The reliability and mechanical performance of electronic integrated circuit (IC) package is studied based on the finite element analysis. The flip chip on board (FCOB) package -one of the most current plastic encapsulated IC packages -consists of a die, solders, a printed circuit board and underfill resin encapsulated between the die and PCB, which is subjected to a typical thermal cycle experienced during the manufacturing process. The effects of viscoelastic properties of underfill resin on mechanical responses and residual stresses of FCOB packages are specifically studied and the results are compared with those obtained for a thermo-elastic underfill resin. The viscoelastic properties of underfill resin are characterized using the time-temperature superposition (TTS) technique. Both the rigid and flexible PCB substrates are considered.It is found that the elastic model underestimates significantly both the thermal stress and the accumulated creep strain in the solder joints, whereas it overestimates the stress concentrations in the underfill fillet around the die corner than the viscoelastic model. When the package is subjected to an extended dwell time at an elevated temperature, a moderate loss of mechanical coupling between the die and organic substrate is expected due to the relaxation of underfill resin. The implication is that the viscoelastic nature of underfill material should be properly taken into account if the failure of solder joints and the lifetime of the package are to be accurately estimated. This study shed an insight into the importance of correct material property inputs in the design and theoretical studies of plastic packages.

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