In this study, a deep analysis of the reliability of lead-free (SnAgCu) solders in comparison with tin-lead solders have been done for a particular aerospace application. Creep properties of this solder composition have been investigated, and results have been validated with a thermal shock test. An 0805 resistor has been considered to simulate creep deformation, an electronic board has been used for a thermal shock test to check continuity of the circuit for the specified mission, and finally a detailed metallographic study has been performed. Main constitutive relations have been implemented in a commercial finite-element analysis software (ABAQUS 6.5), to predict creep strain accumulation under thermal loads. Effects of implementing different constitutive relations in compare with life prediction models have been investigated. Overall, both solders are highly reliable with this number of thermal cycles; however, SnAgCu shows higher life time under this type of loading.