The consumption of electroless Ni-P and its effect on the failure mechanism of solder joints duringelectromigration under a current density of 1.0×10⁴ A/cm² at both 150 and 200℃ wereinvestigated using line-type Cu/Sn/Ni-P solder joints. Before the electroless Ni-P was completely consumed,the microstructural evolution of the Sn/Ni-P interface (cathode) was the formation of Ni₂SnP and Ni₃P accompanied by the consumption of the electroless Ni-P. Ni atoms diffused from the electroless Ni--P into the Sn solder under electron current stressing. Most Ni atoms precipitated as (Cu, Ni)₆Sn₅ or (Ni, Cu)₃Sn₄ in the Sn solder, and few Ni atoms could arrive at the opposite Cu/Sn interface (anode). After the electroless Ni-P was completely consumed, the microstructural evolutions of the Sn/Ni-P interface (cathode) were the formation of voids and the transformation from Ni₃P to Ni₂SnP. Furthermore, cracks that resulted from the propagation of voids significantly increased the current density through solder joints, and thus greatly enhanced the Joule heating of solder joints, resulting in the failure of solder joints by the fusing of Sn solder.