Correlation of thermo-mechanical properties of adhesives with reliability of FC interconnections

This report presents the results of the evaluation of isotropic and anisotropic conductive adhesives for flip chip applications. Samples consist of bumped test chips mounted on fine pitch rigid and flexible substrates. The finest pitch of the rigid FR4 substrates is 150 mu m and for the flexible substrates 100 mu m. Promising candidate for adhesive joining technique are the isotropic conductive adhesives. As they conduct electricity equally in all directions, the material has to be applied precisely onto the points to be connected, and is not allowed to now and short circuit between circuit lines. The anisotropic conductive adhesive materials are prepared by dispersing electrically conductive particles in an adhesive matrix. The concentration assures reliable conductivity between the substrate and the IC electrodes but insulation between adjacent bumps. Several adhesives both commercial and experimental products - 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 curing time. The reliability evaluation was performed with special regard to the degradation and to the interface reactions between polymers and metal surfaces in adhesive contacts. 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. Data obtained from reliability testing were correlated with investigated thermo-mechanical properties. Thus we found some simple criteria for adhesive selection for flip chip applications.