Underbump metallurgy

Zhang et al. reported the reconfiguration of AuSn4 in the bulk solder to a more thermodynamically stable Cu-Sn-Au phase at interfacial intermetallic compound formations after aging at 150°C. The conversion of CugSns to a Cu-Sn-Au phase was followed by the formation of a Cu-Sn-Au-Ni phase with continued aging and the addition of Ni from the underbump metallurgy (UBM). A faster rate of reaction was reported in Sn-Pb solders compared to Sn-Ag-Cu solders with either 0.7 or 1.2 wt.% Au content [16]. 

FUp-chip attachment with Pb-free solders is shown to present unique assembly and reliability issues pertaining to the underbump metallurgy. Miniaturization trends in the electronic industry continue with ever-smaller interconnection schemes. The implications of compositional changes have been demonstrated to have associated reliability consequences. 

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