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 Ni12P5 + Ni3P mixed upper layer and a Ni3P lower layer. No ternary Ni-Sn-P layer was observed in the initial stage. After subsequent reflow for 60 s, a ternary Ni2SnP layer (containing a small amount of the Ni3P phase) was formed between the Ni3Sn4 and P-rich Ni layers (Ni3P + Ni12P5 + Ni).