Abstract: Provided is a flux, flux residue of which keeps a predetermined viscosity during reflow and after a reflow step without hindering wettability of a solder alloy, the flux being compatible with an underfill in an underfill hardening step. The flux contains a thermosetting resin, a hardening agent, an organic acid and a solvent in which the flux contains 3% or more by mass to 8% or less by mass of the resin, 1% or more by mass to 5% or less by mass of the hardening agent to remain within a limit of an additive amount of the resin, 5% or more by mass to 15% or less by mass of the organic acid and a remainder of the solvent.