Evaluation of lead-free SnAg solder ball deposition and reflow processes for flip chip applications

For flip chip technology solder balls, SnAg and SnAgCu are the most promising materials to replace standard SnPb solder, which will be restricted in the future because of environmental reasons [Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment, Official Journal of the European Union, 13.2.2003, pp. L37/19-23]. Electrochemical processes are commonly used for the solder deposition, but it is difficult to adjust the composition ratio in the deposited binary and ternary metal compounds with this method. In this paper, we report the evaluation of different metallization schemes with Ni- or Cu-studs beneath the solder material. Special efforts were put on the investigation and optimization of selected unit processes and hardware setups required for the whole bump formation process, like resist patterning, electroplating, resist stripping, under bump metallization (UBM) etching and the reflow process.