Abstract: In order to improve the wettability of Sn-Ag-Cu lead-free solder paste, surfactants are added to the flux to reduce the interfacial tension between the solder and Cu substrate. The effects of DDBAC and SDBAC contents on the contact angle, solder joint morphology and microstructure of SAC305 solder paste are studied by means of contact angle measuring instrument, SEM and EDS. The results show that the addition of surfactants can decrease the contact angle of solder on the Cu substrate, and that with the addition content increases, the contact angle decreases first and then becomes stable. When the addition contents (mass fraction) of SDBAC and DDBAC are 0.6%, the contact angles are 10.0° and 12.2°, respectively, with the solder joints round and full, therefore, both surfactants make the solder paste show good wettability. With the increase of the DDBAC and SDBAC additions, the intermetallic compounds (IMC) layer thickens obviously for the surfactant significantly improves the diffusion of molten solder through surface modification.