Conduction Cooling for an LSI Package: A One-Dimensional Approach

The introduction of LSI packaging has significantly increased the number of circuits per silicon chip, and at the same time has greatly increased their heat flux density. In comparison to earlier MST (monolithic systems technology) products, the heat flux which must be removed from the new multi-chip substrates (100 or more chips) has increased by an order of magnitude or more. This paper discusses an innovative conduction-cooling approach using He gas encapsulation which has been developed in response to the new LSI technology requirements. Background is provided on the liquid-encapsulated-module technology which preceded the new approach, and the basic challenges encountered in building a thermal bridge from individual chips to the module and cold plate are described. The underlying theory of operation is presented using one-dimensional mathematical and discrete analog models. The effects of various factors such as geometry, chip tilt, He concentration, air leakage, and materials are illustrated using these models. A thermal sensitivity analysis is performed to determine variations in junction temperatures and the contributions of the major parameters. The companion paper by Oktay and Kammerer which follows this one treats the more general “multi-dimensional” approach using numerical analysis techniques.