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Theoretical and experimental methods are used to predict the thermal properties of an air-cooled integrated-circuit package with a large LSl type of chip. A previously reported three-dimensional solution to the differential equation for steady-state heat transfer has been extended to include composite media with up to four layers of unequal thickness and thermal conductivity. The theoretical results are used to calculate the detailed thermal characteristics of a 1.3 X 0.6-in package with a 0.17-in square chip. Geometric and physical considerations include the effects of ceramic thickness and thermal conductivity, chip dimensions, lead conduction, and heat-sink variations. An empirical technique is used to determine the forced-convection heat-transfer coefficients for a variety of extruded aluminum heat sinks. The effects of fin length, air velocity, and number of fins per inch are measured.