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

摘要

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