In order to explore the mechanism of reactive wetting occurred between solder and Cu substrate, the wetting behavior of high-Pb, say, Pb-xSn solder (x = 0, 0.7, 1.4, 2.1, 2.8, and 3.5 wt.%, respectively) was investigated using the wetting balance technique. It was found that small amount of addition Sn in Pb could improve the wettability of Pb-xSn solder on Cu severely. Since the current reactive wetting theory suggested that the first intermetallic compound formed during the wetting reaction could influence the wetting properties, the driving forces of the intermetallic, compounds formation under metastable equilibrium were calculated using Thermo Cale software. And the first intermetallic compound was confirmed to be Cu3Sn by both theoretical computation and EDX analysis. The reactive wetting theories was employed, and combined with the analysis of wetting balance test results, to discuss the wetting phenomena herein. (c) 2005 Elsevier B.V. All rights reserved.