Soldering
Flux use and residue

An improperly selected or applied flux can cause joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an unsound joint.

For electronic work, flux-core solder wire is generally used, but additional flux may be used from a flux pen or dispensed from a small bottle with a syringe-like needle.

Some fluxes are designed to be stable and inactive when cool and do not need to be cleaned off, though they can if desired. If such fluxes are used, cleaning may merely be a matter of aesthetics or to make visual inspection of joints easier in specialised 'mission critical' applications such as medical devices, military and aerospace. For satellites, this will also reduce weight, slightly but usefully. In high humidity, since even non-corrosive flux might remain slightly active, the flux may be removed to reduce corrosion over time.

Some fluxes are corrosive and flux residue must be removed after soldering. If not properly cleaned, the flux may corrode the joint or the PCB. Water, alcohol, acetone, or other solvents compatible with the flux and the parts involved are commonly used with cotton swabs or bristle brushes.

In some applications, the PCB might also be coated in some form of protective material such as a lacquer to protect it and exposed solder joints from the environment.
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