The aim of the paper is to present the estimation of thermal time constants of various substrates of integrated circuits. The investigation leads to unexpected results, which show that materials having good thermal conductivity are recognized as not so good in aspect of thermal dynamics. The theoretical procedure carried out step-by-step leads to unsteady state temperature computation and thermal time constants calculation. It can be helpful for determination whether a chip is able to overcome so big deal of energy dissipated in expected short period of time. The comparison of thermal time constants of substrates or heat sinks made of various materials is included.