|Industry is moving towards lower cost, increased functionality, and miniaturized electronics placed in ever closer proximity of higher operating temperatures, typical of harsh environments. In this thesis, three aspects of harsh environment thermo-mechanical reliability of fine-pitch electronics has been studied. (1) Effect of underfills on reliability enhancement of high I/O area-array packages on two board finishes including immersion silver, and Hot Air Solder Level under harsh thermo-mechanical environment. (2) Relative comparison of thermo-mechanical reliability of Sn3Ag0.5Cu, Sn3.5Ag, Sn1Ag0.5Cu, SAC-X, SAC-X-plus has been studied on chip-scale packages under harsh thermo-mechanical environment. (3) Comparison of thermo-mechanical reliability of five interconnect systems including Microperal SOL, Sn3Ag0.5Cu, 90Pb10Sn, 63Sn37Pb, and Copper Column. High-lead packages used in the study are the control group. In each case the reliability models have been developed to validate that the measured response of the experimental assemblies is consistent with the expected inherent trends. Failure analysis of the assemblies has been conducted to understand the failure mechanisms.