Soldering a through-hole joint
Solder is pulled into a hot joint, not pushed on with an iron. Everything else on this page — the temperature, the counting, the order you take things away — is in service of that one sentence.
The joint you are trying to make is not a blob of metal stuck to a board. It is an alloy: the solder wets the copper pad and the component lead and bonds to both. That only happens if both of them are hot enough, which is why the iron goes into the corner between them and not onto one or the other.
What you need
| Soldering iron | Temperature-controlled if you can. A 30 W fixed-temperature pencil works and gives you no way to fix it when it does not. |
| Solder | 0.7 mm or thereabouts. Lead-free is the sane default; leaded flows better and melts cooler, which is why the people who are allowed to use it do. |
| Flux | Already inside the solder wire. A separate pen is for rework, where the original flux is long gone. |
| Wick or a pump | Not optional. You will get one wrong. |
| Stand and a damp sponge | The tip has to be wiped clean every few joints or it stops carrying heat. |
Ventilation matters and it is not about lead — the fumes are burning flux, and they are the irritant. A window and not leaning directly over the board covers it.
Set the temperature once
320–370 °C for almost everything through-hole. Lead-free wants the top of that; leaded is happy at the bottom.
Colder than 300 °C and the pad never reaches solder temperature inside the few seconds you are willing to hold the iron there, so you get a cold joint that looks made. Hotter than about 400 °C and the flux burns off before it can pull the solder anywhere, and the copper pad starts to lift off the board — which is a repair rather than a rework.
If joints are coming out badly, the tip is a likelier culprit than the number. A tip that has gone dull and black carries almost no heat regardless of what the display says. Wipe it, tin it with fresh solder, and try again before you turn anything up.
The order, and why it is the order
- Insert the part and bend two leads outward slightly so it stays put while the board is upside down.
- Iron into the corner where the lead meets the pad, touching both. Count three.
- Feed solder into the joint, on the far side from the tip. Count three. It should be drawn in and flow around the lead by itself; if you have to wipe it off the tip, the joint is not hot enough and nothing you do next will fix that.
- Take the wire away first, leaving the iron there. Count three.
- Take the iron away straight up, and do not move the board while the joint goes from shiny-wet to solid. That takes about a second.
Never onto the tip. Solder melted on the tip has already used up its flux by the time it reaches the pad, and flux is the thing that makes it stick.
On a header with a lot of pins
Solder the first and last pin only, then look at the part from the side. A header that is going in crooked can still be pushed straight with the iron on one of those two joints; after the third pin it is set.
Then work along the row. If you are going fast enough that the board is getting warm to the touch, you are going too fast — heat spreads sideways through the copper, and a warm board takes much longer to make each joint.
Trim last
Cut the excess leads flush after all the joints on that part are done, not between them. Cutters twist the lead slightly as they close, and a lead twisted inside a joint that has just solidified is the other way to make a cold joint without noticing.
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