Phase transformation of the Ni₃P phase at the interface between Sn and the Ni₃P/Ni substrate

The interfacial reaction between the Sn and stable Ni₃P phase layers was examined via scanning electron microscopy and transmission electron microscopy. Stacked samples (Sn and Ni₃P/Ni substrate) were heattreated at 300 °C for 30 min, in order to observe the phase transformation of the stable Ni₃P phase during the soldering process. The examination revealed that Ni atoms diffuse into the solder from the stable Ni₃P phase, thereby resulting in the transformation of the Ni₃P phase to the Ni₂P phase. Moreover, the subsequent diffusion of Sn atoms into the Ni₂P phase resulted in the formation of the ternary Ni₂SnP phase. This study confirmed that the ternary Ni₂SnP phase, which formed in the uppermost region of the Ni₂P layer, resulted from a reaction between the Ni₂P phase and the internally diffused Sn.