Soft solder, flux and the soldering process for joining sheet metal.
Soldering is joining two metal pieces with a filler metal (solder) that melts at a lower temperature than the base metals. The base metal does not melt.
Soft solder
- An alloy of tin and lead (lead-free solders use tin with silver/copper).
- Melting range about 183 °C – 250 °C.
- Common grades: 60/40 (tin/lead) for electrical work, 50/50 for general sheet metal.
Flux
Flux cleans the surface, prevents oxidation during heating and helps the solder flow.
| Flux | Type / use |
|---|---|
| Zinc chloride (killed spirit) | Active (corrosive) – steel, copper, brass. Wash the joint after soldering. |
| Hydrochloric acid (dilute) | Active – galvanised iron sheets |
| Resin / rosin | Non-corrosive – electrical work |
| Tallow | Lead and pewter |
Soldering process
- Clean the surfaces thoroughly (file, emery, wire brush) – solder will not stick to dirt or oxide.
- Fit the parts closely.
- Apply flux.
- Heat the soldering iron (copper bit) and tin its face with solder.
- Heat the joint, then apply the solder – it flows into the joint by capillary action.
- Let it cool without moving the parts; clean off the flux.
Safety Flux fumes and lead are harmful – work in a ventilated area, wash hands after soldering, keep the hot iron on its stand.
Quick Revision
- Base metal does not melt in soldering.
- Soft solder = tin + lead, melts ~183–250 °C.
- Clean surface + flux = good joint.
- Zinc chloride is active flux; resin is for electrical work.
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