The liquid most commonly used before soldering is flux.
While the provided reference mentions vinegar and salt or baking soda and water as potential cleaning agents, these are not the typical or recommended liquids used before soldering as flux. They might serve a rudimentary cleaning function, but they don't provide the key benefits of flux.
What is Flux and Why Use It?
Flux is a chemical cleaning agent. It serves several crucial purposes in the soldering process:
- Removes Oxidation: Metals, especially at soldering temperatures, readily oxidize. This oxidation prevents the solder from properly bonding to the metal surface. Flux chemically reacts with these oxides, removing them and exposing clean metal.
- Prevents Re-oxidation: Flux creates a barrier that shields the cleaned metal from further oxidation during the heating process.
- Promotes Wetting: Wetting refers to the solder's ability to flow and spread evenly across the metal surface. Flux reduces surface tension, allowing the molten solder to "wet" the joint and create a strong bond.
Types of Flux
Flux comes in various forms, each suited to specific applications and materials:
- Rosin-based Flux: Commonly used in electronics soldering. It's available in paste, liquid, and cored solder forms (where the solder wire contains a core of flux). Rosin flux is generally considered non-corrosive and electrically non-conductive after soldering, although cleaning is still often recommended.
- Acid-based Flux: Stronger fluxes designed for soldering metals like copper pipes, brass, and other non-electronic applications. Acid fluxes are corrosive and must be thoroughly cleaned after soldering to prevent future corrosion and potential failure of the joint.
- No-Clean Flux: Designed to leave minimal residue after soldering, theoretically eliminating the need for cleaning. However, in critical applications, even "no-clean" flux residues are often removed for optimal reliability.
Cleaning Before Soldering (Beyond Flux)
While flux is applied immediately before soldering, other cleaning steps may be performed prior to applying flux. These might include:
- Mechanical Cleaning: Using abrasives like sandpaper, steel wool, or wire brushes to remove heavy oxidation, dirt, or grease.
- Solvent Cleaning: Using solvents like isopropyl alcohol (IPA) or acetone to remove oils, grease, and other contaminants.
However, these steps are distinct from the use of flux before soldering. Flux is always the last step applied to the joint surfaces before the soldering iron or heat source is applied.
Example
Imagine soldering a wire to a circuit board pad. The steps would be:
- Lightly clean the wire and pad with isopropyl alcohol if visibly dirty.
- Apply a small amount of rosin flux to the pad.
- Heat the pad and wire with a soldering iron and apply solder.
- Remove the heat and allow the solder to cool.
- Clean off any flux residue with isopropyl alcohol (optional but recommended).
