The effects of printed circuit board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between Small Outline J (SOJ) leads and Sn-37Pb solder were investigated. The thickness of the IMC layer formed initially at as-soldered SOJ/Sn-Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. The parabolic TMF cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of total IMC layer at SOJ/Sn-Pb interface over Ni/Au PCB surface finish is the same as that over OSP PCB surface finish and, thus, the total IMC layer at the SOJ/Sn-Pb interface over Ni/Au PCB surface finish is thicker than that over OSP PCB surface finish. Using the Cu-Ni-Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the CU3Sn layer while increasing the initial growth of the CU6Sn5 layer can be addressed. (C) 2004 Elsevier B.V. All rights reserved.