Definition

Flux removes oxides from metal surfaces and promotes solder wetting. Types: rosin-based (traditional, leaves residue), no-clean (low residue, most common), water-soluble (aggressive, requires cleaning). Flux in solder paste activates during reflow's preheat/soak phase. Too little flux causes poor wetting; too much causes residue problems. No-clean flux residue is designed to be benign but can cause problems under conformal coating or in high-humidity environments. Water-soluble flux requires thorough cleaning to prevent corrosion. Flux classification (ROL0, REL1, ORH1) indicates activity level and residue type.