The morphological and compositional evolutions of intermetallic compounds (IMCs) formed at three Pb-free solder/electroless Ni-P interface were investigated with respect to the solder compositions and reflow times. The three Pb-free solder alloys were Sn3.5Ag, Sn3.5Ag0.75Cu, and Sn3Ag6Bi2In (in wt.%). After reflow reaction, three distinctive layers, Ni₃Sn₄ (or Ni-Cu-Sn for Sn3.5Ag0.75Cu solder), NiSnP, and Ni₃P, were formed on the electroless Ni-P layer in all the solder alloys. For the Sn3.5Ag0.75Cu solder, with increasing reflow time, the interfacial intermetallics switched from (Cu,Ni)₆Sn₅ to (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄, and then to (Ni,Cu)₃Sn₄ IMCs. The degree of IMC spalling for the Sn3.5Ag0.75Cu solder joint was more than that of other solders. In the cases of the Sn3.5Ag and Sn3Ag6Bi2In solder joints, the growth rate of the Ni₃P layer was similar because these two type solder joints had a similar interfacial reaction. On the other hand, for the Sn3.5Ag0.75Cu solder, the thickness of the Ni₃P and Ni-Sn-P layers depended on the degree of IMC spalling. Also, the shear strength showed various characteristics depending on the solder alloys and reflow times. The fractures mainly occurred at the interfaces of Ni₃Sn₄/Ni-Sn-P and solder/Ni₃Sn₄.