Characteristics of intermetallics and micromechanical properties during thermal ageing of Sn–Ag–Cu flip-chip solder interconnects

Sn–3.8 wt.% Ag–0.7 wt.% Cu solder was applied to Al–1 wt.% Cu bond pads with an electroless nickel (Ni–P) interlayer as an under bump metallisation (UBM). The microstructure and micromechanical properties were studied after ageing at 80 °C and 150 °C. Two types of intermetallic compounds (IMCs) were identified by electron back-scattered diffraction (EBSD), these being a (Cu, Ni)6Sn5 formed at the solder–UBM interface and Ag3Sn in the bulk solder. The (Cu, Ni)6Sn5 layer grew very slowly during the ageing process, with no Kirkendall voids found by scanning electron microscopy (SEM) after ageing at 80 °C. Nano-indentation was used to analyse the mechanical properties of different phases in the solder. Both (Cu, Ni)6Sn5 and Ag3Sn were harder and more brittle than the β-Sn matrix of the Sn–Ag–Cu alloy. The branch-like morphology of the Ag3Sn IMC, especially at the solder–UBM interface, could ultimately be detrimental to the mechanical integrity of the solder when assembled in flip-chip joints.