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

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.