The effect of lead addition on the improvement of wettability of Sn-3.5Ag lead-free solder has been investigated by the electrochemical reaction of soldering flux. Among the elements used in solder and copper base metal, tin in solder forms the most stable oxide film, therefore, the preferential dissolution of tin by electrochemical contact with copper base metal is beneficial both for removal of the oxide film and the enhancement of wettability. However, Sn-3.5Ag solder has similar electrode potential to copper base metal in soldering flux, therefore, the accelerated anodic dissolution of tin by contacting copper base metal is only a little. To enhance the wettability, the reduction of electrode potential of Sn-3.5Ag might be effective for the enhancement of preferential dissolution of solder by the contact with copper. Therefore the effect of lead addition of 2-10% has been investigated by electrochemical measurement in soldering flux. The addition of lead lowered the electrode potential of Sn-3.5Ag-0-10Pb solder which increased the potential difference between solder and copper base metal resulted in the increased contact current between them. The improved wettability is confirmed by adding lead to Sn-3.5Ag, all lead added solders showed larger spread area, i.e., smaller contact angle than Sn-3.5Ag after spreading test. The work proposed the electrochemical mechanism affecting on the wettability; the degree of contact corrosion current based on the potential difference between solder and base metal infuluences on wettability.