Browse Prior Art Database

ALUMINUM SILICON CARBIDE PHASE CHANGE HEAT SPREADER

IP.com Disclosure Number: IPCOM000009338D
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-16
Document File: 5 page(s) / 203K

Publishing Venue

Motorola

Related People

John M. Waldvogel: AUTHOR

Abstract

An RF power module for telecom applications is typically built-up on a carrier plate, and multiple modules are attached to a heatsink to produce a power amplifier. In general, RF modules cannot be built-up directly on current heatsinks as the coeffi- cient of thermal expansion (CTE) mismatch between the RF circuit material and the heatsink material would yield prohibitively high thermally induced stresses. Therefore, the function of a carrier plate in this application is to match the CTE of the RF circuit and provide a good thermal path from the RF power devices to the heatsink. With the increase in power dissipation and the decrease in footprint for newer RF power devices, the temperature rise from the heataink to the device will increase to the point that the device maximum temperature can be exceeded.

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Developments Technical 0 M MOTOROLA

ALUMINUM SILICON CARBIDE PHASE CHANGE HEAT SPREADER

by John M. Waldvogel

BACKGROUND

  An RF power module for telecom applications is typically built-up on a carrier plate, and multiple modules are attached to a heatsink to produce a power amplifier. In general, RF modules cannot be built-up directly on current heatsinks as the coeffi- cient of thermal expansion (CTE) mismatch between the RF circuit material and the heatsink material would yield prohibitively high thermally induced stresses. Therefore, the function of a carrier plate in this application is to match the CTE of the RF circuit and provide a good thermal path from the RF power devices to the heatsink. With the increase in power dissipation and the decrease in footprint for newer RF power devices, the temperature rise from the heataink to the device will increase to the point that the device maximum temperature can be exceeded.

  Carrier plates can be constructed of solid graphite composite or Aluminum Silicon Carbide (AlSiC). These materials adequately match the CTE of the RF circuit material; however, a large tempera- ture gradient is established in the carrier plate when the heat input area (device footprint) is small because the thermal conductivity of the carrier plate is too low to provide an efficient thermal path to the heatsink.

SOLUTION

  The basic idea in this design concept is to pro- duce a hollow AlSiC carrier plate which is partia.lly

filled with a liquid and sealed. The dissipated heat in the device boils the contained liquid. The vapor travels by natural convection within the cavity to the side of the carrier plate which is attached to a heatsink. The vapor condenses at this surface, trans- ferring heat to the heats& The liquid returns to the heat input area, establishing a continuous heat exchange loop. With this/approach, the temperature drop across the carrier plate is reduced as the boiling and condensation processes occur at nearly the same temperature.

  The invention is similar to common heatpipe solutions which take advantage of phase change of a fluid. However, this concept is unique in that the fluid is contained within an structure constructed of a material with CTE matched to the RF circuit mate- rial. Heatpipes are traditionally constructed of met- als, to which RF circuits typically cannot be reliably bonded. The use of AlSiC material also allows the designer to mold details in the carrier plate for mechanical alignment and for heat transfer augmen- tation.

  Two variations of this cooling approach are shown. In Figure la, the heat dissipating device(s) is mounted on the outside surface for the phase change heat spreader. In F&me lb, the heat dissipat- ing device(s) is mounted inside of the phase change

heat spreader.

  The following numbered bullet items further describe the concept and highlights its unique ele- ments.

QMOMola.Inc.1599 226 June 1999

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