Iron temperature: how to set it without guessing

There is no universal setpoint

Ask five soldering trainers what temperature to use and you will get 320, 340, 350, 370 and "it depends". The first four are habits inherited from whichever line the trainer learned on. The fifth is the only correct answer, and it is unhelpful until you know what it depends on.

A soldering iron is a heat source with a fixed ability to deliver energy, and a joint is a heat sink with a fixed appetite. The setpoint is the compromise between the two. Change the joint — a 0.3 mm pad versus a ground plane — and the correct number changes with it, even though the iron and the operator have not.

This is why a single setpoint across a mixed-product line produces two complaints at once: some joints are cold, and some boards are scorched. The number is not wrong. Applying one number to every joint is.

Start from the joint, not the dial

Work backwards from what the joint needs. Solder has to reach its melting point, wet the pad and the lead, and form a fillet — all before the flux is exhausted. That entire sequence should take one to three seconds on a normal through-hole joint. If it takes longer, the tip is not delivering enough heat, and the question is why.

There are only four answers: the setpoint is low, the tip is too small for the joint, the tip is not seated properly in the heater, or the face is oxidised and not transferring heat at all. Only the first is a temperature problem. The other three are geometry, fit and maintenance — and raising the setpoint to compensate for any of them makes the real cause worse.

For lead-free alloys, start about 30 °C above the melting range as a baseline and adjust from there. For SAC305 that puts you near 350 °C. For tin-lead, 300–320 °C is usually enough. These are starting points, not standards.

What running hot actually costs

Running 40 °C above what the joint needs feels free, because the joints still look good. The bill arrives in three places:

• Oxidation. The face blackens faster, which means more cleaning, which means more mechanical contact with the plating.
• Iron loss. The tin–iron reaction accelerates with temperature, so the working layer thins faster. This is the mechanism that ends tip life, and it is invisible until the face pits.
• Flux burn-off. Flux activates in a temperature window. Above it, the activator is gone before the solder flows, and the joint skins over. Operators read this as "needs more heat" and turn the dial up again.

The third one is the trap. A tip that is too hot produces cold-looking joints, because the flux died before it could work. The natural response makes it worse.

Idle, sleep and the stand

An iron spends most of its working day not soldering. On a typical bench, contact time is well under a fifth of the shift. Everything else is idle time at whatever temperature the station is set to, and idle time is when oxidation happens, because there is no solder on the face to exclude oxygen.

Sleep or standby modes drop the setpoint to 150–200 °C after a set delay and recover within a second or two of contact. On a station without sleep, the practical alternative is to park the iron in a stand with a damp sponge and re-tin before the next joint. Both work. Leaving a dry tip at 380 °C between joints for an hour does not.

If your line runs 24 hours and you replace tips on a calendar schedule rather than on condition, sleep mode is the first thing to check. In most cases it is either disabled or set to a delay so long that it never triggers.

It is worth knowing what the display is actually telling you. A station measures its heater, usually with a thermocouple or a resistance reading, and holds that value steady. The face of the tip is cooler than the heater by an amount that depends on the tip’s mass, its length and how well it seats. A short chisel may sit 20 °C below the setpoint at idle; a long, thin conical tip can sit 60 °C below it. Two operators on identical stations, using different geometries, are therefore soldering at different temperatures even though both displays read 350. Set the number against the tip you are actually holding, not against the number on the front panel.

A method that survives an audit

A setpoint that cannot be explained is a setpoint that will drift. Write it down with the reason attached:

1. Measure or estimate the joint mass — a pad on a ground plane is not the same joint as a pad on a thin signal layer.
2. Pick the largest geometry that fits the pad without touching a neighbouring joint.
3. Set the lowest temperature that wets in one to three seconds, and record it against the product, not against the station.
4. Verify seating: the tip should slide in with light resistance and no play. A rocking tip is a thermal bottleneck.
5. Re-check when the alloy, the board or the flux changes. Any of the three can move the number by 20 °C.

One number cannot cover a mixed line, but a short table can. Most plants that stop chasing tip failures end up with three or four setpoints tied to product families, written on the station, and reviewed when the product mix changes.

Common questions