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

This paper presents an 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 pitches are 150 μm for rigid FR4 substrates and 100 μm for flex substrates. Isotropic conductive adhesives are promising candidates for adhesive joining. As they conduct electricity equally in all directions, the material must be applied precisely to the points to be connected, and not allowed to flow and short circuit between circuit lines. Anisotropic conductive adhesive materials are prepared by dispersing electrically conductive particles in an adhesive matrix. The concentration assures reliable conductivity between substrate and IC electrodes but insulation between adjacent bumps. Several adhesives, both commercial and experimental products, were investigated. A detailed thermo-mechanical analysis was used to characterize the materials according to their physical properties. This analysis method was also used to optimize the curing profile, i.e. to shorten curing time. The reliability evaluation was performed with special regard to the degradation and to 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. Reliability test data was correlated with the thermo-mechanical properties. We thus derived simple criteria for adhesive selection for flip chip applications