Design of nanowire anisotropic conductive film for ultra-fine pitch flip chip interconnection

As the prediction that the I/O pitch will decrease from 60 um in 2004 to 20 um beyond 2010 by ITRS roadmap, flip chip interconnection by traditional ACF containing conductive particles with micro-meter size will face more and more challenges. One of many possible solutions is using high aspect-ratio metal posts or flake instead of conductive particles for electrical interconnection between chip and substrate. But this interconnection by metal posts is less reliable compared with elastic conductive particles. Therefore we develop a new type of conductive film composed of nanowires and polymer. Unlike some other composed material by blending nanowires, tubes, powders in polymer, the arrangement of nanowires in polymer is highly ordered in X, Y, and Z direction for anisotropic conductance. In order to achieve high reliability performance of this novel package, the structure design of flip chip package constructed by nanowires/polymer conductive film was evaluated by stress simulation and related D.O.E analysis. In this research, series of finite element models were established based on the D.O.E. (design of experiment) matrix. The four factors including thickness of nanowires/polymer composed film, volume ratio of nanowires in nanowires/polymer composed film, CTE and Young's modulus of polymer were used in this D.O.E. matrix. The full factorial DOE matrix was applied to optimize the response of peeling stress. These results indicated that volume ratio of nanowires was the major factor. The other important factor was film thickness. Besides the above stress analysis, we also demonstrated the production of nanowires/polymer composed film. Now we can obtain the silver nanowires/polyimide composed films with diameter of nanowire about 200nm and maximum film thickness up to 50 um. The X-Y insulation resistance is about 4~6 GOmega and Z-direction resistance including the trace resistance (3mm length) is less than 0.2Omega