Sn-3.5Ag solder reacted with heat-treated Cu films, and the characteristics of Kirkendall void formation at the Cu3Sn/Cu interface were investigated. Heat-treatment for the suppression of Kirkendall void formation in Sn-3.5Ag/Cu solder joints was conducted at varying temperature (T=400, 500 and 600 degrees C). The results showed that S segregation on the void surfaces were effectively suppressed by heat-treatment, and Kirkendall voiding at the Cu3Sn/Cu interface decreased as the heat-treatment temperature was increased. According to the solid solubility limit of sulfur in Cu, S in Cu film was in solid solution at all heat-treatment temperatures, so the amount of sulfur segregation was reduced. (C) 2013 Elsevier B.V. All rights reserved.