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In this paper, a representative material cell containing a single microvoid is used to investigate void growth under combined vapor pressure and thermal stress. The plastic IC packaging material is assumed to be transversely isotropic and its strain energy includes a power-law reinforcing model has been analyzed. Using the theory of cavity formation and unstable void growth in incompressible hyper-elastic material, we gained an analytical relation between the applied traction (moisture-induced vapor pressure and thermal stress) and void volume fraction in forementioned materials. Numerical analysis is given of such polymers electronic packaging materials occurred "popcorn" critical load burst. Numerical results indicate that the critical stress for unstable void growth is very sensitive to the initial porosity. Through numerical simulation, we also obtained the critical stress is related to the hardening exponent n and the strength of the fiber reinforcement parameter alpha.