Abstract
Au was used in an electronic package to protect the conductor from oxidation. However, Au dissolved into solders and reacted with the Sn-rich phase to form AuSn 4 during soldering. After aging, Au diffused from AuSn 4 toward the solder/metallization interface. If Ni 3 Sn 4 formed at the soldering interface, a layer of AuSn 4 was redeposited on Ni 3 Sn 4 . In contrast, Au diffused into Cu 6 Sn 5 -based intermetallic compounds (IMCs) to produce either (Cu,Au) 6 Sn 5 or (Cu,Ni,Au) 6 Sn 5 , while Cu 6 Sn 5 or (Cu,Ni) 6 Sn 5 was formed at the soldering interface. Gibbs free energy evaluation revealed that both (Ni,Au) 3 Sn 4 and (Cu,Au) 6 Sn 5 were more thermodynamically stable than AuSn 4 . The maximum amount of Au diffused in Ni 3 Sn 4 was 4.6 at.%, while the maximum dissolution of Au in Cu 6 Sn 5 was 24.3 at.% at 150°C. Thus, dissolution of Au in Ni 3 Sn 4 was limited, and residual Au rereacted with Sn to produce the layer-type AuSn 4 . If Cu 6 Sn 5 formed at the interface, most of the Au in AuSn 4 diffused into Cu 6 Sn 5 . Consequently, AuSn 4 formation could be inhibited by controlling formation of Cu 6 Sn 5 in solder/under-bump metallization (UBM) assemblies. © 2007 TMS.