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The test vehicle is the FCOB with the chip size of 4mm*4mm*0.52mm, 0.32mm pitch and an I/O array of 13*13. Different solder height and UBM height are considered. The analysis is performed by a 2-D plane strain finite element model using Ansys 10.0 software. The Sn-3.5Ag solders are examined for their reliability by accelerated thermal cycling test with temperatures ranging from -40°C to 125°C. Two different constitutive models namely state variable elastic-plastic-creep and viscoplastic analysis are used to simulate the machanical property of Sn-3.5Ag solder joints under temperature cycle. The life prediction is evaluated through the Darveaux energy model. The simulation results indicate that the outmost solder joint has the largest equivalent strain energy density. The equivalent inelastic (plastic+creep) strain energy density and viscoplastic strain energy density extracted from the elastic-plastic-creep and viscoplastic analysis results respectively, at the critical solder joint location, are used as a failure parameter for solder fatigue model employed. For the non-underfill FCOB assembly, the outermost solder joint has an inelastic strain energy density six~ten times higher than the inneer solder joint, for both the elastic-plastic-creep and viscoplastic analysis. While in underfilled FCOB assembly, the inelastic strain energy density at the outermost solder joint and inner solder joint are reduced by twenty times and two~three times, respectively, for both elastic-plastic-creep and viscoplastic analysis. The inelastic strain energy density decreases with the increase of solder height and UBM height. The introduction of underfill to the FCOB assembly couples and restricts the CTE mismatch between the silicon die and the FR4 board and increase the fatigue life.