The trend to copper UBM for lead-free area-array packages subjected to drop impact means that the properties of the intermetallic that forms at the solder/copper interface are major determinants of joint reliability. The transformation of the Cu6Sn5 from the close packed hexagonal η to the monoclinic η’ at 186°C can reduce the impact resistance and long-term reliability of the solder joint.
In this presentation, the authors report new studies on the effect of Ni over the range 0-9 percent on the stabilization of the η phase. The distribution of Ni in the Cu6Sn5 and its crystal structure, lattice dimensions and thermal stability have been studied using SEM/EDS, TEM/EDS, synchrotron micro-XRF mapping techniques, synchrotron XRD, DSC and dilatometry. The mechanical properties of the intermetallic as a function of nickel content have been measured by nanoindentation. The data collected provides a basis for lead-free alloy formulation for area-array attachment.
