The feasibility of the Al/Mo/Ni/(Pb-Sn) as a solder bump system on a silicon wafer was investigated in terms of metals interdiffusion. The interdiffusion study was conducted at 150 degrees C for up to 900 h. The solder bump structure investigated comprises a 300 or 600 nm Mo film with a fixed thickness of 500 and 200 nm for the Al and Ni films, respectively. The results of Auger electron spectroscopy depth profile and scanning electron microscopy elemental analysis revealed that Mo is valid for retarding the interdiffusion between Al and Pb-Sn. Mo may diffuse outward to the Pb-Sn solder, while Al never penetrates the Mo layer. The interdiffusion occurrences are interpreted in terms of thermodynamical behaviour.