Abstract The liquid structuer of two molten lead-free solder alloys, Sn-0.7Cu and Sn-2Cu (wt.%), were studied at 260, 330 and 400篊 by using high temperature X-ray diffractometor. Only short range order (SRO) structure was detected in molten Sn-0.7Cu solder under all temperature. A pre-peak was found in the low Q part of the structure factor S(Q) of Sn-2Cu tested under 260篊, implying that there exists not only SRO structure but also medium range order (MRO) structure in molten Sn-2Cu solder. The appearance of pre-peak is related to the existence of Cu6Sn5-phase-like MRO clusters in molten solder. But the amount of Cu-Sn clusters decreased sharply with increasing temperature, and the pre-peak disappeared finally at 400篊 indicating that the temperature is high enough to destroy the bond in Cu-Sn correlations. The correlation raduis rc and the coordination number Nmin of the two liquid solders were obtained by calculation, and then the changing trend of the two parameters with temperature was set out. The result of Gaussian decomposition of the radial distribution function (RDF) shows that the size of Cu-Sn correlations in molten Sn-Cu solders increases with increasing Cu content.
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