Effect of reflow numbers on the interfacial reaction and shear strength of flip chip solder joints

The effects of multiple reflows on the microstructural variation and joint strength of the flip chip solder joints were investigated. Conventional Sn–37Pb and a representative Pb-free Sn–3.0Ag–0.5Cu were used, and a popular low cost under bump metallurgy (UBM) of immersion Au/electroless Ni–P plating was employed. In case of Sn–37Pb, only Ni3Sn4 intermetallic compound layer was formed during multiple reflows. In case of Sn–3.0Ag–0.5Cu, (Cu,Ni)6Sn5 intermetallic compound layer was observed after a single reflow, while (Ni,Cu)3Sn4 layer was additionally settled at the interface between the (Cu,Ni)6Sn5 layer and Ni–P UBM. The thicknesses of the intermetallic compound layers increased and coarsened. The shear force of the both joints slightly decreased during the multiple reflows. The decrease of the joint strength was caused by different softening phenomena between the two cases, and that is discussed in the main text.