We present a general criterion for determining the dynamic instability of a
thin polymer film that is strongly confined to a substrate. When such a po
lymer film is heated above its glass transition temperature, it dewets from
the substrate by a spinodal dewetting scenario, in which the onset of the
instability is governed by dispersion force and residual thermal stress. It
turns out that the thermal stress plays an important role when there is a
special interaction between a polymer and a substrate.