Electroplating Characteristics of Sn–Bi Microbumps for Low-Temperature Soldering

In this paper, the characteristics of eutectic Sn-Bi microsolder bumps fabricated by electroplating are investigated. The underbump metallization (UBM) layers on a Si chip consist of Al, Cu, Ni, and Au, sequentially from bottom to top. The desired Sn-Bi bump size has a diameter of 22 μm and pitch of 44 μm. In order to obtain the optimal conditions for the eutectic Sn-Bi solder bumps, the polarization curves of Sn, Bi, and Sn-Bi electrolytes are analyzed, and the variation of the Sn- Bi composition as a function of the current density is measured. Experimentally, from the polarization curve, Bi and Sn start to deposit below -0.12 and -0.54 V, respectively, and Sn-Bi codeposition occurs below an electropotential of -0.54 V. The Bi content of the electroplated bumps decreases from 92.4 to 38.2 wt% when the current density is increased from 20 to 50 mA/cm², and near-eutectic composition of the Sn-61 wt% Bi bump is obtained by plating at 40 mA/cm² for 5 min. The surface of the Sn-Bi microsolder bumps show plate-like structures with acicular shapes, and the grain size increases with increasing current density. An intermetallic compound layer, estimated as AuSn⁴ with a thickness of about 0.5 μm, was observed between the UBM layers and the as-plated Sn-Bi bumps.