This thesis demonstrates the feasibility of power cycling a "Flip Chip" assembly for reliability assessment. The assemblies studied were Si on Si Multi-Chip Modules (MCMs) that were mounted on either an organic FR4 or a metallic (copper) substrate. The aim of the work was to investigate how anisothermal temperature distributions caused by local power inputs could influence the reliability of devices that would not be expected to be effected by thermal cycling. This work was performed using two complementary techniques: physically manufacturing assemblies in order to perform "real" power cycles, and utilising Finite Element Analysis (FEA) to perform "virtual" cycles. The MCMs consisted of "heater chips" into which electrical power could be dissipated to heat the device locally. These heater chips were flip-chip bonded to Si carrier chips by solder interconnections and the entire assembly was then mounted onto a substrate. The thermal performance of the MCMs as a result of power input was characterised under steady state and cycling conditions using a number of techniques including thermal imaging. In addition, many devices were power cycled to evaluate their reliability. In addition to the evaluation of real devices, a three dimensional finite element model was developed of the same structures. The model initially provided thermal data that was validated against that obtained from the real devices operating under the same environmental and power input conditions. In addition, it enabled the stress level within the solder joints to be evaluated so that insight to the long-term reliability of the assemblies could be gained. The results of the experimental and modelling work have shown that the thermal performance and reliability of the devices depend strongly on the substrate onto which the MCMs are bonded. It was found that, using a copper substrate, the temperatures reached within the assemblies were greatly reduced and that the reliability during power cycling was enhanced.