Soldering basics: iron temperature, flux, tinning and good vs cold joints

A good solder joint means getting the component lead and the copper pad hot enough that solder flows into both of them, not dropping blobs of hot solder on top. This lesson gives you starting temperatures, explains what flux does, shows how to look after the tip, and how to tell by eye which joints are good and which need redoing. It ends with fumes and lead, which beginners tend to skip.

The work here is through-hole soldering, such as adding header pins to an ESP32 board or a sensor breakout, with a temperature-controlled iron.

What you need on the bench

• Temperature-controlled iron with a stand. An iron without a set temperature runs at whatever it runs at.
• Tip cleaner: a damp sponge or brass wool.
• Rosin-core solder in a gauge that suits the work.
• Separate flux (pen or gel), a big help with lead-free solder and large pads.
• Safety glasses: clipped leads and solder splashes fly.
• Something to hold the work (helping hands or a vice) so you never hold hot parts.
• A fan or fume extractor.

Leaded vs lead-free solder

	Leaded 63/37 (Sn/Pb)	Lead-free SAC305
Melting point	183 °C (melts at a single point)	217–220 °C
Iron setting found in guides	about 300–350 °C	about 340–380 °C
Flow	flows easily, forgiving	needs more heat and flux
Tip wear	slower	tip oxidises up to about 5× faster
Watch out for	contains lead, wash hands	no lead, flux fumes still matter

The temperature ranges vary between sources because they depend on the station, the tip size and the joint. SparkFun suggests a medium heat of about 325–375 °C for general work. What iron makers agree on is use the lowest temperature that still makes a good joint: extra heat wears the tip and burns the flux.

Don’t mix the two alloys in one joint. When reworking an old board of unknown origin, remove the old solder first.

Worked example: a starting temperature

The rule of thumb from iron makers: the solder at the joint should be about 50 °C above its melting point, and the iron is set a further 70–100 °C higher because the work pulls heat out of the tip. A station with fast thermal recovery needs the smaller margin; a slower one needs the larger.

Leaded 63/37

1. 183 + 50 = 233 °C at the joint
2. Fast-recovery station: 233 + 70 = 303 °C
3. Slow-recovery station: 233 + 100 = 333 °C

Lead-free SAC305

1. 220 + 50 = 270 °C at the joint
2. Fast recovery: 270 + 70 = 340 °C
3. Slow recovery: 270 + 100 = 370 °C

Treat that as a starting point. If the solder flows slowly and you have to hold the iron there a long time, go up 10 °C at a time. If the flux smokes hard or the tip blackens quickly, go down. A large pad tied to a ground plane soaks up heat; a bigger tip often helps more than a higher temperature.

What flux does

Every metal surface carries a thin oxide layer, and solder won’t stick to oxide. Flux cleans the surface as it heats, so solder spreads and bonds (wetting). Rosin-core solder already has flux inside, enough for ordinary leads. Add more when:

• you use lead-free solder, which flows less readily
• the pad is large or connected to ground
• you rework an old joint whose flux has burnt away

Clean leftover flux with isopropyl alcohol and a soft brush.

Tinning the tip

A tip with a thin coat of solder passes heat into the work well. A black, oxidised tip barely passes heat at all, which tempts people to turn the temperature up.

1. With the iron hot, wipe it on the damp sponge or brass wool.
2. Touch solder to the tip until it is shiny all over.
3. Add a little solder every time before returning the iron to its stand.
4. Before switching off, leave a slightly thicker coat on; don’t wipe it off.

One joint, step by step

1. Push the lead through the hole and hold the work steady.
2. Touch the side of the tip to both the pad and the lead at once.
3. Give it a moment to heat, then feed solder into the joint on the side opposite the iron. The solder should melt from the heat of the work, not from touching the tip.
4. Once solder has flowed around the lead and over the pad, pull the solder away first, then the iron.
5. Don’t move the work until the solder sets.
6. Clip the excess lead, covering it with a finger or tape so it doesn’t fly.

Good joints and joints to fix

What you see	What it means	Fix
Concave, cone-shaped like a volcano, covering lead and pad	Good joint, both sides wetted	Nothing
Rough, grainy lump	Cold joint, solder never fully melted	Reheat until it flows; add flux if needed
Large round ball, edges not touching the pad	Too much solder, may not be bonded	Draw off the excess with the iron, or use wick
Thin solder, pad showing	Solder-starved, may crack later	Reheat and add solder
Solder on the lead but not the pad	Heat never reached the pad	Put the tip at the base touching the pad, add solder
Two neighbouring joints joined	Bridge, a short circuit	Drag a hot tip between the leads, or use wick
Brown burnt residue around the joint	Too hot or too long	Clean with isopropyl alcohol; lower the temperature next time

When you finish, use continuity mode to check neighbouring pins for bridges (see the multimeter lesson).

A joint that looks solid is not necessarily connected. A big blob that never wetted the pad can conduct today and fail tomorrow.

Fumes and lead: make it a habit

• Flux fumes. Breathing rosin flux fumes repeatedly can cause occupational asthma or make existing asthma worse, and they irritate the eyes and airways. Work in a ventilated space and keep your head to the side of the work, not above it. Without an extractor, a small fan blowing from behind you across the bench moves fumes away from your face; don’t aim it at someone else.
• Lead. The main route into the body is swallowing it from contaminated hands. Wash with soap and water after every session and never eat or drink at the bench. Keep solder scraps and old sponges out of the ordinary rubbish.
• Heat. Always return the iron to its stand, never the bench top. Switch it off when not in use, keep flammables away, and wear glasses.
• If your eyes or airways feel irritated while soldering, stop and get fresh air. If symptoms don’t settle, see a doctor.

Common mistakes

• Melting solder on the tip and smearing it onto the joint. The flux burns off before it reaches the work; you get a cold joint every time.
• Turning the temperature up instead of cleaning the tip. A black tip usually just needs re-tinning.
• Touching only the lead, not the pad. Solder balls on the lead and the pad underneath stays bare.
• Blowing on the joint or moving the work before it sets. Rough surface, cracked joint.
• Soldering a board that is powered. Unplug USB and batteries first, always.

Next: Breadboards and jumper wires

Sources

• How to Solder: Through-Hole Soldering — SparkFun Learn
• Adafruit Guide to Excellent Soldering: Common Problems — Adafruit
• Soldering Guidelines — University of Toronto Environmental Health and Safety
• Lead-Free Soldering, Derek’s Tech Tips Issue 16 — Hakko UK
• Soldering tip oxidation and care — Hakko UK
• Solder Melting Point Guide: Leaded vs. Lead-Free — LCSC