COMPARISON OF NEAR-EUTECTIC SnPb AND SnAg SOLDER PLATING FOR WLP APPLICATIONS
Authors: Bioh Kim, Charles Sharbono, Tom Ritzdorf, and Dan Company: Semitool, Inc. Date Published: 7/7/2006
Abstract: The plating characteristics of near-eutectic SnPb and SnAg solders were compared for wafer-level packaging applications. The neareutectic SnPb process showed a typical polarization behavior due to the close proximity of reduction potentials of the two metals. No large change in surface morphology and alloy composition was observed with varying current density. In the case of the near-eutectic SnAg deposition, silver had significantly higher reduction potential and much lower concentration than tin. This resulted in a very unique polarization behavior, showing two separate plateaus. A significant change in surface morphology and alloy composition was observed with increasing current density. A displacement reaction also occurred between the tin anode and the silver ions in the bath. With the waferscale deposition of SnPb solder, we observed that (i) organic components had a major impact on alloy composition and surface morphology, (ii) growth shape changed significantly with deposition conditions, and (iii) bath composition, bath aging, deposition rate, and waveform significantly impacted the probability of abnormal growths. With the wafer-scale deposition of SnAg, we observed that (i) alloy composition was a function of various deposition conditions such as current density, silver concentration, flow conditions, and pattern parameters, (ii) the surface morphology at low current densities was dendritic, and (iii) gas generation from insoluble anodes was one of the major rate-limiting factors. With both bumping processes, we achieved a similar controllability (good uniformity and repeatability of thickness and composition) at high deposition rates, and reflown bumps had spherical shapes and smooth surfaces with no voids.
Key words : near-eutectic, SnPb, SnAg, plating, wafer-level packaging