Direct evidence for a Cu-enriched region at the boundary between Cu6Sn5 and Cu3Sn during Cu/Sn reaction

The dominant mechanism for the growth of Cu6Sn5 and Cu3Sn in the early stages has been investigated using Sn4.0Ag0.5Cu solder reflowed on Cu. Contrary to previous studies, a thin layer of Cu3Sn (80-90 nm) was observed between Cu6Sn5 and Cu, even at temperatures as low as 217.4 degrees C. High-resolution transmission electron microscopy identified Cu-enriched regions at phase boundaries between Cu6Sn5 and Cu3Sn. 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 Cu6Sn5. 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)Sn5 and Cu3Sn in the early stages of soldering reactions. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.