Effects of the Ni(P) plating thickness on microstructure evolution of interfacial IMCs in Sn–58Bi/Ni(P)/Cu solder joints

Effects of the Ni(P) plating thickness on interfacial reaction in the Sn–58Bi/Ni(P)/Cu joint system were revealed. It was found that the interfacial reaction was significantly influenced by the thickness of Ni(P) plating, and 0.1 μm Ni(P) plating completely transformed into Ni₂SnP layer after soldering. This Ni₂SnP layer not only provided a large number of diffusion channels but also reduced the solder joint reliability, demonstrating that 0.1 μm Ni(P) plating was not efficient in inhibiting the diffusion process between solder and substrate. However, the Ni(P) plating with thickness more than 0.5 μm could effectively inhibit atomic diffusion, and the Sn–Ni interaction would dominate the interfacial reaction instead of Cu–Sn phases. Although the Ni(P) plating with thickness of 0.5 μm partly transformed into Ni₂SnP layer, the growth rate of compound layer was suppressed. In addition, the Ni₃Sn₄ would transform into (Cu,Ni)₆Sn₅ since Ni₂SnP layer provided channels for Cu diffusing toward the solder/Ni₃Sn₄ interface. The Ni(P) plating with thickness of 1.5 μm remained integrated even after aging for 240 h, nonetheless, the excessive thickness of this barrier was unnecessary. Thus, it could be concluded that the appropriate thickness of Ni(P) plating should be controlled at 0.5–1.5 μm.