Electrodeposition of ternary near-eutectic SnAgCu solders with an alkaline bath

The electrodeposition of near-eutectic SnAgCu solders was investigated with an alkaline bath. The polarization behavior, surface morphology and film composition were examined to understand the mechanism of ternary alloy deposition. Pattern plating was tested with several types of patterns and reflow tests were conducted. Ternary alloys were obtained only when the current density was driven beyond the mass transfer limitation of noble metals (silver and copper), which seemed to cause the transition of surface morphology and film composition with increasing current density. With increasing current density, the morphological transition occurred through 4 stages (dendrites, suppression of dendrites, nodules and columns/dendrites) and the content of noble metals in the film tended to drop. Process results obtained with patterned wafers seemed to be comparable to those of PbSn alloy plating from the viewpoints of surface morphology, shape evolution, thickness and compositional uniformity, deposition rate and process repeatability. Two types of in-situ stack plating were successfully performed; Cu/SnAgCu and Ni/SnAgCu. Reflow tests conducted with near-eutectic SnAgCu solders on copper studs showed proper shapes with smooth surfaces.