Abstract
The dominant mechanism for the growth of Cu 6 Sn 5 and Cu 3 Sn in the early stages has been investigated using Sn4.0Ag0.5Cu solder reflowed on Cu. Contrary to previous studies, a thin layer of Cu 3 Sn (80-90 nm) was observed between Cu 6 Sn 5 and Cu, even at temperatures as low as 217.4 °C. High-resolution transmission electron microscopy identified Cu-enriched regions at phase boundaries between Cu 6 Sn 5 and Cu 3 Sn. Such regions were about 3-6 nm thick due to a large lattice mismatch. Similar Cu-enriched regions were also observed at the grain boundary of Cu 6 Sn 5 . The existence of these Cu-enriched regions suggests that grain boundary and phase boundary diffusion of Cu atoms were the dominant mechanisms for the growth of Cu 6 Sn 5 and Cu 3 Sn in the early stages of soldering reactions. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.