Characterization of interfacial reaction layers formed between Sn-3.5Ag solder and electroless Ni-immersion Au-plated Cu substrates

The interfacial reaction between a eutectic Sn-3.5wt.%Ag solder and an electroless nickel-immersion gold-plated Cu substrate during reflow was examined by transmission electron microscopy (TEM). During the initial reflowing, the amorphous, electroless Ni (P)-plated layer crystallized into two P-rich Ni layers: a Ni ₁₂P ₅ + Ni ₃P mixed upper layer and a Ni ₃P lower layer. No ternary Ni-Sn-P layer was observed in the initial stage. After subsequent reflow for 60 s, a ternary Ni ₂SnP layer (containing a small amount of the Ni ₃P phase) was formed between the Ni ₃Sn ₄ and P-rich Ni layers (Ni ₃P + Ni ₁₂P ₅ + Ni).