The more refined model proposed here and its experimental verification provides a consistent picture of the complex mechanism for strain relief and defect propagation in Si/SiGe/Si heteroepitaxial stacks used in HBT technology. We have identified and quantified the relevant phenomena to predict the coherency and relaxation behavior of more complicated heteroepitaxial structures and can precisely predict the equilibrium critical thickness for a defect-free Si capped SiGe epilayer on Si substrate. The results allow us to optimize the device design for high con strained layer configurations and to determine the latitude in process margin.