---
title: "Soldering process and fluxes"
subject: "Workshop Technology"
chapter: 3
key: "semester_2/workshop_technology:soldering-process-flux"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/soldering-process-flux
site: BCT Engineering
---

# Soldering process and fluxes

Workshop Technology · Chapter 3: General Hand Working Operations

- Asked in: year not given, 6 marks
- Video: [Soldering and Brazing Explained | Tools, Flux, Types & Comparison | Mootechie](https://www.youtube.com/watch?v=mYZW2jlrj3k) — mootechie
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/soldering-process-flux
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-3

## Question

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.

## 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).

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What distinguishes soldering from welding?
   A. The parent metals are melted and fused
   B. A filler melting below 450 °C is used and the parent metals are not melted
   C. It needs no cleaning
   D. It uses only high temperatures above 900 °C
   Answer: B. In soldering only the filler (solder) melts, forming the joint by wetting, not by fusion of parent metals. (Key point: Soldering principle)

2. In soldering, what should be heated directly?
   A. The solder only
   B. The flux only
   C. The work, not the solder
   D. The iron only
   Answer: C. The work is heated; the solder is touched to the hot joint and drawn in by capillary action. (Key point: Heating the joint)

3. What is the correct order for the early steps?
   A. Apply flux, then clean, then heat
   B. Clean the surfaces to bright metal, then apply flux
   C. Heat the joint, then clean it
   D. Apply solder, then flux
   Answer: B. Surfaces are cleaned of oxide, dirt and oil first, then flux is applied before fitting and heating. (Key point: Order of steps)

4. Which is a function of flux?
   A. Raises the melting point of solder
   B. Dissolves oxide film and lowers surface tension of molten solder so it wets
   C. Cools the joint rapidly
   D. Hardens the solder
   Answer: B. Flux removes oxide and prevents re-oxidation, and lets the molten solder spread and wet the surface. (Key point: Functions of flux)

## Related in Workshop Technology

- [Temporary and permanent joints; soldering, brazing, welding](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ajoints-soldering-brazing-welding-compare) — Workshop Technology, chapter 10

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