Temperature profiles for MCM-D flip chip assemblies at cryogenic conditions

A high-power flip chip thermal test vehicle was fabricated and powered up in liquid nitrogen under saturated pool boiling conditions and in an unsaturated cryo chamber held at 90 K. The test vehicle had 12 1/4"×1/4" thermal test chips bonded with conductive epoxy onto to a Si substrate and packaged in a ceramic flatpack. Under pool boiling conditions, this test vehicle generated over 60 W with a chip thermal resistance of 0.36 K/W in the nucleate boiling regime. Under the same conditions but with a hermetic lid on the package, an additional 2.78 K/W chip thermal resistance was measured. Vertically oriented chips could dissipate 2.5 to 3 W/cm² (chip area) more than horizontally placed chips due to bubble scrubbing. Even in the absence of conductive bumps the thermal resistance of the gas in the package cavity forces most of the heat to flow down through the substrate