Higher amount of flux increases soldering quality. Rosin is best known passive flux. It comes in cakes or pieces, you can use
any. Avoid using rosn for bow musical instruments - there is lime in it.
Flux can be liquid - it is rosin solution (with alcohol or acetone). This flux is still passive. Always wash active fluxes
throughly. Leaved on tracks, active flux is causing their corrosion.
Flux is washed with solvent with rag, wadding or cotton. Industrial production uses ultrasonic bath or indelible fluxes that
form protective coating on tracks. There also fluxes for aluminium and steel soldering.