This paper focuses on the thermal fatigue reliability of fine pitch flip-chip BGA packages employing multi-layer organic built-up substrates with an integral heat spreader. For high density FPGA devices, the die size and package footprint are larger than the prevailing industry standard, this poses a significant challenge for meeting component level temperature cycle condition B (-55/spl deg/C to +125/spl deg/C) reliability requirements. FEM is a key component for identifying high stress regions and identifying solutions to long term reliability problems. FEM was used to study two different material combinations where the heat spreader/stiffener attach adhesive and thermal interface materials (TIM) were changed. Temperature cycle tests were performed on both material sets. A secondary goal of this study was to evaluate the effects of the substrate land pad geometry and moisture preconditioning. The temperature cycle test lots were built with both solder mask defined (SMD) and non-solder mask defined (NSMD) pads and the test lots were subjected to two different levels of moisture preconditioning (JEDEC levels 3 and 4). Temperature cycle reliability results have been presented and substantiated by the FEM prediction and observed failure modes.