Liquid encapsulated MCM package-thermal demonstration

Abstract

This experimental study was undertaken to demonstrate the capability of liquid-encapsulated system in its simplest form of implementation to cool a multichip module (MCM) package. The MCM package was made of kovar (dimension: 58/spl times/35/spl times/11 mm). Two silicon thermal test chips were bonded to an FR-4 board and housed inside the metal package. Tests were performed on both dry (no liquid filled) and liquid-filled packages, In the liquid-filled situation, either a pure dielectric liquid or a dielectric liquid mixture was employed. The MCM package was externally cooled by either free-air or forced-air (1.0 to 2.54 m/s of air flow). Heat transfer history from single-phase, through nucleate boiling, to film boiling was documented. The overall improvement for the liquid-filled package was 2-4 times compared to the dry package, due to the superior thermal properties of dielectric liquids compared to air. The maximum power dissipation in the liquid-filled package at 2.54 m/s external air flow was 18 W (based on junction temperature maximum of 125/spl deg/C). In the liquid-filled package, both the single-phase and boiling heat transfer were enhanced by varying the external boundary conditions from free-air to forced-air. The total power dissipation limit in the liquid-filled package is strongly influenced by the ability to condense the vapor in the package. By changing the boundary conditions from free-air to forced-air (2.54 m/s), the condenser (top lid) efficiency is raised, thus raising the maximum power dissipation by 2.5 times. Results also indicated that for any given external boundary condition, the allowable power dissipation did not vary much under different liquid conditions: nondegassed liquid, degassed liquid at slightly above ambient pressure, and degassed liquid at ambient pressure. When the system was sealed, increasing power in the package created large liquid pressure and raised its boiling point. The dies remained in the single phase regime and no boiling occurred.