Phase reaction in Sn-9Zn solder with Ni/Au surface finish bond-pad at 175 deg C ageing

摘要

gamma-Au_3Zn_7 and gamma_2-AuZn_3 formed as band-shaped phases in solder after reflow process. During 175 deg C ageing, gamma and gamma_ transformed to gamma_3-AuZn_4 and finally (Zn) phase precipitated out next to e-AuZn_8. Ni_5Zn_(21) formed in the bond-pad interface. There were (Zn)-free zones near intermetallic compound (IMC) regions after certain ageing period. Ni could diffuse through the (Zn)-free zone and react with Au-Zn IMC and formed a new (Ni, Au)Zn_4 ternary phase. The rest part of Au-Zn IMC such as gamma_3-AuZn_4 transferred toward Au-rich IMC as gamma_2-AuZn_3 and then gamma-Au_3Zn_7 phase. According to the results of this study, the existence of reflowed eutectic (Zn) phase is the key factor to maintain the different phase equilibrium in the Sn-Zn solder.