Characteristics of Sn–Cu Solder Bump Formed by Electroplating for Flip Chip

Sn–Cu near eutectic solder bump was fabricated by electroplating for flip-chip, and its electroplating and bump characteristics were studied. A Si-wafer was used as a substrate and the under bump metallization (UBM) comprised 400 nm of Al, 300 nm of Cu, 400 nm of Ni, and 20 nm of Au sequentially from bottom to the top of the metallization. The electrolyte for plating Sn–Cu solder consisted of ${\hbox {Sn}}^{+2}$ (concentration of 30 g/L) and ${\hbox {Cu}}^{+2}$ (0.3 g/L) solutions with methasulfonic acid and deionized water. The experimental results showed that the plating ratio of the Sn–Cu increased from 0.25 to 2.7 $\mu/{\hbox {min}}$ with increasing current density from 1 to 8 ${\hbox {A/dm}}^{2}$. In this range of current density, the plated Sn–Cu maintained its composition nearly constant level as Sn-(0.9∼1.4)wt% Cu. The solder bump of typical mushroom shape with 120-$\mu{\hbox {m}}$ stem diameter and 75-$\mu{\hbox {m}}$ height was formed by plating at 5 ${\hbox {A/dm}}^{2}$ for 2 h. The mushroom bump changed its shape to the hemispherical type of 140-$\mu{\hbox {m}}$ diameter by air reflow on a hot plate at 260 °C. The homogeneity of element distribution in the solder bump was examined, and Sn content in the mushroom bump appears to be uneven changed to more uniform after the air reflow. The highest shear bond strength of the Sn–Cu hemispherical bump showed 113 gf by reflowing at 260 °C for 10 s.