Residual stresses during cool down process after curing are evaluated on the basis of thermoviscoelastic finite element analyses with material properties dependent on both time and temperature. Temperature distributions in packages are predicted by solving the heat conduction equations. The effects of cool-down history on thermomechanical responses of a lead-on-chip (LOC), thin small outline package (TSOP) are investigated. Three linear cool down temperature histories are considered. Numerical results show that residual stresses in a LOC TSOP are significantly influenced by the manufacturing temperature history. Residual stresses in plastic packages are strongly time-temperature dependent due to the thermoviscoelastic behavior of molding compounds. Substantial residual stresses arise when a LOC TSOP has cooled to room temperature and rapid cool down permits small viscoelastic effects.