Microstructure, shear strength, and nanoindentation property of electroplated Sn–Bi micro-bumps

• We electroplate eutectic Sn–Bi solder on Si chip with Ti/Cu UBM.
• Cu6Sn5 was formed between eutectic Sn–Bi solder and Cu UBM after reflow.
• Shear strength of eutectic Sn–Bi bumps increased with increasing reflow time up to 15 min.
• Elastic moduli of eutectic Sn–Bi soder and Cu6Sn5 by nanoindentation were 53.5 and 121.1 GPa.
• Hardness of eutectic Sn–Bi soder and Cu6Sn5 by nanoindentation was measured to be 0.43 and 6.67 GPa.

In order to investigate the microstructure and mechanical properties of small sized Sn–Bi bump, the eutectic Sn–Bi bumps with a diameter of 25 μm and a height of less than 20 μm after reflow were fabricated by electroplating and reflow. The reflow temperature of the Sn–Bi bumps was 170 °C, and the reflow times were varied from 5 to 20 min. The experimental results showed that a eutectic Sn–Bi composition was obtained by plating at a current density of 30 mA/cm2 for 15 min. The average height and diameter of the bumps reflowed for 5 min were 16.1 ± 0.7 μm and 25.2 ± 0.7 μm, respectively. The microstructure of the reflowed bumps consisted of Sn- and Bi-rich phases. The thickness of the IMC of Cu6Sn5 increased from 1.17 to 2.25 μm with increasing reflow time from 5 to 20 min. The shear strength of the reflowed Sn–Bi bump increased with increasing reflow time, and reached approximately 11 gf at 15 and 20 min. The elastic modulus and hardness of eutectic Sn–Bi bump by nanoindentation were 53.5 and 0.43 GPa. Those of Cu6Sn5 were found to be 121.1 and 6.67 GPa.