Flip chip reliability was evaluated using thermal stress tests at 150 degreesC. Electrical failures of flip chip devices were found to occur at the solder/under-bump-metallization interface by forming a porous amorphous chromium layer. The formation of the porous amorphous layer responsible for electrical failures resulted from the outdiffusion of copper atoms from a copper-chromium co-deposit, used as one of the under-burrip-metallization layers. A strong interaction of Cu with the Sn component of the solder is the driving force of the Cu outdiffusion. (C) 2003 Elsevier Science B.V. All rights reserved.