Thermal design of an advanced multichip module for a RISC processor

Multichip module (MCM) technology is attracting attention from designers who need high-speed interchip connections and a reliable package for their circuit applications. This technology is being applied to realize a 1-ns cycle time 32-bit RISC processor, using 50 GHz AlGaAs/GaAs heterojunction bipolar transistor (HBT) technology and triple-level full-differential current mode logic (CML), at Rensselaer. The processor is partitioned into multiple chips due to the high power consumption and low integration level of the technology. There are several key challenges associated with this module. It has to provide high-bandwidth (≤10 GHz) and high-density (≤40 μm pitch) interconnect, and dissipate nearly 250 W of power. Maximum heat flux on the MCM surface is 2.0×10⁵W/m². Poor heat conduction ability of GaAs chips make it tough to dissipate this heat. A methodology is developed to design a thermally stable module using a multilayer substrate with parylene as a low dielectric constant insulator, and fine pitch copper for high bandwidth lines. The design of the MCM and the thermal simulation results are presented.