, Volume 38, Issue 6, pp 828-833
Date: 14 Apr 2009

Viscosity and Surface Tension of Liquid Sn-Cu Lead-Free Solders

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The viscosity of Sn-0.7wt.%Cu and Sn-2wt.%Cu lead-free solders was measured using a torsional oscillation viscometer from 503 K to 773 K. For both melts the viscosity decreased with increasing temperature, and the values dropped abruptly at certain temperature ranges, i.e., 553 K to 573 K for Sn-0.7Cu and 573 K to 603 K for Sn-2Cu. Correspondingly, two segments, a low-temperature zone and a high-temperature zone, were found on both ηT and ln η–1/T plots, where η is the viscosity and T is the absolute temperature. The activation energy ε and the unit volume of flow ν m were obtained by applying the parameters of linear fitting for each temperature zone. The surface tension of the solders was calculated using the measured viscosity values, and the results were checked by measuring the wetting angles of metallurgical reactions between the solders and the Cu substrates. Both viscosity and surface tension are closely related to the liquid structure of the solder alloy.