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A deep analysis of the reliability of lead-free (SnAgCu) solders in comparisonwith tin-lead solders have been done for a particular aerospace application,SwissCube's motherboard (MB). Since 2006, using Pb in consumer electronicshas been banned. Aerospace applications are not included in this legislation,however, sooner or later aerospace industry should comply with the generaltrend in the electronic industry because it is dependent on the whole indus-try for its components. Searching for a lead-free solder which presents high-reliability properties is still continued. SnAgCu solders have become popular inthe electronics industry, because of the price and their mechanical properties.The creep properties of this solder composition, as the most important deforma-tion mechanism under thermal loads, has been investigated in this thesis, andresults have been validated with a thermal shock test. To simulate creep de-formation in 2-dimension, one small electronic package, resistor, of SwissCube'smotherboard was selected. Main constitutive relations have been implementedin a nite-element analysis software, to calculate creep strain evolution underthermal loads. The results of simulation suggest using, the new kind of solder, SnAgCu, would be even better for this type of package. For 3-dimensionalanalysis, one of the largest packages of the SwissCube's motherboard was simulated, using a conservative assumption in terms of constitutive relation from 2-dimensional analysis. The risk of failure for this package is not high, and itwas not predicted to see a total failure after one thousand test cycle. Again,both solders show a good resistance to creep deformation, although SnAgCushows a higher number of life cycles to failure from simulation results. Finally,a thermal shock test was performed to evaluate the reliability of solder joints practically. The test was performed for one thousand cycles. Temperature andoutput current were monitored. No cut in voltage was seen, while temperaturefollowed a very close trend to thermal simulation result. Microstructural analysis has also been performed using metallography process and optical microscopy.Results of this investigation also shows that both solders are safe to be used upto one thousand cycles. Overall, both solders are highly reliable with this number of thermal cycles, however, SnAgCu shows higher life time under this typeof loading.