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. •A metastable eutectic consisting of βSn+Ag3Sn+NiSn4 was found to grow during soldering of Sn–3.5Ag to Ni or ENIG substrates.•NiSn4 is found to decompose into Ni3Sn4 and βSn during ageing.•Coarsened NiSn4 was found on the fracture surface after impact shear testing.