NiSn4 in solder joints between Sn-3.5Ag and Ni, ENIG or ENEPIG

Sn-3.5Ag solder is commonly used with ENIG (electroless nickel, immersion gold) and ENEPIG (electroless nickel, electroless palladium, immersion gold) platings. Most research has focused on the interfacial reactions with the substrate and the microstructure development. In the present study we describe a new phenomenon that has been missed in past research: the formation of metastable eutectic βSn+Ag₃Sn+NiSn₄ instead of the equilibrium βSn+Ag₃Sn+Ni₃Sn₄ eutectic during solder bulk solidification in Sn-3.5Ag/Ni, Sn-3.5Ag/ENIG and Sn-3.5Ag/ENEPIG joints. It is shown that metastable NiSn₄ transforms during ageing at 150°C and that the transformation occurs in two stages (i) coarsening of NiSn₄ eutectic sheets into tile-like crystals and (ii) decomposition of the coarsened NiSn₄ crystals into Ni₃Sn₄ and βSn. It is further shown that Pd additions of ~2at% can stabilize NiSn₄, preventing decomposition into Ni₃Sn₄. However, Pd from the ENEPIG substrate also causes a relatively large volume fraction of NiSn₄ to form which coarsens rapidly at 150°C.