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The thermo-mechanical properties of the polyimide film used in chip-on-film (COF) packaging have a significant effect on the inner lead bonding (ILB) mechanism. Furthermore, specifying an appropriate bonding force is essential in establishing a suitable compromise between a good bonding strength in the ILB process and a suitable gap size for the subsequent resin potting process. This paper commences by performing a series of experimental micro-force tensile tests to investigate the stress-strain relations of two polyimide films, namely Kapton-EN and Espanex-M. The tests are performed using formfitting specimens from actual COF products at various temperatures ranging from 25degC to 200degC . Based on the experimental results, constitutive equations are developed to model the temperature-dependent stress-strain characteristics of the two films. A finite element model of the COF/system comprising the thermo-compression tool, the polyimide film, the copper lead and the gold bump is constructed and is used to simulate the ILB process under various bonding forces. A good agreement is found between the experimental and numerical results obtained for the deformed profile of a single polyimide/copper lead following the bonding process and the variation of the bump sinking value with the bonding force, respectively. The numerical results are used to construct COF/ILB parameter design charts for the Kapton-EN and Espanex-M polyimide films which enable suitable values of the bonding force to be selected for given gap size and bump sinking values. In general, the results show that Kapton-EN has a broader working range than Espanex-M as a result of its higher elastic stiffness. When performing the ILB process using Espanex-M polyimide film, it is necessary to carefully control the bonding force to ensure that the minimum potting gap size is maintained.
Date of Publication: Aug. 2009