Co-included Sn-Bi-based high-strength lead-free low-temperature welding flux

The invention discloses a Co-included Sn-Bi-based high-strength lead-free low-temperature welding flux. The welding flux comprises, by mass, 54% to 58% of Bi, 0.5% to 4.0% of Ag, 0.1% to 1.0% of Cu, 0.005% to 0.08% of Co, 0.002% to 0.1% of P and the balance Sn, and the sum of the mass percent of the components is 100%. The melting point of the welding flux is only 138-149 DEG C. Compared with prior Sn-Bi eutectic welding fluxes, the welding flux has the advantages that in the same conditions, the wetting power is improved, and the strength is improved by 9% to 23%.