With greater integration of VLSI circuits, power consumption and power density have increased dramatically resulting in high chip temperatures and presenting a heat removal challenge. To effectively limit the high temperature inside a chip, thermal specific approaches, besides low power techniques, are necessary at the chip design level. In this work, we investigate the power and thermal management of System-on- Chips (SoCs). Thermal analysis is performed in a SPICE simulation approach based on the electrical-thermal analogy. We investigate the impact of inter- connects on heat distribution in the substrate and present a way to consider temperature dependent signal delay in global wires at early design stages. With the aim of reducing high local power density in hotspots, we propose two placement techniques to spread hot cells over a larger area. The proposed methods are compared in terms of temperature reduction, timing and area overhead to the general method, which enlarges the circuit area uniformly. A case study analyzes the design of Floating Point Units (FPU) from an energy and a thermal perspective. For the division operation, we compare different implementations and illustrate the impact of power efficient dividers on the energy consumption and thermal distribution within the FPU and the on-chip cache. We also characterize the temperature dependent static dissipation to evaluate the reduction in leakage obtained from the decrease in temperature.