Intermetallic compounds formation and joints properties of electroplated Sn–Zn solder bumps with Cu substrates

Sn–Zn solder bumps are prepared using electrodeposition method, and the interfacial compounds formation and joints properties of solder bumps with Cu substrates are also investigated in this paper. The DSC results indicate that the melting temperature of electroplated Sn–9Zn solder bumps declines markedly with Zn alloying element added into pure Sn solder. XRD results suggest that the content of Zn on the surface of solder bumps decreases significantly after solder reflowing. EPMA results indicate that in Sn–9Zn/Cu interconnection joints, Zn diffuses from the surface of micro-bumps to the interface during solder reflowing. The BSEM images and EDX results of the solder bumps reveal that there was only one layer of Cu–Zn intermetallic compound(IMC) formed at the interface which is safely assumed as Cu₅Zn₈. Under the condition of aging tests at 150 °C for 400 h, joints interface between electroplated Sn–9Zn solder bumps and Cu substrate was Cu₅Zn₈ layer invariably, and no other significant phase transition occurs. However, interfacial intermetallic compound between pure Sn bump and the Cu substrate changes from Cu₆Sn₅ to CuSn₃ under the same condition of aging tests. The shear strength of electroplated Sn–9Zn solder joints is higher than that of the pure Sn solder joint due to small Cu₅Zn₈ sizes and thin IMC layer, and shear strength of the solder joints decreases with the increase of aging time.