Reliability study of flip chip on FR4 interconnections with ACA

This report presents the results of the evaluation of anisotropic conductive adhesives for flip chip applications. Samples consist of bumped test chips mounted on rigid substrates with pitches down to 150 mu m. The dies were prepared with different bumps - electroless Ni, mechanical and electroplated Au. Several commercially available adhesives were selected to be investigated in this study. A detailed thermo-mechanical analysis was used to characterize the materials according to their physical properties. This kind of analysis method has also been used to optimize the curing profile, i.e. to shorten the cure time. The influence of process parameters like pre-baking, cure temperature and heating ramps on the properties and appearance of the adhesive layer has been investigated. We found also a strong influence of the bump metallurgy and shape onto the reliability of the electrical interconnection. The reliability evaluation was performed with special regard to the degradation of the adhesives and weakening of the adhesive joints. The electrical and mechanical performance of the adhesive bonds were studied by evaluating initial contact resistance and mechanical adhesion as a function of temperature and humidity. The reliability results of Flip Chip interconnections on FR4 were compared to those on flexible substrates obtained in former studies.