The assemblies PoP (Package on Package) can greatly increase the integration density of microelectronic circuits and systems, by vertically combining discrete semiconductor elements. The interconnections of these systems suffer the stresses never reached before. We were able to identify, characterize, model and simulate the potential failure mechanisms of these assemblies and their evolution: * The warpage in the assembly phase and thermomechanical stress of "PoP" are more serious than the individual components. An original analytical model has been built and put online for pre-estimating this warpage. * The hygroscopic and hygromechanical behaviors are simulated and measured by an original method. The assembly "PoP" absorbs more moisture than the sum of the individual components, but its hygromechanical warpage and stress are smaller. * Two types of accelerated aging tests are performed to study the reliability of "PoP" at the board level: the thermal cycling and the testing under current and temperature. In both types of tests, assembly a component "top" on another component "bottom" to form a "PoP" increases the risk of failure. * The microstructure evolution depending on the type of aging is compared by the physical and physico-chemical analysis. The cracks are always located in the interface substrate/balls, which corresponds to the critical areas predicted by the simulations.
