Sn-3.5Ag-xFe (x = 0.1, 0.5, 1.0 and 2.0 wt.%) solders were reacted with electroplated Cu under bump metallization and the characteristics of Kirkendall void formation at the solder joints were investigated. An electroplating process was carried out in a Cu sulfate bath with an additive. The results show that the formation of Kirkendall voids at the solder joint decreased with the Fe content due to the scavenging of S originating from the additive. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.