Absfract-A multichip module (MCM) package was constructed that uses integral immersion cooling to transfer heat from the chips to a final heat transfer medium outside the package. The package is a miniature immersion cooled system with a pin-fin condenser that can be operated in either the submerged or vapor-space condensing mode. Sixteen chips were bonded on a 57-mm-square alumina substrate carrying copperlpolyimide thin-film interconnect. Tests of the thermal performance of the system show that it is capable of handling over 160 W power with chip thermal resistances, based on chip area, as low as 2 K-cmz/watt provided by the immersion cooled portion of the thermal path. Tests have been performed with the module fully powered and with subsets of the chips powered. The results indicate that the heat transfer coefficient is similar in all partially powered modes. Data taken with condenser temperatures ranging from 20°C to 50°C were used to obtain a performance map delineating the heat transfer regimes in the module and the liiits imposed by critical heat flux and condenser performance. The fluid used in the module enclosure was Fluorinert FC72.
Index Terms-Immersion cooling, multichip module, partially powered module, boiling performance map, vapor space.
The thermal conductivity of printed wiring boards depends -strongly upon the copper content of the signal and power planes. For boards having continuous power and ground planes, it is adequate to use simple lumped conductivities based on the volume percentage of copper. However, nearly order of magnitude errors can occur when this approach is used with boards having discontinuous or significantly perforated power and ground planes.We describe a statistical approach to modeling the copper connectivity in wired signal planes, based on the probability distribution of wire lengths in the X and Y directions. Using this approach we have constructed finite element models of six-layer printed wiring boards which approximate functional boards with a wide range of wiring density, via density, and power/ground plane perforations. The finite element results show that connectivity of the wiring, vias, and powedground planes plays an important role in establishing the board's average thermal conductivity. Although there is some scatter in the results, due to wiring details, we find that the percentage of copper in the printed wiring board can still be used as an initial indicator of thermal conductivity. We present an empirical fit to the models suitable for design applications.
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