The influence of alloying elements on metastable NiSn4 in Sn-Ag solders on Ni-containing metallizations

Sn-Ag based solders are a popular Pb-free alternative in consumer electronics and have a relatively long history with a large body of dedicated research. Past research has primarily focused on the formation of interfacial intermetallic (IMC) layers and the growth of ßSn, and little attention has been paid to Ni-containing IMCs that form in the bulk solder when Sn-Ag alloys are soldered to Ni-containing metallizations. The present study illustrates a new phenomenon: formation of metastable NiSn₄ as a primary and a eutectic phase in solder joints between Sn-Ag solders and Ni-containing surface finishes. The metastable NiSn₄ was demonstrated to transform into ßSn and equilibrium Ni₃Sn₄ IMC phase during ageing at 150°C. Impact shear tests suggested a significant drop in solder joint mechanical response when larger than 2μm NiSn₄ crystals were present in the solder bulk. Further exploration of the influence of alloying elements on metastable NiSn₄ in solder joints yielded three scenarios: (i) Au, Pt, Pd and Co promoted NiSn₄ formation and increased its volume fraction by substituting for Ni atoms in NiSn₄; (ii) Bi, In, Fe and Pb additions appeared to have little or no discernible effect on NiSn₄ and (iii) Cu was found to promote equilibrium Ni₃Sn₄ phase formation that replaces metastable NiSn₄ at Cu levels above 0.3wt%.