Flip chip is one of the most promising electronic packaging interconnection technologies. Because flip chip realizes more I/Os than Wire-bonding or TAB does. Electroless Ni-P UBM and Screen-printing bump process are used broadly for the benefit of low cost process. Currently, there are a number of legislations that intended to ban Pb use. Then there are many studies of Pb-free solders that alternate thoroughly with eutectic Pb/Sn. The most common Pb-free solder alloys are based on the Sn-rich alloy with alloying elements of Bi, Cu, and Ag.
In this study, interfacial reactions between electroless Ni-P UBM and Sn3.5Ag, Sn3.5Ag4.8Bi, and Sn58Bi were investigated, focusing on identification of IMC phases and IMC growth kinetics with Bi contents. These three type solders have respectively the Bi contents of 0wt%, 4.8wt% and 58wt%. As well, bump shear were investigated, focus on bump shear strength and failure modes with Bi contents.
IMCs contained Bi were not observed at interface and in the bulk of solders during reflow and aging. By the way, $Ni_3Sn_4$ IMC phase were observed in the bulk of Sn3.5Ag4.8Bi solders during reflow. However, $Ni_3Sn_4$ IMCs were not observed in the bulk of Sn3.5Ag(contained 0wt% Bi) or Sn58Bi(contained 58wt% Bi). Interfacial reaction rates of each solder increased in the order of SnAg(≒SnAgBi) and SnBi during reflow. On the other hand, $(Au, Ni)Sn_4$ IMC phase were observed in the bulk of Sn3.5Ag4.8Bi solders during aging. Interfacial reaction rates of each solder increased in the order of SnBi and SnAg(≒SnAgBi) during aging. These results will be discussed in the view of Bi contents.
Bump shear strength of each solder increased in the order of Sn3.5Ag4.8Bi, Sn58Bi, and Sn3.5Ag after reflow. On the other hand, bump shear strength of each solder increased in the order of SnAgBi, SnAg, and SnBi after aging Shear failures arose inside solder bumps in case of Sn3.5Ag, but at interfaces between solder and IMCs in case of Sn3.5Ag4.8Bi and Sn58Bi after reflow and aging. These differences of shear strength and shear modes will be discussed in the view of Bi contents and heat treatment.
