Metastable NiSn4 in Sn-3.5Ag/ENIG joints

Inhalt:
Sn-Ag solders are a popular Pb-free choice in power electronics. While significant research has focused on the formation and growth of an interfacial Ni3Sn4 intermetallic layer, little work has considered the Ni-based intermetallics that form in the bulk solder during soldering of Sn-Ag alloys to Ni-based substrates. In the present study we demonstrate an important phenomenon that have been missed in past research: the formation of metastable Sn-Ag3Sn- NiSn4 eutectic throughout the solder bulk when Sn-3.5Ag is soldered to ENIG (electroless nickel, immersion gold) or Ni surface finishes. It is shown that the metastable NiSn4 eutectic phase rapidly coarsens and transforms into Ni3Sn4 and betaSn during ageing at 150deg C. Primary NiSn4 crystals or coarsened NiSn4 eutectic particles can significantly compromise solder joint reliability as demonstrated by impact shear tests. In addition to its metastable nature, NiSn4 is a brittle intermetallic compound with a weak interface with the betaSn matrix that serves as the path for crack propagation during shear tests. It is further shown that Cu additions to Sn-3.5Ag solder promote equilibrium Sn-Ag3Sn-Ni3Sn4 eutectic growth.