How to solder: the basics that actually matter

Why most soldering goes wrong

Solder is not glue. It only bonds when both surfaces, the pad and the component lead, are hot enough for the molten tin to wet them and form an alloy layer. Melting solder on the tip and dropping it onto a cold joint gives a blob that looks connected and is not. Almost every bad joint comes from that one mistake.

Set up the iron

• Temperature: 320 to 350 °C for leaded solder, 350 to 380 °C for lead-free. Hotter burns the flux before it works and can lift pads.
• Tip: use the largest tip that fits the pad. A 2 mm chisel covers most through-hole work. A fine conical tip transfers heat badly; beginners pick it and struggle.
• Tin the tip: a clean, shiny, tinned tip transfers heat. Wipe it on brass wool and add a little fresh solder before each joint.

The technique

1. Clean the tip and add a small drop of solder to it. That drop is a heat bridge, not the joint.
2. Touch the tip to the pad and the lead at the same time.
3. After one to two seconds, feed solder into the joint from the opposite side. It should flow around the lead by itself.
4. Remove the solder wire, then the iron. Total time: three to four seconds.
5. Do not move anything for two seconds while the joint solidifies.
6. Cut the lead just above the joint with a flush cutter.

Read the joint

A good through-hole joint is shiny (dull is normal for lead-free), concave like a small volcano, and you can see the outline of the lead. A ball sitting on the pad, a dull grainy surface or solder that does not reach the edge of the pad means the joint did not wet: add flux and reheat.

Flux is the secret

The flux inside the solder wire burns off in a couple of seconds. When a joint needs rework, add fresh flux from a pen. Most problems beginners blame on the iron are a lack of flux. Clean the residue afterwards with isopropyl alcohol and a brush if it is sticky.

Common mistakes

• Carrying solder to the joint on the tip.
• Iron too hot, tip too small.
• Moving the part before the solder solidifies.
• Reheating the same joint again and again without adding flux.
• Pressing hard with the iron. Pressure does not transfer heat; contact area does.

Kit list

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