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This paper describes results of the computer-analysis pottion of a research program which was conducted to study the thermal characteristics of microcircuits in high temperature environments. A special IC chip, bonded to an alumina chip carrier, was modeled for these simulations. It was found that thermal resistance values and thermal time constants nearly double when the chip carrier temperature is increased from 70 to 257°C. For a chip power dissipation of 1.5 W, the peak junction temperature increased from 138 to 385°C, an increase of 247°C, while the chip carrier only increased by 187°C. The thermal time constant of the junction peak temperature rise, measured relative to the chip carrier, increased from 15 to 26 µs over the same temperature range.