What is soldering?
Soldering is a technique for permanently joining metal parts by adding a filler alloy, called solder, that bonds to both of them. The solder melts at a lower temperature than the parts being joined, so the parts themselves do not melt. That is the key difference with welding, where the base metals fuse together at much higher temperatures. Dissimilar metals can also be soldered. In Dutch the process is called solderen.
Soft soldering and brazing
- Soft soldering. Tin-based solders working below 450 °C. Used on zinc gutters and roof work, lead, and copper water pipes with capillary fittings. Done with a soldering iron or gas torch.
- Hard soldering (brazing). Filler alloys containing copper and silver, working above 450 °C. Produces stronger joints that also cope with higher temperatures. Done with a brazing torch.
Never use lead-based solder on drinking water pipework: use lead-free solder.
Application
- Clean the joint. Abrade both surfaces to bright metal with abrasive pad or wire wool. Grease and oxide stop the solder from wetting.
- Apply flux. A thin coat of flux suited to the metal and solder prevents oxidation while heating.
- Heat the work, not the solder. Heat the joint until solder melts on contact with the metal. In a capillary fitting it draws itself into the gap.
- Let it cool undisturbed. A joint that moves while solidifying turns dull, grainy and weak.
- Clean off residue. Wipe away leftover flux, which can cause corrosion.
- Work safely with flame. Use a heat mat near timber fascias or roofing, keep an extinguisher at hand and check for smouldering afterwards.
Common mistake: pointing the flame at the solder rather than the work. The solder drips off and never properly bonds to the parts.
Related terms
- Welding
- Folded zinc
- Gutter
- Sheet lead
- Leaded glass
- Pressure pipe
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