbers.
Soldering joins metal parts with a filler metal that melts below the parts’ own melting point: it wets the clean surfaces, flows into the joint and freezes into a bond that is both mechanical and electrical. Almost every electronic assembly depends on it, and the difference between a joint that lasts for decades and one that fails in a year is mostly heat, time, cleanliness and flux. This guide takes the subject in ten pages: how soldering works, alloys and flux, irons and tools, through-hole joints, surface-mount parts, reflow, wires and connectors, rework, joint quality and failure, and safety. Every worked number (energy per joint, solder volume, reflow time above liquidus, fatigue life) is computed so that you can check your own bench. The circuits and the board design are in Electronics Fundamentals and PCB Design and Layout; making whole boards is in PCB Prototyping.
Start here: what do you need?
| I need to… | Go to |
|---|---|
| understand why solder sticks, or why a joint looks dull and fails | How Soldering Works |
| choose leaded or lead-free solder, flux, wire or paste | Solder Alloys, Flux and Paste |
| choose an iron, a tip, a temperature, or a hot-air station | Irons, Stations, Tips and Hot Air |
| solder a through-hole part, or work out how much solder it takes | Through-Hole Soldering |
| solder chips, QFPs or fine-pitch parts, or stop tombstones and bridges | Surface-Mount Soldering |
| set up a reflow profile, a toaster oven or a hot plate | Reflow Profiles, Ovens and Hot Plates |
| solder wires, connectors and cable, or heat-shrink a splice | Wires, Connectors and Cables |
| remove a part, clear a hole, or repair a lifted pad | Rework, Desoldering and Repair |
| judge a joint, find the cause of a defect, or understand fatigue | Joint Quality, Inspection and Failure |
| extract fumes, handle lead, avoid burns and protect parts from static | Soldering Safety, Fumes and ESD |
A suggested order
Start with How Soldering Works and Solder Alloys, Flux and Paste, then Irons, Stations, Tips and Hot Air and Soldering Safety, Fumes and ESD before you pick up an iron. Through-Hole Soldering is the first skill; Surface-Mount Soldering and Reflow Profiles, Ovens and Hot Plates follow; Wires, Connectors and Cables, Rework, Desoldering and Repair and Joint Quality, Inspection and Failure make you reliable.
- How Soldering Works: Wetting, flux, heat and time, the joint and its intermetallic layer, and the energy a joint needs.
- Solder Alloys, Flux and Paste: Leaded and lead-free alloys, melting ranges, wire sizes, flux types and activity, solder paste and how much to use.
- Irons, Stations, Tips and Hot Air: Power and thermal mass, temperature control, tip shapes and care, hot-air tools and the bench set-up.
- Through-Hole Soldering: The four-step joint, hole and pad sizes, barrel fill, ground-plane pins and a worked solder budget.
- Surface-Mount Soldering: Chip sizes, tack-and-reflow, drag soldering fine-pitch, hot air and paste, tombstones and bridges.
- Reflow Profiles, Ovens and Hot Plates: The reflow profile and its numbers, toaster ovens and hot plates, moisture-sensitive parts and cooling.
- Wires, Connectors and Cables: Tinning, splices, solder cups, heat-shrink, strain relief, magnet wire and the resistance of a joint.
- Rework, Desoldering and Repair: Wick, pump and hot air, removing chips and connectors, lifted pads, jumpers and how many cycles a board survives.
- Joint Quality, Inspection and Failure: What a good joint looks like, defects and causes, inspection, voids, whiskers, thermal fatigue and cleanliness.
- Soldering Safety, Fumes and ESD: Fume extraction, lead, burns and fire, static discharge and the safe bench.
The idea in one page
Three things make a good joint: clean metal (flux removes the oxide so that the solder can touch the copper), enough heat (the whole joint, not just the iron tip, must pass the solder’s melting point) and a still joint while it freezes. A typical joint needs a few joules, which a small iron delivers in a second or two (see How Soldering Works).
Quick reference: temperatures and numbers
| Topic | Number | Page |
|---|---|---|
| Sn63/Pb37 solder | melts at 183 °C (eutectic); tip about 320 to 350 °C | Solder Alloys, Flux and Paste |
| Lead-free SAC305 | melts at 217 to 220 °C; tip about 350 to 380 °C | Solder Alloys, Flux and Paste |
| Time on a joint | 1 to 3 s; over 5 s risks lifted pads and burnt flux | Through-Hole Soldering |
| Energy per joint | 7 J small pin; 37 J with a ground plane; 97 J large lug | How Soldering Works |
| Hole size | lead + 0.2 to 0.3 mm | Through-Hole Soldering |
| Barrel fill | at least 75 % (IPC class 2), 100 % (class 3) | Joint Quality, Inspection and Failure |
| Reflow, lead-free | peak 235 to 250 °C, 60 to 90 s above 217 °C, ramp 1 to 3 °C/s | Reflow Profiles, Ovens and Hot Plates |
| Rework cycles | a board survives about 3 reflow cycles | Rework, Desoldering and Repair |
Related guides and tools
- Electronics Fundamentals: the components you are soldering, and the Design, Testing and Troubleshooting guide for finding faults.
- PCB Design and Layout and PCB Prototyping: pads, stencils and board making.
- Workshop Safety: fume extraction, lead, fire and first aid for burns.
- Shop Tools: the bench, light and holding.
- PCB Trace Width Calculator and the Battery Runtime and Pack Calculator.
Safety first
- The iron is at 350 °C or more: keep it in its stand, never leave it on unattended, and cool burns under running water for 20 minutes.
- Extract the flux fume (it causes asthma) and wash your hands after handling leaded solder (see Soldering Safety, Fumes and ESD).
- Wear safety glasses: solder splashes and clipped leads fly.
- Work on unplugged equipment, and discharge capacitors before touching a power supply (see Electrical Safety).
FAQ
What temperature should I solder at?
About 320 to 350 °C for leaded solder and 350 to 380 °C for lead-free, with the tip hot enough that a joint is done in one to three seconds. See Irons, Stations, Tips and Hot Air.
Leaded or lead-free solder?
Leaded (Sn63/Pb37) is easier and more forgiving for hobby work and repair; lead-free is required for most commercial products and RoHS compliance. See Solder Alloys, Flux and Paste.
Why is my solder joint dull and grainy?
Usually a cold joint: the joint moved while it froze, or it did not get hot enough. Reheat it with a little fresh flux. See Joint Quality, Inspection and Failure.
Is soldering fume dangerous?
The flux smoke is a sensitizer and causes occupational asthma; use an extractor or a fan to move the plume away. Lead does not evaporate at soldering temperature but gets to the mouth on the hands. See Soldering Safety, Fumes and ESD.
Where did the old Soldering sections go?
All of them are in this guide with much more depth: the station, the steps, the temperatures, the wires and connections, the safety and the troubleshooting each have pages of their own.
References
- IPC J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies; IPC-A-610, Acceptability of Electronic Assemblies; IPC J-STD-004 (flux) and J-STD-006 (solder alloys).
- Klein Wassink, R. J., Soldering in Electronics, Electrochemical Publications; Lee, N.-C., Reflow Soldering Processes and Troubleshooting, Newnes.
- Manko, H. H., Solders and Soldering, McGraw-Hill; Horowitz, P. and Hill, W., The Art of Electronics (practical soldering).
- NASA-STD-8739.3 (soldered electrical connections) and Directive 2011/65/EU (RoHS).
Last reviewed: November 2026. Temperatures, times and energies are typical: follow your solder, flux and component makers’ data sheets.