Interfacial microstructure and bonding strength of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn–3Ag–0.5Cu–0.5Ce–xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn3 intermetallics and Zn atoms, formed in the Sn–3Ag–0.5Cu–0.5Ce–xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu–Sn intermetallics growth in Sn–3Ag–0.5Cu–0.5Ce–xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn–Ag–Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.