Thermal performance of an MCM flip-chip assembly in liquid nitrogen

A complementary metal-oxide-semiconductor (CMOS) microprocessor module could operate faster and more efficiently in liquid nitrogen due to high heat removal, faster gate speeds, and increased material conductivity. A high-power flip-chip test vehicle was fabricated consisting of 12 0.40 cm/sup 2/ thermal test chips each containing a heater and thermal diodes bonded with conductive epoxy onto a Si substrate and packaged in a 1.5-in cavity ceramic flatpack. Under pool boiling conditions in liquid nitrogen with no lid, the thermal resistance was 0.15 K/W in the nucleate boiling regime up to a total power of 50 W where film boiling began. However, it is undesirable to have bare chips in any microelectronic equipment due to reliability considerations. A standard hermetic lid increased the overall thermal resistance to nearly 4 K/W due to the presence of a 10 mil gas gap between the lid and the flip-chips. This was unacceptably high for cryogenic operation. This thermal resistance was lowered by an order of magnitude with the help of thermally conductive interface materials to fill this gap and by the addition of a pin fin heat spreader to retard the onset of film boiling. In this configuration, the test vehicle could dissipate over 115 W (24 W/cm/sup 2/ on a chip area basis) with only a 50 K rise in temperature above the saturation temperature of liquid nitrogen. This level of thermal performance will enable a CMOS multichip module (MCM) processor module with a protective lid in place to operate at high power while taking advantage of the speed-enhancing characteristics of cryogenic operation.