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 μ m and pitch of 44 μ m. 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 AuSn ₄ with a thickness of about 0.5 μ m, was observed between the UBM layers and the as-plated Sn-Bi bumps.