Abstract: A solder paste according to an aspect of the invention includes, within a flux, an activator that has a dibasic acid with a molecular weight of 250 or less, a monobasic acid with a molecular weight of 150 or greater and 300 or less, and a dibasic acid with a molecular weight of 300 or greater and 600 or less; and at least one resin additive selected from the group consisting of high-density polyethylenes and polypropylenes. The solder paste has the resin additive in an amount of 4% by weight or greater and 12% by weight or less when the total amount of the flux is taken as 100% by weight, and has a viscosity of 400 Pa·s or greater at 80° C.

Abstract: A flux for a lead-free solder that is able to reduce “void(s) appear within solder joint”, “void(s) appear within solder joint”, and “solder-balling” without impairing general solderabilities. The flux for a lead-free solder comprising 6-55 wt % of a polyvalent alcohol ester of a resin acid (a1) as a base resin (A). Further, a solder paste comprising the said flux for a lead-free solder and a lead-free solder powder provides a very small number of void(s) or a very small amount thereof.