Lead-free soldering

Abstract

Due to the global trend of green manufacturing, lead-free becomes the main stream soldering choice of electronic industry. SnAgCu alloys are the prevailing choices, with SnCu(+Y), SnAg(+Y), and BiSn(+Y) families also being adopted, where Y represents minor additive elements. The soldering processing window is narrower than that of Sn63, mainly due to the elevated melting temperature of SnAgCu solder and the limited high temperature tolerance of components and board. The high surface tension of Sn aggravates the difficulty in wetting, while the high reactivity of Sn puts more constraint in contact time allowed between molten solder and base metal or solder container. The creep rate of SnAgCu is slower at low stress, but faster at high stress than Sn63. This results in a longer temperature cycling life at low joint strain applications, but a shorter cycling life at high joint strain applications. Higher Cu content stabilizes IMC structure at interface between SnAgCu solder and NiAu. The high rigidity of SnAgCu solders enhances the fragility of joints, although significant improvement has been accomplished via low Ag or high Cu content or doping approaches. © 2009 Springer-Verlag US.