In this paper, the creep behavior of a flip-chip package under a thermal load was investigated by using nonlinear finite element technique coupled with high density laser moire´ interferometry. The real-time moire´ interferometry technique was used to monitor and measure the time-dependent deformation of flip-chip packages during the test, while the finite element method was adapted to analyze the variation of stresses at edges and corners of interfaces with time by considering the viscoelastic properties of the underfill and the viscoplastic behavior of the solder balls. The results show that the creep behavior of the underfill and the solder balls does not have significant effect on the warpage of the flip-chip under the considered thermal load due to their constrained small volume. The variation of the time-dependent deformation in the flip-chip package caused by the creep behavior of the underfill and the solder balls is in the submicro scale. The maximum steady-state U-displacement is only reduced by up to 6.7 percent compared with the maximum initial state U-displacement. Likewise, the maximum steady-state V-displacement is merely reduced by up to 10 percent compared with the maximum initial state V-displacement. The creep behavior slightly weakens the warpage situation of the flip-chip package. However, the modeling results show that the localized stresses at corners and edges of interfaces greatly decrease due to the consideration of viscoelastic properties of the underfill and the viscoplastic properties of the solder balls, and, thereby, effectively preventing interfaces from cracking. In addition, the predicted deformation values of the flip-chip package obtained from the finite element analysis were compared with the test data obtained from the laser moire´ interferometry technique. It is shown that the deformation values of the flip-chip package predicted from the finite element analysis are in a fair agreement with those obtained from the test.
