Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

Abstract

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn–37Pb and Sn–3.0Ag–0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni 3Sn 4 intermetallic compound (IMC) layer formed at the Sn–37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn–3.0Ag–0.5Cu solder, (Cu,Ni) 6Sn 5 and (Ni,Cu) 3Sn 4 IMCs were formed at the interface. The shear force of the Pb-free Sn–3.0Ag–0.5Cu flip chip solder bump was higher than that of the conventional Sn–37Pb flip chip solder bump.