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Soldering process and fluxes

Short answer

Soft soldering uses a tin-lead filler melting below 450°C. Steps: clean surfaces, apply flux, fit with small gap, heat the joint, touch solder to let capillary action draw it in, cool undisturbed, and clean residue. Flux removes oxides, prevents re-oxidation, and improves wetting. Rosin flux for electronics; zinc chloride for sheet iron and brass plumbing (must be washed).

Explain the soft soldering process with its steps. State the functions of a flux and name two fluxes with the joints where they are used.

Soldering is a joining process in which a filler metal (solder) melting below 450 °C is melted into the joint; the parent metals are not melted. The joint forms by wetting and a thin alloy layer, not by fusion.

Soft solder and equipment

  • The common soft solder is tin-lead (for example 60% Sn, 40% Pb), which melts at about 183 to 190 °C; other tin-lead grades melt at up to about 250 °C. Lead-free tin-copper and tin-silver solders are also used.
  • Tools: soldering iron (copper bit), blow-lamp or gas torch, flux, file or emery paper.

Steps

  1. Clean the surfaces to bright metal with a file or emery cloth to remove oxide, dirt and oil.
  2. Apply flux to the clean surfaces.
  3. Fit and hold the parts in position with a small gap (0.05 to 0.15 mm) for capillary action.
  4. Heat the joint with the clean, tinned iron or a torch. Heat the work, not the solder.
  5. Touch the solder to the heated joint; it melts and is drawn into the gap by capillary action.
  6. Remove the heat and let the joint cool without movement.
  7. Clean off the flux residue, especially corrosive flux, with water or a solvent.

Functions of flux

  • Dissolves and removes the oxide film from the metal surface and prevents fresh oxidation during heating.
  • Lowers the surface tension of the molten solder so that it spreads and wets the surface.
  • Helps heat transfer.

Examples

  • Rosin (resin) flux: non-corrosive; for electrical and electronic joints.
  • Zinc chloride (killed spirit) or ammonium chloride: active and corrosive; for sheet iron, brass and copper plumbing. It must be washed off.

✓ Answer checked by Claude Opus 5.5 (high effort)

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