The evolution of interfacial microstructure of Sn3.5Ag solder bump with Cu under-bump metallization

Low cost electrodeposited Sn3.5Ag solder bumps fabrication technology was developed for three-dimensional packaging applications. Effect of reflow time on interfacial reaction of electroplated Sn3.5Ag solder bumps with Cu under-bump metallization (UBM) was investigated. The microstructure of eutectic Sn3.5Ag is composed of β-Sn phase and Ag 3 Sn IMCs in the solder region. At the interface between solder and Cu, scallop-like Cu-Sn IMCs were formed toward solder region and were identified as Cu 6 Sn 5 IMCs by compositional measurement of EDX. XRD measurement identified that Cu 6 Sn 5 and Ag 3 Sn formed after reflowing for lmin, and as the reflow time extended the quantity of Ag 3 Sn almost remained the same while the quantity of Cu 6 Sn 5 increased. No Cu 3 Sn was detected, even after reflowing for 20min. EDX analysis was consistent with the XRD result.