Effect of multiple reflows on mechanical strength of the interface formed between Sn-Zn-Bi solder and Au/Ni/Cu bond pad

In this work, the shear strengths and interfacial reactions of Sn–8Zn–3Bi andSn–8Zn–1Bi (wt%) solders with Au/Ni/Cu ball grid array (BGA) pad metallizationwere systematically investigated after multiple reflows. The peak reflow temperaturewas fixed at 230 °C. After the shear test, fracture surfaces were investigated using ascanning electron microscope equipped with an energy dispersive x-ray spectrometer.Cross-sectional studies of the interfaces were also conducted to correlate with thefracture surfaces. Two failure modes, ball cut and pad lift, were assessed for thedifferent solders and reflow cycles. It was found that the shearing forces of both theSn–Zn–Bi solder joints tended to increase slightly with an increase in the number ofreflow cycles due to augmentation of the shearing area. A layer-type spalling of theinterfacial intermetallic compounds (IMCs) was observed very early in the liquid-statereaction for the solder alloys. The active nature of the Zn confirmed an instant reactionzone at the interface to maintain the bonding between the solder and the substrate.