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+Ag
₃
Sn+NiSn
₄
instead of the equilibrium βSn+Ag
₃
Sn+Ni
₃
Sn
₄
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 NiSn
₄
transforms during ageing at 150°C and that the transformation occurs in two stages (i) coarsening of NiSn
₄
eutectic sheets into tile-like crystals and (ii) decomposition of the coarsened NiSn
₄
crystals into Ni
₃
Sn
₄
and βSn. It is further shown that Pd additions of ~2at% can stabilize NiSn
₄
, preventing decomposition into Ni
₃
Sn
₄
. However, Pd from the ENEPIG substrate also causes a relatively large volume fraction of NiSn
₄
to form which coarsens rapidly at 150°C.