Electrodeposition of near-eutectic SnAg solders for wafer-level packaging

The electrodeposition of near-eutectic SnAg solders for wafer level packaging was investigated with Shipley-Ronal's EXP-0700 SnAg bath. Fundamental studies including polarization behavior, morphological transition, and compositional change were performed to investigate the alloy deposition mechanism. It was proven that the mass transfer limitation of silver ions at low potential drove the drastic change of morphology and composition with increasing current density. The morphological transition occurred through four stages (dendrites, suppression of dendrites, facets, and dendrites) and the content of silver in the deposit dropped with increasing current density. The deposition mechanism of SnAg alloys with this bath looks similar to that of SnAgCu alloys with an alkaline bath due to a similar polarization behavior. Pattern plating results including studs, mushrooms, and stacks can be summarized as fine-grained surface, high deposition rate, good uniformity, and repeatability. Two types of in situ stack plating, Cu/SnAg and Ni/SnAg, were successfully performed with no defects between two layers. Reflow tests conducted with near-eutectic SnAg solders on copper studs showed spherical shapes with smooth surfaces, where the average melting point of whole bumps in a wafer was 223.7degreesC, which is close to the melting point (221degreesC) of the eutectic composition. (C) 2003 The Electrochemical Society.