|The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In this work, the effects of isothermal aging on solder mechanical behavior has been investigated using a carefully-designed test matrix of aging temperatures, aging times, and Sn-Ag-Cu (SAC) solder alloys. Stress-strain and creep tests have been performed on four “standard” SAC alloys (SAC105, SAC205, SAC305, SAC405) with varying silver content. For the alloys studied, five different aging conditions have been used including room temperature (25 °C), and four elevated temperatures (50, 75, 100, and 125 °C). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) have been observed as a function of aging. In addition, mathematical models have been developed to predict the variation of the stress-strain and creep properties with aging time, aging temperature, and SAC alloy composition. The existence of creep cross-over points between the SAC alloys and eutectic 63Sn-37Pb under various aging conditions has been investigated in detailed. In a second portion of this work, the mechanical behavior and physical properties of several mixed formulation solder alloys have been investigated. Seven mixed solders with different mixture ratios of 63Sn-37Pb and SAC305 solder materials have been formed. For the various percentage mixtures, the melting point, pasty range, stress-strain curves, mechanical properties and creep behavior have been characterized. The variations of the mechanical properties and creep rates with aging at room temperature (25 °C) and elevated temperature (100 °C) have also been measured. Finally, the microstructures of these mixtures have been examined and correlated to their mechanical properties. The results for the mechanical and physical properties show a very complicated nonlinear dependence on the mixture ratio.