The IBM ES9000 system family of computers utilizes advanced multilayer packaging technology and material sets, combining polyimide copper thin film technology with glass ceramic substrate to provide high wiring density with excellent electrical properties. The author describes a series of simulations done using finite element analysis to calculate the thermally induced stresses in the structure and to develop design guidelines for the mechanical integrity of the multilayer system. Using a simple thin film redistribution structure, stresses in a typical polyimide film are calculated as a function of film thickness and the diameter of the copper via. It is shown that, for copper via geometries, the aspect ratio should be less than 0.5. For a reliable polyimide-glass-ceramic interface, the film thickness has to be less than 50 mu m, but by using low expansion polyimides the total film thickness (hence the number of layers) can be increased to twice as much. Tapering of via walls reduces stress levels by about 10 to 15% near discontinuity. A comparison of feature sizes in multilevel thin film hybrid modules from the literature shows excellent agreement with calculations of via aspect ratio.
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