Metastable eutectic in Pb-free joints between Sn-3.5Ag and Ni-based substrates

Microstructure development in solder joints between Sn-3.5Ag and Ni-based substrates has been widely reported. However, in the present study we illustrate a new phenomenon: that during soldering of Sn-3.5Ag to Ni or ENIG (electroless nickel, immersion gold), the bulk solder solidifies to contain a metastable eutectic consisting of βSn+Ag3Sn+NiSn4 instead of the βSn+Ag3Sn+Ni3Sn4, expected of equilibrium solidification. It is shown that metastable NiSn4 coarsens and then decomposes into Ni3Sn4 and βSn during aging at 150 and 200 °C and that coarsened NiSn4 particles deteriorate impact shear properties.