Inhibiting AuSn4 Formation by Controlling the Interfacial Reaction in Solder Joints

[[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 AuSn4 during soldering. After aging, Au diffused from AuSn4 toward the solder/metallization interface. If Ni3Sn4 formed at the soldering interface, a layer of AuSn4 was redeposited on Ni3Sn4. In contrast, Au diffused into Cu6Sn5-based intermetallic compounds (IMCs) to produce either (Cu,Au)6Sn5 or (Cu,Ni,Au)6Sn5, while Cu6Sn5 or (Cu,Ni)6Sn5 was formed at the soldering interface. Gibbs free energy evaluation revealed that both (Ni,Au)3Sn4 and (Cu,Au)6Sn5 were more thermodynamically stable than AuSn4. The maximum amount of Au diffused in Ni3Sn4 was 4.6 at.%, while the maximum dissolution of Au in Cu6Sn5 was 24.3 at.% at 150°C. Thus, dissolution of Au in Ni3Sn4 was limited, and residual Au rereacted with Sn to produce the layer-type AuSn4. If Cu6Sn5 formed at the interface, most of the Au in AuSn4 diffused into Cu6Sn5. Consequently, AuSn4 formation could be inhibited by controlling formation of Cu6Sn5 in solder/under-bump metallization (UBM) assemblies.[[fileno]]2020323010239[[department]]材料科學工程學