Soldering is not gluing. You are not sticking two parts together with molten metal — you are heating both of them until they are hot enough that the solder alloys with their surfaces and flows into the joint on its own. Every rule below follows from that one sentence.
Which is why the commonest failure is not too little solder. It is heating the solder instead of heating the joint.
Flux is not optional, and it is not glue
Copper grows a thin oxide layer within minutes of being exposed to air, and molten solder will not wet an oxide — it beads up and sits on top like water on a greasy plate. Flux is a mild acid that dissolves that oxide at soldering temperature and then keeps air off the surface while you work.
- Ordinary electronics solder has flux inside it, in cores running down the middle of the wire. That is enough for a clean new joint.
- Extra flux, from a pen or a tin of paste, is for everything else — reworking an old joint, desoldering, anything tarnished, and all surface-mount work.
- Never use plumbing flux or acid paste on electronics. It is far more aggressive, it does not stop working when the joint cools, and it will quietly eat the copper track over the following months.
Before you switch on
- 1
Set the temperature, if the iron lets you
About 330 to 350 degrees C for ordinary leaded solder, 360 to 380 for lead-free. Hotter is not better — above 400 the flux burns off before it can do its job, and you get a dull joint on a scorched board.
- 2
Tin the tip
Melt a little solder onto the tip and wipe it on a damp sponge or brass wool. A bright, silver, wetted tip carries heat into the joint; a dull grey oxidised one barely conducts at all. Most complaints of a weak iron are a dirty tip.
- 3
Get the board still and the parts held
Two hands are already busy holding the iron and the solder. A stand, a clip or a lump of putty is what holds the third thing. Trying to manage it all at once is where burns come from.
- 4
Sort out the air
The smoke is vaporised flux, not lead, and it is an irritant you should not be breathing. Work in a ventilated room and keep your face out of the plume. A small fan pulling the smoke sideways is enough.
The joint itself
A through-hole joint takes about five seconds. Longer than that and you are cooking the component and lifting the pad; shorter and the joint never got hot enough.
- 1
Touch the iron to the pad and the lead at once
Both, not one. Angle the tip so it bridges the two, and use the flat side of the tip rather than the point — contact area is what transfers heat, and a needle point touching one edge heats almost nothing.
- 2
Count one, two
Let them come up to temperature together. Do not add solder yet.
- 3
Feed solder into the joint, on the far side from the iron
Not onto the tip. Touching solder to the tip lets it melt and drip without either surface being hot, and that is precisely how a dry joint is made. If the joint is hot enough it will pull the solder in on its own and you will see it flow.
- 4
Remove the solder, then the iron
In that order, about a second apart.
- 5
Let it cool without moving
Blowing on it or nudging the part while the metal is freezing produces a crystallised, grainy joint that is weak and high in resistance. Two seconds of stillness is all it needs.
Reading a joint
| What you see | What it means | Fix |
|---|---|---|
| Shiny, smooth, a concave slope from pad to lead | Correct. The solder wetted both surfaces. | Nothing. |
| A round ball sitting on the pad, with a visible edge | Dry joint. Only the solder melted; the pad never got hot. | Add flux, reheat both surfaces properly, add a little fresh solder. |
| Dull, grey, grainy | Moved while cooling, or the iron was far too hot. | Reheat, then leave it alone until it is solid. |
| Solder bridging two adjacent pads | Too much solder, or the tip dragged across. | Add flux and drag the tip across the bridge, or lift it with wick. |
| A crater or a lump with a hole in it | Something boiled off — usually flux, or a damp board. | Reflow with fresh solder. |
| The pad has lifted off the board | Too much heat for too long. The glue under the copper let go. | Serious. You will need a wire link to the next point on the track. |
Taking parts off again
Two tools, and they suit different jobs. Neither works without flux.
- Desoldering braid — copper wick that draws molten solder in by capillary action. Best for flat pads, bridges and cleaning a hole out. Lay the braid on the joint, press the iron on top of the braid, and wait for the solder to climb into it. Then lift both together; letting it cool welds the braid to the board.
- A vacuum pump — a spring-loaded plunger that sucks the joint clear. Best for through-hole pins on a multi-pin part. Melt the joint, get the nozzle over it, and fire while the solder is still liquid.
- 1
Add fresh solder to the old joint first
This sounds backwards and it is the trick that makes desoldering work. Old solder has spent flux and a high melting point. Fresh solder brings new flux in and gets the whole joint flowing at a lower temperature.
- 2
Clear each pin, then check the part is free
Rock the component gently. If a pin is still anchored, clear that one again rather than pulling harder.
- 3
Never pull a part out while a pin is still soldered
That is how a pad comes off, and a lifted pad is a much larger repair than the one you started with.
Leaded or lead-free?
Leaded solder — 60/40 tin-lead, or 63/37 — melts at about 183 to 190 degrees C, flows beautifully, and is far more forgiving to learn on. Lead-free melts around 217, needs a hotter iron, and looks duller even when the joint is perfect, which makes reading your work harder.
For learning and for project work, leaded is the sensible choice and is legal for that purpose in India. Commercial products sold into markets with RoHS rules must be lead-free. Whichever you use, the hazard is the same and it is not the fumes: it is lead transferring to your hands and then to your mouth. Wash your hands.
| Item | Why |
|---|---|
| A temperature-controlled iron | A fixed 25 W iron is either too cold for a ground plane or too hot for a small part, and you cannot tell which. |
| A stand with a sponge or brass wool | Safety first, tip life second. |
| 0.8 mm flux-cored solder | Thin enough for ICs, thick enough to feed by hand. |
| A flux pen | Turns every rework and desolder job from a fight into a few seconds. |
| Desoldering braid and a pump | Different jobs, both cheap. |
| Side cutters | Trim leads after soldering, close to the joint, never during. |
| Isopropyl alcohol and a brush | Removes flux residue so you can inspect the joint and so it does not attract dirt. |