Skills · skill
Soldering
Joining · Beginner
- Difficulty
- Beginner
- Safety
- Fumes are flux, not lead, but ventilate anyway. Lead solder: wash hands, no food at the bench. The iron is 350 °C and looks the same cold.
- Done When
- The joint is bright, smooth and concave, wets both parts, and a gentle tug on the lead moves nothing.
- Tools
- Temperature-controlled iron (330–370 °C for lead-free), 0.5–0.8 mm rosin-core solder, flux pen, brass tip cleaner, tweezers, extractor fan.
- Category
- Joining
- Summary
- Joining electrical parts with a low-melting tin alloy that wets copper and freezes into a conductive, mechanically weak joint.
Steps
Tin the tip: melt a little solder on it and wipe.
Touch the tip to both the pad and the lead at once — heat the joint, never the solder.
After a second, feed solder into the far side of the joint; it should flow toward the heat.
Remove the solder, then the iron; hold still until it dulls and sets.
Inspect: a shiny concave fillet climbing both surfaces.
Clip the lead above the fillet.
The Science
The iron heats the joint, not the solder: when copper reaches about 250 °C, molten tin dissolves a whisker of it and forms an intermetallic layer — a metallurgical bond, not glue. Flux strips the oxide off first so the tin can wet the metal; without it the solder balls up. Heat conducts through the joint by Fourier's law, which is why a big ground plane needs a hotter, bigger tip.
How It Fails
A cold joint — grey, lumpy, a ball sitting on the pad — from heating the solder instead of the joint, or from movement while it froze. A bridge between neighbouring pads from too much solder. A lifted pad from too much heat for too long.
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