THE STABILITY WINDOW AND MECHANICAL PROPERTIES OF THE INTERFACIAL INTERMETALLIC IN LEAD-FREE SOLDER JOINTS

摘要

The trend to copper as the preferred UBM for Pb-free area array packages likely to be subjected to drop impact means that the behaviour and properties of the intermetallic that forms at the solder/copper interface are major determinants of joint reliability. The transformation of the Cu_6Sn_5 from the close packed hexagonal to the monoclinic form at 186°C that was previously overlooked has now been identified as a significant disruption to the integrity of the intermetallic layer that can reduce the impact resistance and long term reliability of the solder joint. There is evidence that the intermetallic formed from solders microalloyed with nickel does not undergo this transformation but the range of nickel levels and cooling rate over which the hexagonal close packed form of Cu_6Sn_5 can be retained has not been know. And the stability of that phase in long term service at elevated temperature was uncertain. The fact that the hexagonal close packed crystal has many more slip planes and directions available for deformation than does a monoclinic crystal would be expected to have implications for the toughness and compliance of the solder joint of which the intermetallic layer is a part. But to the extent that the nickel in substitutional solid solution might affect interplanar spacing and dislocation mobility those mechanical properties might be expected to vary with the level and distribution of the nickel in the hexagonal close packed Cu_6Sn_5. In this paper the authors will report new studies on the effect of the nickel level over a range from 0 - 9% in the Cu_6Sn_5 that grows at the solder/copper interface, Cu_6Sn_5 that precipitates from the melt during solidification and Cu_6Sn_5 made by mixing the constituent metals in the appropriate stoichiometric ratio. The distribution of the Ni in the Cu_6Sn_5 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. On the basis of the data collected in this study it should be possible to prepare some guidelines on Pb-free alloy formulation for area array attachment.