The delamination at the epoxy/copper interface adversely affects the reliability of IC packages and this is a common failure mode during the qualification process. One of the factors governing the interfacial delamination is the moisture content at the interface. This study has developed an experimental measurement procedure for interfacial moisture diffusion using the Fourier Transform Infrared-Multiple Internal Reflection (FT-IR-MIR) technique with a new calibration method. In this study, interfacial moisture was detected by FT-IR-MIR on an isothermal gloptop epoxy system at selected locations on the epoxy/copper interface of samples which went through 1000 h of 85% relative humidity at 85 degrees C pre-conditioning. By comparing the FT-IR-MIR results for epoxy samples with different pre-conditioning times, the interfacial moisture contents at different positions in the epoxy sample were obtained and a comparison was made. This study has demonstrated that the seepage mechanism along the epoxy/copper interface is the major driver for interfacial delamination under moisture pre-conditioning. It is the prevailing mechanism as compared to the established bulk diffusion mechanism in the epoxy molding compound. Surface analyses conducted on fracture surfaces of button shear test specimens show that interfacial moisture diffusion affects the cuprous oxide content in the epoxy/copper interfacial region and this leads to adhesion degradation. (C) Koninklijke Brill NV, Leiden, 2009