Abstract: Effects of active agent,paste and acidity regulator of Sn1. 0 Ag0. 5 Cu solder paste flux on spreading ratio and post-soldering residual corrosion were studied. The results showed that the increase of active agent content is beneficial to the spreading ratio of solder paste when the content of active agent was below 8%（ mass fraction,the same below）. The post-soldering SIR value decreases as the content of active agent increased. The post-soldering SIR value decreased to 1. 6×10⁹Ω at the content of active agent of 9%. The spreading ratio is 81. 4% and less post-soldering residual corrosion can be achieved when PEG2000 and modified epoxy resin was used as paste for the solder paste. The SIR value of this type of solder paste remains at 1. 5×10¹¹Ω after soldering for 10 days. The spreading ratio is 82. 6% and the SIR value was 4. 0×10⁹Ω when rosin was used as paste for the solder paste. The spreading ratio of the solder paste increases while the p H of flux and the postsoldering SIR value were decreased after triethanolamine being added as acidity regulator. The post-soldering SIR value reaches up to the maximum of 1.6×10¹²Ω at the content of triethanolamine of 1. 0%.