Abstract: A finite element model of high density flip chip packages was developed,in which multi-linear kinematic hardening material model and viscoplastic material model were used to describe the material behavior of the underfill and Pb-free solder paste. The stress-strain of solder bump was simulated to investigate its stability and the results were validated by experimental data. The results show that,the maximum equivalent stress appears at the outmost solder bump which can be considered as the most easy to be failure solder bump. Based on this solder bump,the maximum plastic shear strain appears at the chip side and it decreases from the outside to the inside which demonstrates a crack may initiate from the edge of the solder bump and propagate to the inside. This conclusion is also validated by experimental results. Besides,the failure mechanism was investigated by analyzing the behavior of plastic strain of the solder bump corner.