Interfacial reactions and electromigration in flip-chip solder joints

Soldering has been one of the most important assembly and interconnection technologies for electronic products since the beginning of the electronic age. As electronic devices become more complicated and smaller, the number of solder joints on each device increases while the size of these joints decreases-several chemical and physical processes become ever more threatening toward the small joint reliability. These processes include chemical reactions, metal dissolution, diffusion driven by chemical potential gradient, electromigration, Joule heating, thermomigration, and stress migration. There are two threatening issues arise from the combined effect of these process: (1) excessive intermetallic formation and (2) excessive under bump metallurgy (UBM) consumption. In this chapter, we would first discuss the chemical potential gradient-induced dissolution and chemical reaction kinetics between the substrate materials (e.g., Cu and Ni) and the Sn-based solders (e.g., eutectic Sn-Ag and Sn-Ag-Cu). Emerging reliability issues like (1) chemical interaction/cross-interaction between Cu and Ni; (2) effect of alloying with active elements on the reaction; and (3) effect of small solder volume on the solder joints are presented in the section. Next, electromigration issues like effects of electron current stressing on solder and UBM are highlighted. Lastly, mitigation strategy against electromigration is presented. © Springer Science+Business Media New York 2013