A simple method has been developed to estimate cracking of plastic packages during reflow soldering process. A criterion for estimating package cracking has been introduced by comparing the mold resin fracture toughness and the maximum value of the stress intensity factor in the package. The criterion equation for package cracking was made to an empirical form. The real-time measurement of package deformation is effective for providing information on the difference of the fracture mode. The difference of the delamination in the package causes the different shape of the deformation curve. The absorption characteristic of the package obeys Fick's law even for thinner packages.