Phase transformation of the phosphorus-rich layer in SnAgCu/Ni–P solder joints

The interfacial morphologies for the Sn–3Ag–0.5Cu/Ni–P joint were investigated using a field emission electron probe microanalyzer (FE-EPMA). As the Ni–Sn–P formed, several layers of P-rich layers including Ni3P, Ni12P5, and Ni2P were observed. The relationship between Ni–Sn–P formation and the evolution of P-rich layers was quantitatively analysed by FE-EPMA and by using the phase diagram of binary Ni–P. It was also proposed that once the Ni2P phase was formed, Sn atoms diffused from the solder matrix into Ni2P. As a result, the original Ni2P transformed to Ni2SnP due to the in-diffusion of Sn.